Clausius-Clapeyron Scaling of Extremely Heavy Precipitations: Case Studies of the July 2017 and July 2018 Heavy Rainfall Events over Japan

Abstract

The usefulness of Clausius–Clapeyron (CC) scaling in explaining extremely heavy precipitations is explored in the present-day climate and in pseudo-global-warming (PGW) conditions. This is analyzed by conducting regional-scale numerical simulations at 1-km grid resolution for two recent extreme rainfall events that occurred in Japan: the case in northern Kyushu during 5–6 July 2017 and the case in Shikoku Island during 5–8 July 2018. The Weather Research and Forecasting (WRF) model was used for the simulation, and the data samples were collected at each grid point individually for each hour over the two regions. We found that the frequency and intensity of extremely heavy precipitation associated with the two events are increased under PGW conditions. The extremely heavy precipitations (> 50 mm h⁻¹) followed CC scaling for the temperatures up to 22°C in the present-day climate, while those under the PGW conditions followed CC-scaling up to 24°C. The peak intensity of the extremely heavy precipitations in the precipitation-temperature relationship is found as ∼ 140 mm h⁻¹ at 25°C in the present-day climate, while the same with PGW conditions is projected as ∼ 160 mm h⁻¹ at 27°C. The increasing rate of the extremely heavy precipitations in the present-climate condition is noticed as ∼ 3 % °C⁻¹ and that under the PGW conditions is anticipated as ∼ 3.5 % °C⁻¹. The increase in peak precipitation intensity and the rate of precipitation increase against temperature in future warming climate are attributed to the decrease in temperature lapse rate and increase in atmospheric water vapor and convective available potential energy. To our knowledge, this is one of the first quantitative investigations of CC scaling of extremely heavy precipitations based on case studies.