Characteristics of convection over the Arabian Sea during a period of monsoon onset

Abstract

The organization of convection during the pre-onset, onset, and post-onset phases of monsoon has been studied using a linear stability model of convection which parameterized the latent heat released in preexisting cumulus convection, and the special thermodynamic and wind soundings collected during the Summer Monsoon Experiment (SMONEX) of 1979. The motivation for this study came from several satellite and research aircraft observations during SMONEX, which revealed that convection was organized into mesoscale bands over the Eastern (EAS), Central (CAS), and Western (WAS) Arabian seas. These bands varied in size from 20 to 100 km in length. Some (called longitudinal) were oriented parallel to the vertical shear of the base state winds; some (called transverse) were perpendicular. The linear stability model performed a normal mode analysis of convection and examined the factors governing the orientation of bands; longitudinal bands resulted when precipitation efficiency was high, and transverse, when the efficiency was low. The phase propagation of bands was reduced when more moisture was available. The linear stability model utilized a One Dimensional Steady State cloud (1DSS) model to estimate the latent heat released in preexisting cumulus convection. The 1DSS model developed Cumulus (Cu) clouds based on the observed temperature and humidity soundings in various parts of the Arabian Sea during different phases of the monsoon. Although this simple 1DSS model cannot reproduce faithfully all the characteristics of cumulus convection in the monsoon, some of the results, such as estimates of cloud tops, agreed well with those observed. Based on the 1DSS results and the kinematically computed mesoscale vertical velocities, three composite vertical cross sections (one for each): pre-onset, onset, and post-onset phases of the monsoon were constructed. Highlights of the cross sections were (1) during the pre-onset stage, WAS developed moderately strong Cu convection while EAS was inversion dominated; (2) during the onset stage, both CAS and EAS developed vigorous Cu convection while WAS did not show strong convection; and (3) during the post-onset stage, both CAS and EAS continued to develop Cu convection while WAS was inversion controlled. The growth of Cu convection seems to be in accord with satellite and research aircraft observations.