Study of a Monsoon Depression (II), Dynamical Structure

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Abstract
This is a second part of a paper on the structure of a westward propagating monsoon depression. In this study, we examine the dynamical structure of the disturbance. We note that the meridional variation of potential vorticity showed an inflection point in its profile. This implies that the disturbance is imbedded in a region where the necessary condition for the existence of the combined barotropic-baroclinic instability is satisfied. In order to examine this question one step beyond the necessary condition, we carried out a quasi-geostrophic baroclinic prediction experiment. The energy exchanges of this experiment confirmed that the eddy kinetic energy over a domain, enclosing the disturbance, did increase by the combined barotropic-baroclinic processes. The westward propagation speed of the monsoon depression was examined in a number of simple numerical prediction experiments. The barotropic non-divergent model and quasi-geostrophic model were found inadequate to account for the westward phase speed. A multi-level primitive equation model was used to carry out a 48 hour forecast. The model physics includes features such as air-sea interaction, parameterization of cumulus convection, large scale condensation, heat balance of the earth's surface and smoothed orography. A somewhat reasonable 48 hour real data forecast was carried out in this study. The forecasted fields include the motion, thermal mass and moisture variables. A discussion of the calculated versus the observed rainfall rates and associated heating function is presented. Finally, we present the energetics of the monsoon depression based on the primitive equation prediction and those based on observa-tions. The primary result here is that the disturbance is primarily driven by cumulus convection.