Meso-scale convergence rainfall and surface fluxes during the Indian summer monsoon

Meso-scale convergence and vertical velocity were computed using Bellamy technique from rawindsonde and pibal data obtained from three stations. These stations formed the vertices of a meso-scale triangle around Kharagpur (22.3° N, 87.2° E) - a region of moist convection located near the eastern end of the monsoon trough, where the monsoon trough boundary layer experiment (MONTBLEX), was conducted during the Indian summer monsoon 1990. We find that the total rainfall over the meso-triangle depends on the proximity and intensity of the large-scale pressure system forming in the head Bay and moving W/NW towards the land during the onset and active phase of the monsoon. When the large-scale flow showed cyclic circulation extending to mid-troposphere, the rainfall over the triangle does not seem to be affected by the meso-scale divergence. However the rainfall at Kalaikonda (KKA) -an airforce station near Kharagpur (KGP) correlates with meso-scale divergence/convergence. Surface heat flux estimated using sonic anemometer at 8m AGL at KGP was large during meso-scale convergence. It increased with wind speed while the momentum flux showed large variation.