Some Physical Processes that Govern Inversion and Analytically Modelled Convection Over the Arabian Sea

The Arabian Sea troposphere during a Summer Monsoon Season is characterized by convection. While deep, well organized, banded convection is observed over the Eastern Arabian Sea, shallow convection capped by an inversion is present over the Western Arabian Sea. Observations indicate that this inversion layer rises from Western to Eastern Arabian Sea during the monsoon onset and post-onset periods. Moderately intense convection tends to appear wherever this inversion layer is lifted. The main goal of the research is to investigate the physical processes associated with the growth, maintenance and dissolution of this inversion. The processes that maintain the inversion are warm air advection from Saudi Arabia, and subsiding air from Iran. The processes that weaken an inversion is radiative cooling from a cloud top. The processes that promote convection include sensible warming from the sea surface, and latent heat release due to cumulus cloud. A 33-level analytical model of convection uses a linear system of an elastic perturbation equations to determine the solutions such as growth rate, phase speed, and orientation of the most unstable mode. A one-dimensional cloud model is employed to determine the vertical distribution of latent heating. A simple bulk aerodynamic formula is applied to evaluate the surface heat fluxes. The stability indicies are computed to evaluate the stratification over the Arabian Sea. Finally, LOWTRAN-7 computer code is engaged to estimate the radiative cooling rates at the cloud top. Results indicate that moderately deep convection is favored in the west during the pre-onset with intensely deep convection in the east during the onset and post-onset phases. The warm air advection from Saudi Arabian Desert is associated with the low level strong westerly flow during the onset and post-onset phases. Computed vertical velocities revealed ascent over the Western and Central Arabian Seas during the pre-onset phase and descent causing an inversion over the Western Arabian Sea during the onset and post -onset phases. Stationary transverse and stationary longitudinal modes are favored for the Central and Eastern Arabian Sea, respectively, during the entire monsoon. The most unstable mode for the deepest convection is found to be stationary longitudinal. The cloud bands are generally oriented in the east-west direction over the Eastern Arabian Sea agreeing with Bombay radar data. The surface sensible heat fluxes are about one fourth of the latent heat fluxes. The impact of radiative cooling at the cloud top is found to be significant in the growth of convection.