A deeper philosophy of airflows

Most textbooks on dynamic meteorology, the study of atmospheric motions as solutions to a set of governing equations, tend to be rather formal and dry. John Green's Atmospheric Dynamics is a welcome exception. Composed from a set of lectures he has delivered to students for many years, it emphasises fundamental derivations and thought processes over perfunctory discussions of complicated computer simulations. Written with wit, peppered with personal anecdotes, and illustrated with simple physical problems, this text is a pleasure to read and acts as a useful reference book as well.

The coverage is fairly broad. After a discussion of how meteorologists describe atmospheric phenomena, a very brief exposition of symbols and coordinate systems, and a concise review of the fundamental equations that determine how air moves, Green focuses on the analysis of the equations and their application to weather systems. When doing the latter, one is compelled to make simplifications to the equations, a move that leads to approximations that allow certain types of wave motion that owe their existence to the compressibility of the atmosphere, to the earth's rotation, and to the nature of the boundary conditions assumed. The discussion therefore includes Rossby waves, inertial-gravity waves, sound waves, and Kelvin waves. Gravity waves and aspects of their propagation in the face of obstacles (ie mountains) are treated nicely, although it would have been helpful to have included a few photographs of wave clouds and related phenomena. He then reviews theories of instabilities in the flow as a result of shear.

Vertical convection (by cumulus and cumulonimbus clouds) is summarised much too quickly; what is presented in 11 pages in chapter seven is usually treated as a whole text. A discussion of "boundary-layer" dynamics, which is squeezed into the beginning of the chapter on mesoscale motion, is rather brief. In addition, the exposition of the dynamics of mesoscale features such as groups of convective storms, tropical cyclones, and the sea breeze, which are dominated by neither gravity waves or

quasi-geostrophy, is also abbreviated.

The strongest sections are the chapters on large-scale motions, the evolution of baroclinic waves, and the properties of wave motion in a barotropic atmosphere.

Only the most fundamental aspects of modelling are presented. Students are well advised to look elsewhere to learn about actual modelling. However, Green carefully discusses the philosophy of modelling and has the reader consider why one uses models and why one should simplify the models as much as possible so as to preclude spurious effects that may be hidden by complexity.

Transport and mixing are discussed in the penultimate chapter, which smoothly makes the transition to the final chapter on the general circulation.

I recommend this book mainly to students who have already completed a formal course in dynamics. Since it was written for students "already familiar with the subject", it is not suitable as a first text at the undergraduate level. There are no problems or exercises listed, the history of topics is mostly ignored, and there are few references. The forte of Green's book lies in its ability to help a student who has already had introductory courses in the subject enhance his or her understanding of the subject and to delve more deeply into the philosophy of dynamic meteorology. Graduate students in meteorology who are studying for qualifying exams would benefit substantially from this book.

Some problems in atmospheric science are interdisciplinary. Radiation, air chemistry, precipitation and cloud physics, oceanography, and hydrology all play important roles in the general circulation of the atmosphere and climate. In recent years significant improvements in our understanding of the energy balance of the earth and of the hydrologic cycle have been made possible by advances in computer technology and the technology of remote sensing of the earth from satellites. A more active dialogue has been initiated among scientific disciplines that had previously remained isolated from each other. Global Energy and Water Cycles , edited by K. A. Browning and R. J. Gurney, is a collection of short reviews by prominent, internationally recognised scientists that brings together in one volume the results of research in the spirit of the movement towards an "integrated earth system science". The text is based on contributions to a conference held in London in 1994 on the Global Energy and Water Cycle Experiment (Gewex), an international project sponsored by the World Climate Research Program.

R. Rosen, a former student of V. P. Starr and E. N. Lorenz, two pioneers in the study of large-scale atmospheric processes, leads with a thorough review of the basic aspects of the global energy cycle. He brings the reader up to date on research in this area and describes the challenges for future investigations. A parallel treatment of the global water cycle follows by T. Oki.

The scope of the volume is wide. Computer modelling of the atmosphere, coupled atmosphere-ocean models, and how to create an accurate estimate of the initial conditions for the models are well covered. There are detailed discussions of radiation and how it is affected by clouds, water vapour, and aerosols.

The outline of the different types of precipitating cloud systems, could be improved, however. The treatment of mid-

latitude convective storms is overly simplified in places. For example, in the few lines devoted to supercell storms it is incorrectly implied that veering of the vertical shear is necessary.

Techniques for estimating precipitation remotely from earth and space, and air-sea fluxes, are thoroughly reviewed. Important aspects of oceanographic processes, turbulent air-land-sea interaction, vegetative processes important in water exchange between the ground and the air, and hydrology, are also comprehensively treated.

This ambitious and handsomely produced book succeeds in explaining the achievements and challenges of studies in the sub-disciplines of global energy and water cycles. Enough information is provided so that readers not familiar with a sub-discipline can appreciate what the basic physics and the problems are. Reference lists are provided so that specialised researchers can broaden their horizons. Graduate students and researchers in meteorology, oceanography, and hydrology will find this volume very useful.

Since it is based on a conference held six years ago, there will inevitably be some gap between what we know now and what we knew then. Although in several years this book will be a decade out of date, the reviews are sufficiently comprehensive to be of use. My main criticism is that some of the reviews are less comprehensive than others, and that some reference lists make use of textbooks, rather than a few important original articles. Overall, however, I commend the editors and authors for skilfully assembling a wide range of material.

Howard B. Bluestein is professor of meteorology, University of Oklahoma in Norman, United States.