An Analytical Study of the Dependence of Orientation and Propagation of the Arabian Sea Convection Bands on Wind Shear, Static Stability, and Preexisting Convection

Abstract

Organization of convection into bands over the Arabian Sea during a monsoon onset was studied analytically. Observed vertical profiles of temperature, moisture, and wind velocity were used as base states by a linear model of convection to deduce the most unstable mode in the mesoscale. The analytical characteristics of bands such as width, length, and orientation were then compared with in situ aircraft observations and satellite imagery. A unique feature of this study is the estimation of latent heating through a cloud model and the employment of such heating by a linear model to find the most favored bands. The cloud model computed the depth of the cloud and allowed variations of its radius and supplied a diffusion coefficient to the linear model. The cloud model showed the dependence of cloud growth on the conditionally unstable stratification, while the linear model showed the sensitivity of the band orientation to the wind profile. The linear model further scrutinized the impact of moisture accession and latent heat release on the emergence of transverse and longitudinal bands. It was also shown that the phase speeds of bands are dependent on precipitation efficiency and that the low-level flow transfers energy to the bands under certain circumstances contributing to their longitudinal orientation. Support for our results came when the computed band orientations in various parts of the Arabian Sea were in accord with the aircraft observations. Some support also came from the land-based radar that showed the prevalence of longitudinal bands in the eastern Arabian Sea.