The Interaction of Waves and Convection in the Tropics

Abstract

Interest in tropical waves and their interaction with convection has been rekindled in recent years by the discovery, using satellite infrared data to track high clouds, that such waves closely display the dispersive properties of linear, inviscid wave theory for an atmosphere with a resting basic state and equivalent depths between 12 and 60 m. While several current approaches focus on internal modes in the atmosphere, this is inconsistent with the absence of internal modes in the atmosphere, which is characterized by a single isolated eigenmode and a continuous spectrum. It will be shown, using an extremely simple approach to convection, that the observed properties of waves are consistent with a continuous spectrum. The approach assumes that the total convection is determined by mean evaporation, but that the convection is patterned by zero-averaged perturbations to triggering energy following the recent approach of Mapes. This is, perhaps, the simplest hypothesis that can be applied. The observed convection associated with the migrating semidiurnal tide is used to calibrate the time scale for the convective response to patterning, which is the only adjustable parameter in this formulation. It is shown that this time scale leads to not only the observed phase of the semidiurnal heating but also the observed phase lead of low-level convergence in tropical waves vis-à-vis the convective heating. Finally, it is shown that this phase is sensitive to the equivalent depth, which it is suggested is the basis for the selection of equivalent depth. Reasonable simulations of observed waves are readily obtained.