Development and Structure of Winter Monsoon Cloud Clusters On 10 December 1978

Abstract

Data from the Winter Monsoon Experiment (WMONEX) are used to study cloud clusters that occurred over the South China Sea on 10 December 1978. These clusters underwent life cycles in which they began as groups of intense convective cells and then developed into mesoscale systems consisting partly of convective cells and partly of stratiform precipitation. In the cellular regions of clusters, ice particle concentrations (at the 8 km, or −17°C, level) were found to be of the order of hundreds per liter, local convective updrafts of 4–17 m s−1 were observed, and the dominant ice-particle growth mechanism appeared to be riming. In the stratiform regions of clusters, the ice particles appeared to grow by vapor deposition and aggregation and weaker but more widespread mesoscale updraft motion was indicated at mid to upper levels by the observed ice-crystal structures, while unsaturated mesoscale downdraft motion was indicated at mid to lower levels by sounding data. The ice-particle concentrations at 8 km in stratiform regions were one to two orders of magnitude lower than in the convective regions; in areas of weaker stratiform precipitation the concentrations were 1–10 per liter, while in areas of stronger stratiform precipitation they were 20–70 per liter. Substructure within the stratiform regions was indicated by variations in observed ice-crystal habits over horizontal distances of 10–100 km. The dominant cluster observed on this day was initiated when a group of convective cells formed just off the Borneo coast, apparently in response to land-breeze convergence. During the mature stage of this cluster, the stratiform rain area became nearly surrounded by a broken line of convective cells of various intensity. The greatest concentration of intense cores was maintained in a generally stationary region on the southeast side of the cluster where the land-breeze convergence was located, while convection on the northwest side of the cluster propagated out to sea. The stratiform precipitation area of this cluster appeared to be formed and maintained by a combination of three processes: dying convective cells being transformed into stratiform structures, hydrometeors being advected from the tops of active cells into the stratiform precipitation region and condensation in the mesoscale updraft contributing to the growth of hydrometeors falling as stratiform rain. About 46% of the total precipitation from this cluster fell as stratiform rain. Other cloud clusters that occurred on this day generally formed around the periphery of the large land-breeze generated cluster and were of a smaller size. Convective downdraft outflows found in the regions between the old and new clusters may have been involved in triggering the new clusters.