On the vorticity budget and vertical 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 on the Bay of Bengal . The depression, while intensifying, progressed westward across India with a speed of about 5SUP 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 nondivergence (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 cylonic vorticity increases, not only in the lower troposhere but also in the middle and upper troposhere while progressing westward. It has been shown that the presence of deep convective cloud acivity in the western sector provides the necessary process to compensate the negative vorticity tendency in the middle and upper troposphere. Through a simple parameterisation 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 cylclonic vorticity by deep convective clouds is found to be essential for the intensification as well as for the westward movement of the monsoon depression. (A)