ミャンマー沿岸における沖合・沿岸の降雨コントラストの原因メカニズム

The diurnal cycle of rainfall plays a crucial role in shaping monsoon variability along the west coast of Myanmar (WCM), where steep orography and strong land sea contrasts influence rainfall. This study examines the spatial and temporal characteristics of diurnal rainfall using 22 years of TRMM 3B42 V7 precipitation data and ERA5 reanalysis data set. Rainfall days with maximum rainfall exceeding 3 mm h^-1 are identified and selected for the analysis. These days are classified into eight regimes based on the timing of daily maxima (morning, afternoon, evening and nocturnal peaks) and their location (offshore and onshore). Results reveal a sharp distinction between offshore and onshore regimes. Offshore rainfall is dominated by morning and nocturnal peaks, forming broad latitudinal bands linked to low level convergence, cyclonic vorticity and moisture convergence over the ocean, along with enhanced ascending motion. Similarly, afternoon and evening offshore regimes are also driven by dynamics. Conversely, onshore rainfall is strongest during the afternoon and evening, confined to a narrower zone associated with maximum vertical gradients of moist static energy, associated with instability generated by surface heating. However, the nocturnal and morning maxima onshore regimes are controlled by dynamic forcing rather than thermodynamic instability. Offshore rainfall regimes are consistently controlled by large scale dynamic processes, whereas onshore regimes vary with the dominant forcing. Thermodynamic instability drives afternoon and evening peaks, while dynamic forcing governs nocturnal and morning maxima. This contrast indicates the importance of distinguishing land ocean differences when studying diurnal variability. The results provide critical insights for improving model representation of diurnal rainfall and enhancing the prediction of monsoon extremes in the WCM.