Evaluation of Multi-Satellite TRMM Derived Rainfall Estimates over a Western State of India

Abstract

The Tropical Rainfall Measuring Mission (TRMM) blended rainfall product (3B42-V6 data set) has been used for inter-comparison with observations from a high density rain-gauge network over the western state of Maharashtra, India. The state of Maharashtra was chosen for the inter-comparison exercise primarily because of its diverse rainfall regime. The rain distribution over Maharashtra, characterized by a pronounced longitudinal gradient is closely linked to the north-south oriented orography of the region known as the Western Ghats. The validation exercise was carried out for 7 monsoon seasons from June to September from 1998 to 2004 at various space and time scales. The results are consistent at daily, monthly and seasonal timescales. The comparisons among stations and over geographically similar climatic zones demonstrated that the performance of 3B42-V6 product varies over the different climatic regimes. In terms of spatial reproductivity, the existence of the west-east rainfall gradient along the west coast is captured by the satellite product, but the orographic effect (rainfall maxima is over the Western Ghats as captured by the rain-gauge) is not reflected by 3B42-V6 product. The 3B42V6 product shows rainfall maxima at the coast. The satellite estimates of rainfall amounts over the state were found to be most accurate over regions of moderate rainfall and mainly inaccurate in regions of sharp rainfall gradient. In terms of magnitude of the rainfall amounts, over the windward side of the Western Ghats the 3B42V6 product was unable to resolve the heavy orographic rainfall amounts and over the leeward side the rainfall amounts in the immediate rain-shadow region were overestimated. One of the key results obtained from the daily rainfall intercomparison exercise is the ability of the 3B42-V6 estimates to detect the wet and dry phases of monsoon over most parts of the state (except the leeward side). Though the rainfall amounts estimated by the satellite product were sometimes under/over estimated, the timing of the rain events as estimated by the satellite product was generally coincident with the gauge observations over most of the regions except in the immediate rain-shadow region of the state. The TRMM 3B42-V6 estimates therefore could have tremendous potential to be used for intraseasonal studies over most regions of the state.