Analysis of large‐scale conditions associated with convection over the Indian monsoon region

This paper examines the thermodynamics of the atmosphere in relation to occurrence of convective rainfall over the Indian region. Various thermodynamical and kinematic parameters such as convective available potential energy (CAPE), convective inhibition energy (CINE), equivalent potential temperature and relative vorticity field are computed based on model analysis field of the limited area model of India Meteorological Department. The data period of this study is from 15 March 2001 to 28 February 2002. The study shows that the spatial distribution of CAPE in pre‐monsoon and monsoon season exhibits a belt of high CAPE along the east coast extending north up to Gangetic West Bengal and another zone of high CAPE along the southwest coast. During post‐monsoon and winter seasons, the belt of maximum CAPE shifts over the extreme southern Peninsula. The annual variation of CAPE over all stations is bi‐modal in nature with twin peaks each at the beginning and at the end of the monsoon season. Magnitude of CINE on the other hand is nearly zero during the monsoon season. The result shows that presence of strong thermodynamic environment is not sufficient for the occurrence of deep convection. Factors like proper dynamic conditions play a very important role in controlling the occurrence of deep convection. The surface warming during the pre‐monsoon season destabilizes the atmosphere, but large‐scale moist convection, generally, does not take place until CINE becomes close to zero (in the monsoon season). Preceding the monsoon season, the land sea contrast heating strengthens circulation in the lower atmosphere, which in turn leads to the moistening and destabilization of lapse rate. This process starts about a month prior to the actual start of the monsoon rainfall, allowing moisture to build up to lead to the widespread monsoon rainfall. During winter months, rainfall occurs over northwest India in association with high negative value of CINE and over southern Peninsula in association with high value of CAPE. Substantial changes are noticed in the equivalent potential temperature (θe) and vorticity profiles between days with and without rainfall over northwest India (Delhi). The results suggest that the increase in moisture in association with the occurrence of rainfall over Delhi is of advective origin. Where as, other regions (Kolkata, Mumbai and Thiruvananthapuram) display relatively smaller changes in the vertical profiles of θe, indicating an in situ presenc