Spatial rainfall patterns associated with Indian northeast monsoon derived from high resolution rainfall estimates of Chennai DWR

Abstract

The Indian northeast monsoon (NEM) season benefits the southeastern parts of peninsular India during the period October-November-December (OND).  In this study, which is a first of this type for the Indian  region, certain new and salient features of the NEM rainfall (RF) have been derived utilising the very high resolution (333 m × 333 m) radar estimated rainfall (RERF) data generated by the Doppler Weather Radar (DWR) at Chennai for the 12 year period (2002-13), over a circular area of 100 km radius spreading over both land and ocean. More than 2.8 lakhs of grid point data per day have been processed. Rain gauge measured rainfall (RGRF) data of 34 inland stations has also been used. Monthwise spatial distributions of RERF for October, November and December and for the entire season OND have been generated. It is shown through rigorous analysis that RERF is heavier closer to the coast and for a given longitude over land, southern latitudes receive 10-15% more RF than the northern latitudes. Decrease of RF eastwards into Bay of Bengal (BoB) is gradual whereas westwards over inland it is sharp and almost linear.  By and large, the climatological features of NEM derived from historical analysis of RGRF data are well-captured by the analysis based on RERF data. A few new features of monthly and seasonal RF have also been identified. For the 34 stations, 12 year data set for OND, the mean RERF and RGRF values are 629.8 mm and 627.4 mm respectively yielding a difference of just 2.4 mm but with a substantial mean absolute deviation of 69.2 mm. RERF during pre-NEM days of Oct contributed to 10% of the seasonal OND total. RERF in the area of study, during days of cyclonic disturbances (CD days) is nearly twice over outer oceanic areas of BoB than over land.  It has been shown that during the onset to withdrawal period of NEM, RERF is heavier over areas close to the coast (75 cm) than oceanic areas (68 cm) within the 100 km radius of the DWR. High RF zones approximately extending 25-30 km westwards into land and around 30-40 km eastwards over the ocean have been delineated. Spatial distributions of RERF during the various phases of NEM, viz., dry, weak, normal, active and vigorous as identified from the RGRF data have been generated, critically analysed and results drawn.  In the case of vigorous, active and vigorous (AV) NEM days excluding CD days, a relatively high daily RERF patch of 5-6 cm located approximately 5-10 km west of the coast inland and in the SW sector of Chennai DWR has been identified. During post-NEM withdrawal days of December, oceanic areas of eastern sector are shown to receive highest RF compared to land areas, a feature consistent with the withdrawal pattern of NEM. The instrumental limitations and artifacts of radars contributing to errors in RERF have been discussed.