This chapter gives a quick sketch of some of the material to be covered in this book. We start in Section 1.1 with an outline of some of the more important physical processes that occur in the Earth's atmosphere. To interpret atmospheric observations we need to develop physical and mathematical models; they are briefly discussed in Section 1.2. In Section 1.3 two simple models are introduced; the second of these is a very basic representation of the greenhouse effect, which can be adapted to give some insight into aspects of global warming. In Section 1.4 we present a selection of observations of atmospheric processes, together with simple physical explanations for some of them. In Section 1.5 we briefly mention some ideas on weather and climate.

The atmosphere as a physical system

The Earth's atmosphere is a natural laboratory, in which a wide variety of physical processes takes place. The purpose of this book is to show how basic physical principles can help us model, interpret and predict some of these processes. This section presents a brief overview of the physics involved.

The atmosphere consists of a mixture of ideal gases: although molecular nitrogen and molecular oxygen predominate by volume, the minor constituents carbon dioxide, ozone and water vapour play crucial roles. The forcing of the atmosphere is primarily from the Sun, though interactions with the land and the ocean are also important.

In keeping with the emphasis on atmospheric physics in this book, the purpose of the present chapter is to illustrate the use of basic physical principles in the study of some aspects of atmospheric chemistry, rather than to provide a comprehensive treatment of atmospheric chemistry as a whole. We therefore focus on stratospheric chemistry, which provides some simple yet important applications of the basic principles and also some examples of interactions between chemistry and dynamics.

In Section 6.1 we outline some of the basic thermodynamics of chemical reactions, while in Section 6.2 we introduce some elementary aspects of chemical kinetics, including the concepts of reaction rates and chemical lifetimes. In Section 6.3 we focus on bimolecular reactions and show how physical reasoning can give an expression for the reaction rate. The process of photodissociation is introduced in Section 6.4. Once these basic ideas have been established, we apply them to stratospheric ozone in Section 6.5, first describing the Chapman theory (which involves oxygen compounds only) and then introducing the effects of catalytic cycles. The principles of chemical transport by atmospheric flows are discussed in Section 6.6, with a qualitative description of the main global-scale meridional transport structures in the middle atmosphere. Finally, in Section 6.7, we bring several of these ideas together in a general description of the processes implicated in the formation of the Antarctic ozone hole.

Atmospheric physics has a long history as a serious scientific discipline, extending back at least as far as the late seventeenth century. Today it is a rich and fascinating subject, sustained by detailed global observations and underpinned by solid theoretical foundations. It provides an essential tool for tackling a wide range of environmental questions, on local, regional and global scales. Although the solutions to vital and challenging problems concerning weather forecasting and climate prediction rely heavily on the use of supercomputers, they rely even more on the imaginative application of soundly based physical insights.

This book is intended as an introductory working text for third or fourth-year undergraduates studying atmospheric physics as part of a physics, meteorology or environmental science degree course. It should also be useful for graduate students who are studying atmospheric physics for the first time and for students of applied mathematics, physical chemistry and engineering who have an interest in the atmosphere.

Modern scientific study of the atmosphere draws on many branches of physics. I believe that a balanced introductory course in atmospheric physics should include at least some atmospheric thermodynamics, radiative transfer, atmospheric fluid dynamics and elementary atmospheric chemistry. Armed with some understanding of these topics, the interested student will be able to grasp the essential physics behind important issues of current concern, such as the amplification of the greenhouse effect and associated questions of climatic change, the Antarctic ozone hole and global depletion of ozone, as well as more familiar processes such as the formation of raindrops and the development of weather systems.

In this chapter we consider a small selection of techniques for observing the atmosphere. These techniques have been chosen for two main reasons: (a) they illustrate the use of physical principles, including principles introduced earlier in this book; and (b) they provide crucial data on atmospheric phenomena modelled elsewhere in this book, such as Rossby waves, gravity waves and the Antarctic ozone hole. The topics considered are all examples of remote sounding; we do not attempt to present a balanced account of all observational methods.

In Section 7.1 we briefly list some of the main atmospheric observational methods. In Section 7.2 we outline the principles of remote sounding of the atmosphere from space, focusing on methods that rely on thermal emission from atmospheric gases and on scattering of solar radiation by atmospheric gases. Then in Section 7.3 we discuss three types of ground-based remote sounding, namely the Dobson spectrophotometer, radars and lidars. We omit the details of the instruments' optical and electronic systems, the technicalities of signal processing and the sophisticated statistical methods that may be required in order to extract meaningful physical quantities from the raw measurements.

Atmospheric observations

Quantitative observations of the atmosphere are made in many different ways. Routine meteorological measurements of ground-level temperature and wind are made with simple thermometers and anemometers, respectively, and routine measurements of temperature and humidity through the depth of the troposphere are made with balloon-borne instruments (radiosondes) that transmit information back to the surface by radio.

This chapter describes some aspects of energy transfer by electromagnetic radiation in the atmosphere. In Section 3.1 we introduce the Planck function, the solar spectrum and the concept of local thermodynamic equilibrium. In Section 3.2 we list some formal definitions of radiometric quantities and then derive and solve the radiative-transfer equation, which describes the way in which radiative power is affected by extinction and emission of radiation. In Section 3.3 we present some basic aspects of molecular spectrosopy and give some of the properties of spectral line shapes. In Section 3.4 we introduce the concept of transmittance, the fraction of radiative power that survives propagation from one point to another. In Section 3.5 we consider the absorption and emission of infra-red radiation and the absorption of ultra-violet radiation by gases in the atmosphere. This absorption and emission lead to heating and cooling; the principles of the calculation of heating rates are outlined in Section 3.6. In Section 3.7, we revisit the greenhouse effect, investigating a more realistic model than that described in Section 1.3.2. Finally, in Section 3.8, we discuss a simple model of atmospheric scattering.

The solution of the radiative transfer equation also plays an important role in certain aspects of atmospheric remote sounding. This will be covered in Chapter 7.

It is an unfortunate fact that quantitative calculations of radiative heating rates, for example, involve considerable geometric and algebraic detail, which tend to distract attention from the basic physics of the processes.

The atmosphere is a fluid in which a wide variety of flows occurs. This chapter introduces the basic fluid-dynamical laws that govern these atmospheric flows. The length scales of interest range from metres to thousands of kilometres; these are many orders of magnitude greater than molecular scales such as the mean free path, at least in the lower and middle atmosphere. We may therefore average over many molecules, ignoring individual molecular motions and regarding the fluid as continuous. ‘Local’ values of quantities such as density, temperature and velocity may be defined at length scales that are much greater than the mean free path but much less than the scales on which the meteorological motion varies.

In Section 4.1 we derive the mass-conservation law (often called the continuity equation) for a fluid. In Section 4.2 we introduce the concept of the material derivative and the Eulerian and Lagrangian views of fluid motion. An alternative form of the mass conservation law is given in Section 4.3 and the equation of state for the atmosphere (an ideal gas) is recalled in Section 4.4. Then in Section 4.5 Newton's Second Law is applied to a continuous fluid, giving the Navier–Stokes equation. The Earth's rotation cannot be ignored for large-scale atmospheric flows, so its incorporation into the Navier–Stokes equation is discussed in Section 4.6. The full equations of motion for a spherical Earth and for Cartesian tangent–plane geometry are given in Section 4.7. Simplifications of these equations for large-scale flows are introduced in Section 4.8.

PROLOGUE

This chapter deals with Reynolds number effects in turbulent shear flows with particular emphasis on the canonical zero-pressure-gradient boundary layer and twodimensional channel-flow problems. The Reynolds numbers encountered in many practical situations are typically several orders of magnitude higher than those studied computationally or even experimentally. High-Reynolds-number research facilities are expensive to build and operate, and the few that exist are heavily scheduled with mostly developmental work. For wind tunnels, additional complications due to compressibility effects are introduced at high speeds. Likewise, full computational simulation of high-Reynolds-number flows is beyond the reach of current capabilities. Understanding turbulence and modeling will therefore continue to play vital roles in the computation of high-Reynolds-number practical flows using the Reynolds-averaged Navier–Stokes equations. Because the existing knowledge base, accumulated mostly through physical as well as numerical experiments, is skewed toward the low Reynolds numbers, the key question in such high-Reynolds-number modeling as well as in devising novel flow control strategies is, What are the Reynolds number effects on the mean and statistical turbulence quantities and on the organized motions? Understanding the Reynolds number effects is important for flow control on two counts:

1. A passive or active control device developed in a low-Reynolds-number facility may perform quite differently at high Re.

2. For reactive control, coherent structures are targeted.

PROLOGUE

There is no doubt that rational design (i.e., based on first principles) of flow-control devices is always preferable to a trial and error approach. Rational design of course is not always possible owing to the extreme complexity of the equations involved, but one tries either analytically or, more commonly to date, numerically. The search for useful compliant coatings, discussed in Chapter 7, is a case in point. The window of opportunity for a successful coating is so narrow that the probability of finding the right one by experimenting is near nil. Fortunately, the analytical and numerical tools to guide the initial choice for a transition-delaying compliant surface are currently available. On the other hand, the flowfield associated with a typical, deceivingly simple vortex generator for airplane wings is so complex that its design is still done to date more or less empirically.

The proper first principles for flow control are those for fluid mechanics itself. The principles of conservation of mass, momentum, and energy govern all fluid motions. Additionally, all processes are constrained by the second law of thermodynamics. In general, a set of partial, nonlinear differential equations expresses those principles, and, together with appropriate boundary and initial conditions, constitute a wellposed problem.

PROLOGUE

The subject of flow control is broadly introduced in this first chapter, leaving much of the details to the subsequent chapters of the book. The ability to manipulate a flowfield actively or passively to effect a desired change is of immense technological importance, and this undoubtedly accounts for the subject's being more hotly pursued by scientists and engineers than any other topic in fluid mechanics. The potential benefits of realizing efficient flow-control systems range from saving billions of dollars in annual fuel costs for land, air, and sea vehicles to achieving economically and environmentally more competitive industrial processes involving fluid flows. In this monograph both the classical tools and the more modern strategies of flow control are covered. Methods of control to achieve transition delay, separation postponement, lift enhancement, drag reduction, turbulence augmentation, and noise suppression are considered. The treatment is tutorial at times, which makes the material accessible to the graduate student in the field of fluid mechanics. Emphasis is placed on external boundary-layer flows, although applicability of some of the methods discussed for internal flows as well as free-shear flows will be mentioned.

PROLOGUE

Boundary layer manipulation via reactive control strategies is now in vogue. The payoffs are handsome, but the difficulties involved are daunting. This topic is deferred to the last chapter of the book. There are, however, much simpler alternatives to such sophisticated flow alteration devices, and the present chapter discusses one such alternative: passive compliant walls. We particularly review the important developments in the field of compliant coatings that took place during the past decade or so. During this period, progress in theoretical and computational methods somewhat outpaced that in experimental efforts. There is no doubt that compliant coatings can be rationally designed to delay transition and to suppress noise on marine vehicles as well as other practical hydrodynamic devices. Transition Reynolds numbers that exceed by an order of magnitude those on rigid-surface boundary layers can readily be achieved.

PROLOGUE

A particular control strategy is chosen based on the kind of flow and the control goal to be achieved. Flow-control goals are strongly, often adversely, interrelated, and there lies the challenge of making the tough compromises. There are several different ways for classifying control strategies to achieve a desired effect. Presence or lack of walls, Reynolds and Mach numbers, and the character of the flow instabilities are all important considerations for the type of control to be applied. All these seemingly disparate issues are what places the field of flow control in a unified framework. They will be discussed in turn in this chapter.

Control Goals and Their Interrelation

What does the engineer want to achieve when attempting to manipulate a particular flowfield? Typically he or she aims at reducing the drag; at enhancing the lift; at augmenting the mixing of mass, momentum, or energy; at suppressing the flowinduced noise; or at a combination thereof.