Tropical Meteorology (Revised Edition) by G.C. Asnani - Chapter 1

CHAPTIiRl Special Features of Tropical Meteorology Contents 1.1

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History of Meteorology and Monsoon Studies (Pages 1-2 to 1-16) Ancient History; Meteorology after 1600 AD.; some notable meteorological characteristics of the tropics; Rossby waves on synoptic charts; the first numerical experiment on electronic computer; satellites enter the scene; joint impact of computers and satellites; numerical analysis; parameterization of physical processes; dynamical instability; trapped waves; atmospheric tides; pressure-wind adjustment; special data collection expedition; weather modifications; laboratory simulation of atmospheric processes; atmospheric pollution; energy; weather consciousness in society.

Special analysis for tropics (Pages 1-37 to 1-49) Object of the analysis; 24-hour change charts in other elements; additional levels for constant-pressure analysis; streamline analysis; asymptotes; some additional suggestions about wind analysis in the tropics; isotach analysis; wind analysis in frontal zones;

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Pressure~wind adjustment

(Pages 1-66 to 1-84) Definition of the problem; practical importance of the adjustment problem; outline of theoretical treatment; Obukhov's linearized theory; Obukhov's non-linear theory; simplified concept of linearized theory of adjustment; theoretical problem of adjustment and its practical importance; Obukhov's (1949) linearized theory; Obukhov's (1949) non-linear theory; Temperton' s (1973) simplified treatment of linearized theory;

Special features of tropics; Monsoons

(Pages 1-16 to 1-37) Tropical region; quasi-geostrophic approximation; temperature gradients; seasonality of weather; diurnal cycle; 1.2.1 Definition of tropical monsoon and tropical monsoon region 1. Introduction, definition of monsoon 2. Traditional definition of tropical monsoon region; new definition 3. SWAMP-1990 4. Characteristics of monsoon climate 5. Complexity in the understanding of monsoon 6. Histograms of monthly rainfall in north, central, and south America Rainfall pattern in India-Sri Lanka monsoon region; rainfall pattern in south and central America; rainfall in south and central America, north of the equator; VAMOS; 1.3

velocity and diabatic heating; synoptic-scale waves, planetary~scale waves; quasi-balance model of Stevens et aI., 1990; summary;

Scale analysis Cor tropics (Pages 1-49 to 1-66) Introduction; synoptic-scale migratory waves; magnitudes of vertical velocity and diabatic heating; planetary-scale quasi-stationary seasonal motions; magnitudes of vertical

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Atmospheric tides (Pages 1-84 to 1-97) Historical background; 1. Pressure observations at the surface; 2. Observations at higher level; 3. Seasonal variation of Pt and P2' 4. Outline of Chapman-Lindzen theory; 5. Laplace's tidal equation 6. Vertical structure equation 7. Boundary conditions M. Solutions dependent on forcing functions 9. Semi-diurnal and diurnal tides Comparison between Chapman-Lindzen theory and observations; remedy suggested for Chapman-Lindzen (1970) theory;

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Diurnal variation of precipitation

(Pages 1-97 to 1-110) Diurnal (24 hour) cycle of precipitation over tropical land stations; Synoptic-scale systems; Meso-scale systems; interaction between meso~scale and large-scale systems; 1.7.2 Diurnal (24 hour) cycle of precipitation and cloudiness over the oceans 1.7.3 Semi-Diurnal (12-hour) cycle of precipitation and cloudiness over land and oceans; 1.7.4 Summary of diurnal variation of precipitation in the tropics. 1.7.1

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Summary (Pages 1-110 to 1-114)

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1.1 History of Meteorology and Monsoon Studies

1.1 History of Meteorology and Monsoon Studies

Ancient History Weather has affected man in most of his activities. In every part of the world, the weather patterns have determined the traditional patterns of food, clothing, housing, agriculture, social festivals, etc. The results of some of the worst wars in the world have been significantly affected by the weather (Neumann, 1975). The D-day operations during World War II definitely influenced the course of the war. Naturally, man has always liked to know how the weather will change in course of a few hours, a few days, a few months and a few years ahead. More recently, questions are even being asked: "Is the whole pattern of climate on the earth changing? Is man's activity responsible for the anomalies of weather which we are witnessing? Can we do something about it?" Even in the absence of recorded history, we can safely state that ever since man started walking on this earth, he has always attempted to understand and to forecast the weather of tomorrow. In ancient literature, we have the evidence that at least 5,000 years B.C., the Rig Veda of India contains several references to the seasons of northwest India including the arri val and withdrawal of the monsoon. In the Yajurveda of India, there are references to different types of rainfall.The great scholar Panini of 5th century B.C., refers to the measurement of rainfall and assigns a unit for the same. In the 4th century B.C. Kautilya of India wrote his famous treatise on socio-economics, the "Arthashastra", in which he stresses the economic importance of the measurement of rainfall and prescribes units and methods of measurement of rainfall. He also indicates the amount of rainfall suitable for various crops in different parts of the country. In "Manu-Smriti" dating 2nd century A.D., it is stated that "the sun generates rain" (Adityat Jayate Vrishti). This has bcen adopted as the inscription in the official crest of the India Meteorological Department. The monsoon clouds were poetically described in the 3rd century A.D. in Sanskrit classic "Meghdoot" by the great poet Kalidas who hails the monsoon

cloud as messenger of love. In this work, the first day of the month of Asadha (middle of June) was given as the date of onset of monsoon over central India, which nearly coincides with the current normal date of onset of monsoon over central India as given in the publications of India Meteorological Department. In the 6th century A.D., the erudite scholar Varahmihira compiled "BrihatSamhita". In this compilation, he not only describes a raingauge and the wind vane but also gives detailed instructions for correct observations. He was also perhaps the first scholar to describe changes in seasons and associated rainfall with the changes in the behaviour of animals, birds and insects. The monsoon winds were utilized by the commercial sailors of ancient India, China, Arabia and Egypt for steering their ships on high seas. Recorded history gives evidence of how even the knowledge of local land-sea breeze saved Greece from a disastrous defeat at the hands of the invading Persian Navy of King Xerxes in 480 B.C. (Neumann. 1975). Thentistocles, the Greek naval commander so directed the hour of beginning of the naval combat and so arranged the course of the fighting that the heavy and clumsy Persian warships found themselves in the narrow straits of Salamis, off Athens, at a time of the day when Athens sea breeze made the waters choppy and the large unwieldy Persian vessels found it difficult to maneuvre in the narrow straits. As already planned, Themistocles ordered the nimble and easily maneuvrable Greek ships to attack the unwieldy Persian vessels just at this time and the Greek navy succeeded in annihilating the Persian armada and rescuing Greece from a possible destruction. Towards the middle of the fourth century B.C., Aristotle wrote the book "Meteorologica". This book, followed by the work of his pupil and successor Theophratus, showed the systematic scientific study of the atmosphere and the weather. They summarized all that was known about weather and winds at that time . This suminary remained the reference work for nearly two thousand years. The Arabs brought the contents of "Meteorologica" to the knowledge of medieval Europe. Dante's treatment of weather

1.1 History of Meteorology and Monsoon Studies in La Divina Comedia was based on those writings of Aristotle and Theophratus. In the early fourteenth century, William Merle, Rector of Driby in England, kept daily records of local weather for about seven years. With the Renaissance in Europe and the adventurous voyages of Columbus and others, the need for systematic meteorological information was keenly felt. Meteorology after 1600 A.D. The air thermometer was invented in 1600, probably by Galileo. His pupil Castelli standardized a raingauge in 1639. Another of his pupils, Torricelli, invented the barometer in 1644. At about this time, different forms of hygrometer and anemometer also appeared. In 1653, Ferdinand II of Tuscany arranged to establish a network of seven meteorological stations in northern Italy and four more stations outside Italy. Instrumentation, observation and experimentation in the laboratories and in the free atmosphere led to the deeper understanding and discovery of the physical laws of nature operating in the atmosphere. In 1659, Robert Boyle enunciated his famous pressure law pV =constant when temperature is kept constant. Then came Charles' Law relating volume and temperature of gas at constant pressure. Combination of Boyle's Law and Charles' Law gave the now well-known equation of state p a = RT. Halley (1686) presented a detailed and methodical account of the trade winds as observed in the tropical oceanic regions and also sought a common law governing these air motions. He rejected an earlier notion that due to its lightness, the air simply could not keep up with the earth's surface in its daily rotation. He attributed the northeasterly and the southeasterly directions of the trade winds to the tendency of the air to converge from north and south and rise up near the most strongly heated regions i.e. at the equator. Foneasons which are not clear, he further assumed that the cumulative effect of the afternoon tendency of the air to move towards the warmer west would outweigh the morning tendency of the air to move towards the east and hence a general component of trade winds from east to west.

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Hadley (1735) accepted the idea of Halley (1686) that solar heating maximum at the equator would lead to horizontal convergence of Northerlies and Southerlies and vertical upward motion near the equator but he rejected Halley's idea that motion towards warmer region would lead to a net motion also from east to west. He suggested that the absolute velocity of the earth's surface from west towards east is highest at the equator. Hence an air parcel moving towards the equator, either from north or from south and attempting to conserve its original absolute velocity from west to east would lag behind an observer sitting at the equator. Hence to observers fixed on the earth, air moving meridionally towards the equator would also appear to be moving zonally from east to west; hence the occurrence of northeasterly and southeasterly trades in the tropics. Quantitative calculations showed that if there were no other retarding forces operating, an air parcel starting from 200 N would attain an easterly component of 67 ms-I which was too high compared to the observations which showed wind speeds only of the order of 10 meters per second. Hadley attributed this lower zonal velocity to the operation of frictional forces. Hadley also rightly concluded that air converging horizontally towards the equator in the lower layers would rise up, then would be diverging and moving away from the equator in the upper layers. He postulated something like a closed meridional cell with upward motion near the equator and downward motion in the higher latitudes. It may be mentioned here that Hadley was not correct in assuming conservation of absolute velocity for.an individual air parcel; it should have been conservation of absolute angular momentum around the polar axis. The Meteorological Society of Mannheim started in 1780 and established a network of 39 weather observing stations (14 in Germany, 4 in U.S.A. and the rest in other countries), all equipped with comparable and calibrated instruments like barometer, hygrometer, raingauge and wind vane and also standard instructions for their use. Lavoisier in 1783 and Dalton in 1800 published their findings concerning the nature and composition of air. The genius Lavoisier,

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1.1 History of Meteorology and Monsoon Studies

who coined the word oxygen, fell victim to the French Revolution. The day after his execution, his mathematician friend, Lagrange observed; "It required but a moment to sever that head. Perhaps a century will not suffice to produce another like it" .

maintaining weather log books. On 14th August 1872, in Leipzig, there was a meeting of a number of leading meteorologists. They arrived at an agreement on standardised methods of observation, uniform set of weather symbols and methods of chart analysis. They prepared the ground for holding the First International Meteorological Congress in Vienna next year from 2nd to 16th September, 1873. 32 representatives of 20 governments who met in Vienna set up a Permanent Committee to stimulate and organize voluntary international co-operation and uniformity in observations and analysis. Buys Ballot was the first President of this Committee. In a slightly modified form, the permanent Committee continues till today with the name ofWMO'S Executive Committee. The Congress also proposed the formation of an International Fund for the establishment of Meteorological Observatorie's "on islands and at distant points of the Earth's surface". In September 1874, a decision was taken that there should be the publication of synchronous observations from 1st January 1875 by various national meteorological departments. This was about the time when national meteorological departments were organized in several countries. A number of them, including India and U.S.A., celebrated their centenaries around 1975. The International Meteorological Congress actively participated in the observational and analysis programme of the First International Polar Year (1882-1883). Towards the end of the nineteenth century, the noted dynamic meteorologist, V. Bjerknes, enunciated his famous circulation theorem;

The first systematic attempt at preparing a weather map appears to have been made by H.W.Brandes, in Leipzig, in 1820, using the meteorological data assembled by the Meteorological Society of Mannheim in 1783. Later, he prepared weather maps showing some of the storms which affected Europe in 1820 and 1821. Almost at the same time, W.C.Redfield of New York prepared the first series of charts showing the rotatory and translatory motions of the American hurricanes. Within the next twenty years, J. P. Espy of Philadelphia and Piddington and Reid of England were able to establish the existence of characteristic patterns of pressure, wind and weather associated with cyclones and anticyclones. They also formulated empirical rules for their development, movement and decay. But all these interesting findings were based on observations collected long after the occurrence of the event, far too late for forecasting. A silent revolution took place in the field of communications when Samuel Morse invented the electric telegraph and in sheer delight transmitted between Washington and Baltimore his famous message (1843); "WHAT HATH GOD WROUGHT !" The first weather maps based on telegraphic transmission of meteorological data were publicly displayed in Washington, D.C. in 1850 and in France in 1855. This display aroused public interest and also a demand for weather de = _" adp forecasting. dt 'Y , Necessity was felt for the formation of an international cooperative organization. The First clearly distinguishing between the barotropic International Meteorological Conference took fluid of the then classical hydrodynamics and the place in Brussels in August,1853, the countries baroclinic fluid like the atmosphere. being represented mostly by naval officers who Also, towards the close of the nineteenth realized the importance of meteorology in their century and in the beginning of the twentieth maritime operations of commercial or military century, the network of surface observatories nature. The conference emphasized the increased all over the globe, a few pilot balloon importance of international co-operation, observatories were started and even a few standardisation of observations and uniformity in meteorograph measurements were initiated.

1.1 History of Meteorology and Monsoon Studies Balloons filled with hydrogen would rise up due to buoyancy and be drifted horizontally by winds of varying speed and direction. Their visual tracking by telescopes would give a measure of horizontal winds at various levels. The sensitive barographs and thermographs attached to some of these balloons would trace curves of pressure and temperatures. Ultimately, these balloons would burst up in the atmosphere and descend down, along with their "precious payload" over cities, forests, rivers, valleys and oceans. These carried a request written in local language, to return the same to the head office of the national meteorological department at government cost plus a token prize to the person who returns the instrument. The curves traced by the barographs and the thermographs would then be deciphered. An important finding was that the temperature of the air does not continuously decrease as we go up. Above what is known as the tropopause, the temperature does not decrease as we go up but it increases up to a certain height. Incidentally, it has been subsequently established that the coolest temperatures in the whole atmosphere occur not near the poles but near the equator at the tropopause level, nearly 17 km above the sea-level. Meteorological observations and messages became "secret" possessions of each nation during the First World War (1914-1918). Observations increased during the war period but were not internationally exchanged during the period. Even inside each country, these were transmitted in heavily guarded secret meteorological codes. The experiences gained during the war from the improved network of observations led to the formulation of simple models of weather sequence in relation to the extra-tropical low pressure systems seen moving on the daily weather charts. Concepts of air masses which had been introduced in the middle of the nineteenth century became clearly defined; frontal model of Solberg and J. Bjerknes (the illustrious son of the illustrious father V. Bjerknes) became popular with the operational forecasters although the dynamical theory for its formation and movement was clearly in need of improvement. In the 1920s, pilot balloon observations with visual telescopes (theodolites) were started

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on a routine basis in several countries. It is worthwhile to mention here the name

of Sir Napier Shaw who embarked on the ambitious task of completing a comprehensive "Manual of Meteorology". This was eventually published in four vol urnes over the years 1926-31, including a complete rewrite of volume 4 that had first been published in 1919. He followed this "heavyweight" publication with a somewhat lighter-weight popular text, "The Drama of Weather" (Shaw, 1933) which was published in 1933, when Shaw was 79, and went into a second edition six years later. Napier Shaw enjoyed weather and shared his enjoyment with others. In the thirties, the occasional pressure and temperature observations in the free atmosphere with meteorographs gradually gave place to routine measurements with radiosonde instruments which telemetred the observations while the balloon was rising with its instrumented package. Norwegian school from which had come V. Bjerknes, J. Bjerknes, Solberg and other meteorologists continued to lead in the field of theoretical meteorology. In 1939, C.G.Rossby came up with a simple and elegant model of large-scale atmospheric waves, now-a-days called Rossby Waves. Also see section 5.1. The Second World War which started in 1939 again brought about a black-out in the international exchange of meteorological observations but gave a great impetus to the expansion of meteorological observations and services within each country. The upper air observations of wind, pressure and temperature were indispensable for planning of air force operations of bombing and transporting military cargo. Whenever there have been national and international disasters like wars, floods, famines, the Governments of the affected countries have generally realized that more investments in meteorological activities are worth the money put m. Some Notable Meteorological Characteristics of the Tropics : As stated earlier, a number of meteorological departments had started functioning around 1875, some of these being in the tropical region. The chief forecasters in these

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1.1 History of Meteorology and Monsoon Studies

tropical meteorological services usually came from extra-tropical regions with their experience of frontal systems of analysis and forecasting. They saw fronts everywhere, even at the centres of tropical cyclones. It was after the Second World War, in the late forties and early fifties of 20th century, thatthe extension of frontal systems deep into the tropics on a routine basis was seriously questioned and nearly given up. Nevertheless, uplo the time of World War II, significant differences were recognized in the structure and behaviour of tropical weather systems as compared to extra-tropical weather systems and special techniques were devised to handle the same. Some of these differences are sketched below: a) 24·hoor Pressure Tendency: Soon after the introduction of barometer in tropical latitudes, it was realized that daily pressure variations in tropical regions were quite different from those in extra-tropical regions. In the extra-tropical regions, the barometric variations were essentially caused by the successive passages of extra-tropical cyclones, their period being of the order of 5 days and amplitude being of the orderof25 millibars(hPa). With kitchen barometers, one could almost forecast the pattern of local weather that was coming. On the other hand, in the tropics, there was a very regular double pressure wave pattern being observed every day, showing very little variation from one day to another. This pressure wave seemed to have little or no relationship with the local weather. What seemed to be of some use for weather forecasting was the small residual pressure variation which was obtained after eliminating the relatively large regular daily double pressure wave in the tropical region. In practice, this was obtained by getting 24-hour pressure tendency at each tropical statiun and plotting 24-hour pressure tendency charts for tropical region. b) Departure-from-nonnal charts : It was also found that unlike extra-tropical regions, the seasonal quasi-stationary pressure patterns and flow patterns dominated the daily synoptic charts. These caused typical seasonal weather patterns of dry seasons, wet seasons and transitional seasons. People of the region were familiar with seasonal weather. What the clients

of meteorological services in these regions wanted were the departures of rainfall and other weather elements from the 'seasonal', 'normal' or 'long-term-average' conditions. The necessity

of preparation of anomaly or 'departure-from -normal' charts was immediately felt. At first, normal charts were prepared for each calendar month. Subsequently, the period was reduced to 5-day unit (pentad) in respect of some of the elements like rainfall and surface pressure. Such charts were found very useful for separating the dominant seasonal quasistationary waves from the relatively feeble migratory waves, the latter being associated with deviations from the seasonal weather. c) Tropical Cyclones: These were differnt from the extra-tropical cyclones, being less frequent, smaller in horizontal extent and with comparatively shorter span of life but much more intense and much more devastating than the extra-tropical cyclones. The visit of a tropical cyclone was a disaster, some times taking as many as 200,000 human lives in one sweep. A vigilant warning system was devised, as much as could be achieved through the technology of those times and the administrative capabilities of the respective regions. Climatology of these tropical systems in terms of their tracks was compiled for different regions and different seasons. Surface features of these systems were well known. d) Easterly Waves : It was known that while the middle-latitude stations were visited by migratory waves coming from the west, the tropical stations were visited by migratory waves coming from the east. It was also appreciated that unlike the westerly waves of the middle latitudes, the easterly waves of the tropical latitudes were weak compared to the seasonal quasi-stationary waves of the region and as such had to be identified through charts giving elements like 24-hour pressure tendency, departure of surface pressure from normal, fluctuations in rainfall and cloudiness and through minor but significant changes in winds of the lower troposphere. The end of Second World War was the beginning of significant developments in the field of meteorology in general and tropical meteorology in particular.

1.1 History of Meteorology and Monsoon Studies Rossby Waves on Synoptic Charts : Rossby and collaborators (1939) were the first to have identified that the wave patterns seen on the routine synoptic weather charts belonged to a particular class of waves. This class of waves had been known earlier in classical hydrodynamics as Hough's oscillations of the Second Class in a barotropic fluid but it had not been appreciated until Rossby pointed out that the daily weather charts showed their existence in the atmosphere and that these waves were the most important ones for meteorological forecasters. Reference is invited to section 5.1 in Chapter 5. Chart analysis helped Rossby to score over others. He also emphasized the importance of vorticity (relative vorticity and coriolis parameter) in the dynamics of large-scale atmospheric motions. His treatment of the dynamics of this type of waves was simple, straight-forward and easy in application. Rossby's (1939) paper created a new line of thinking. A very simple form of vorticity equation seemed to explain the movement of migratory cyclonic storms and quasi-stationary planetary-scale waves seen on the daily charts and also on the time-averaged charts of the middle latitudes. Circulation theorem and vorticity equation are intimately related to the gradient wind equation and the transport capacity of curved isobaric channels. In the very first issue of the Journal of meteorology (1944), Bjerknes and Holmboe attempted to explain the structure and development of extra-tropical cyclones on the basis of vorticity equation and transport capacity of curved isobaric channels. In the text book "Dynamic Meteorology" by Holmboe, Forsythe and Gustin (1945), one finds systematic exposition of this idea in chapter 10. This line of thinking was substantially different from the frontal theory of extra-tropical cyclones which was in the field before the Second World War. The development of the extra-tropical cyclones was explained in terms of phase difference between the pressure wave and the temperature wave. With the advent of Numerical Weather Prediction, synoptic meteorology has been confined to back benches. As such, synoptic meteorologists are an endangered species of meteorologists. It needs to be stated that without familiar contact with

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daily synoptic charts, the theoretical meteorologists including numerical modelers are likely to miss the simple and correct interpretation of some of the theoretical or computer results. It is advisable to have a good combination of synoptic meteorology, dynamic meteorology and numerical modeling. Before Rossby (1939) discovered and got the world renown and credit for identifying Rossby waves on the daily synoptic charts, he had fortunately got a sound background in dynamic meteorology and also experience of chart analysis, and organizing and teaching operational synoptic meteorology (Persson and Phillips, 2001 : "C.G. Rossby's Experience and Interest in Weather Forecasting." Bull. Amer. Met. Soc., 82., 2022-2026). A well-advanced theoretical treatment of what are now known as Rossby waves, had been given by Hough (1897, 1898), Lamb (1932) and even a couple of years earlier by Haurwitz (1937), but they did not know the presence of these waves in the atmosphere, and seen on daily weather charts. Rossby won the credit. After the Second World War: In 1947, there appeared two notable papers, one by Charney (Journal of Meteorology, 1947) and the other by Sutcliffe (QJRMS, 1947), both being landmarks in the theory of development of extra-tropical cyclones. Both papers emphasized the importance of the vorticity equation and the associated divergence and vertical motion in different sectors of the extra-tropical wave pattern. Charney's treatment was more general and it followed earlier classical method of analysis of hydrodynamic instability, now applied to synoptic-scale systems of the atmosphere. The pre-war idea of narrow frontal zones of the extra-tropical latitudes being the cause of formation and development of extra-tropical cyclones was replaced firmly by the new idea that the large-scale broad and extensive westerlies of extra-tropical latitudes were baroclinically unstable; this baroclinic instability gave rise to intensification of an initially weak wave perturbation. Deepening of the wave brought in juxtaposition, cold air masses from the polar regions and warm air masses from the sub-tropics and caused the fronts. Thus, the fronts were not the cause of extra-tropical cyclones but the resuIt

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1.1 History of Meteorology and Monsoon Studies

of the deepening of the waves. The work of Rossby. Charney and Sutcliffe opened a gateway towards modern thinking and action on numerical weather prediction. It is relevant here to refer to the work of L. F. Richardson who made the first effort, under very strange circumstances, to make a weather prediction, by numerical method. Vision of L. F. Richardson: In the introduction to his famous book "Weather Prediction by Numerical Process", Richardson wrote on October 10, 1921: "The investigation grew out of a study of finite differences and first took shape in 1911 as the fantasy ... Serious attention to the problem was begun in 1913...The arithmetical reduction of the balloon and other observations, was done with much help from my wife. In May 1916, the manuscript was communicated by Sir Napier Shaw to the Royal Society. The manuscript was revised and the detailed example of Chapter IX was worked out in France in the intervals of transporting wounded in 1916-1918. During the battle of Champagne in April 1917 the working copy was sent to the rear, where it became lost, to be re-discovered some months later under a heap of coal... The whole work has been thoroughly revised in 1920, 1921". He worked out pressure change for 6-hour period 0400-1000 GMT on 20th May, 1910 over central Germany. He obtained a rise of 145 mb (hPa) in 6 hours, whereas in fact there was practically no change. Assuming that a computer might work about ten times as fast as he had done, he estimated that it would need 64000 computers to complete the calculation of the new distribution for the whole globe just before the time to which it referred. He remarked: "Perhaps some day in the dim future it will be possible to advance the computations faster than the weather advances and at a cost less than the saving to mankind due to the information gained. BUT THAT IS A DREAM". In 1961, Prof. Charney, in acknowledging the award to him, by the Royal Meteorological Society, of the Symons Gold Medal (its highest honour), remarked: "...to the extent that my work in weather prediction has been of value, it has been a vindication of the vision of my distinguished predecessor, L. F. Richardson".

At this stage, we can disregard the error in Richardson's forecast. The cause of the error was more or less correctly diagnosed by Richardson himself. Such a result was due to the method used by Richardson for computing pressure changes from the reported pilot balloon observations in the lower atmosphere. But a beginning had been made. A person of great vision had gazed into the future. It was the dream of a great man. Richardson himself called it a dream. In a relaxed mood after hard work, he wrote : "After so much hard reasoning, may one play with a fantasy? Imagine a large hall like a theatre, except that the circles and galleries go right round through the space usually occupied by the stage. The walls of this chamber are painted to form a map of the globe. The ceiling represents the north polar regions, England is in the gallery, the tropics in the upper circle, Australia on the dress circle and the Antarctic in the pit. A myriad computers are at work upon the weather of the part of the map where each sits, but each computer attends only to one equation or part of an equation. The work of each region is coordinated by an official of higher rank. Numerous little "night signs" display the instantaneous values so that neighbouring computers can read them. Each number is thus displayed in three adjacent zones so as to maintain communication to the North and South on the map. From the floor of the pit a tall pillar rises to half the height of the hall. It carries a large pulpit on its top. In this sits the man in charge of the whole theatre; he is surrounded by several assistants and messengers. One of his duties is to maintain a uniform speed of progress in all parts of the globe. In this respect he is like the conductor of an orchestra in which the instruments are slide- rules and calculating machines. But instead of waving a baton he turns a beam of rosy light upon any region that is running ahead of the rest, and a beam of blue light upon those who are behindhand. Four senior clerks in the central pulpit are collecting the future weather as fast as it is being computed, and despatching it by pneumatic carrier to a quiet room. There it will be coded and telegraphed to the radio transmitting station". When we look at a computer console of 1970s and 1980s in the midst of a series of

1.1 History of Meteorology and Monsoon Studies machines and magnetic tapes making whispering sounds around, one feels surrounded by Richardson's "assistants and messengers" assiduously performing their duties to maintain a uniform speed of progress in all parts of the world. Advanced Parallel Computing Systems of 1990s, though more silent, also give similar signals. The First Numerical Experiment on Electronic Computer: After Richardson's first numerical computation experiment by hand, extending over a period of a few years, the next experiment on record was a barotropic forecast made by Charney and his collaborators in March, 1950 on the first major electronic computer ENIAC. A beautiful account of this experiment is given by Platzman(l979) in his V. P. STARR Memorial Lecture at M.LT. in October, 1978 and published in April, 1979 issue of the Bulletin of the American Meteorological Society. About 20 important figures in the field of Meteorology including Rossby, Starr, Von Neumann, Wexler, Haurwitz, Namias and Charney met on Aug. 29 and 30, 1946 at the Institute for Advanced Study in Princeton, New Jersey and formulated a project with the objective of investigating "the theory of Dynamic Meteorology in orct"er to make it accessible to high-speed, electronic, digital, automatic, computing". The Minutes of the Meeting refer to "some rather abstract problems suggested by Dr. Charney". This meeting was a sequel to Von Neumann's proposal, given a few months earlier, to create a Meteorological group within Electronic Computer Project for re-assessment of the Meteorological theory so as to make weather forecasting possible with the help of Electronic Computer. On the first Sunday of March, 1950, a band of 5 meteorologists (Charney, Fjortoft, Freeman, Smagorinsky and Platzman) started a scientific experiment in Meteorology, combined with vision and hope. The work started at 12 p.m. Sunday, March 5, 1950. It continued round the clock, 24 hours a day, for 33 days and nights, with only brief interruptions. Charney had maintained somewhat detailed log book recording, day by day, the stages of progress and regress in the ENIAC operations, interspersed with occasional

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expressions of gaiety and anguish. Rossby was feeling somewhat excited about the future possibilities which were being opened up by the new developments in dynamical Meteorology and Electronic Computing. In a letter dated 8th May, 1949, to Platzman, he wrote: "It seems to me that we now must go on... to a systematic test and extension of Charney's method so as to get rid of the horrible subjectivity which still characterises all, or almost all forecast efforts... "I must confess that I have an extremely strong feeling that we are standing at the threshold of a new era in Applied Meteorology and that we must push this line to the point where it can be put in general operation..." Rossby himself visited the ENIAC complex while the experiment was progressing and saw for himself the progress as well as the anguish of the experimenting. Sometimes, they felt delighted at the progress of computation and their success in taking good decisions to correct errors of computer coding; sometimes they felt distressed to see the errors of computation growing near some region of the boundary and the handicaps of a new computing machine. Finally, they felt happy and delighted to see a surprisingly good 24-hour forecast from the map of 31 st January, 1949. The barotropic model experiment had been successful. A new ground had been covered. With hope and confidence, they could look upward to scale the higher heights inviting them ahead. Climatology undergoes changes in Scope and Content: The subject of climatology underwent a major qualitative change. Until late forties, climatology consisted mainly of collection of data and their organization in terms of averages, with a little emphasis on standard deviations. Phillips' (1956) experiment of General Circulation was a turning point in the study of climate; now we could simulate some features of the climate of the earth itself. Subsequent General Circulation Experiments of the 1960s and 1970s greatly increased the degree of success in climate simulation. Even seasonal cycles could be simulated, along with broad features of the well-known summer monsoon of the southeast

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1.1 History of Meteorology and Monsoon Studies

Asia. At the same time, it became apparent, what had been earlier conjectured, that ocean was exerting a continuous, persistent, significant influence on weather over periods more than a few weeks. It was considered not only desirable but almost essential to incorporate ocean-atmosphere interactions in the G. C. Models. This meant not only greater demand on the memory and speed of the computers but also familiarity with the science of oceanography. Meteorologists who are experts in the science of the atmosphere are generally much less familiar with the science of the oceans. Although oceanography is being introduced in the curricula of many universities along with courses in atmospheric science, yet the familiarity of meteorologists with oceanography is much less than is required for management of oceans in the G. C. models. Rudiments of oceanography are getting introduced into G. C. models. Ocean-atmosphere coupling in G.C. models is helping in the simulation of climate several thousand years before present, as also the likely changes in climate several thousand years hence. CO2 increase has set the ground for speculations, somewhat backed by G. C. model simulations about the climate changes likely to take place in the coming century. In dealing with climate of the past several millenia and future millenia, particularly after the general support of astronomical theory of climatic changes has suddenly underlined the necessity of associating scientists of other disciplines like atomic physics, geology, biology and chemistry with the SUbject of climatology. Detailed analysis of climatic parameters brought a substantial change in the theory of mixing processes in the atmosphere. Turbulence in non-rotating fluids is yielding place to geostrophic turbulence in rotating fluids. The classical concept of cascading of energy into smaller and smaller scales of motion has certainly given place to energy going not only to smaller and smaller scales (positive viscosity) but also at the same time going into larger and larger scales of motion (negative viscosity). This development has all the potentialities of influencing human thought and action. The concept of late nineteenth century that we are progressively going into a state of increasing random motions,

what is often referred to as a state of "disorder" is yielding place to the concept of orderliness in the apparent disorder in the universe, tendency towards organization into meaningful and aesthetical1y beautiful patterns, as if there is beauty and its appreciation at the back of the universe. When this concept gets wider acceptance, as it is destined to, perhaps after a rude shock from the philosophy of disorder, meteorology would have made a major contribution in influencing not only man's external environment but also his internal make up. Negative viscosity is going to have a very positive influence on human history. Climate is recognized as an important element of environment. Climate impact studies have been adopted by UNEP as part of its important programmes. Sahel disaster of early 1970s, worldwide weather anomaly of 1972, fear of early 1970s that the earth might slip into an ice age, necessity of tuning agricultural operations to meet the food requirements of growing population in the world, the rise of oil prices forcing the world to look for alternate sources of energy (rain, sunshine and wind) have all raised the status of climatology. Climate is not only to be suffered or tolerated but can be used and should be used as a resource to be pressed into the service of mankind. Just as water should not be al10wed simply to caUSe flood and to flow into the oceans but it has to be stored and used for irrigation and energy; similarly, strong winds are to be harnessed for smal1-scale industry. Above all, the energy received from the sun is to be used for production of usable energy. The deserts of the world may one day become the greatest power-generating places of the world, converting the day-time scorching sunshine into life- saving energy. Satellites Enter the Scene: When the first Sputnik went up in space in 1957, it was a thrill for many, challenge for some but the beginning of a new stage for human thought and action. Taking of cloud pictures was the immediate obvious application of this costly programme. For more than a hundred years, meteorologists had seen rough pictures of cloudiness on the daily synoptic weather charts. Now one could view a substantial part of the earth, with one look, and also the clouds

1.1 History of Meteorology and Monsoon Studies organized in various patterns-beautiful, enchanting, challenging. With polar-orbiting satellites we could get pictures of cloud around the whole earth at different times. Their path was made sun-synchronous sO that at every point on the earth we could get at least two observations at about the same local time every day. In the beginning when only visible range TV cameras were available on the satellite, cloud pictures could be taken only during the day light portion of the earth. Within a couple of years, infra-red cameras were mounted on the orbiting satellites, infra-red photographs were compared and calibrated with the visible-range photographs, colour scheme suitably adjusted so that infra-red picture could be read as conveniently as visible-range picture. Now cloud pictures could be obtained in the infra-red range over the dark night portion of the earth as conveniently as in the visible range over the daylight portion of the earth. Multiple-channel sensing was soon introduced. Using the differences in transparency of the atmosphere in different wave-lengths due to -different absorption properties of the atmospheric constituents, it became possible to measure parameters of the atmosphere which one hardly thought possible to measure, a few years back. One can now measure, with reasonable confidence, vertical distribution of temperature, ozone, sea-surface temperature, total water vapour content in a vertical column, growth rate of clouds, horizontal speed of clouds, etc. Experiments are nearly successful to infer the rate of precipitation from the clouds, soil moisture content of the earth surface, roughness of the sea-surface and hence the strength of the surface winds over the ocean, vertical distribution of the aerosols, etc. The polar-orbiting satellites have been joined by geostationary satellites. Standing at a height of about 36000 km above the earth surface, the satellites look at nearly half the surface of the earth round the clock, all days of the year, in various channels. Five geostationary satellites placed around the equator and two orbiting satellites together can watch the full earth day and night, with reasonable accuracy. In less than five decades, their achievements have been remarkable. One can hardly imagine the limit of

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the achievements of the satellites during the decades to come. Joint Impact of Computers and Satellites: We are living in the age of fast computers and of satellites. We have seen these two developments from their beginnings. It is thrilling to see the transition in the field of Meteorology from pre-1945 stage to the present one. Before the Second World War, one could hardly imagine that Meteorology was going to see such vast and rapid progress. The numerical modelling, made possible by computer facility, has inspired many young meteorologists to seek methods of exact mathematical understanding and forecasting variations in meteorological systems. For this, there was a need for global observing system, to cover vast areas of the oceans, the deserts, forests, mountains and land areas where it was difficult to organize regular meteorological observations. Satellites came on the scene in time to assure the

meteorologists that there shall be no dearth of observations. Numerical Analysis: The numerical work on the computers was not as easy and simple as it was first thought to be. The problems of errors due to replacement of differential coefficients by finite differences, although somewhat known earlier, were better

appreciated after seeing the results of fast repeated numerical computations on the digital computers.

Meteorologists soon became familiar with earlier developments of methods of numerical analysis and themselves contributed to the methods of numerical analysis by devising numerical schemes which could conserve some of the main integral properties of differential equations and differential coefficients. 4·dimensional Data Analysis : Quick reception, vastness of data coverage and continuous (asynoptic) observations by satellites underlined the necessity of designing techniques for objective analysis of synoptic as well as asynoptic data. If untouched, the asynoptic data create something like shock waves in the computer prediction model. This problem of4-dimensional analysis (3-dimensions of space and the additional fourth dimension of time) has been more or less successfully tackled by the meteorologists.

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1.1 History of Meteorology and Monsoon Studies

Parameterization of Physical Processes: Digital computers, however big and fast, have to use a grid system in space. In the real atmosphere, there will always be subgrid physical processes which are too small in spatial extent to be directly caught by the computer model grid-system. These sub-grid scale processes have either to be ignored or incorporated through some approximate artifice. This latter method of incorporating sub-grid scale physical processes in an approximate way in terms of parameters available on the grid scale is called parameterization of physical process. It has three major areas; parameterization of radiation, cloud condensation and boundary layer processes. Quite a bit of success has been achieved in parameterizations but a good deal more still remains to be done. Dynamical Instability: Atmospheric systems grow and decay. Scientific understanding of the process requires analysis of dynamical instability, the dynamical process which leads to the growth of small perturbations into major meteorological systems. Great advances had been made during the 19th century through linearized theory of instability of simple physical systems. The problem of atmospheric systems posed problems of the following types: a) Rotation of the earth b) Continuous variation of density of the atmosphere in space and time c) Sphericity of the earth and d) Complicated physical processes. Remarkable progress has been achieved in developing theories of barotropic instabilities, baroclinic instabilities, inertial instability, Conditional Instability of Second Kind (CISK), shear instabilities of Kelvin-Helmholtz type for small-scale motions, etc. Originally starting with linear analysis, advances have since been made in development of non-linear theory of finiteamplitude perturbations. Borrowing from other physical sciences, meteorologists have recently made a good start in interpreting the results of earlier studies on instabilities in terms of over-reflection. Meteorologists have also started developing concepts of pulse asymptotics to explain the observed preferential areas of cyclogenesis.

In this stability analysis, meteorologists have generally adopted analytical techniques. When the problem becomes too difficult to be tackled analytically, computer facility has been pressed into service to perfonn time integration and see the growth of perturbations in time. Trapped Waves: In 1940s, attention was given to zonal propagation of Rossby-type waves. This continued during 1950s with some attention to the influence of orography in generating waves which had also substantial component of movement in the vertical. In the early and middle 1960s, attention was given to trapping of wave energy in the vertical and also in the meridional directions. It was also realized that gravitational waves set up by orography and differential heating of the earth-surface contributed towards the large-scale energetics of the atmosphere. The concepts of Kelvin waves and mixed Rossby-gravity waves trapped in the nearequatorial region were developed. An important discovery was made in 1960 - the discovery of QBO in the lower tropical stratosphere. Within a few years, considerable work was done on observations in the troposphere and stratosphere to detect the vertical and horizontal extent of QBO. Spectral analysis technique for meteorological time series came handy. Surprisingly, QBO was detected almost in every meteorological element, practically throughout the globe, in the troposphere as well as in the stratosphere, although the QBO found in the winds of the lower tropical stratosphere remained the dominant signal of the phenomenon. The theory of trapped waves also came handy to explain the large-scale features of QBO in the tropical atmosphere. During late 1960s, a quantitative theory of QBO was offered along with numerical simulation of QBO in the tropical stratosphere. Ever since that time, vertical and meridional trapping of Rossby wave energy has been regarded as an important phenomenon in the atmosphere. The theory of waves has become an important subject by itself in atmospheric sciences. Atmospheric Tides: Ever since the first barometric observations were taken in the tropics, the semi-diurnal pressure wave has fascinated many

1.1 History of Meteorology and Monsoon Stodies

scientists in meteorology and other allied subjects. Of all the atmospheric phenomena, the semi-diurnal pressure wave is perhaps the most regular, precise and steady phenomenon. Sensitive barometers have been able to detect it even within the field of a tropical hurricane! It stands majestically in all tropical barograms. Some of the best mathematicians and hydrodynamicians which the world has known during the last more than hundred years, have tackled this problem. Still there are doubts whether the problem can be taken as solved. Seeing the extreme regularity and similarity of this phenomenon to that of the ocean tides, the scientists called this as a phenomenon of atmospheric tides. Tht! theory of ocean tides was applied to atmospheric tides and a search began for an equivalent depth of the atmosphere for which the period of oscillation would be 12 hours. Kelvin (1882), hypothesised that if the atmosphere has a free period of oscillation very close to 12 hours, then the sun-generated semi-diurnal gravitational tide in the atmosphere would get enhanced about 70-fold by resonance and we would have then resolved the problem of semi-diurnal pressure wave in the atmosphere. Research began to see if we could find an equivalent depth of 7.84 km for the atmosphere. In 1885, the Krakatao eruption caused an atmospheric wave which was so powerful that it could travel on the earth-sphere right upto the antipode and come back to Krakatao and travel back. Computations for this wave suggested that the atmosphere has also another equivalent depth of lOA km. It is interesting to read the research papers of outstanding mathematicians like Margules, Jeffreys, Bartels and Taylor, arguing with one another about the correct value of equivalent depth of the atmosphere. In 1936, Taylor said that there is a double infinity of equivalent depth values for the atmosphere, one pair for each vertical profile of temperature of the atmosphere. In 1937, Pekeris came out with a brilliant calculation to show tl'at for the vertical profile of temperature known at that time, there were two equivalent depths 7.84 and lOA km! This appeared to solve all the problems of the semi-diurnal pressure wave and to confirm Kel vin' s hypothesis of 1882. Till late I940s, Taylor- Pekeris theory of resonance was

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accepted as a satisfactory theory for the observed semi-diurnal pressure wave. Post-war observations with new techniques of atmospheric sensing and re-calculation of equivalent depths with different plausible vertical profiles of temperature aroused serious doubts about the validity of Taylor-Pekeris resonance theory. The theory of trapped waves along with availability of computers made it relatively easy to show that an alternate to resonance theory was possible. Chapman-Lindzen theory soon established itself in late 1960s and early 1970s. This theory lays emphasis on thermal heating of the atmosphere through great depths, treating sun's gravitational tide and the resonance altogether unimportant. Here is a great contrast. The Taylor-Pekeris theory which was held in very high esteem at one time has been totally rejected about three decades later. It is not altogether impossible that Chapman-Lindzen theory would also receive severe jolts in time to come. Pressure-Wind Adjustment: After the war, interest in the tropics increased. At first, it was thought that quasi-geostrophic approximation would have no place in the tropics. But closer studies revealed that about 5 degrees of latitude away from the equator and beyond, quasi-geostrophic approximation was not too bad; it was of great help in drawing the isobars on sea-level charts and the height contours on constant pressure charts at higher levels. There is some adjustment between the pressure field and the wind field on synoptic and planetary scales in the tropics outside 5 degrees from the equator, so that stream-lines and pressure contour lines run nearly parallel to one another although not as much as in the middle latitudes. Fundamental question of pressure-wind adjustment was examined for all latitudes. Rossby's pioneer work done in late I930s was extended considerably by Obukhov in late 1940s, which showed that in the long run outside the near- equatorial region, pressure and wind fields, tend to remain in geostrophic balance. Initially, if the fields, on synoptic and larger scales, are unbalanced geostrophically, then either the pressure field adjusts itself to the given wind field, or the wind field adjusts itself to the given pressure field or

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1.1 History of Meteorology and Monsoon Studies

both undergo considerable adjustments to get into geostrophic balance. In the extra-tropical latitudes, it is more likely that winds will adjust themselves to get into geostrophic balance with a given pressure field while the reverse is the case in the tropical latitudes. This conclusion has greatly influenced the methods of meteorological analysis in the tropics as distinguished from those in the extra-tropics. Wind field has to be analysed and that too with great care in the tropics. This theory of pressure-wind adjustment has also found applications in 4-dimensional analysis of meteorological data.

Special Data Collection Expeditions: After the second world war, for development and validation of quantitative models of atmospheric processes which are considered important for weather phenomena on different scales of space and time, a number of national and international ventures have been undertaken to collect special data over many parts of the world. Some of the important projects in this respect have been the Thunderstorm Project, BOMEX, HOE, GATE, FGGE, TOGA. AMEX, TAMEX, FIRE and INDOEX. These experiments, particularly since 1980s, have highlighted the importance of oceans and other water-surfaces. It has also been appreciated that climate variability should be tackled at international level with dynamical models. These ventures have provided valuable data which have contributed appreciably towards the development of Meteorology in general and Tropical Meteorology in particular. Additionally, these experiments have offered an occasion for arranging close co-operation between the different Governments in collection, communication, analysis and archiving of the data and finally in their utilisation to improve the quality and to extend the period of weather forecasts. Some tele-connections have been discovered and more are being discovered to show that weather over a region is connected with

weather over other regions separated from one another by several thousand kilometers in space and by several months and even years and decades in time. This discovery has clearly indicated the possibility of forecasting of major anomalies in weather a few months and years in advance, by use of statistical correlations. When

we have a full set of global data of the atmosphere and of the ocean surface, then with the help of modern computers, it would be possible to understand the physical processes underlying the statistical correlations, and tele-connections. Weather Modification: In the late 1940s, it became clear that by artificial seeding of clouds, it is possible to accelerate the growth of clouds and induce them to give rain locally. This generated commercial interest in rain-making, with aggressive salesmanship. Several private agencies started making money through rain-making. Difference arose between the scientists and the commercial interests. Scientists asserted that time was not yet ripe for commercial operations in rain-making but the commercial interests felt that they could use the new technology of rain-making for the benefit of those that were in need of rain water. The commercial operations provided an opportunity to realise that for sound advancement of the subject, even for subsequent commercial operations, it was essential to have beller understanding of the physical cloud processes operating in the atmosphere. This had a negative contribution also. The whole subject of rain-making got into disrepute. Legal complications also arose due to legal claims from

those who thought that by artificial seeding of clouds on the up-stream side, somebody had deprived them of their normal share of rain which would have come to them by natural growth and downstream movement of the clOUds. The same technique of cloud-seeding has also been applied by non-commercial government agencies in SOffie countries for hail-suppression and for fog- dispersal over busy air-fields. This artificial seeding of the clouds also helped in realising that in addition to the cold process of rain-drop growth, the atmosphere also showed evidence that considerable precipitation developed in the tropics inside clouds which did not reach the freezing level at all. This was a new scientific discovery, particularly for rain in

tropical latitudes. Efforts have subsequently been made to study the possibilities of rain-fall enhancement by a different type of seeding of warm clouds in the tropics. In the search for physical processes, which

1.1 History of Meteorology and Monsoon Studies

control the growth and precipitation of rain drops, attention has also been given to the role of aerosols in the atmosphere on one hand and of atmospheric electricity on the other. The subject of aerosols has advanced considerably during the last three decades. The subject of atmospheric electricity has also progressed but at much slower pace due to inherent difficulties in measurement of electrical parameters in the atmosphere under natural conditions on a scale small enough to reveal the conditions under which rain-drops develop different electrical characteristics. Realizing the importance of cloud electrification, the American Meteorological Society devoted the whole August 1994 Issue of Monthly Weather Review (Vol. 122, No.8) to the subject of Thunderstorm Electrification and Lightning. Questions are being asked: a) Can there be sufficiently large rain drops forming below the freezing level ("Warm Rain" Problem)? b) Why do we get copious monsoon rainfall in the tropics on several occasions, without

conventional lightening and thunder? These questions will be touched upon in Chapter 4 on Physics and Dynamics of Monsoon and in Chapter 10 under Thunderstorm. Laboratory Simulation of Atmospheric Processes:

The science of meteorology has registered progress also in the field of laboratory simulations of atmospheric processes ranging in dimensions from the diameter of a growing rain-drop (of the order of a fraction of a millimeter) to the size of planetary scale motions of Rossby waves. Fultz- Hide experiments constitute an important land-mark in the history of laboratory simulations of Rossby waves. Between the scales of the rain-drops and Rossby waves, we have recently seen production of tornado-type vortices in the laboratory. All these models help the Meteorologists to develop better understanding of the atmospheric processes. Attempts are simultaneously made to produce numerical simulations of the results obtained in the laboratory. Another advantage of laboratory experiments is to generate and study phenomena which are seen on very few occasions in the atmosphere, and when seen in the real

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atmosphere they last for such a short period that it is not possible to organise adequate observations for understanding the phenomena. For instance, the subsidence in the centre of severe tornadoes has been occasionally recorded in the atmosphere but its understanding has come mainly through laboratory experiments. Atmospheric Pollution: After a century of rapid industrialisation, it has been realised that atmospheric pollution caused by industrial waste products in the atmosphere constitutes a hazard for human health, a cause of inadvertent change in earth's climate and the cause of soil degradation on the downwind side. Anew awareness has developed in society that industrial units must observe the rules of maintaining pollution-free atmosphere. Old archaeological monuments must also be saved

from the attack of avoidable pollution in the atmosphere. Energy: Industries first used charcoal on a large-scale as a source of energy. Soon, oil came handy. Subsequently, oil became costly. Search began for utilisation of perennial sources of energy like rain, wind and sun-shine. These are meteorological parameters. As such, the search for alternative energy sources has emphasised the importance of meteorology for the well- being and development of human society. Weather Consciousness in Society: Every country having television network is displaying, on a regular basis, meteorological charts and satellite pictures of clouds to explain the weather situation around the region of interest for the viewers. People are getting interested to

understand the cause of day-to-day changes in the weather. Meteorology is finding its applications in many fields of activities of man over land. sea, in air and even in space. Governments are interested in long-range weather forecasting and in past climatological records for building airports, industrial complexes, multipurpose dams. food-storage warehouses, tourism, off-shore oil drilling and for increasing food production. Meteorology has influenced not only the daily life of a citizen, but also has proved crucial at critical stages of history-making war-operations. D-day operations in the Second world War proved to be an important event in the history of

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1.2 Special Features of-Tropics; MOllIiOOns

the world. These D-day operations depended on the meteorological forecasts. Individuals, Govemments and United Nations Agencies are getting more and more interested in meteorology. 1.2 Special Features of Tropics; Monsoons

Tropical Region : From geometrical considerations, the latitude of 23 1;2 oN is called tropic of Cancer and the latitude of 23 1;2°S is called tropic of Capricorn. The region between these two latitudes on the two sides of the geometrical equator is called the tropical region. However, the weather systems of the two hemispheres are not geometrically fixed like the geometrical latitudes. These move with the sun. During the northern(southern) summer season, the weather systems of both the hemispheres shift towards the north (south). The centre of the weather systems of the two hemispheres is to the north of the equator during the northern summer and to the south of the equator during the southern summer. This centre of the weather systems is called the meteorological equator. In the lower troposphere, Inter-tropical Convergence Zone (ITCZ) is often referred to as meteorological equator. In several respects, meteorological systems of the tropical region are different from those of the extra-tropical region. In this respect, we can speak of tropical meteorology as a distinct subject. The differences between tropical and extra-tropical systems are indicated below. Quasi-geostrophic Approximation: In the equation of motion dV dt+ f

kx V=- \'

1.2(1)

the acceleration term d V/dt is generally an order of magnitude smaller than f kx V in the extra-tropical regions; the two terms are comparable in magnitude in the tropical regions. In other words, Rossby Number Ro - 0.1 in extra - tropics - I. 0 in tropics where

Ro =

Id V / dt I/ I fk xV I

The consequence is that quasi-geostrophic approximation is generally valid in the extra-tropical regions for synoptic-scale systems while it has serious limitations in the tropics. In these lower latitudes, one has to use primitive equation (P.E.) models. Experiments are in progress to test whether non-linear and linear balance models have reasonable validity in the tropics. Theory of geostrophic adjustment (Monin andObukhov, 1959; Washington, 1964) suggests that in the extra-tropical region, wind field adjusts itself to pressure field leading to quasi-geostrophic balance. The period of adjustment is relatively small. In the tropical region, the pressure field adjusts itself to the wind field leading to ultimate quasi-geostrophic balance; also the period of adjustment is large. Due to these differences in the periods of adjustments, the 4-dimensional assimilation of data is not difficult in extra-tropical latitudes. Such assimilation is creating serious problems in tropical regions and hence there are more difficulties in NWP work in the tropics. Due to quasi-geostrophic balance in the extra-tropics, one can work with quasi-geostrophic models in which one can start with pressure field and infer the wind field. Pressure-contour analysis by itself without wind analysis is adequate On many occasions. In tropical regions, on the other hand, the wind analysis (stream-line-isotach analysis) and the pressure analysis are both essential. Temperature Gradients: Meridional temperature gradients are very strong 'in extra-tropical regions compared to those in tropical regions. Meridional motions in extra-tropics bring, in juxta-position, air-masses with substantial temperature differences along one and the same latitude circle. As a result, we get highly baroclinic frontal surfaces. A temperature difference of 10 K within a distance of 100 km in middle latitudes is common. In tropical regions, such temperature contrasts are uncommon; temperature gradients are weak. Consequently, middle-latitude frontal concepts based on large temperature contrasts are not quite useful for forecasting in the tropical regions. Here relative humidity contrasts and wind

1.2 Special Features oQlIBpics; Monsoons

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discontinuities are of primary importance for forecasting purposes. Air-mass contrasts are more through humidity than through dry bulb temperatures in the tropics. For the same reason, surface temperature changes during 24 hours are very small ( -1 K) in the tropics but are substantial (-5K) in the extra-tropics. Temperatures determine the density and hence the surface pressures. In extra-tropical regions, pressure gradients are steep and 24-hour pressure changes are of the order of 10mb (hPa). In the tropical regions, pressure gradients are weak and the 24-hour pressure changes are of the order of 1 mb (hPa) only. Surface temperature changes of the order of 5K may occur locally in the tropics in association with thunder Showers on hot summer days. But this cooling also is confined to shallow layer below the cloud base; hence the pressure change associated with such cooling is also of the order of 1mb (hPa) only. Seasonality of Weather : In extra-tropical regions, cyclones with their attendant cycle of weather, affect a station almost throughout the year. Rarely does one get a long spell of dry weather and cloud-free skies, say for a period of two to three weeks at a stretch. In the tropics, there are dry seasons and wet seasons. For several days in the wet season, one may not see the sun and for several days in the dry season one may not see a speck of cloud. The wet and dry seasons come with almost clockwork regularity. Again, each season, in the tropical regions, has its well marked diurnal cycle of weather. One may not feel surprised to find that for three or four days in succession, a rain shower starts at 3 p.m. local time every day with a margin of ± 5 minutes! This seasonality of weather with its own

sity, with periods which are much larger than the periods ofthe migratory systems. In the interactions with seasonal quasi-stationary systems, the feeble migratory systems undergo considerable changes in intensity, shape, structure and speed of movement to such an extent that quite often, it is difficult to detect their movement or eVen their presence in a region which is experiencing sudden changes in the daily cycle of seasonal weather. Quite often, two synoptic charts of isobars and winds look very much alike and still the weather is very different. Undoubtedly, there are differences but these are so small that they elude an analyst's eye. Ta separate the perturbation from the

daily cycle makes "persistence" principle very use-

sea"sonal pattern, it is useful to subtract the seasonal

ful in 24-hour forecasting in the tropics. Changes in weather of a tropical region from one day to another are brought about by changes in the intensity and the position of three types of systems: i) Migratory synoptic scale disturbances of extra-tropical regions on the borders of tropical

'normal' values from the total values and prepare 'anomaly' charts. 24-hour change charts are also very useful in this context. The 24-hour changes and anomalies are again a mixture of the changes and anomalies associated with migratory and quasi-stationary systems. The horizontal and vertical scales as well as the time-periods and phases of the two systems are

regions;

ii) Migratory synoptic scale disturbances of the tropical regions; and iii) Quasi-stationary seasonal lows, highs, troughs and ridges. The extra-tropical migratory synoptic scale disturbances are of sufficient intensity and can be followed easily on the daily synoptic charts. Their movement produces oscillations in the position. and

intensity of the quasi- stationary seasonal lows and highs, troughs and ridges, extending from sub-tropical latitudes into the tropics. These extra-tropical systems also induce formation of 'cold' lows in the adjoining tropical latitudes at particularly favourable spots and these lows move along their climatological tracks along with their characteristic sequence of weather.

The migratory synoptic weather systems of the tropics are generally weak. As they move, they get mixed up with the quasi-stationary seasonal lows and highs which are comparatively stronger than the migratory ones. The horizontal and vertical scales of seasonal systems are much larger than those of the migratory ones. The seasonal lows and highs also undergo oscillations in their position and inten-

1-18

1.2 Special Features of Tropics; Monsoons

quite different. As a logical step to synoptic system of weather forecasting, we should split up the observed 24-hourchanges and anomalies into different scales and identify their respective periods. phases and phase velocities. We should then forecast the position and intensity of each system, taking into account their mutual interactions.

Unfortunately, due to lack of adequate observational material and suitable analysis, this logical step has not yet been evolved to a regular operational level. This step has been substituted by 'experience' of a synoptician over limited regions. By experience, a synoptician comes to frame a set of thumb rules so that by a look at his charts, he can forecast the weather over a particular region for which he has acquired experience and skill. Differences in the skill scores of individual forecasters arise from their inability to separate the migratory from the quasi-stationary systems and to forecast their respective changes and mutual interactions. The success of numerical weather prediction models in extra-tropical regions is largely due to the large intensity of migratory systems which dominate the weather. Errors inherent in numerical model of forecasting are then small compared to the changes which the model predicts. In the tropical region, these model errors have to be extremely small in absolute magnitude in order to be smaller than the magnitudes of the pressure and wind changes which the model is going to predict. In other words, the accuracy demanded from NWP models is much higher in the tropical regions than in the extra-tropical regions. Diurnal Cycle: As stated earlier, tropical regions have fairly regular daily cycle of weather. This cycle manifests itself in almost all meteorological

elements like pressure, temperature, wind, relative humidity, cloud type and cloud amount. To detect a real change in the position and intensity of a tropical synoptic system, an analyst must examine

24-hour changes in the respective meteorological elements. In particular, an analyst looks for 24-hour pressure changes in the tropics and for 3-hour pressure changes in the extra-tropics. As will be shown in section 1.6, the daily pressure wave is dominant in the tropical regions; here 3-hour pressure tendency associated with the daily cycle is much larger

than the 3-hour pressure tendency associated with the movement or change in intensity of synoptic system. This is unlike the case in extra- tropics. This daily cycle is not confined only to individual stations on a meso-scale. It is observed in synoptic-scale and planetary-scale systems as well. It looks as though the whole tropical atmosphere responds actively to the daily solar cycle. For instance, the daily wind cycle, the so-called land-sea breeze, caused by the daily cycle of temperature gradient due to land-sea contrast is not confined to a few tens of kilometres near the coast. The effect extends several hundred kilometres deep inside the continents. The real maritime air may not be able to reach stations several hundred kilometres inland in this daily cycle, but the pattern of flow certainly penetrates that deep inland. Morning-to-afternoon changes in wind vector at stations near the coast and stations far inland exhibit a continuous pattern to confirm that the so-called land-sea breeze near the coastal boundary of a tropical continent is a manifestation of a much larger circulation system. The intensity and extent of such circulation is much less in the extra-tropics. Other differences between the tropical and extra-tropical regions are shown below: 1.2.1 Definition of Tropical Monsoon and Tropical Monsoon Region 1. Introduction: Traditionally, monsoon has been restricted to Southeast Asia, North Australia and tropical Africa. Recent observations and analyses have changed this traditional view; it is now established that there is monsoon also in North, Central and South America. In fact, monsoon prevails over the entire global tropical region land area as well as ocean area. Defmition of Monsoon: Monsoon is defined as alternation of relatively dry and wet seasons. This point is highlighted with the help of histograms of monthly rainfall over land stations including island stations in mid-oceans. Obviously, alternation of dry and wet seasons is linked to the north-south oscillation of ITCZ. As such, the region in which ITCZ oscillates helps in delineating the boundaries of the Tropical monsoon region.

1.2 Special Features of Tropics; Monsoons Extra-tropics i) In general, extra-tropical atmosphere is convectively stable. ii) Amounts of rainfall in 24 hours are of the order of! cm. iii) Rainfall generally shows one maximum and one minimum during the year. iv) With respect to dry adiabatic process, atmosphere is baroclinically unstable. v) In dry adiabatic processes, different layers along the vertical in the troposphere are strongly coupled. vi) Atmosphere loses westerly momentum due to friction at earth-air interface in middle latitudes. It receives momentum from tropical regions. vii) Through combined effect of radiative processes and heat exchanges at earth-air interface, extra-tropical regions would cool continuously. The balance of heat is maintained through import of heat from the tropical regions. viii) For meridional transports of heat and momentum, quasi-horizontal eddies are more effective than the mean meridional circulation (Ferrel Cell). ix) Mean meridional circulation (Ferrel Cell) is indirect.

x) Extra-tropical cyclones are cold-core systems in the troposphere. Their horizontal extent is large but their intensity is much less than that of tropical cyclones. The extra-tropical cyclones do not have an eye, generally. xi) Extra-tropical regions are affected mostly by extra-tropical westerly waves and hardly by tropical easterly waves.

xii) Above the tropo-pause, there is a deep isothermal layer in the lower stratosphere. xiii) There is no pronounced Quasi-Biennial Oscillation (QBO) in the stratospheric winds. xiv) Relative vorticity < coriolis parameter!

ITCZ is itself a region of wind discontinuity. Hence, change of wind direction with change of dry and wet seasons gets implied automatically whether alternation of wind direction is explicitly mentioned or not in the definition of the monsoon. Through international discussions and studies, this type of definition of "Monsoon" is now universally accepted (Trenberth et aI., J.

1-19

Tropics i) In general, tropical atmosphere is convectively unstable in the lower and middle troposphere. ii) 24-hour rainfall amounts are larger, of the order of3 cm. iii) Rainfall generally shows two maxima and two minima during the year, particularly within 5-10 degrees from the Equator. iv) With respect to dry adiabatic process, atroosphere is baroclinically stable. Condensation is essential for dynamic instability on synoptic-scale. v) In dry adiabatic processes, different layers along the vertical in the troposphere are weakly coupled. Strong vertical coupling comes through moist processes.

vi) Atmosphere gains westerly momentum due to friction at earth-air interface in the region ofeasterly trades. It exports momentum to extra-tropical regions. vii) Through combined effect of radiative processes and heat exchanges at earth-air interface, tropical regions would be warming up continuously. The balance of heat is maintained through export of heat to the extra-tropical regions. viii) For meridional transports of heat and momentum, mean meridional circulation(Hadley Cell) is at least as effective as the quasi-horizontal eddies. ix) Mean meridional circulation (Hadley Cell) is direct. x) Tropical cyclones are warm-core systems. Their horizontal extent is smaller but their intensity is much larger than that of extra-tropical cyclones. xi) Quite often, the tropical cyclones have an eye. Tropical regions are affected by both the westerly and the easterly waves, though more by tropical easterly waves. xii) Above the tropopause, the temperature sharply rises with height. There is a typical tropical tropopause. xiii) There is a very pronounced QBO in the stratospheric winds. xiv) Relative vorticity - coriolis parameter! Clim., Nov.2000, pp. 3969-3993). 2. Traditional Definition of Tropical Monsoon Region; New Definition: Defirtition of Tropical Monsoon Region given by Ramage (1971) had been traditionally accepted, confining the Tropical monsoon to the area between latitudes 25°S and 35~ and longitudes 300W and 1700 E, shown in Fig 1.2( I). Ramage stated as follows :

1.2 Special Features of Tropics; Monsoons

1-20

"I define the monsoon area as encompassing regions with January and July surface circulations in which; 1. The prevailing wind direction shifts by at ieast 120° between January and July. 2. The average frequency of prevailing wind directions in January and July exceeds 40 %.

This definition of tropical monsoon is presently not satisfactory. Looking at i) Global Nature of annual pressure and wind oscillations, and ii) Global Nature of ITCZ Oscillation to which monsoon is linked in Afro-Asian region, Asnani (January 1993, Tropical Meteorology, Vol. 1, Page 270) gave revised definition of monsoon region as follows: "It is that region which lies between the lat,itude 5°N of the northern- most (July) surface position of ITCZ, and the latitude 50 S of the southernmost (January) surface position oflTCZ, on climatological basis. As such, it runs round the whole earth in the tropics." Monsoon is also linked to Sub - tropical Trough Lines which meet ITCZ and separate quasi - stationary sub - tropical Anticyclones. It is these Sub-tropical Trough Lines, which bring summer monsoon to Southeast China and also to Southwest USA. This subject is further elaborated in Chapters 2 (Sub-tropical Anticyclones), 12 (Monsoon of China) and 15 (Monsoon of North, Central and South America). The equatorward extension of these

3. The mean resultant winds in at least one of the months exceed 3 ms- 1 and

4. Fewer than one cyclone-anticyclone alternation occurs every two years in either month

in a 5° latitude-longitude rectangle. The only region satisfying all parts of my monsoon definition boundary and squaring off, I can enclose the monsoons between 35°N and 25°S and between 300 W and l70oE." This has also been the traditional concept of restricting Tropical monsoon to Southeast Asia, North Australia and Tropical Africa. Following this traditional concept, World Meteorological Organization (WMO) and International Council of Scientific Unions (ICSU) also planned the International Monsoon Experiment-1979 (MONEX), considering this as a monsoon region.

•,

:

L

I

I

I

,

,'.

!

,

I

I

I

'

!

!

I

,

I

I

-l--'-

I

,

I

PCl l JANUARY1

AG. 1.2(3a) : Schematic picture of ITCZ and subtropical trough lines during January and July SPCZ = South Pacific convergence zone SACZ = South Atlantic convergence zone

relatively dry and wet seasons. b) Alternation of dry and wet seasons is linked to the annual north-south oscillation of ITCZ. Weather is relatively wet in the neighborhood of ITCZ. c) ITCZ itself is a zone of wind discontinuity. Hence, with north-south oscillation of ITCZ, a place near the ITCZ also experiences change of wind direction. d) ITCZ, which runs nearly east west is also linked to sub-tropical trough lines at some places. These sub-tropical trough lines separate adjacent sub-tropical High-pressure cells. The High-pressure cells lie roughly in east-west direction along sub-tropical Ridgeline. The sub-tropical trough lines run almost in north-south direction with slight tilt towards east or west depending on the land configuration. These sub-tropical trough lines are cyclogenetic airmass discontinuities, almost like ITCZ. They shift north - south along with the ITCZ and also shift east-west in response to the position and intensity of sub-tropical High pressure cells on their two sides. Fig. 1.2(3a) schematically shows the association of sub-tropical trough lines with ITCZ and sub-tropical High-pressure cells in the northern and the southern hemispheres. The annual cycle of change in position and intensity of these sub-tropical trough lines brings alternation of wet and dry seasons in sub-tropical regions also. Notable examples are : i) Southeast China having wet season in summer ii) Mexico and SW USA having wet season in summer

iii) SPCZ in southern Pacific iv) SACZ in southern Atlantic v)There is also NW-SE running convergence zone (SIOCZ) in SW Indian Ocean during Southern summer as seen in Fig 1.2 (3b) based on satellite pictures presented in Climate Diagnostics Bulletin (USA) January 2003; this convergence zone becomes active in the rear of migratory westerly extra-tropical cyclones. This figure also shows SPCZ in southwest Pacific Ocean and SACZ in southwest Atlantic Ocean. There is also a weak convergence zone near West Australia. 5. Complexity In the understanding of Monsoon On the Polar side of sub-tropical anti-cyclones, rains occur mostly in winter season in association with migratory synoptic-scale extra-tropical cyclones, while summer seasons are relatively dry. The opposite occurs on the equator-ward side of sub-tropical anticyclones, where rains occur mostly in summer season while winter seasons are relatively dry. This is illustrated in Table 1.2(1) for South America. Since there is alternation of dry (summer) and wet (winter) seasons on the polar side of the sub-tropical anti-cyclones, we should accept that there is Monsoon on the polar side of sub-tropical anti-cyclones also. This is shown in Table (1.2(1» for land 0 o area between latitudes 30 S & 4l S and 0 longitudes 70 W and 73°W. Alternation of wet and dry seasons in this sub-tropical land area is pretty clear; its rainy season is winter and its dry

season is summer.

1.2 Special Featlll'ts'of Tropics; Monsoons

1-24

60N 50N 40N 30N 20N

10N

EO 105 2CS

JC'S

'lOS

C

L

._l .

50

200

100

o

900

'80 300

500

400

FIG. 1.2(3b) Estimated rainfall (mm) for January 2003 using the special Sensor Microwave/Imager (SSMII) precipi.a.ion Index (Ferraro 1997,1. Geophys. Res., 102, 16715-16736). Contour interval is 100 mm. (Source: CLIMATE DIAGNOSTICS BUL\..ETIN (USA) JAN 2003).

TABLE 1.2(1) : Normal monthly rainfall (rom) for some sub~tropical stations in South America Chos-

SanCarlos

Puerto

Malal

Debarilache

Montt

37 S

37 S

41 S

41 S

71 W

73W

70W

7.W

73W

2

2

17

9

37

90

9

2

3

21

10

12

139

,6808Wj 100

-

-

90

l~~

70 60

l50

f---

~60

H

-f-

f-

20 10

350

f---

r ~

.-

J

-

150 100

o

o

-r

f---

rr--

.rll

MEAN RAINFALL (!lIM) STATION NAME :YACUIBA (ELEVATION: 5801I}(22 Ol~ 6343\11)

~

100 80 6lJ 40 20 OJ

1- -

JJASONDJFIlAM MONTHS

r-

_

f-

........

~120

~

If-.-

lASOND1FMAM MONTHS

200 180 ~ 160 ~140

f-

50

50

10

MEAN RAINfALL (14M) STATION NAME: BELO HOmONTE (ELEVATION 91514)(19 56S, 4356\11)

200

~

!~

JJASONDJFMAM MONTHS

300

r-

~40

c-

O

o

MEAllRAINFALL (liM) STATION NAME: ORURA (ELEVATION: 3,7l:llIl) (17 58S, 6707\11)

f---

. r-

•

-

JASOND1F14AM MONTHS

FIG. 1.2 (6c): (Sources: (i) World Survey of Climatology, Vol. 12, "Climates ofCeutraland 80uth America" edited by Werner Schwerdtfeger. (ii) WMO Climatological Normals (CLINO), for the period 1961-1990, WMO No. 847, 1996).

1.2 Special Features of Tropics; Monsoons

1·33

MEAN RAINFALLrMlI\

STATIONNAlIE : SALTA

.Ml:AlI RAINFALL (!JM) STATION NAME: T)JCUMAN (!LEVATION ......48l1.l) (26 48S 6512W)

(ELEVATION: 1,226 11)(24 SIS, 65l9Vi) 180 100

.-

fIII

f..-

~1411

1

r-

r

411

"",120

~10 0

f..-

J~

~

l00 . 80' 60

0

J

.-'-

48

~

J o

A

N

D

P \I

hA

e-

10 1--1

M

O

MEA1l)WNFALL (1II.l)

90 80 70

-

J:

f-

i-~

sON D 1ot0liDll

~

I

!l

~

!:

c--

A

-

Iso

~

~OO

A M

STAnONI!AMl:CATAM~A (ELtvATION .541M)(2826S, 65 46W)

100

1

r.t

1Il0liDll

MEAN RAINFAU. (MIl) STATION NAlIE :SANTLAGO DEL ESTERO (ELEVATION: 199M) (2146S,6418W) 90 80 10

,

o N D

A

1I01ITlll

30 20 10 I--, o )

e-

48

20

o)

.-'-

160

~

120

,

200

A M

20 18

o

f--

c-~

r

"'-

IASOND

,

n-

III

A III

IIt11!1l

FIG. 1.2 (6d) : (Sources: (i) World Survey of Climatology. Vol. 12, "Climates of Central and South America" edited by Werner Schwerdtfeger. (ii) WMO Climatological Normals (CLINO), for the period 1961-1990, WMO No. 847, 1996).

1.2 Special Features of Tropics; Monsoons

1-34

MEAN RAINFALL (MM) STATION NAIiI :PEURTO AYACUCHO (ELEVATION: 99MH05 41 N,61381'1)

MEAN RAINFALL (MM)

STATION NAI.IE :ST. IGNATIUS

(ELEVATlON9jMH0321N,5948W)

40IU

450 40[ 350

f-

350 L50 ~150

-

100

50 0

J~:

-

~200

r

l JJAS

lIoms

OJFIIAMJJASOND

lIoms

MEAN RAINYALL(IIlM) STATION NAliE :BARCELONA (ELEVATION: 1M) (10 O1N, 64 41W) 140

350

1

25

-

~

120

f-

1

[

-

~150 ~Ioo

o

-

100

~:lJ[

50

-

J

OND

IiIAN RAIl!FALL (IIlM) STATIONIWIE: FERNANDO (ELEVATION .13M) (0153N,61261'1) ):I[

-:-

50

!h-

.r

f-

-

100

f--'-

JFllAM

-

1300

r-

~300

~80

f-_

l:

--- -

~

iL

JFMAIIlJJASOND MONTHS

:lJ

-

r-

-

h

°JFMAMJJASOND MONTHS

FIG. 1.2 (7a): (Sources: (i) World Survey of Climatology, Vol. 12, "Climates of Central and South America" edited by Werner Schwerdtfeger. (ii) WMO Climatological Nonnals (CLINO), for the period 1961-1990, WMO No. 847,

1996).

1-35

1.2 Special Features of Tropics; Monsoons

MEAN RAIHALL (!.III) STATIOHNAME ,SAN SALVADOR (ELEVATION ·100 II) (lJ 4J H, 89121'/)

MEAN RAlNALL (MM) STATION NAIlE, MANAGUA (I:LEVATION l6M) (12 Il8lf, 86111'/)

JlIJ

JO 0

~

.--

JO( ~

0

I-

0

r-

r-

0 r-

rL

0 JFIlAIlJJASOHD 1l0HTHS

2l1J

JO 0

.--

r-

r-

200

.-l-

0

h

DIFMAMIIASOND MONTHS

MEAN RAINALL (1111) STATIOH NAill>. PASO REAL (I:LI:VATlON· 41M) (ll JlN, 8J 20 I'/)

IlEAH RAIHALL (Mil) STATION HAM!: ,GUATI:MAL.I CITY [l:LEVATIOH . I,JOOll) (15 J9 H, 9016 I'/) ~

J

l0

..-

l-

I--

I-

I-

....l

lIJ ~

0

h

JFIlAMJIASOHD MONTHS

Ol-l--r o

I

I

,• M A M

I

h-

IASOND MONTHS

Fl'G. 1.2 (7b): (Sources: (i) World Survey of Climatology, Vol. 12, "Climates of Central and South America" edited b~ Werner Schwerotfeger. (ii) WMO Climatological Normals (CLINO), for the period 1961-1990, WMO No. 847,

1996).

1.2 Special Features of Tropics; Mousoons

1-36

TABLE 1.2 (2): WMO (1996) Climatological Normals 1. 2. 3. 4. 5. 6. 7. 8.

Puerto Baquerizo Pichilingue Guayaquil Barra Do Corda Rernanso Porto Nacional Cuzco Formosa

00° 54'S

01° 02° 05° 09° 10° 13° 15°

06'S 12'S 30'S 41'S 31'S 31'S 32'S

in South America, south of the Equator, is to emphasize the point that this region has Monsoon-type altemalion of wet and dry seasons; as such, it must be recognized as part of tropical monsoon region.

Unlike in Figures 1.2 (4), the left end of X-axis in Figures 1.2 (6a, b, c, and d) is June, and right end is May. This is to highlight that summer season (December-March) is rainy season in Central-South America. Rainfall pattern is closely associated with the position and north-soulh movement of ncz. Rainfall in South and Central America, north of the Equator In South America, south of the Equator, rainy season is local summer (December-January- February). In the same South America, north of the Equator, rainy season is (June-July-August) which is the local summer there. As such, in both regions, north and south of the Equator, rainy season is the local summer season. Both are having summer monsoon rains. This point is highlighted through Table 1.2 (3) and corresponding Figures 1.2 (7a, b).

TABLE 1.2 (3) 1. 2. 3. 4. 5. 6. 7. 8.

Ingatius Puerto Ayacucho Fernando Barcelona Managua San Salvador Guatemala City

Paso Real

3° 21'N 5° 4l'N 7° 53'N 10° OTN 12°08'N 13° 43'N 15 0 29'N 22° 35'N

59° 48'W 67° 38'W 67° 26'W 64° 41'W 86° ll'W 89° 12'W 90° 16'W 83° 20'W

Fig. 1.2 (6) shows distribution of mean

9.

10. 11. 12. 13. 14. 15. 16.

16° 30'S 17° 58'S Belo Horizonte 19° 56'S 22° OI'S Yacuiba 24° 51'S Salta Tucuman 26° 48'S Santiago del Eastero 27° 46'S Catamarca 28° 26'S LaPaz

Orura

monthly rainfall of the following stations in South and Central America, north of the Equator, arranged latitude-wise, Equator to north. X-axis is again arranged with month January at the left end, month of December at the right end, as in Fig. 1.2(4) for India and Sri-Lanka. In Table 1.2 (3), the stations are arranged latitude-wise, latitude increasing downwards. In Fig. 1.2 (7 a,b), the extreme left-hand month is January and the extreme right-hand month is December to highlight maximum rainfall during the northern summer. (June-July-August). Looking at rainfall histograms given in Figures 1.2 (4,6, & 7), it will be appreciated that the overall pattern of rainfall in South America is similar to the pattern in India, in respect of the following two aspects: i) There is alternation of wet and dry seasons, summer being the rainy season. il) Rainfall is associated with the north-south oscillation of ncz. Since India and Sri Lanka are recognized to have monsoon rains, South America has to be recognized as having monsoon rains. Note: These monthly rainfall histograms are prepared on the basis of rainfall data published by WMO (CLINO), India Meteorological Department and World Survey of Climatology (Vol. 12, Climates of Central and South America), "Definition of Monsoon and Monsoon Region". From the comparison with the classical monsoon characteristics of climate over southeast Asia, Zhou and Lau (1998, Journal of Climate, May 1998, pp. 1020- 1040) also came to the conclusion that a monsoon climate does exist over South America. According to them, the summer monsoon circulation is a robust climate feature of South America.

1.3 Special Analysis For Tropics VAMOS (Variability of the American Monsoon Systems) The International Meteorological Community has also recently accepted the existence of Monsoon in North, Central and South America (see Chapter 15). The International Clivar programme has an important component named VAMOS (Variability of the American Monsoon Systems). The VAMOS panel has called North American component of VAMOS as NAME (North American Monsoon Experiment). This is counter-part of MESA (Monsoon Experiment in South America). (Ref: "CLIVAR Exchanges", Dec 2000, pp. 1-3.) More and more information is now appearing in literature on American Monsoon, particularly on Monsoon of North America (Saleeby & Cotton, 2004). Summary: 1. Tradi tionally, Monsoon has been restricted to Southeast Asia, North Australia and tropical Africa. This concept must be revised on. Monsoon prevails over the entire tropical region. 2. Traditional definition of monsoon region is presented. 3. During late 1980s, there was a feeling that the summer rains in Mexico and southwest USA should also be classified as summer monsoon rains. A major observational project called Southwest Area Monsoon Project (SWAMP - 1990) was mounted in North America with co-operation between USA and Mexico. It took extensive observations in Mexico and southwest USA, for one month beginning 7th July 1990. This clearly revealed features of "monsoon rains" in Mexico and Southwest USA.

4.Characteristics of monsoon climate throughout tropical region are brought out :(a) There is alternation of wet and dry seasons. (b) Correspondingly, there are seasonal changes in atmospheric circulation; in particular, there is north-south oscillation of ITCZ. (c) There are also changes in intensity and position of sub-tropical trough lines separating sub-tropical anticyclones, These changes are associated with: i) Monsoon in China.

1-37

ii) Monsoon in Mexico and Southwest USA. iii) SPCZ in southern Pacific iv) SACZ in south Atlantic 5. It is pointed out that rainfall in extreme southern sub-tropical region of South America between 300 S and 41 Os shows characteristics of monsoon rainfall in winter season, rather than in summer season. There are also monsoon-type "winter" rains in some other sub-tropical areas, poleward of sub-tropical Highs.. 6. At present, it is universally accepted that there is monsoon over India. But textbooks have so far generally excluded the whole of American continent from monsoon. To emphasize that tropical and sub-tropical regions are certainly in monsoon region, we have presented monthly rainfall histograms for 8 stations between latitudes 6°N and 24°N in India and Sri Lanka in Fig. 1.2(4), for 16 stations between Equator and 29°S in South America in Fig 1.2(6) and for 8 stations between 3"N and 23°N in South and Central America in Fig 1.2(7). Histograms of South and Central America show the same pattern of alternation of wet and dry seasons as shown by stations in India and Sri Lanka, This alternation of wet and dry seasons is in association with north-south movement of ITCZ following the position of the sun relative to the earth. The monsoon prevails over the tropical oceanic area also. Where ITCZ is, there monsoon is.

1.3 Special Analysis For Tropics Object of the Analysis: The object of the analysis is understanding of the atmosphere for the purpose of forecasting. The periods of forecasting range from a few minutes to as long a period as possible, may be even a few years if that would be possible. At present, the differences between tropical and extra-tropical techniques of weather forecasting are mainly in forecasting for the periods of the order of a couple of days. In any case, this is the class of weather forecasting in which meteorological forecasting offices are

1-38

1.3 Special Analysis For Tropics

mainly engaged in, at the moment. For this period of forecasting, one depends essentially on the synoptic charts which are prepared, as a routine, in all meteorological forecasting offices. Numerical Weather Prediction (NWP) models which weoe in research mode in early 1990s are now in operational mode in the tropics also. We have already pointed out that in the tropics, the weather is seasonal with a dominant daily cycle, that chang~ in weather from one day to another are brought about by the changes in position and intensity of the migratory extra-tropical and tropical disturbances and the quasi-stationary tropical systems; that we need to isolate the 24-hour pressure changes and anomalies associated with the migratory and the quasi-stationary systems; that the logical course has not yet been evolved quantitatively to the operational level. In this section, we shall indicate a few additional aids in tropical analysis and forecasting. 24-hour change charts in other elements: In addition to 24-hour pressure changes, it is useful to prepare 24-hour change charts in respect of : i) Wind vectors at a few specified levels; ii) Minimum temperature; and iii) Maximum temperature. Anomaly Charts: Anomaly charts in respect of these elements are also equally useful. These charts are possible if we have 'Normal' charts based on very long periods of observations at each station. Hence we need observations over long periods. How long should this period be ? It is generally believed that the period should be atleast thirty years long, the longer the better. Further, different stations should have observations over the same period of 30 years or more. It has been discovered that the atmosphere has inter-annual cycles of various periods ranging from two year~ to several thousand years. Hence all 'long' periods are not statistically similar periods. Within the periods of standardised meteorological observations, we cannot cover

full periods of all cycles which are present in the atmosphere. As next best, let all the stations cover the same period of 30 years for calculation of normals. Even this is not always easy to achieve

in the tropical regions. Let us assume that we have 30-year data for a number of stations. We can then prepare the arithmetic averages and call them 'normals'. For which calendar days of a calendar year should we combine the observations? Shall we prepare normals for each calendar day of the year, e.g. 1st January, 2nd January and so on? With our present day knowledge of the subject, it is not considered essential or even worthwhile tb calculate daily normals. At present, two types of normals are in use:

a) 5-day normals b) monthly normals. Five-day normals are in use more in the tropics than in the extra-tropics. Five-day periodicity was detected in the tropical weather systems towards the end of nineteenth century soon after regular and extensive surface observations became available for the tropical regions; hence the preference for 5-day normals. One month appears to be too long a period to be considered homogeneous. Monthly normals have their utility if we wish to appreciate the evolution 0'1" weather patterns which have the period of about a year or so. Shorter period normals and departures-from-normal are necessary if we wish to study the evolution of patterns which have the shorter periods of say a few months or a few weeks. In such cases, five-day normals would perhaps be useful but such normals are not available except for a few surface meteorological elements like rainfall, maximum

temperature

and

minimum

temperature. Preparation of five-day normals for other elements, particularly for upper air observations, will pose additional problem of sufficiency of the number of observations. For one observation a day, we have five observations for a particular five-day period in each calendar year. For thirty yetirs, we have 150 observations. Can we consider this number as adequate for calculation of normals? On the other hand, for the calculation of monthly normals, we have 900 daily observations over a period of thirty years. A critical statistical analysis on this topic is lacking at the moment. It is felt that we should now have half-monthly upper air normals in the tropical region in place of monthly normals. A

- - - - - - - - - - - - - - -- - - - - - - - - - -

1.3 Special Analysis For Tropics month appears to be too long a period to be considered homogeneous even for the study of those phenomena which have the cycle period of a year. Rainfall has two maxima and two minima in near Equatorial region. Hence six-month period comes to be important. Six points in a wave period or wave-length do not give a satisfactory representation of the wave phenomenon. Twelve points give a fairly satisfactory representation. Thus, there is a strong case for half-monthly normals. (p,l) Charts: These are also called vertical time section charts. Such charts are prepared, one for each station which takes upper air observations. Pressure p is plotted on logarithmic scale as y-axis. Time I is plotted as x-axis. It is useful and convenient to plot time increasing in positive x-direction if the station is affected by meteorological synoptic systems moving from east to west. This is applicable to most stations in the tropics. If , on the other hand, the station is affected by synoptic systems from the west as in sub-tropical region, then time I should increase in negative x-direction. At each (p,l) point, the plotting model is the same as for any upper air chart. This system of plotting preserves the trough-ridge appearance on (P, I )chart as on constant pressure charts. This wind representation is illustrated in Fig. 1.3(1) (a,b). At the bottom of the chart, one can plot the weather in surface weather plotting model. In Fig.I.3( 1a), a station in northern hemisphere is affected by an easterly wave. In Fig 1.3(1b), the station is affected by a westerly wave. In Fig' 1.3(1c), the data of Fig. 1.3(1a) are plotted with time I increasing westwards. The sequence of observations at the station for different values of time I is exactly the same in (a) as in (c). Only the direction of plotting is different in (a) & (c). Alt=2,6,l0 and 14, troughs in the easterlies are passing over the station. The appearance of these troughs on the vertical time section chart(a) is the same as on the conventional synoptic charts at any level, like the troughs in the easterlies. If we look at vertical time section chart(c), the appearance at 1=2,6,10 and 14 is as if ridges of the easterly waves are passing over the station at these times. It is convenient to plot I increasing eastwards for

1-39

101

I

(I

I

,

..

II'

••

,-

II

10

'"

'-.-.J: ........... ,-.kJ , I.......... ---1 '-. J I

I

;

II

14

liS

I'

,-p _ f>a _ f>T _ PaT

au au ~ +u au ax +vay+waz- -fv=-aax

.. at a = - (Ca ., ax + Ca) y ay

II)

'12

~--PIu-_Lfu f>p=

W-

av av av av - + u- + v- + w=- + fu at ax ay az

ay

~

-l -1

Number

Magnitude of

ay

v) ax - pfv - gDfv

Cha.racteristic depth

f=

a~ - ~ ax ax

Iu--a~--~ I i; -

speed

Characteristic Corialis parameter

1.4(36)

1.4(29)

= -

1.4(3 I)

an ay

a= 1.4(32) 1.4(33)

as _ 10-3 e

1.4(40)

Magnitudes of vertical velocity and diabatic heating: We again look at the thermodynamic equation

(~at + u ~ax +

v

~) (In (I) + w ~ (In (I) = ~ ay az CpT

1.58

1.4 Scale Analysis for Tropics

;t (In 0)

= - ( Cx

_ Cx 80 Lx 0 U

a:

+ Cy ~ ) (In 0) 0

dX

•

1.4(42)

0

Lx

1.4(43)

w ~ (In 0) - W ~ - 10-5 W dZ D

1.4(44)

CpT

ff-

p

iv)

0.25 °C/day

p

lL_ 2 .5

Q 10-\-1 CpT-

f

11 appears to us that

Cp

aV2

aU2

v2

U2

ax

ily

Lx

Ly

1.4(51)

v) Sphericity ofthe earth will be unimportant for

C d'Ivergence IOf

U

tan = 35° 26'. This is the nodal latitude in the northern and the southern hemispheres. Pressure change on the equatorial side has the opposite sign of what it has on the polar side; when the pressure rises on the equatorial side, it falls on the polar side and vice versa. It may also be remarked that this analytical representation fits the observations pretty well in the polar latitudes but not so well in the equatorial latitudes(Wilkes, 1949). The Greenwich mean time expressed in degrees in the above formula can be replaced in terms of local time in degrees by using Ie = t - A where A is the longitude of a station to which the local time refers. Hence the semi-diurnal polar pressure wave is

(P2)polar = 0.0425 (3 sin 2q> - I) sin (21 - 2)" + 118°)

1.6(3b) The maximum occurs 56 minutes or approximately one hour before mid-day and midnight Greenwich mean time. 2 2, The Observations at higher levels: FIG. 1.6(4) : 5, (P) Equilines: Amplitude (A" unit 10The great bulk of tidal analysis has been mb (hPa)) and phase (E ,) (Chapman and Lindzen, 1970; made for surface data because these data are most Haurwitz, 1956; Asnani. 1993). vii) Analytical representation of readily available. Amongst the surface semi-diurnal pressure wave S2:- As stated meteorological parameters, pressure is measured earlier, it consists of the travelling equatorial most accurately. Above the surface, pressure wave and the standing polar wave. Haurwitz's measurement is not very accurate; wind (1956) formula for the travelling equatorial wave measurement is relatively more dependable, but the observations are not plentiful. To catch 12IS hourly wave in the free atmosphere, we should 3 (P'),quam,'al = 1.l6cos q> . sin (2 1+ 158°) 1.6(2) have at least 4 observations in 12 hours, if not more. Such frequent observations are generally where p, is expressed in millibars; I is the local not available. However, at some stations, at some time expressed in degrees, 360° representing 24 levels, there have been observations which give hours. The amplitude at the equator is 1.16 us some idea of the diurnal and semi-diurnal mb(hPa), decreasing to zero at the poles. The waves at higher levels in the atmosphere. These maxima occur when 2/+ 158°= 90°, 450°,810°... will be discussed later during comparison i.e. at 9 hr. 44 min. a.m. and p.m. between theory and observations. The standing polar wave can be expressed as 3, Seasonal variation of P J and p, : 2 Haurwitz and Cowley (1973) carried out (p, ),,,'a, = 0.0425 (3sin

50

I

\

\ \

.. ... .... .... " . ~

60

~

eo .> 20 km), mainly diurnal wave and little or no evident vertical phase progression. At such levels and in a relatively small area, one does not expect such differences in behaviour. These authors concluded that observations of Group I stations did show some resemblance to some of the results of Tidal Theory (Chapman and Lindzen, 1970) but additional data sets are required to pronounce firm judgement. We have seen the fate of Taylor-Pekeris theory of semi-diurnal pressure wave. At one time, particularly during 1940s, hardly anyone dared to voice doubts about the validity of this theory, even though it had the following 3 weaknesses; i) It depended critically on the values of equivalent depths calculated from the then available vertical profile of temperature in the atmosphere. ii) It expected a phase-reversal of semidiurnal pressure wave near 30 km level; observations did not support the existence of this phase-reversal. iii) The existence of polar standing semi-diurnal pressure wave was well established by observations, but Taylor-Pekeris theory had no satisfactory explanation for this pressure wave. Presently, the idea of resonance has been given up and Chapman-Lindzen theory has completely replaced Taylor-Pekeris theory. We have to remind ourselves that Chapman-Lindzen theory also suffers from weaknesses (ii) and (iii) mentioned above. namely phase-reversal and polar standing wave. As it always happens in science, we should not rest satisfied until these 2 weak points are also rectified. The effort in this direction might even open the door to some new discovery in geophysical science! Some speculation on this point is given in Chapter 16, connecting

1-97

Atmospheric Tides, particularly semi-diurnal pressure wave to Solar wind.

1.7 Diurnal Variatinn of Precipitatinn We have already referred to relatively larger diurnal variation of temperature over tropical land region than over tropical ocean region. In addition, over the land region, there are many irregularities of terrain of various horizontal and vertical dimensions. Each of these irregularities of terrain causes its own local air circulation, air generally tising up the sloping surfaces during day and sinking down the sloping surfaces during night. Under favourable conditions, these local circulations can attain considerable intensities. These diurnally varying vertical circulations also contribute to the diurnal variation of precipitation/cloudiness. Some of these circulations of meso-scale dimensions will be dealt with in Chapter 10. Here, we shall concern ourselves with these meso-scale circulations only to the extent that we consider them contributing to the diurnal variation of precipitation in the tropics. Historically, the ideas about diurnal variation of precipitation in the tropics, over land and ocean, have been substantially influenced by Hann's (1901) classification of the patterns of diurnal variation of precipitation for the whole globe, as given below: a) Continental Climates: Most precipitation falls in the form of convective showers in the afternoon. b) Maritime and Coastal Climates : Most precipitation occurs at night or during early morning. c) Regional Peculiarities : In addition to the broad classification by continental and maritime climates as mentioned above, there are pronounced seasonal variations in some regions. For instance, over much of western Europe, winter precipitation shows night maximum while summer precipitation shows afternoon maximum; in tropical monsoon season, there is a tendency for morning maximum. We shall deal with this topic under three sub-headings: Diurnal (24 hours) cycle of precipitation over tropical land stations, diurnal (24 hours) cycle of precipitation and cloudiness

1-98

1.7 Diurnal Variation of Precipitation

over the oceans and semi-diurnal (12 hours) cycle of precipitation and cloudiness over land and oceans.

1.7.1 Diurnal (24 hours) Cycle of PrecipiJation over Tropical Land Stations: It has been appreciated that in every continent, there are regions which do not fall into Hann's (1901) simple classifications (a) and (b) above. There are far too many "regional peculiarities" inland over the continents and

along the coasts so much so that classifications (a) and (b) may almost look misleading; e.g. Kincer (1916) showed that: i) At many stations in the central and north central United States, mOTe precipitation occurs

during the night than during the day; and ii) coastal stations in southeast United States show a well-marked afternoon maximum. There have been further studies for USA by Means (1944), Rasmusson (1971) and Wallace (1975); for Europe by Hann and Suring (1939); for Asian region by Ramage (1952, China), Raman and Raghavan (1961,India) and Prasad(l970, India); for West Africa by Hamilton and Archbold (1945) and Jeandidier and Rainteau (1957); for East Africa by Thompson(1957), Tomsett(1975) and Asnani and Kinuthia(l979); for Sudan and Ethiopia by Pedgley(1969, 1971) and for Southern Africa by Hastenrath (1970). These observational studies show a wide variety of patterns of diurnal variation of precipitation over the land stations spread around the globe not falling in categories (a) and (b) above. In a survey of diurnal variation of precipitation over' tropical land stations, Atkinson(1971) came to the conclusion that "contrary to popular opinion, many tropical land stations do not show a rainfall maximum during the afternoon period associated with maximum surface heating; instead, many tropical continental stations show rainfall maximum during the night time hours". In later surveys of tropical rainfall, Wallace (1975) and Gray and Jacobson (1977) also emphasized the existence of a large variety of patterns which do not fall into. Hann's Classes(a) and (b) above. Physical-cum-dynamical explanations have been offered for such deviations, by different authors (e.g. Gentry and'Moore, 1954; Franket aI., 1967;

Pielke, 1974; Wallace, 1975; Schwartz and Bosart, 1979). These ideas and explanations are briefly given below. Diurnal variation of precipitation over

tropical land stations has two principal causes: i) Reduction of Static Stability: This occurs through afternoon heating from below or through night-time radiative cooling of cloud tops from above; similar de-stabilization of the atmosphere and consequent diurnal cycle of precipitation could also take place if there be advection of warm air in the lower layers and/or advection of cold air in higher layers at preferred times of day or night. The afternoon insolalion causes bubbling up of the hot air which is in contact with the hot ground. Some of the large hot bubbles end up as large Cu or Cb clouds. Convective instability of the tropical atmosphere helps in the fQrmation of large Cu or Cb clouds. At night time, the cloud tops cool more than the cloud bottoms; static stability gets reduced. This favours vertical over turning. While this process and advective processes mentioned above have been quoted as contributing to diurnal cycle of precipitaion, it is doubtful if their contribution is so significant as to cause a precipitation maximum during night time. ii) Release of convective instability

through

organised low level convergence:

This may be associated with one of the following three factors: Synoptic-scale systems. Meso-scale systems. Interaction between meso-scale and large-

scale systems. SYNOPTIC-SCALE SYSTEMS a) Migratory Synoptic-scale Systems: We can assume that there is no particular bias regarding the time of the day when such systems move across a station, Le. their times of

movement are randomly distributed with respect to the daily clock. As such, by themselves alone, these ntigratory systems are unlikely to cause any significant diurnal variation of precipitation over a station, except in cases where these systems have a tendency to change their own intensity when passing over a region during a partiCUlar time of the day or night.

1-99

1.7 Diurnal Variation of Precipitation b) Non-migratory Synoptic-scale Systems: Tropical region is also characterised by synoptic-scale quasi-stationary systems. The time-invariant component of these systems wjll not cause any diurnal variation of precipitation. c) Diurnal oscillations in the migPlltory and non-migratory synoptic-scale systems: Quite often, there is interaction between the insolation and the synoptic-scale systems. As a result, both the migratory and the non-migratory synoptic-scale systems frequently manifest diumal oscillations in their position and intensity. For example, over peninsular regions, the troughs of low pressure get accentuated in the afternoon while the ridges of high pressure get accentuated late in the night. These will contribute to the diurnal variation of flow and precipitation over large regions like afternoon precipitation being favoured in regions of seasonal troughs. This phenomenon is markedly manifest in peninsular India during pre-monsoon period (April-May) and near the monsoon trough over north India during monsoon period (June-August). MESO-SCALE SYSTEMS Due to orographic. influences over land regions and due to orographic-cum-water surface heating differentials near the coast lines of oceans and lakes, there are intense meso-scale circulations with marked diurnal variations in the tropical regions. Precipitation is favoured in the areas of upward motion within these meso-scale systems and also in the adjacent areas where large CU/Cb clouds would drift before dissipation. Some of the examples of these meso-scale systems with marked diurnal variation are land-sea breezes, land-lake breezes and up-slope! downslope winds. Stations located near the coast line and also in regions of rugged terrain show corresponding diurnal variations of precipitation. Development of afternoon convection is favoured over land regions near the coast line due to land-sea and land-lake breezes. Similarly along the slopes of the rugged terrain, convective activity develops in the afternoon. The steering of the convective clouds by the prevailing winds in the troposphere will extend the phenomenon of diurnal variation of precipitation downwind from the region of their first development. PreCipitation

associated with such meso-scale systems can have very marked diurnal variations. INTERACTION BETWEEN MESO-SCALE AND LARGE-SCALE SYSTEMS This appears to be the most important factor responsible for a wide variety of patterns of diurnal variation of precipitation, not falling into Hann's categories (a) and (b), over land and adjoining sea areas. The "regional peculiarities" mentioned in Hann's (1901) classification- and there are far too many of such peculiaritiesappear to be due to this interaction. The meso-scale wind circulations caused by differential heating and cooling near rugged terrain or near coast line separating land and water surfaces may be regarded as time-varying perturbations superimposed on the nearly time-constant large-scale wind circulations. There is interaction between these two distinct wind circulations. A simple idealized case is illustrated in Fig. 1.7(1). In this illustration, land breeze at night and large-scale easterly trade. winds blowing during day and night are in opposite directions. In this case, near the coast line, the resultant wind will continue to be easterly at night but will be slower and horizontally convergent in the neighbourhood of the coast line. This horizontal velocity

LAND

i

SEA

1

I ·~----I··----

--·1--

-_.,-:.....·----

·.----11----~·----I

1

I

FIG. 1.7(1) : Idealized picture of interacting meso-scale circulation (Land Breeze at night, shown by thin vectors) and large-seale circulation (Easterly trade winds, shown by thick vectors) (Source: Asnani, 1993).

1-100

1.7 Diurnal Variation of Precipitation

convergence in the lower levels will cause vertical upward motion favouring release of convective instability in the neighbourhood of the coast line. The opposite would happen during day time when the sea breeze blowing from east would augment and cause acceleration in the large-scale easterly trade winds, horizontal velocity divergence and subsidence of air in the neighbourhood of the coast Jiae.. Such a situation would lead to a minimum of precipitation in the late afternoon and a maximum of precipitation late in the night or early in the morning. The above illustration is a simple idealized picture presented to stress the role of interaction between the meso-scale and the large-scale circulations. In the real atmosphere, the meso-scale and large-scale circulations have a wide variety of patterns in direction and in intensity, the meso-scale systems varying relatively faster in time. Also, meso-scale circulations arising from different orographic features(e.g. land-sea contrast, sJoping terrain and variation in frictional drag) may operate at the same time and interact among themselves and also with the large-scale wind flow, in different ways. It can easily be appreciated that such ·interactions can cause a very wide variety of patterns of diurnal variation of precipitation and cloudiness. This variety of patterns is expected particularly over rough terrain of the continents and in the neighbourhood of coast lines separating land surface from water surface of oceans or lakes. How far does the coastal effect extend inland and out into the sea? It is difficult to answer this question quantitatively at present but we can say that the influence of the coast line penetrates deeper into the land and also into the sea if: a) Land mass has large areal extent and is surrounded by large water mass; b) Land mass has high mountain elevation; and c) Solar insolation is strong. Some illustrations of such pronounced interactions and corresponding patterns of diurnal variation of precipitation are presented below: i) Florida Peninsula: Burpee(l979) and Schwartz and Bosart (1979) have pointed out that during northern

summer season, an east-west ridge line is often oriented over Florida peninsula in USA, giving large-scale westerly flow over the northern sector of the east coast and easterly flow over the southern sector of the east coast. During the afternoon hours, the sea-breeze tends to come from the east all along the east coast. Along the northern sector of the east coast, the meso-scale sea-breeze and the large-scale winds oppose each other and tend to cause afternoon maximum in precipitation; the reverse tends to happen along the southern sector of the east coast. These conclusions support the earlier analysis of Gentry and Moore(l954) and of Frank et al.(l967) for the same Florida peninsular areas leading to the conclusion that early morning coastal showers were more prevalent near the upwind coast and afternoon deep convective activity was observed near the downwind coast. Such behaviour has also been numerically simulated by Pielke(l974). ii) Hawaii Island: Leopold (1949) showed that diurnal variation of precipitation at several locations in Hawaii island could be qualitatively explained as arising from the interaction between large-scale trade winds and the local diurnal circulations in the form of land-sea breezes. iii) South Peninsula of India: Along the west coast of India in general and near Bombay in particular, during the northern summer monsoon season (June-September), there is maximum rainfall activity during late night/early morning. The large-scale winds are westerly. During late night/early morning periods, there is retardation of these winds due to opposing land breeze augmented by downslope flow from the mountains (Western Ghats). The horizontal velocity convergence resulting from the interaction between the large-scale westerlies on one hand and meso-scale flow (land breeze+downslope wind) on the other hand gives the observed late night/early morning maximum in precipitation. Iv) East Africa (Kenya, Uganda and Tanzania): This region has the following unique features shown in Fig. 1.7(2) : a) It is close to the equator, being located between SON and 11oS. As such, insolation is

1.7 Diurnal Variation of Precipitation

1·101

.. I

I

I'I-----If----\

'.""'.'If

......

--••

r

.•

,,

" ,, ,

• -e.u.....

,,

" ,

•

"

..

• T....

t*.

,

I

I

I

7

,

"

I

, ' i".,.

,

I'

~ •••

,

11;;

II

I

Io,om

..., l;teO m

I...n'

n" I.oom

L•• '

.... ., ilTOCm

LIII. nl'

3.g.0m

, ;'

"L.:-_~:-_~-:-_..J..__L..:,...._~i£""u..-:-_.l.-:-_..l.._J-;:_-c.._-.J,r'I...c"'-"'_~.:J

.z..

'0·

.II·

JI.I-

•• -

••

FlG. 1.7(2): Physical features of East Africa (Asnani and Kinuthia, 1979; Asnani, 1993).

40

41

41

1-102

1.7 Diurnal Variation of Precipitation

strong throughout the year. b) Almost in the centre of this region is the Lake Victoria (area - 60,000 km 2). It is the largest lake in the tropical region. Equator passes right

The interaction between the large-scale easterly trade winds and the intense meso-scale circulations in many directions produce a variety of patterns of diurnal variation of precipitation.

across the lake. This lake gets late-night thundershowers almost on all nights of the year. c) Lake Victoria lies in a valley between two

Hourly rainfall data of about 50 stations in this

mountain ranges running in nearly north-south direction.

observed p-atterns of diurnal variation of

d) Indian Ocean washes the eastern coasts of Kenya and Tanzania. The coast line is almost in north-south direction. After a narrow coastal strip

about 100 km wide, the land rises steeply with Mt. Kenya(5199 m) near the equator and Mt. Kilimanjaro (5895 m) near 3°S. These unique orographic features and intense solar insolation provide a unique setting for intense diurnal meso-scale circulations: land-sea

breezes on the east, upslope.downslope winds towards east and west of the mountain ranges and land-lake breezes on the shores of Lake Victoria. Large-scale winds over the region are easterly

trades throughout the year, with a southerly component during the northern summer and a northerly component during the northern winter.

region were examined by Asnani and Kinuthia

(1979) who identified ten zones on the basis of the precipitation on yearly basis (Table 1.7(1)). The authors attributed these patterns mainly to interaction between large-scale easterly trades and the meso-scale diurnal circulations induced by land-sea breezes, land-lake breezes and upslope-downslope winds. Schematic diagram of diurnal meso-scale circulation in the vertical(x,z) plane is shown in Fig. 1.7(3). The diagram shows afternoon flow minus morning flow. Arrows at X,z grid network show the direction and magnitude of the east-west components of V 12 - V6 · VIZ represents afternoon (1200 GMT) wind and V6 represents morning (0600 GMT) wind. Shaded region shows topography above sea level. On the western slopes of the highlands, there is a combined influence of lake breeze and

TABLE 1.7( 1) ; Patterns of diurnal variation of rainfall (annual) in East Africa (Asnani and Kinuthia, 1979; Asnani, 1993). NO.

ZONES

PERIOD

1.

East coast

Maximum Minimum

Morning between 6 a.m. & noon Evening between 6 p.m. & midnight

2.

Eastern slopes of highlands of Kenya and Tanzania

Maximum Minimum

Between midnight & early morning Around local noon

3.

Highlands of Kenya and Tanzania

Maximum Minimum

Afternoon 3.00 to 6.00 p.m. Morning 7.00 to 11.00 a.m.

4.

Northeast coast of Lake Victoria

Maximum Minimum

Afternoon 3.00 to 6.00 p.m. Morning 7.00 to 11.00 a.. m.

5.

North and northwest coast of Lake Victoria

2 Maxima

Early morning and early afternoon

6.

West coast of Lake Victoria

Maximum Minimum

Morning 7.00 to 11.00 a.m. Afternoon 3.00 to 6.00 p.m.

7.

Southern coast of Lafe Victoria

2 Maximum

Early morning and early afternoon

8.

Lowlands of Uganda and Tanzania

Maximum Minimum

Afternoon 3.00 to 6.00 p.m. Morning 7.00 to I LOO a.. m.

9.

Highlands of Uganda and Northwestern Tanzania

Maximum Minimum

Afternoon 2.00 to 4.00 p.m. Morning 7.00 to 4.00 a.m.

iD.

Lowlands of Kenya

Mixed Patterns

1.7 Diurnal Variation of Precipitation

~ ~ ~

.. -;;"z . ~

~

0

~

Z

0

....

~

•

0

C Z

;< ::E

m



'" '" 0

~ ~

0 ~

I I

Z

~

N

0 N

" 0

~

0 0

LAKE VICTORIA

x_

INDIAN OCEA.N

SCALE OF WIND ARROW: -; = 5 knot FIG. 1.7(3) : Schematic Diagram of (Afternoon minus morning) flow in the vertical (x,z) plane between Indian Ocean and Lake Victoria. Arrows show direction and magnitude of east -west component of (V12 - V6) at centre of arrow. Shaded region shows topography above sea level. (Scale of wind arrow as shown.). (Asnani and Kinuthia, 1979; Asnani, 1993).

the upslope wind. On the eastern slopes of these highlands, there is combination of sea breeze and upslope flow. Sea-breeze circulation is shallower and weaker than lake-breeze circulation_ A few typical patterns of diurnal variation of precipitation over east Africa are shown in Figs. 1.7(4) to 1.7(9). In these figures, time is shown in East African Standard Time (EAST) which is 3 hours ahead of GMT. Y-axis shows annual rainfall (mm), hour by hour. East coast stations [Fig. 1.7(4)] show a marked minimum in the evening when the easterly trades are augmented by the sea breeze. Eastern slopes of the highlands [Fig. 1.7(5)] show a maximum late in the night when the easterly trades are opposed by the land breeze and the down-slope flow. The highland stations [Fig. 1.7(6)] show a well-marked peak in the afternoon/evening. Figures 1.7(7), 1.7(8) and 1.7(9) show varying and interesting features around Lake Victoria. Kisumu [Fig. 1.7(7)] on the north-eastern shore of Lake Victoria shows a late evening maximum and mid-morning

1-103

minimum. Bukoba [Fig. 1.7(8)] on the west coast of Lake Victoria shows just the opposite, a maximum in the mid-morning and a minimum late in evening_ Kisumu and Bukoba oscillations are just in opposite phases! Kampala [Fig. 1.7(9)] which lies between Kisumu and Bukoba along the coast of Lake Victoria shows two maxima and two minima. These observed patterns need to be quantitatively modelled but in the meanwhile, these leave us in no doubt that it will be mis-leading to try to classify them as continental or marine/coastal types. These arise mainly from the interaction between large-scale and meso-scale circulations. 1.7.2 Diurnal (24.hour) cycle of precipitation and cloudiness over the oceans There have been a number of investigations to see if there is any significant diurnal (24-hour) cycle of precipitation and cloudiness over the oceans. In general, precipitation observation is more quantitative and more dependable than cloud observation. However, for precipitation observation, we need stationary observation platforms. Only stationary ships in the ocean, if any, could provide this information. We have had practically no stationary ships on the open seas in the tropics for reasonably long periods of time to give dependable estimates of the diurnal variation of precipitation. Only recently, during GATE experiment, we had some stationary ships, though for a relatively short period, to give some estimates. The earlier accepted concept has been that due to solar heating of the tropical ground during the day and the presence of convective instability in the tropical atmosphere, there is maximum precipitation over land during afternoon/evening and minimum during the night Such preference of time may not occur over the oceans. On the other hand, after Hann's (1901) classification. it has been believed for a long time that tropical oceanic cloud and rain observations do exhibit maximum amounts and frequency during the hours of darkness, compared to the hours of daylight (Brier and Simpson, 1969). Observationally, the existence of the diurnal cycle as stated above had generally been accepted. However, subsequently, there have

1.7 Diurnal Variation of Precipitation

1-104

EAST COAST ZANZIBAfl OAA-ES-SAlAA"I

MQhlBASA

LAMU MAl.1IHH

'60

~~.

'40

'20 E E foa

," ~_~_-_c_--"_._"--'"lt"_w.:(:;,-: ~

80

60

....... -_.~--t".-.:;'

...."'

40

""'~'

······11..... «..~ ......"(, " --"1'•

I