Mesoscale characteristics and prediction of an unusual extreme heavy precipitation event over India using a high resolution mesoscale model

Abstract

Numerical prediction experiments using a high resolution mesoscale model NCAR MM5 were performed to simulate an unusual extreme precipitation event that occurred over west coast region of India on 26 July 2005. During this event, unprecedented precipitation of 90–100 cm was recorded over northeast parts of Mumbai City, India causing enormous losses while southern parts received only 10 cm. Model prediction with analysis nudging for 12 h followed by 36 h of integration produced the best simulation with 55 cm of precipitation in 24 h and with the location over north Mumbai agreeing with the observations. Model diagnostics of the vorticity, divergence, vertical velocity and lower tropospheric moisture convergence show the mesoscale characteristics of the convective system with a horizontal extent of 50 km2 and of a sudden cloud burst for 3–6 h followed by few shorter rain spells. The model simulates the veering of the wind with height due to warm air advection to favor convection but a moist layer at lower levels capped by dry air inhibited convection. The simulated circulation features indicate that a mesoscale convective system formed in the monsoon westerlies due to passage of a synoptic disturbance across the east coast strengthening the monsoon flow, and dry air incursion at middle levels suppressed convection and contributing to increase of potential instability at lower levels. All this helped the sudden initiation of deep convection and cloud burst with heavy precipitation rate. Stretching term associated with vorticity contributes most for the increase of cyclonic vorticity indicating the interaction of convection with mesoscale circulation.