hree dimensional structure of a monsoon depression developed over the Bay of Bengal during summer MONEX is examined by the use of aircraft dropwindsonde and conventional upper-air observations. Wind, temperature and relative humidity data are interpolated objectively at 1° latitude/longitude grid points over the area of 11°-24°N and 80°-93°E and at 25mb pressure levels from the surface to the 500mb level for the period 3-8 July 1979.
The depression formed over the Bay of Bengal at about 19°N and 90°E on July 6 and moved westward with about 2° day-1 speed and reached a coast line of India on July 8. The developed depression has the maximum vorticity of about 1.5×10-4s-1 and a warm core slightly to the east of the depression center. Horizontal convergence and rising motion occur to the west of the depression center where active convective clouds exist. The horizontal axis of the depression is inclined from southwest to northeast. The depression transports heat and momentum northward and gains its kinetic energy from the mean tonal flow.
The analysis during the pre-formation period shows that there exists large positive vorticity of about 5×10-5 s-1 in the area over the bay which is close to the area of depression formation. This large positive vorticity is caused by both the existence of a weak trough and the meridional gradient of the zonal flow.
The latitude-height distribution of the zonal wind averaged over the Bay of Bengal for the pre-formation period shows that the necessary condition for instability of the zonal flow is satisfied. A stability analysis of the zonal flow averaged in the lower troposphere below 500mb indicates that the flow is barotropically unstable with the maximum, growth rate at about 3, 500km. The unstable wave has several similar characteristic features to those observed.
Meteorological Society of Japan