Estimating the rate of change in heavy rainfall intensity for flood control planning in Japan

Heavy rainfall leading to disasters is projected to intensify and become more frequent as the average global temperature continues to rise. To manage flood risk, flood control plans that account for future climate change are urgently needed in Japan. We have estimated the ratio of the intensity of heavy rainfall in the future to that in the past (hereinafter referred to as “rainfall change rate”), using large-ensemble climate model predictions. The results show that the average rainfall change rate across Japan is 1.1, with an increase in temperature of 2 °C. This phenomenon is higher in Hokkaido, the northernmost island in Japan. This seems to be due to the large increase in sea surface temperature around Hokkaido as well as the high rate of increase in vapour pressure in lower-temperature areas, based on the Tetens equation. The Japanese government has set the target rainfall change rate for the flood control planning to 1.15 in Hokkaido and 1.1 for Japan excluding Hokkaido.