Climatological characteristics of nocturnal eastward-propagating diurnal precipitation peak over South India during summer monsoon: Role of monsoon low-level circulation and gravity waves

This study explores the climatological diurnal cycle of precipitation during the summer monsoon season over southeast India using the Tropical Rainfall Measuring Mission (TRMM) 3G68 precipitation product from 2000 to 2014. The primary area of focus is the leeward side of the Western Ghats. We found that the climatological diurnal cycle over this area exhibits a bimodal peak with an afternoon/evening peak near the eastern slopes of the Western Ghats and a nocturnal peak near the southeast coast. This nocturnal peak near the southeast coast propagated offshore in the eastward direction with a phase speed of 18-20 m s(-1) and attained an offshore peak in the late night/early morning. The afternoon diurnal peak is associated with daytime convection and propagates eastward over southeast India. The nocturnal peak is due to the convergence of intensified monsoon low-level westerly winds along the land-sea boundary over the leeward side and supports the nocturnal diurnal peak to cross the coast at late night. The eastward-propagating gravity waves support further eastward offshore propagation of nocturnal diurnal precipitation patterns. Altogether, the leeward-side climatological diurnal peaks and diurnal variability in southeast India are controlled by complex precipitation types and their formation mechanisms due to monsoon low-level westerly winds and gravity waves.