Spatiotemporal variation of rainfall over the central Himalayan region revealed by TRMM Precipitation Radar

The rainfall-elevation relationship in the central Himalayan region (CHR) for pre-monsoon and monsoon seasons is analyzed utilizing the 11-year (1998-2008) high-spatial-resolution TRMM PR 2A25 near-surface rainfall data. The results indicate a strong relationship between rainfall and elevation during both seasons. The investigation reveals a relatively large amount of rainfall over higher elevations during pre-monsoon season. Interestingly, two significant rainfall peaks appear over the southern slope of the Himalayas during summer monsoon season. The first primary peak appears along the Sub-Himalayas (similar to 500-700 m above MSL), while the second appears along the Lesser Himalayas (similar to 2,000-2,200 m above MSL). The former rainfall peak is attributed to fewer heavy rainfall events, and the latter to frequent, weak, but persistent rainfall. It is suggested that the atmosphere is insufficiently moist to trigger convections during the pre-monsoon season, and sufficiently moist during summer monsoon season. The convections over the Sub-Himalayas may moisten the middle layer, and the water vapor in the atmosphere condenses because of the forced lifting along the slope, forming the second rainfall band. The total rain amount is primarily determined by the frequency of rain. The rain-conditioned rain rate along the slope monotonically decreases with elevation. This shows that the precipitation occurs because of forced lifting. In addition, our results show that seasonal variation of rainfall is rather similar to the variation of rainfall characteristics observed during active and break periods.