Physical mechanism of diurnal variability of onshore and offshore summer monsoon rainfall along the west coast of India

The west coast of India (WCI) experiences a distinct rainfall maximum during the summer monsoon, characterised by significant spatial and temporal variability. This study analyses the diurnal patterns and underlying mechanisms responsible for the variations over the WCI using TRMM_3B42 V7 precipitation dataset. Rainfall days are categorised into three types - onshore, offshore, and uniform - based on the distribution of rainfall aggregate within their respective regions, as determined by area-averaged daily rainfall time series analysis. Onshore days exhibit maximum rainfall over the windward mountain slopes, offshore days bring peak rainfall over the ocean near the coast, and uniform days show rainfall maxima over both land and adjacent oceans. Onshore and uniform days are characterised by afternoon rainfall peaks, while offshore days feature a morning rainfall maximum. The zonal wind and moisture transport play a critical role in modulating these rainfall patterns in addition to orography. During offshore days, strong zonal winds prevail over the Arabian Sea, whereas uniform and onshore events are associated with an eastward extension of the core of low-level westerlies. Enhanced zonal winds, moisture transport towards land, and strong ascending motion contribute to increased rainfall in these regions. Thermodynamic analysis reveals an unstable atmosphere extending to mid-levels, with a positive moist static energy anomaly indicating significant moisture transport into the region. This study highlights the combined influence of dynamic and thermodynamic processes, particularly the role of zonal winds and moisture transport, in shaping the diurnal and spatial distribution of monsoonal rainfall along the WCI.