Impacts of Evaporative Cooling from Raindrops on the Frontal Heavy Rainfall Formation over Western Japan on 5–8 July 2018

Abstract

This study presents the impacts of evaporative cooling from raindrops on precipitation over western Japan associated with the Baiu front during a heavy rainfall event from 5 to 8 July 2018. We conducted analyses of the dynamic and thermodynamic features of the stationary Baiu front using the Japanese 55-year reanalysis. During this period, large amounts of water vapor were continuously transported to the stationary Baiu front, supporting the record-breaking rainfall. The 299K isentropic surface was identified as the frontal surface. Along the isentropic surface, warm moist air adiabatically ascended, became saturated at an altitude of approximately 500 m, and initiated active precipitation systems. We found that the diabatic cooling near the tip of the frontal surface played an important role in retaining the position of the frontal surface and stalling its northward retreat. Additionally, numerical sensitivity experiments were conducted to examine the impacts of evaporative cooling and topography on heavy rainfall formation using a cloud-resolving non-hydrostatic numerical model (the Japan Meteorological Agency Non-Hydrostatic Model: JMA-NHM) with a horizontal resolution of 3 km. A heavy precipitation area extending from the Chugoku region to central Kinki was simulated regardless of whether the terrain was flattened or not. Precipitation formed primarily as a result of updrafts above a frontal surface at a potential temperature of 300 K. This precipitation area shifted northward by more than 100 km when raindrop evaporation was excluded from the experiment. Raindrop evaporation suppressed the northward retreat of the frontal surface by maintaining cold airmass amounts below the frontal surface.