Quasi-Geostrophic Diagnosis of the Monsoon Depression of 5–8 July 1979

Abstract

Quasi-geostrophic diagnosis of the fields of vertical motion and streamfunction tendency, based on wind analysis, was undertaken for the monsoon depression of 5?8 July 1979, a uniquely well-observed case during Summer MONEX. Wind analysis, in terms of streamfunction and velocity potential, was performed for data averaged in three layers of equal pressure depth front 1000 to 100 mb. The thermal stratification near the depression above 850 mb was approximately moist-adiabatic. The wind analyses showed that the basic zonal currents, westerly in the lower troposphere and easterly in the middle levels, weakened as the depression formed. Central vorticity increased rapidly on 5?6 July to a magnitude three times the Coriolis parameter. The center tilted toward the southwest with elevation. The quasi-geostrophic omega-equation was solved at the interfaces between layers, for a range of stabilities from the full dry-adiabatic value to 1% of it. Ascent west of the center for the near-neutral stability was sufficient to account for the storm-scale average observed precipitation. Divergences for each layer were combined with vorticity advections to calculate the quasi-geostrophic streamfunction tendencies. The observed slow westward motion of the system was reasonably well accounted for, but the temporally increasing meridional slope of the system was somewhat exaggerated. The strong increase in central vorticity during development could not be accounted for by divergence arising from advections of temperature and vorticity.