Mesoscale Organization and Cloud Microphysics in a Bay of Bengal Depression

Abstract

Airborne radar and cloud microphysical data were obtained throughout a monsoon depression observed over the Bay of Bengal on 3–8 July 1979 during the Summer Monsoon Experiment of the Global Atmospheric Research Programme. The precipitation in the disturbance occurred in mesoscale rain areas 100–300 km in horizontal dimension, with the largest areas tending to be elongated in the east-west or northwest-southeast direction. These mesoscale precipitation features resembled the rain areas of equatorial cloud clusters. Each mesoscale precipitation feature contained intense convective cells but the area covered by rain was predominantly stratiform. The convective cells in the mesoscale features were sometimes arranged in rapidly moving, rather narrow arc-shaped east–west lines, with the stratiform precipitation trailing the line. In other cases, the convective cells were more randomly embedded in the stratiform rain. Ice particle images obtained at flight-level temperatures of +5 to −25°C indicated the precipitation mechanisms in the stratiform regions of the mesoscale precipitation features. Crystals were growing in habits determined by the ambient temperature and were drifting downward. Liquid water was virtually absent above the 0°C level. Pristine shapes, including needles, columns, plates and dendrites were observed in maximum concentration about a kilometer below the altitude where their growth habit was determined. Large dendritic crystals apparently aggregated to form large snowflakes. As ice particles fell below the 0°C level, they abruptly melted to form the drops that constituted the stratiform rain. These observations of precipitation growth and fallout imply that mesoscale updraft motion strong enough to promote ice particle growth, but not strong enough to prevent sedimentation of the particles or to maintain liquid water in the presence of ice, was prevalent at upper levels in the stratiform regions of the mesoscale precipitation features.