Annual Variation of Extreme Precipitation Intensity in Japan: Assessment of the Validity of Clausius-Clapeyron Scaling in Seasonal Change

The annual variation of extreme precipitation in Japan was analyzed using hourly data at 981 stations for 36 years (1979-2014). Except over the Nansei Islands, the intensity of hourly extreme precipitation (P-ext) defined by upper 0.1% cases is found to show a unimodal annual variation with a peak in midsummer, unlike total precipitation amount which has a bimodal annual variation with two peaks of Baiu and Shurin. The dependence of P-ext on mean temperature is close to or slightly lower than the Clausius-Clapeyron (CC) rate of 6-7%/K, indicating that the annual variation of extreme precipitation is strongly linked to that of saturation vapor amount. On the other hand, P-ext is different according to regions by several tens of percent for a fixed temperature. This fact suggests the influence of geographical factors on its spatial distribution. As for 24-hour precipitation, P-ext shows larger deviation from the CC scaling and larger regional differences than that of hourly precipitation, implying stronger control of synoptic effects in generating long-lasting heavy rainfall.