Wednesday,
Revised January 28, 2009
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1. Introduction
Copyright 2009, David A. Randall
The nature of the subject
The atmosphere circulates. The circulation is global in extent (Fig. 1.1). It transports dry air, energy, three phases of water, momentum, and too many additional quantities to list here. In the Earths atmosphere, the circulations of energy and water are closely linked. The circulation is constantly fighting against friction, but it is sustained by thermal forcing, which ultimately comes from the Sun. On the average, the Earth intercepts about 340 W m-2 of incident solar energy, of which a tiny fraction is converted into the kinetic energy of the general circulation. Additional, primordial energy leaks out of the Earths interior, but this rate is only about 0.08 W m-2 (Sclater et al., 1980; Bukowinski, 1999). The externally imposed thermal forcing is strongly influenced by the atmospheric circulation itself, e.g., as clouds form and disappear. This coupling between the circulation and the heating that drives it is a major complication that makes the general circulation much more interesting than it would be otherwise. The conservation equations that govern the behavior of the atmosphere can be used to formulate balance requirements that the general circulation must satisfy. For example, in a time average, the net energy flux at the top of the atmosphere must vanish, the rates of evaporation and precipitation must balance, and the total angular momentum of the atmosphere-ocean-solid Earth system must be invariant, apart from gravitational interactions with the Moon and other extraterrestrial bodies. This check-book approach to the general circulation emphasizes the sources, sinks, and transports of energy, moisture, and angular momentum. We will discuss the general circulation from this classical perspective. It is important to supplement this discussion, however, with descriptions and analyses of the man...
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