Tropical Intraseasonal Oscillation, Super Cloud Clusters, and Cumulus Convection Schemes. Part II: 3D Aquaplanet Simulations

Abstract

Chao and Lin?s work on tropical intraseasonal oscillations, super cloud clusters, and cumulus convection schemes is extended from a 2D model setup to a 3D aquaplanet setup. It is found that super cloud clusters can be simulated in a 3D model and that the 3D setup has more stringent requirements on the cumulus convection scheme than the 2D setup does for a successful simulation of super cloud clusters. Three cumulus convection schemes are compared in experiments simulating super cloud clusters. In the more successful experiments, individual cloud cluster pairs in the meridional direction, once generated near the equator by the cloud cluster tele-induction mechanism, assume a poleward movement while exhibiting weak zonal movement. The combination of two or three successive cloud cluster pairs (i.e., vortex pairs) straddling the equator gives rise to westerly wind burst events of sizable longitudinal range and duration. Thus, the westerly wind burst, as appeared in the model, is really a part of the super cloud cluster structure. The evaporation?surface wind feedback mechanism is found to be unnecessary for the existence of the super cloud clusters. However, it does make the latter more robust. The need for improvement in cumulus parameterization for tropical simulation is discussed.