Direct simulation of convective adjustment and other ensemble effects

Abstract

Convective adjustment is studied using results from a two‐dimensional numerical model (of a baroclinic, dry, rotating fluid) that, in contrast to parameterization approaches, uses the full dynamic equations in spectral form to determine interactions among large‐scale quasi‐geostrophic waves and small‐scale eddy ensembles. Both the large‐scale and the small‐scale eddies result from physical instabilities. The spectral formulation has desirable energy propagation and conservation properties, and also has the advantage (not implemented in this early study) of allowing one to determine interactions between different scale eddies without having to represent intermediate scale eddies. The primary results are that: (i) there is a clear spectral gap in interactions affecting largescale eddies and (ii) the large‐scale vertical heat transport by the smallscale eddy ensembles varies significantly with time. This variation appears at least partly associated with ensemble group velocities in a space‐varying large‐scale environment, rather than with time variations of the large‐scale environment (as would be consistent with the parameterization assumption that large‐scale fields determine nonlinear transports by unresolved eddies).