Morning Peak of Meiyu/Baiu Rainfall Over the East China Sea and Western Japan

Abstract Long-term (1998–2024) satellite-derived precipitation and spaceborne precipitation radar data with high spatiotemporal resolution indicate that the region from the eastern East China Sea (ECS) to Kyushu during the Meiyu/Baiu season (June–July) is one of the areas in East Asia where the diurnal precipitation cycle is most apparent. This area experiences higher rainfall intensity and higher rainfall frequency with exceptionally high convective-type rainfall within the Meiyu/Baiu rainband, leading to the highest rainfall totals over East Asia during the season. The maximum (minimum) rainfall in the rainband north of ~ 30°N, extending from the ECS/Kyushu to the Pacific Ocean, occurs during dawn to noon (evening–midnight). The diurnal cycle and convergence of water vapor flux are the main factors driving the diurnal cycle of precipitation. From midnight to dawn, the anomalous southwesterly water vapor flux with clockwise rotation enhances the total southwesterly water vapor flux, which reaches its maximum around the ECS/Kyushu. Subsequently, water vapor flux convergence increases, resulting in the rainfall peak during the morning. We discussed possible mechanisms for the diurnal cycle in water vapor flux around the ECS/Kyushu, focusing on inertial oscillation driven by terrestrial atmospheric boundary layer processes and land–sea thermal contrast.