Precipitation characteristics over the steep slope of the Himalayas in rainy season observed by TRMM PR and VIRS

Abstract

Characteristics of the precipitation in rainy season over the steep Himalayas and adjacent regions, including four selected sectors of the flat Gangetic Plains (FGP), foothills of the Himalayas (FHH), the steep slope of the southern Himalayas (SSSH), and the Himalayan–Tibetan Plateau tableland (HTPT), are investigated using collocated satellite datasets from the TRMM PR and VIRS at pixel level during May–August of 1998–2012. Results indicate that the rain frequency increases significantly from the FGP via FHH to the lower elevations of the SSSH (~ 2.5 km), then decreases as the elevation further increases up to the HTPT, and reaches the minimum over the HTPT. Along with such spatial variation of the rain frequency, mean rain rates (RRs) are the heaviest over the FGP (4 mm h⁻¹) and the FHH (5.5 mm h⁻¹), medium over the SSSH (2–4 mm h⁻¹), and the weakest over the HTPT (less than 2 mm h⁻¹). More than 60% of precipitation over the FGP, FHH, and HTPT is produced by ice-phase topped clouds, while more than 70% over the SSSH is from mixed-phase topped clouds.

Analysis suggests that the highest rain frequency over the SSSH in rainy season may be caused by a strong upward motion over the SSSH as warm moist air monsoon flow interacting with the terrain of the Himalayas, while the heaviest RR over the FHH may result from low-level convergence where the air flow is blocked by the SSSH. The elevation and relief effects have linear relationships with precipitation over the south sub-region of the SSSH, which indicates that both effects play important roles on precipitation over complex plateau topography.