Spatial Coherence of Water Vapor and Rainfall over the Indian Subcontinent during Different Monsoon Seasons

Spatial coherency in atmospheric water vapor and rainfall and their association have been studied over the Indian subcontinent utilizing high spatiotemporal resolution data. Total column water vapor (TCWV) values derived from the Indian National Satellite (INSAT) system series (INSAT-3D) are first evaluated against data from an International GNSS Service (IGS) GPS receiver at Bangalore and the Global Precipitation Measurement Microwave Imager (GMI). The bias, correlation coefficient, and RMSE of TCWV between INSAT-3D and GMI show that, except for the south Bay of Bengal, the datasets compare well. The seasonal mean TCWV shows large values with lower standard deviation during the southwest monsoon (SWM) than in the northeast monsoon (NEM). Different temporal scales that contribute to the TCWV variance at a given point are quantified, and the variability due to 30-60-day oscillations is found to be dominant during both the monsoon seasons. TCWV and rainfall show good correspondence over the whole Indian subcontinent during both monsoon seasons except over the Arabian Sea and southern Myanmar regions, where large TCWV values show less rainfall during the SWM. On the whole, the spatial homogeneity and intergrid correlations in TCWV and rainfall are higher in NEM than in SWM. The decorrelation distance d(0) for TCWV is found to be 10 times larger than that for rainfall, indicating that the rainfall homogeneity is generally limited to smaller areas. The large d(0) values of TCWV are mainly due to the occurrence of source and sink processes at large spatial scales over those regions.