Analysis of the 26 July 2005 heavy rain event over Mumbai, India using the Weather Research and Forecasting (WRF) model

Abstract

A case-study of the 26 July 2005 Mumbai heavy-rain event that recorded 944 mm rainfall in 24 h with significant spatial variability was carried out using the Weather Research and Forecasting (WRF) model. The event was poorly forecasted by operational models and resulted in large human and economic losses. The present results indicate that the WRF system was able to reproduce the heavy rainfall event and the associated dynamical and thermo- dynamical features. A number of experiments were conducted with the WRF model that suggest the highly localized, heavy rain was the result of an interaction of synoptic-scale weather systems with the mesoscale, coastal land-surface features. These experiments indicate that the large-scale rising motion over the Mumbai region was synoptically forced. Analysis of the model-simulated intense, but short-lived, convective rain cells forming in the large-scale rising motion over Mumbai traces their moisture source to the north and northwesterly flow from the Arabian Sea. Synthetic sensitivity simulations without topography and without a land surface (land replaced with water) show that the large-scale synoptic flow positioned the low-pressure system over the Arabian Sea, while the mesoscale land-surface (including topography and latent heating) feedback modulated the location and intensity of the rain by changes in the winds and regional moisture convergences. Another important feature captured in the high-resolution model analysis is the formation of a mesoscale vortex over Mumbai that appears to have enhanced the conditions for localized, heavy rainfall over Mumbai.