Simulation of a Heavy Rainfall Episode Over the West Coast of India Using Analysis Nudging in MM5 Someshwar Das 1 , Jimy Dudhia 2 , D. M. Barker 2 and M. Moncrieff 2 1 National Center for Medium Range Weather Forecasting, New Delhi – 110003 2 National Center for Atmospheric Research, Boulder, CO - 80307 1. Introduction Several heavy rainfall episodes occurred over the west coast of India during monsoon-2002. Rainfall values as much as 54 cm/day were recorded at some of the places during the period. The T80 global forecast model of the National Centre for Medium Range Weather Forecasting (NCMRWF), hereafter referred to as MRF model, was unable to forecast the intensity of the rainfall as observed. In order to forecast the intensity and structure of heavy rainfall events, a high resolution mesoscale model (MM5) is used at NCMRWF. The MM5 model is run operationally using the input boundary conditions from the MRF model. Presently, the data assimilation is carried out only with the MRF model using a Spectral Statistical Interpolation (SSI) method. The mesoscale model does forecast a better distribution of rainfall than the MRF model, however, sometimes the location of maximum rainfall is misplaced as compared to observations. In order to rectify this problem, experiments have been conducted to investigate the impact of analysis nudging using conventional and non-conventional observations at high resolution in the mesoscale model. Section 2 describes the basic features of the MM5 model used at NCMRWF. The synoptic conditions leading to the heavy rainfall event is described in the third section. Finally, the results and conclusions are presented in the section 4 and 5. 2. MM5 Model Configuration The MM5 model is a 5th generation PSU/NCAR Mesoscale Model (limited area), non-hydrostatic, terrain-following sigma coordinate, designed to simulate or predict mesoscale & regional scale atmospheric circulation (Dudhia et al, 2002). The model has been adapted for real time mesoscale weather forecasting at NCMRWF (Das, 2002). It is run on triple-nested domains at 90, 30 and 10 km resolutions. The model is run using the Grell scheme (Grell et al., 1994) for cumulus parameterization and, a non local closure scheme for the boundary layer parameterization. Explicit treatment of cloud water, rain water, snow and ice has been performed using the simple ice scheme of Dudhia (1996). Cloud radiation interaction is allowed between explicit cloud and clear air (IFRAD=2). The initial and lateral boundary conditions are obtained from the operational global T80 model of NCMRWF. A multiquadric interpolation scheme is used for mesoscale analysis during the four dimensional data assimilation/ nudging (hereafter referred to as FDDA). The multiquadric scheme uses a Thirteenth PSU/NCAR Mesoscale Model Users' Workshop 109