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 compre- hensive 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 character- istics in deserts, dust storms, floods, climate change in deserts, precipitation processes, desertification, the land-surface physics of deserts, numerical mod- eling 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 wasaProfessor in the Department of Meteorology at the Penn- sylvania 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 THOMAS T. WARNER 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 www.cambridge.org Information on this title: www.cambridge.org/9780521817981 © 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-13 978-0-511-18462-8 eBook (NetLibrary) isbn-10 0-511-18462-x eBook (NetLibrary) isbn-13 978-0-521-81798-1 hardback isbn-10 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 page xiv Acknowledgements xvi 1 Introduction1 2 The atmospheric dynamics of deserts7 Some basic concepts of atmospheric structure and dynamics8 Definition of desert 21 Climatological causes of aridity 27 Dynamic feedback mechanisms that may cause and sustain deserts 53 The dynamics of desert heat lows 58 Suggested general references for further reading 60 Questions for review 61 Problems and exercises 61 3 The climates of the world deserts 63 General meteorological characteristics of deserts 68 General physiographic characteristics of deserts 75 General vegetative characteristics of deserts 77 African deserts 79 North American deserts 104 South American deserts 115 Australian deserts 126 Asian deserts 130 European arid areas 149 Ocean deserts 150 Suggested general references for further reading 151 Questions for review 157 Problems and exercises 157 ix x Contents 4 Atmospheric and surface energy budgets of deserts 159 Components of the atmospheric and surface energy budgets 160 Atmospheric and surface energy budgets of deserts 176 Suggested general references for further reading 186 Questions for review 186 Problems and exercises 187 5 Surface physics of the unvegetated sandy desert landscape 189 Introduction to the concepts of land-surface physics 190 Land-surface physics of unvegetated sandy desert 205 Suggested general references for further reading 217 Questions for review 218 Problems and exercises 218 6Vegetation effects on desert surface physics 219 The effects of vegetation on the desert surface energy budget 224 The effects of vegetation on the desert surface water budget 231 The effects of vegetation on desert winds near the surface 238 Suggested general references for further reading 243 Questions for review 243 Problems and exercises 243 7 Substrate effects on desert surface physics 245 Types of desert substrate 247 Effects of desert substrate types on the surface energy budget 256 Effects of desert substrate types on the surface water budget 259 Suggested general references for further reading 263 Questions for review 264 Problems and exercises 264 8 Desert-surface physical properties 265 Albedo 266 Thermal properties 266 Aerodynamic roughness 266 Emissivity 269 Hydraulic properties 270 Suggested general references for further reading 270 Questions for review 270 Problems and exercises 270 9 Numerical modeling of desert atmospheres 271 General concept of numerical weather prediction 272 Some examples of atmospheric model applications in arid areas 273 Suggested general references for further reading 289 Contents xi Questions for review 289 Problems and exercises 290 10 Desert boundary layers 291 Basic concepts of boundary-layer structure 291 Unique aspects of desert boundary layers 297 Suggested general references for further reading 301 Questions for review 301 Problems and exercises 301 11 Desert microclimates 303 Causes of intra-desert microclimates 304 Examples of intra-desert microclimate variability 308 Suggested general references for further reading 325 Questions for review 325 Problems and exercises 325 12 Dynamic interactions among desert microclimates 327 Thermally forced wind circulations in desert environments 328 Oasis effects 344 Suggested general references for further reading 346 Questions for review 346 Problems and exercises 346 13 Desert rainfall 347 Statistical characterization of desert rainfall 353 Causes of desert rainfall, and its modulation 372 Artificial rainfall enhancement in arid areas 378 Dew and fog deposition in the desert 379 Suggested general references for further reading 382 Questions for review 382 Problems and exercises 382 14 Anthropogenic effects on the desert atmosphere 383 Irrigation 385 Water-table deepening 387 Grazing and overgrazing 388 Introduction of non-native vegetation 390 Dryland agriculture 390 Urbanization 391 Off-road vehicle use 391 Deforestation 392 Suggested general references for further reading 393 Questions for review 393 Problems and exercises 393 xii Contents 15 Changes in desert climate 395 Time scales of climate and aridity change 396 A summary of recent climate periods 397 Methods of estimating climate change in arid regions 398 Examples of changes in desert climate during the Late Pleistocene and Holocene 401 Manifestations and causes of climate change in deserts 403 Suggested general references for further reading 416 Questions for review 417 Problems and exercises 417 16 Severe weather in the desert 419 Dust storms and sand storms 420 Rainstorms, floods, and debris flows 439 Suggested general references for further reading 443 Questions for review 443 Problems and exercises 444 17 Effects of deserts on the global environment and other regional environments 445 Global and regional transport of desert dust: background 446 Global climate effects of desert dust 451 Regional and global human-health effects of desert dust 452 Distant ecological effects of desert dust 453 Dynamic effects of deserts on meteorological processes in other regions 453 Suggested general references for further reading 454 Questions for review 455 Problems and exercises 455 18 Desertification 457 What is desertification? 459 The extent of desertification 462 Anthropogenic contributions to desertification 466 Natural contributions to desertification 476 Additional selected case studies and examples of desertification 476 Physical-process feedbacks that may affect desertification 483 Satellite-based methods for detecting and mapping desertification 486 Suggested general references for further reading 488 Questions for review 489 Problems and exercises 489 19 Biometeorology of humans in desert environments 491 The heat balance of the human body in the desert 492 The process of maintaining the heat and water balance 499 Contents xiii Acclimatization to the desert environment 503 Economizing water and reducing the heat load 507 Physical effects of heat stress and dehydration 511 Physical effects of desert mineral dust 515 Psychological effects of deserts 516 Measures of heat stress 517 Electrostatic effects 517 Suggested general references for further reading 517 Questions for review 518 Problems and exercises 518 20 Optical properties of desert atmospheres 519 Mirages 520 Effects of atmospheric dust 521 Scintillation 525 Suggested general