The role of cumulus congestus in the tropical western Pacific

Tropical cloudiness spans a variety of depths but has traditionally been characterized by two leading modes: shallow, boundary layer cumulus that extend in depth to about 2 km, and deep cumulonimbus. The ultimate vertical extent of these cloud types is limited by significant increases in static stability associated with the trade and tropopause inversions, respectively. Radar data collected during the TOGA COARE field campaign (conducted from Nov 1992-Feb 1993) supported the existence of a third cloud mode, cumulus congestus, whose tops were loosely associated with an increase in stability at the 0º C level. During the transition from suppressed to active phase of the intraseasonal oscillation (ISO), these clouds appeared to moisten the tropical atmosphere, which was thought to precondition it for deep convection. Despite the loose association with this stable layer, the definitive mechanisms that dictate the vertical extent of congestus have not been thoroughly investigated. The observational data have suggested, however, a number of hypotheses to explain cloud depth, the origin of the 0º C stable layer, and the ultimate role of cumulus congestus in the overall realm of tropical convection.

In order to explore the role of cumulus congestus in the evolution of the tropical cloud field, we performed a 20-day high-resolution numerical simulation of the western tropical Pacific during TOGA COARE. Preliminary results indicate the presence of congestus clouds, particularly evident when the ISO transitions from suppressed to active phase, along with an appreciable amount of precipitation falling from these mid-level clouds, a point in common with previous studies. Results hint that the distinct trimodal distribution is an oversimplification (at least in the modeling framework), as multilayer cloud structures are very common. We will emphasize the evolution of the cloud distribution over the 20-day simulation and address results in the context of several hypotheses related to tropical cumulus congestus.