Contrasting Features of Monsoon Precipitation around the Meghalaya Plateau in Westerly and Easterly Regimes from a 17-year TRMM Observation

Precipitation features around the Meghalaya Plateau, northeast India, during summer were investigated using a 17-year (1998-2014) TRMM precipitation radar data. Precipitation features around the plateau change drastically between low-level southwesterly and southeasterly flows (westerly and easterly regimes respectively) over Bangladesh that fluctuate on intraseasonal timescales. In westerly regime (WR), strong moist southwesterly wind blows from the Bay of Bengal directly into the southern slope of the plateau through Bangladesh, resulting in very high precipitation along the southern slope due to orographic lifting of moist air. High rain frequency and intensity with high convective rain fraction primary causes the high rainfall. In contrast, in easterly regime (ER) strong moist southeasterly flow can flow out along the Gangetic Plain from northwestern Bangladesh without the orographic barriers, resulting in low precipitation around the plateau due to low rain frequency and intensity. A remarkable diurnal variation of rain frequency is commonly observed around the plateau in both regimes. The process of diurnal variation is crucial to get high rainfall over the southern slope as well as the intraseasonal oscillation. High rain frequency persists from 2100LT up to 1200LT over the southern slope in WR while modest rain frequency appears during 0000-0600 LT in ER. The vertically well-mixed layer due to surface heating below 950 hPa decelerates the prevailing low-level wind speed in the afternoon in both regimes over Bangladesh, while the wind speed accelerates during nighttime above a surface stable layer, resulting in higher wind speed in night than in noon in front of the southern slope. The stronger low-level horizontal wind toward the plateau during late night to early morning provides a favorable condition for frequent occurrence of precipitation along the southern slope. Thus, intraseasonal low-level wind regimes and land-atmosphere interaction over Bangladesh can regulate low-level wind speed normal to the southern slope on intraseasonal and diurnal timescales, forming the contrasting features of precipitation.