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1. Introduction
Among the most important processes at work in the atmosphere is moist convection, which largely sets the vertical temperature structure of the tropical and parts of the extratropical troposphere and which is an important control on the distribution of clouds and water vapor. Yet it is among the most complex of atmospheric processes, involving detailed microphysical and turbulent physics and poorly understood coupling to the boundary layer and to large-scale atmospheric circulations. Perhaps for this reason, it continues to present serious challenges to numerical weather prediction and climate models, and also to conceptual understanding.
With the advent of global, cloud-permitting models, the need to employ parameterizations of convection diminishes, although for some time it will still be necessary to represent in-cloud turbulence parametrically, and cloud microphysical processes will have to be parameterized indefinitely. Yet even with the increasing use of cloud-permitting models, understanding their behavior (not to mention that of the real world) requires a conceptual framework that provides a qualitatively correct and satisfying view of the underlying mechanisms. Understanding of complex phenomena like the Madden-Julian oscillation (MJO) and self-aggregation of convection will not simply emerge from observations, however comprehensive, or numerical simulations, however successful they might be in replicating the phenomenon.
Aside from being the ultimate objective of the scientific endeavor, understanding is usually an important stepping stone to improving applications. In climate and weather prediction, it is the essential ingredient in, for example, the representation of subgrid-scale processes.
It is in this spirit of conceptual understanding that we here present a candidate conceptual model of slow, convectively coupled processes in the atmosphere. By “slow,” we refer specifically to processes whose intrinsic time scale is long compared to time scales associated with internal waves, but nevertheless fast compared to a pendulum day. (The latter is infinite on the equator, so this second limit is rendered irrelevant.) Under these conditions, the weak temperature gradient (WTG) approximation introduced by Sobel and Bretherton (2000) is satisfied and is a cornerstone of the framework described here. In many respects, the present work follows the pioneering paper of Sobel and Bretherton (2000) and Bretherton and Sobel (2002), but differs in its handling of free-tropospheric moisture and also extends that work to other kinds of circulations, including those...