Synoptic Forcing and Control of Deep Convection on Day 261 of GATE

Abstract

An evolutionary view is sought Of a single cloud cluster. This cluster was chosen less for intensity than for comprehensive observations. The aim is to describe the principal outside controls on the cluster, including its relationship with nearby clusters. This is accomplished by combining observations from satellites with those from ships and aircraft. The cluster represented the deepest of four overlapping layers of moist convection present on this day. It?and its neighbors?tended to occur along rings of cumulus clouds, somewhat larger in size, which were formed by the collapse of older clusters. There was no evidence of a migratory cyclonic synoptic disturbance in the lower troposphere. On the contrary, the cluster occurred entirely within southwest monsoon flow. Abruptly, early in the afternoon, as its cumulonimbus towers became aligned across the front face, the cluster accelerated and intensified. It is argued that this change toward squall line structure and behavior was due to strengthening vertical shear in the upper troposphere, which, together with a layer of dry northeasterlies near 600 mb, increased the strength of evaporationally forced downdrafts under the cirrus shield. The change, a kind of metamorphosis, points to more variability in tropical cloud clusters than has commonly been recognized.