Diurnal evolution of orographic precipitating clouds over the southernmost part of the Western Ghats of India during summer and winter monsoons

In the present communication, diurnal evolution of vertical structure of precipitating clouds over the southernmost part (7 degrees-11 degrees N) of the Western Ghats (WG) of Indian region during summer and winter monsoon periods is discussed using 17 years of Tropical Rainfall Measuring Mission (TRMM) precipitation radar measurements. This part of the WG experiences both summer and winter monsoons and provides a unique opportunity to study the orographical processes involved in the formation of precipitating clouds at diurnal scales during two contrasting background environments. During summer monsoon, the lower tropospheric winds are dominated by the presence of strong low-level westerly jets, which interact with steep slopes of the WG whereas during winter monsoon, winds are very weak easterlies and encounter the WG from eastern side. The composite longitudinal distribution of vertical structure of precipitating clouds in terms of frequency of occurrence of radar reflectivities >= 23 dBZ is constructed at four local time intervals. The results are discussed in terms of bands of high frequency of occurrence of precipitating clouds and their longitudinal position with respect to the WG. A band of high frequency of occurrence of precipitation clouds is observed to be moving eastwards and their potential formation mechanism is attributed to the forced lifting over the upslope regions and to the upstream blocking over the offshore during the summer monsoon. During winter monsoon, a two band structure is observed one over the southern BOB, which is attributed to the persistent low-pressure system observed over this region and another over the upslope of the WG from eastern side. Thus the present study for the first time brings out the diurnal evolution of the vertical structure of precipitating clouds during summer and winter monsoon over the southernmost part of the WG and discuss the potential physical processes involved in their formation.