Interannual and Diurnal Variations in the Frequency of Heavy Rainfall Events in the Kyushu Area, Western Japan during the Rainy Season

Abstract

The interannual variation in the number of heavy rainfall events in Japan in 1976-2022, extracted from 3-hour accumulated precipitation (P3H) data exceeding 130 mm, showed a relatively large relationship (correlation coefficient 0.45) with sea surface temperature (SST) around the Japanese Islands. In the Kyushu area during the rainy season (June–July), the correlation coefficient with SST became considerably smaller, while a relatively large relationship (correlation coefficient 0.45) was found with the appearance frequency of 500m-height water vapor flux above 250 g m⁻² s⁻¹, suggesting that the interannual variation could be considerably influenced by the synoptic scale pressure pattern. Diurnal variations in the number of heavy rainfall events, including long-term increasing trends, were also investigated. Although less significant diurnal variations were observed on the annual basis, the events in the Kyushu area were more frequent in the morning (7-9 JST: JST = UTC + 9 hours) during the rainy season. In the Kyushu area, the 47-year long-term trend of heavy rainfall events in 4-9 JST was a 7.47-fold increase in June and July, while it was only a 1.35-fold increase in the other months.