The Structure and Evolution of Convection in a Tropical Cloud Cluster

Abstract

A large cloud cluster which occurred over the data network of the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) on 5 September 1974 is examined. Data from four quantitative shipboard weather radars show that virtually all of the precipitation in the tropical cloud cluster was associated with six mesoscale precipitation features. A prototype for the structure and life cycle of these features is presented which is sufficiently general to describe all six precipitation features, one of which was a tropical squall-line system. These mesoscale features appear to he the primary entitles within which deep tropical convection occurs. In their formative stage, mesoscale precipitation features consist of a line of isolated cumulonimbus cells oriented perpendicular to the low-level wind flow. In the intensifying stage, the rain areas of the individual cells merge when new convective cells develop between and ahead of the existing cells, where the outflow from convective-scale downdrafts enhances low-level convergence. In the upper troposphere an overhang of cloud and precipitation particles extends downwind in the layer of outflow from deep convective updrafts. The mature mesoscale precipitation feature possesses both a region of convective cells along its leading edge and a large area of horizontally uniform precipitation to the rear. The longevity and total rainfall in the area of horizontally uniform precipitation suggest that its maintenance may he due to organized mesoscale uplift in an anvil cloud extending from the 600?700 mb level to the upper troposphere. In the horizontally uniform rain area beneath the anvil cloud, aircraft observations show cold, dry, low-?w air consistent with the presence of a mesoscale unsaturated downdraft maintained by cooling due to the evaporation of failing rain. A pronounced radar bright band at the melting level is further evidence of cooling in this region. In the dissipating stage, intense convective cells cease forming along the leading edge but the area of horizontally uniform precipitation persists for at least several hours longer. Interactions among the six mesoscale precipitation features result in echo mergers that complicate the precipitation pattern of the cloud cluster, and give the cloud cluster of 5 September a distinctive double shape.