Air Weather Service Technical Report 240 Forecaster’s Guide to Tropical Meteorology Second Edition, 1991 by Maj. Gary D. Atkinson, USAF revised by C.S. Ramage * DOES NOT CONTAIN GRAPHICS * 1 April 1971 / Revised August 1991 (Second Draft) PDF version published by Weather Graphics weathergraphics.com No copyright claimed No restriction on distribution PREFACE TO SECOND EDITION As correctly forecast in the Preface to the first edition, Forecasters’ Guide to Tropical Meteorol- ogy was used not only to train Air Weather Service meteorologists, but also as a text for university courses. It was even translated into Chinese. Technological advances in the last twenty years, especially in satellite-based observations, have not seriously affected views expressed in the first edition, but have allowed them to be expanded and refined. Both climatology and synoptic meteorology have benefited, while many hitherto obscure parts of the tropics have been exposed to the global eye. In this edition, satellite data and wide-ranging research have allowed the tropics to be treated more evenly and extensively than before. Sections dealing with Africa, the Americas and south and southwest Asia have been expanded. Satellite pictures illustrate tropical systems and processes. Besides these changes, the following topics have been either added to (bold face) or re- moved from (italics) the first edition. Chapter 1. Terminology; heat lows. Chapter 2. Stress-differential along a coast. Chapter 3. Constant-level balloons; surface reports from buoys; Doppler radar; Profiler. Chapter 4. Low-level jet streams. Chapter 5. Upwelling; tradewinds; low-latitude westerlies. Chapter 6. Fog; character of significant rain in the tropics; interannual variation of rainfall and El Niño; seasonal distribution of rainfall and the monsoons; pentad distribution of rainfall. Chapter 7. A new chapter on diurnal variation, combining the scattered discussions of the first edition, and expanding and adding to them. New sections on vertical mixing, stratocu- mulus and fog, and squall lines. Chapter 8 (formerly Chapter 7). Equatorial waves; monsoon depressions; squall lines; surges; troughs in the upper-tropospheric westerlies; South Pacific convergence zone; superposition of tropical and extra-tropical disturbances; near-equatorial convergence zones Chapter 10 (formerly Chapter 9). Duststorms. Chapter 11 (formerly Chapter 10). Isogon analysis; kinematic frontal analysis. Chapter 12 (formerly Chapter 11). Stability indexes (reduced); preparing to forecast in a new area; 30-60 day cycles; Atlantic tropical storms (seasonal forecasts); preparing to fore- cast in a new area. FIGURES: 137 have been retained and 171 added. REFERENCES: 177 have been retained and 367 added. The range of topics and locations cannot be simply dealt with sequentially; to avoid repeti- tion, extensive cross-referencing has been used. Detailed subject, geographical and author indexes have been added, as well as a fold-out locator map. ACKNOWLEDGMENTS. Many present and former members of Air Weather Service con- tributed to the preparation of this edition, including the sixteen who carefully and con- structively reviewed the first and second drafts, and those who provided weather satellite pictures, bibliographies and publications. I particularly appreciated Garry Atkinson’s comments. Louis Oda drafted and revised the figures, Thomas Schroeder, was responsible for administration. Others who helped included Mike Gentry, William Gray, Barry Hinton, Richard Orville, Patrick Sham, Norman Wagner, J.F. White, Scott Woodruff, and X. Ziang. My wife Alice proofread. C.S. Ramage 1991 TABLE OF CONTENTS CHAPTER 1 - INTRODUCTION CHAPTER 2 - PRIMARY PHYSICAL CONTROLS OF THE TROPICAL CIRCULATION CHAPTER 3 - THE OBSERVATIONAL BASIS CHAPTER 4 - PRESSURE AND WINDS CHAPTER 5 - TEMPERATURE AND WATER VAPOR CHAPTER 6 - CLOUDINESS AND RAINFALL CHAPTER 7 - DIURNAL VARIATIONS CHAPTER 8 - TROPICAL SYNOPTIC MODELS CHAPTER 9 - TROPICAL CYCLONES CHAPTER 10 - SEVERE WEATHER IN THE TROPICS CHAPTER 11 - TROPICAL ANALYSIS CHAPTER 12 - TROPICAL FORECASTING Chapter 1 INTRODUCTION A. General each year, but since 1950 more than 5000 operational and research reconnaissance flights have been made into cyclone centers. The urge to understand tropical meteorol- By contrast, one Arabian Sea mid-tropo- ogy has been driven by a variety of forces: spheric cyclone, one monsoon depression, the devastation caused by tropical cyclones; and two upper-tropospheric cold lows have demands by military operations, e.g. World been reconnoitered. Fifteen single-aircraft War II, the Marshall Islands weapons tests, research traverses have been made across and the conflict in southeast Asia; and the the near-equatorial convergence zone, but belief of influential mid-latitude meteorolo- none into the mysterious weak circulations gists that the tropics may hold the key to that can cause disastrous floods. For most successful forecasts of global weather pat- generating regions, about one hundred terns and climate change. Efforts by World years of daily tropical cyclone positions Weather Watch (WWW) and the Global provide the basis of detailed, authoritative Atmospheric Research Program (GARP), climatologies. Apart from monsoon depres- including the GARP Atlantic Tropical Ex- sions, not even a year of tracks has been periment (GATE) of 1974 and the Winter plotted for other tropical vortexes. and Summer Monsoon Experiments (WMONEX and SMONEX) of December This disproportion, though slightly reduced 1978 and June-July 1979, gave many meteo- by squall-line studies in GATE, Venezuela rologists their first experience of the tropics. and the Amazon, and a 1987 field study of Research papers proliferated, concentrating early-summer rain systems off southeast on the small time and space scales described China (Taiwan Area Mesoscale Experiment by the special observing systems (aerial —TAMEX), means that the meteorologist reconnaissance, rawinsondes, and radars) trying to forecast tropical cyclones is served set up for the experiments. by a variety of subjective, numerical and statistical techniques, by special services But where does all this leave the field fore- from global prediction centers, and by 15- caster? A case study dependent on data not minute satellite updates. For the other 99.9 operationally available, and supported by percent of the time, forecasters are pretty brief, doubtful statistics, doesn’t help much much on their own. This manual tries to in the day-to-day grind of tropical analysis redress the balance. It devotes one chapter and forecasting. to tropical cyclones, while emphasizing Predicting tropical cyclones taps almost all other weather systems, background the resources available to aid the tropical climatologies, and forecasting hints and forecaster. Riehl (1979) devotes a chapter to pitfalls. numerical hurricane prediction and nothing Observational information on the tropics to other tropical forecasting. On an average increased significantly through the 1970s, day in the tropics less than one tenth of one though progress has been patchy over percent of the area is affected by a tropical Africa and the Americas. Weather radars cyclone. Only about 80 cyclones develop operate at many tropical stations, and there is great potential for more aircraft weather of tropical meteorology in much greater reports. Compared to twenty years ago, detail than is possible here. human observations are now fewer; auto- The forecaster is viewed as an applied mated observations, especially those from researcher who can evaluate hypotheses and satellites, are vastly more, and are not yet come up with new ideas. He should realize fully exploited. that in the tropics weather mechanisms are There is an urgent need to synthesize from often not understood. “Authoritative” opin- these diverse observational systems a ions may differ, for example, on the preva- coherent basis for the analysis and predic- lence of easterly waves, on the cause of the tion of tropical circulations and associated quasi-biennial oscillation, on predicting El weather. Our ability to digest and use this Niño, and on generation, movement and information in tropical numerical models dissipation of surges. Even when there is has not kept pace with numerical weather general agreement, such as on tropical prediction in middle latitudes. This is par- cyclone dissipation, diurnal variations along tially due to the greater complexity of a mountainous coast, and the role of surface weather systems and interactions between cooling in causing fog, apparent exceptions various scales of motion in the tropics. Thus, may occur. Chapters 2 through 6, besides although world-wide predictions are being including a chapter (3) on data, focus on the made, at least in the near future, tropical annual cycle and monthly averages, while weather forecasting will continue to depend making some attempt to interpret them in heavily on subjective judgment aided by terms of weather. Chapter 7 covers diurnal selected numerical products such as objec- effects. The remaining chapters concern tive wind analyses, global long-wave predic- weather analysis and forecasting. Here tions and computer-processed satellite data. astronomical control by the annual and diurnal cycles is muted by complex feed- West Pacific typhoons and Atlantic hurri- backs among atmosphere, ocean, and ter- canes are the same phenomenon — they rain. need not be separately described. Thunder- storms also possess the same characteristics world-wide. What is not so well-known