Comparative study of regional rainfall characteristics simulated by RegCM3 and recorded by IMD

• RegCM3 is used to examine the regional and temporal characteristics of ISMR over India and its six homogenous zones.
• The Standardised Precipitation Index, onset and withdrawal of ISMR has also been studied here.
• The climatological ISMR over the country has been overestimated by the model than the observed for the period 1980 to 2000.
• RegCM3 is able to simulate the interannual variation of ISMR and SPI over WCI, CNI, NWI and Hilly region.
• The simulated excess rainfall over Indian subcontinent may be due to the longer duration of rainy season.

The regional and temporal characteristics of the Indian Summer Monsoon Rainfall (ISMR) over the whole country as well as in its six rainfall homogenous zones have been studied using Regional Climate Model Version 3.0 (RegCM3). The model is integrated at 55 km horizontal resolution over India during the years 1980 to 2000 with prescribed lateral boundary forcing from ECMWF 40 Year Re-Analysis (ERA40). The rainfall over the country as a whole has been overestimated by the model in comparison to observed India Meteorological Department (IMD) rainfall dataset. The model is able to capture the rainfall characteristics during 50% of observed Deficient years, 67% of Excess years, 71% of Normal years, 75% of El-Nino years and all of the La-Nina years. Analysis shows that the interannual variation of simulated ISMR bears good correlation coefficients of 0.66, 0.57 and 0.57 significant at 1% and 0.47 significant at 5% with the observed values over four rainfall homogenous zones West Central India, Central Northeast India, North West India and Hilly region respectively. The performance of RegCM3 in simulating various phases of drought conditions using Standardized Precipitation Index (SPI) has also been examined. The SPI is well captured by the model over the same four zones. The model performance has further been evaluated by analyzing the onset, withdrawal dates and Length of Rainy Season (LRS) of Indian summer monsoon during the period of study by normalizing the spatial mean of Vertically Integrated Moisture Transport (VIMT) over Arabian Sea. Results show that the onset has been delayed by 5 days while the withdrawal is delayed by 23 days as compared to NCEP/NCAR reanalysis in the total period of study. The simulated average LRS is 11 days more than the observed. The excess rainfall over Indian subcontinent might be attributed to the surplus moisture flux over the Arabian Sea till the last week of September. As a consequence, RegCM3 has simulated longer duration of rainy season.