Multicloud models for organized tropical convection: Enhanced congestus heating

Abstract

Despite the recent advances in supercomputing, the current general circulation models (GCMs) poorly represent the large-scale variability associated with tropical convection. Multicloud model convective parameterizations based on three cloud types (congestus, deep, and stratiform), introduced recently by the authors, have been revealed to be very useful in representing key features of organized convection and convectively coupled waves. Here a new systematic version of the multicloud models is developed with separate upper- and lower-troposphere basis functions for the congestus and stratiform clouds. It naturally leads to a new convective closure for the multicloud models enhancing the congestus heating in order to better pinpoint the congestus preconditioning and moistening mechanisms. The models are studied here for flows above the equator without rotation effects. First, the new model results consist of the usual synopticscale convectively coupled moist gravity wave packets moving at 15-20 m s-1 but, in addition, these packets have planetary-scale envelopes moving in the opposite direction at about 6 m s-1 and have many of the self-similar features of convectively coupled waves, reminiscent of the Madden-Julian oscillation. Second, when a warm pool forcing is imposed, dry regions of roughly 250 km in extent form "convective barriers" surrounding the warm pool region where only congestus heating survives. Deep convection and moist gravity waves are thus confined within the warm pool region. Finally, linear analysis reveals that, for sufficiently dry mean states, in addition to the inherent synoptic-scale moist gravity waves, the new model supports a planetary (wavenumber 1) standing congestus mode that provides, within its congestus active phase, a region where moist gravity waves evolve and propagate, which results in a Walker-like circulation over a uniform SST background. © 2008 American Meteorological Society.