Elementary Aspects of the Interaction Between Cumulus Convection and the Large-Scale Environment

Abstract

Basic effects of the release of latent heat in a limited region within a stably stratified fluid are reviewed. The response of the environment consists of transient inertia-gravity waves, which are inefficient in transporting heat, and a steady response the nature of which is crucially dependent on the rotational and dissipative properties of the fluid. The vertical propagation of energy associated with the transient gravity waves is emphasized, and the likelihood and nature of effects which feedback positively on the convection are discussed. Conditions under which ensemble averages of cumulus effects may be carried out are examined, with stress placed on the need for a scale separation. The development of ensembles of cumulus clouds is ascribed to two different mechanisms: the destabilization to convection associated with large-scale systems which owe their existence to processes other than convection, and self-exciting cumulus ensembles which require a larger-scale “dynamical flywheel” to organize the cumuli and sustain them against transient influences. A simple model for the ensemble average effect of latent heat release in cumulus clouds is developed and applied to two different “dynamical flywheels”: quasi-balanced vortices (typical cyclones), and inertia-gravity waves. Fundamental theorems regarding the effects of convection in these models are developed and discussed, and we examine the successes and failures of the models in describing observed mesoscale and large-scale convective systems.