Atmospheres in Radiative Equilibrium

Abstract

In this chapter we discuss radiative equilibrium models of the earth’s atmosphere and the closely related radiative—convective models, for which small-scale convection is included in a highly parameterized form. In both cases, heat transports by planetary-scale motions are neglected. Despite their limitations, radiative equilibrium and radiativeconvective studies have provided stimuli for many of the fundamental ideas discussed in this book. Their value is principally heuristic. The radiative equilibrium state is one conceivable state of a planetary atmosphere that may be analyzed so that the implications of parameteric changes can be understood in simple terms (e.g., changes in atmospheric composition, earth orbital elements, solar emission, etc.). The same cannot yet be said of any dynamic model. While numerical solutions are available from general circulation models, their behavior is often no easier to interpret than that of the atmosphere itself. For studies that are not based on the existence of day-to-day observations, radiative equilibrium considerations provide the irreplaceable first step in a number of fields: the atmospheres of other planets, stellar atmospheres, the earth’s primitive atmosphere; and much of the progress in studies of climate change has been based on the simplest energy balance models. In addition to their value in examining general principles, there is a recurrent, although disputed theme that radiative equilibrium has direct relevance to the observed atmospheric structure. This proposition embraces a number of instructive ideas but, before examining them, we consider some of the observational evidence that motivates them. From the earliest days following the discovery of the stratosphere, theoretical workers assumed that the stratosphere, unlike the troposphere, was in radiative equilibrium. The reasoning was that no forms of heat transport, other than radiative, could be important in a highly stable atmosphere. Since nothing was known about planetary-scale motions at that time, this conclusion was premature. If we turn to modern data, Fig. 9.1a presents the observed climatological temperatures in the middle atmosphere, to be compared with radiative equilibrium calculations shown in Fig. 9.1b. Agreement between theory and observation is fairly good except in the region of the polar winter.