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Unformatted text preview: Desert Meteorology Aridity prevails over more than one third of the land area of Earth, and over a significant fraction of the oceans as well. Yet to date there has been no comprehensive reference volume or textbook dealing with the weather processes that define the character of desert areas. Desert Meteorology fills this gap by treating all aspects of desert weather, such as large-scale and local-scale causes of aridity, precipitation characteristics in deserts, dust storms, floods, climate change in deserts, precipitation processes, desertification, the land-surface physics of deserts, numerical modeling of desert atmospheres, and the effect of desert weather on humans. A summary is provided of the climates and surface properties of the desert areas of the world. The book is written with the assumption that the reader has only a basic knowledge of meteorology, physics, and calculus, making it useful to those in a wide range of disciplines. It includes review questions and problems for the student. This comprehensive volume will satisfy all who need to know more about the weather and climate of arid lands. It will appeal especially to advanced students and researchers in environmental science, meteorology, physical geography, hydrology, and engineering. tom warner was a Professor in the Department of Meteorology at the Pennsylvania State University before accepting his current joint appointment with the National Center for Atmospheric Research and the University of Colorado in Boulder. Professor Warner’s career has involved teaching and research in mesoscale meteorological processes and in numerical weather prediction, and he has published on these subjects in numerous professional journals. His recent research and teaching have focussed on atmospheric processes and operational weather prediction in arid areas. Desert Meteorology T H O M A S T. WA R N E R cambridge university press Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge cb2 2ru, UK Published in the United States of America by Cambridge University Press, New York Information on this title: © Thomas T. Warner 2004 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2004 isbn-13 isbn-10 978-0-511-18462-8 eBook (NetLibrary) 0-511-18462-x eBook (NetLibrary) isbn-13 isbn-10 978-0-521-81798-1 hardback 0-521-81798-6 hardback Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. To my mother and father, Dorothy and Tom, who never lost faith, and to my wife Susan “Water, thou hast no taste, no color, no odor; canst not be defined, art relished while ever mysterious. Not necessary to life, but rather life itself, thou fillest us with a gratification that exceeds the delight of the senses.” Antoine de Saint-Exup´ery, Wind, Sand and Stars (1939) Contents Preface Acknowledgements page xiv xvi 1 Introduction 1 2 The atmospheric dynamics of deserts 7 Some basic concepts of atmospheric structure and dynamics Definition of desert Climatological causes of aridity Dynamic feedback mechanisms that may cause and sustain deserts The dynamics of desert heat lows Suggested general references for further reading Questions for review Problems and exercises 3 The climates of the world deserts General meteorological characteristics of deserts General physiographic characteristics of deserts General vegetative characteristics of deserts African deserts North American deserts South American deserts Australian deserts Asian deserts European arid areas Ocean deserts Suggested general references for further reading Questions for review Problems and exercises 8 21 27 53 58 60 61 61 63 68 75 77 79 104 115 126 130 149 150 151 157 157 ix x Contents 4 Atmospheric and surface energy budgets of deserts Components of the atmospheric and surface energy budgets Atmospheric and surface energy budgets of deserts Suggested general references for further reading Questions for review Problems and exercises 159 160 176 186 186 187 5 Surface physics of the unvegetated sandy desert landscape 189 Introduction to the concepts of land-surface physics Land-surface physics of unvegetated sandy desert Suggested general references for further reading Questions for review Problems and exercises 190 205 217 218 218 6 Vegetation effects on desert surface physics 219 The effects of vegetation on the desert surface energy budget The effects of vegetation on the desert surface water budget The effects of vegetation on desert winds near the surface Suggested general references for further reading Questions for review Problems and exercises 7 Substrate effects on desert surface physics Types of desert substrate Effects of desert substrate types on the surface energy budget Effects of desert substrate types on the surface water budget Suggested general references for further reading Questions for review Problems and exercises 8 Desert-surface physical properties Albedo Thermal properties Aerodynamic roughness Emissivity Hydraulic properties Suggested general references for further reading Questions for review Problems and exercises 9 Numerical modeling of desert atmospheres General concept of numerical weather prediction Some examples of atmospheric model applications in arid areas Suggested general references for further reading 224 231 238 243 243 243 245 247 256 259 263 264 264 265 266 266 266 269 270 270 270 270 271 272 273 289 Contents Questions for review Problems and exercises 10 Desert boundary layers Basic concepts of boundary-layer structure Unique aspects of desert boundary layers Suggested general references for further reading Questions for review Problems and exercises 11 Desert microclimates Causes of intra-desert microclimates Examples of intra-desert microclimate variability Suggested general references for further reading Questions for review Problems and exercises 12 Dynamic interactions among desert microclimates Thermally forced wind circulations in desert environments Oasis effects Suggested general references for further reading Questions for review Problems and exercises 13 Desert rainfall Statistical characterization of desert rainfall Causes of desert rainfall, and its modulation Artificial rainfall enhancement in arid areas Dew and fog deposition in the desert Suggested general references for further reading Questions for review Problems and exercises 14 Anthropogenic effects on the desert atmosphere Irrigation Water-table deepening Grazing and overgrazing Introduction of non-native vegetation Dryland agriculture Urbanization Off-road vehicle use Deforestation Suggested general references for further reading Questions for review Problems and exercises xi 289 290 291 291 297 301 301 301 303 304 308 325 325 325 327 328 344 346 346 346 347 353 372 378 379 382 382 382 383 385 387 388 390 390 391 391 392 393 393 393 xii Contents 15 Changes in desert climate Time scales of climate and aridity change A summary of recent climate periods Methods of estimating climate change in arid regions Examples of changes in desert climate during the Late Pleistocene and Holocene Manifestations and causes of climate change in deserts Suggested general references for further reading Questions for review Problems and exercises 16 Severe weather in the desert Dust storms and sand storms Rainstorms, floods, and debris flows Suggested general references for further reading Questions for review Problems and exercises 17 Effects of deserts on the global environment and other regional environments Global and regional transport of desert dust: background Global climate effects of desert dust Regional and global human-health effects of desert dust Distant ecological effects of desert dust Dynamic effects of deserts on meteorological processes in other regions Suggested general references for further reading Questions for review Problems and exercises 18 Desertification 395 396 397 398 401 403 416 417 417 419 420 439 443 443 444 445 446 451 452 453 453 454 455 455 457 What is desertification? The extent of desertification Anthropogenic contributions to desertification Natural contributions to desertification Additional selected case studies and examples of desertification Physical-process feedbacks that may affect desertification Satellite-based methods for detecting and mapping desertification Suggested general references for further reading Questions for review Problems and exercises 459 462 466 476 476 483 486 488 489 489 19 Biometeorology of humans in desert environments 491 The heat balance of the human body in the desert The process of maintaining the heat and water balance 492 499 Contents xiii Acclimatization to the desert environment Economizing water and reducing the heat load Physical effects of heat stress and dehydration Physical effects of desert mineral dust Psychological effects of deserts Measures of heat stress Electrostatic effects Suggested general references for further reading Questions for review Problems and exercises 503 507 511 515 516 517 517 517 518 518 20 Optical properties of desert atmospheres 519 Mirages Effects of atmospheric dust Scintillation Suggested general references for further reading Questions for review Problems and exercises 520 521 525 526 526 526 Appendix A Glossary of meteorological and land-surface terms 527 Appendix B Abbreviations 535 Appendix C Units, numerical constants, and conversion factors 536 Appendix D Symbols 538 Appendix E Maps of the world 541 Hints to solving some problems and exercises 547 References 549 Index 587 Colour plates between pp. 157 and 158 Preface This book is intended as a text and as a reference book for students from a range of disciplines, not just the atmospheric sciences. However, it is expected that students will have had at least an introductory, undergraduate, non-technical course in weather. The equations will have to be interpreted by an instructor for those without preparation in physical sciences and mathematics with calculus. And the short refresher tutorials on various topics at the beginning of some chapters will likely be skipped by those with more rigorous backgrounds in atmospheric sciences. At the end of each chapter are lists of suggested general references for further reading, questions for review, and problems and exercises. The references represent a spectrum of difficulty levels; some are qualitative whereas others may be quite technically oriented. Students without a technical background will need to choose from the qualitative references, of which there are many. The review questions are provided as study aids. The technical discipline of each student, which has served as the motivation for studying this subject, will ultimately determine what material is most germane. Thus, the student and the instructor should add their own study questions to those provided. The problems and exercises are sometimes sufficiently challenging that background reading in other texts in atmospheric sciences may be required. A number of the problems require some mathematics, and these are most appropriate for students with such a background. Hints to solutions of some of the problems can be found at the back of the book. Metric units will be used throughout the book. Exceptions are limited to providing near-surface temperatures in degrees Fahrenheit as well as Celsius. Technical words will often be printed in bold the first time that they are used in the text in order to emphasize that they are important and that their meaning should be remembered. If the bold words are not defined in the text or in a footnote, a brief definition will be found in the glossary in Appendix A. Also provided are lists of symbols employed and their meaning, abbreviations, and conversion factors xiv Preface xv and physical constants (Appendixes B–D). Maps with national boundaries and country names are included in Appendix E to serve as a temporary substitute for a good world atlas, which should be available and consulted frequently. Most city names used in the text are located on the maps. The fact that this is one of the only books exclusively devoted to desert meteorology has served as motivation for trying to make it useful to a broad audience, with both technical and non-technical backgrounds. It has also been the aim to provide as even a treatment as possible for all of the world’s deserts, to make it uniformly useful to students and researchers worldwide. This aim has been occasionally compromised because many good studies of desert meteorology are not readily accessible to a Western author, especially when they are available in report form only. It is nevertheless hoped that subject-area omissions that have resulted from the unavailability of some literature are minor. Throughout the book are inset boxes containing short informal presentations of special topics that generally relate in some way to the subjects treated in the text. The material is primarily of human interest, and is intended to provide the reader with light treatments of some subjects related to arid-land meteorology that do not fit within the more formal text presentations. Even though it has been the primary aim to provide a complete technical treatment of the various aspects of desert meteorology, it is hoped that the reader also develops a subjective impression of the desert environment in terms of its spiritual effects on people, the importance of protecting it from further degradation, and the excitement associated with exploring and appreciating one of the few remaining frontiers on the planet. To facilitate using this book as a text in desert meteorology, course materials are provided free of charge on the author’s web site, which can be accessed through . Included are Microsoft PowerPoint ® files that are used by the author in a desert meteorology course at the University of Colorado, Boulder. Acknowledgements Numerous libraries provided reference material and human resources, including those of the National Center for Atmospheric Research; The University of Arizona, Office of Arid Lands Studies; and the University of Colorado, Boulder. Data were prepared by Hilary Justh, Hsiao-Ming Hsu, and Seth Linden. Technical discussions and manuscript reviews were provided by Elford Astling, Barbara Brown, Toby Carlson, Fei Chen, Brant Foote, Margaret LeMone, Ron Smith, and David Yates. Technical assistance with the manuscript preparation was provided by Inger Gallo, Dara Houliston, Candace Larsen, Carol Makowski, and Carol Park. The graphical design of figures was by Justin Kitsutaka. Encouragement and valuable assistance in many forms were provided by Matt Lloyd, Editor, Jayne Aldhouse, Production Editor, and Lynn Davy, Subeditor, all of Cambridge University Press. xvi 1 Introduction I shall never be able to express clearly whence comes this pleasure men take from aridity, but always and everywhere I have seen men attach themselves more stubbornly to barren lands than to any other. Men will die for a calcined, leafless stony mountain. The nomads will defend to the death their great store of sand as if it were a treasure of gold dust. And we, my comrades and I, we too have loved the desert to the point of feeling that it was there we had lived the best years of our lives. Antoine de Saint Exup´ery, French aviator and writer Wind, Sand and Stars (1939) If one is inclined to wonder at first how so many dwellers came to be in the loneliest land that ever came out of God’s hands, what they do there and why stay, one does not wonder so much after having lived there. None other than this long brown land lays such a hold on the affections . . . once inhabiting there you always mean to go away without quite realizing that you have not done it. Mary Austin, American naturalist and writer The Land of Little Rain (1903) Deserts, in spite of the popular perception of their uniformity over vast distances, often contain within them a complex mosaic of microclimates and local weather. Great contrasts also exist in the climate and surface characteristics from one desert to the next. There are “cold” deserts and hot deserts, deserts with winter precipitation and deserts with summer precipitation, deserts with virtually no precipitation, perpetually foggy coastal deserts and continental deserts with near the maximum possible sunshine, barren deserts and heavily vegetated deserts, sand-dune deserts and deserts with rocky plains. The existence of such variety and complexity in deserts, and their ubiquity as well, are important messages of this book. The expression “desert weather” has a peculiar ring to mid-latitude-centric meteorologists. It even strikes some as a contradiction in terms. This 1 2 1 Introduction preconception is belied by the facts. When it does rain in the desert it sometimes results in a violence that is rarely matched in more temperate places. Dry riverbeds that have lain parched for decades pass a wall of water, after which they may lie parched again for years. In some places, the day–night temperature change is immense, and larger than that associated with most mid-latitude frontal passages. And, most who have experienced a desert sand storm or dust storm would likely tell you that the discomfort level and fear are much greater than, for example, in snow storms. The existence and distribution of desert climates have profoundly shaped historical patterns of human travel, settlement, economic development, and communication, where the deserts seem to have curiously served as both barriers and attractions. Most of the world’s early great civilizations developed at the margins of deserts. Virtually all of the world’s great contemporary religions were born in desert regions: Judaism, Islam, Christianity, Hinduism, and Buddhism. The Aborigines of arid Australia are thought to have the oldest continuously maintained culture and the world’s oldest language family. And the world’s longest continuously inhabited human settlement is in the desert (Jericho, Jordan). Deserts are still significant as physical barriers, even in this age when technology has allowed many of us to become insulated from our environment. In some respects, the greater deserts of the world have been, and still are, more important than mountain barriers or seas in terms of inhibiting surface transportation and commerce. In addition, with a few notable exceptions in hightechnology societies, recent human habitation has not encroached significantly into arid, especially highly arid, areas. It is thus arguable that the deserts represent one of the last terrestrial frontiers in terms of human dominance over, and exploitation of, the natural environment. Most humankind understandably feel culturally, emotionally, and geographically distant from desert environments. However, the green conditions that we take for granted may be ephemeral on time scales shorter than we imagine. Prolonged and unpredictable droughts are natural and frequent occurrences in some areas, and human degradations of the environment have made vast oncegreen places become desert. Our perceptions can quickly change when dust fills the air, and water sources that have been taken for granted for centuries are no longer available. Fig. 1.1 illustrates that only about 3% of Earth’s water is fresh (non-saline), and much of this is not readily usable because it is in the form of permanent ice and snow, and permafrost. Of the remaining water, only 1% is available at the surface in rivers, lakes and vegetation. The remainder is groundwater, some of which is not renewe...
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