CISK-Barotropic-Baroclinic Instability and the Growth of Monsoon Depressions

Abstract

A combined CISK-barotropic-baroclinic instability analysis of the observed monsoon flow has been performed using the quasi-equilibrium assumption for the parameterization of moist convection. Linear perturbation equations for a three-layer quasi-geostrophic model are numerically integrated to get the most unstable mode. A deep cloud model, in which the height of the base of the cloud does not change with time and entrainment occurs for the whole depth of the cloud but detrainment occurs only at the top, is used to parameterize the effects of moist convection. It is found that the maximum growth rate occurs for the smallest scales. The mechanism for scale selection is therefore not clear. The structure and energetics of the computed linear perturbations for a wavelength corresponding to that of the observed monsoon depressions is compared with the observations. The dominant energy transformation for the computed and the observed perturbations is found to be from eddy available potential energy to eddy kinetic energy. The primary source of heating is condensational heating. Reasonable agreements between the structure and the energetics of the computed perturbations and the observed monsoon depressions suggest that CISK may provide the primary driving mechanism for the growth of monsoon depressions.