The Structure and Dynamics of Monsoon Depressions.
This study describes in detail the structure and evolution of two depressions observed during the 1979 Summer Monsoon Experiment with the objective of determining what changes took place during their intensification. The first depression was associated with the onset of the monsoon while the second formed over the Bay of Bengal in early July, 1979. Detailed subjectively produced windfield analyses provided the basis for vorticity, heat and moisture budgets as well as the 4-dimensional description of both depressions. The principal deduction from the analyses was that both depressions had many important features in common. These included a southwest tilt with height, asymmetric distribution of cloudiness and vertical motions, a local maximum in mid -tropospheric temperature just northeast of the center and pronounced thermal advection patterns below 500 mb. Both depressions propagated towards the west during their period of maximum intensity. Vorticity budgets of the rain areas west of each depression's center showed this motion to result from the dominance in the lower troposphere of the divergence term. Horizontal advection of vorticity opposed the divergence term but was smaller, giving a calculated westward motion in close agreement to that observed at and below 700 mb. Above this level discrepancies appeared between the calculated and observed tendencies but did not change the conclusion that vertical transport by cumulus appeared to be of secondary importance to the depression's propagation. Heat budget calculations for the rain areas showed a balance between warm advection and adiabatic cooling at 850 mb, though diabatic heating was large above this level. Rainfall was closely coincident with the maximum of warm advection at lower-middle levels, both increased during cyclogenesis. In the suppressed northeast quadrant of both depressions subsidence warming was approximately balanced by cold advection; here a maximum in mid-tropospheric temperatures resembling a nascent tropical cyclone eye developed in both depressions.
- Pub Date:
- Physics: Atmospheric Science