Diurnal Variation of Precipitation around Western Japan during the Warm Season

Diurnal variations of precipitation around western Japan during the warm season were studied by using the Japan Meteorological Agency (JMA) Radar-Raingauge analyzed precipitation (R/A) dataset from 1995 to 2012. Prominent diurnal variations in monthly mean precipitation were detected in June, with morning maxima (0600-1100 JST) around Kyushu over a large area of both land and sea (30 degrees N-34 degrees N, 128 degrees E-138 degrees E), and afternoon maxima (1200-1700 JST) around the Nansei Islands (26 degrees N-30 degrees N, 126 degrees E-132 degrees E). Case studies of the precipitation maxima around Kyushu and the Nansei Islands in June revealed that both regions were covered by the Baiu cloud band during the days with significant maxima. During this period, cloud systems repeatedly appeared over the East China Sea in the early morning and developed rapidly while traveling eastward. Cloud systems associated with morning precipitation maxima around Kyushu tended to weaken around noon. Afternoon precipitation maxima around the Nansei Islands, however, were accompanied by organized deep cloud systems that continuously developed from the early morning to the afternoon. The JMA 55-year Reanalysis dataset suggested that the June precipitation maxima around Kyushu and the Nansei Islands were both closely related to diurnal variations in low-level southerly winds.