On the vorticity Budget and Vertical Velocity Distribution Associated with the Life Cycle of a Monsoon Depression

Abstract

This paper discusses the results obtained from a diagnostic study of a monsoon depression which formed in the northern part of the Bay of Bengal. The depression, while intensifying, progressed westward across India with a speed of about 5° longitude per day. The computed vertical velocity is in good agreement with the observed asymmetric distribution of rainfall around the depression. The presence of a low level of non- divergence (i.e., around 850 mb) is found to have a significant role in the dynamics of the monsoon depression. The important result of the computed vorticity budget over the period of the intensification of the depression is the detection of a middle and upper tropospheric cyclonic vorticity depletion due to large-scale dynamics in the western sector of the depression. This result is rather unexpected because of the fact that the depression's observed cyclonic vorticity increases, not only in the lower troposphere but also in the middle and upper troposphere while progressing westward. It has been shown that the presence of deep convective cloud activity in the western sector provides the necessary process to compensate the negative vorticity tendency in the middle and upper troposphere. Through a simple parameterization it has been shown qualitatively that the transport of subgrid-scale vorticity by deep convective clouds in the western sector is significant. This mechanism of vertical transport of extremely rich boundary. layer cyclonic vorticity by deep convective clouds is found to be essential for the intensification as well as for the westward movement of the monsoon depression.