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

抄録

 This paper presents a study of 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. First, we conducted analyses on dynamic and thermodynamic features of the stationary Baiu front using the Japanese 55-year Reanalysis (JRA-55). During this period, great amounts of water vapor were transported continuously to the stationary Baiu front, supporting the record-breaking rainfall. The 299 K isentropic surface was identified as a frontal surface. Along the isentropic surface, warm moist air adiabatically ascended, became saturated at around an altitude of 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 keeping the position of the frontal surface without its northward retreat. Next, numerical sensitivity experiments were conducted to examine impacts of evaporative cooling and the topography on the heavy rainfall formation by 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. The precipitation was formed mainly by updrafts above a frontal surface at a potential temperature of 300 K. This precipitation area shifted northward by more than 100 km when the raindrop evaporation was turned off. The raindrop evaporation suppressed the northward retreat of the frontal surface by maintaining cold airmass amounts below the frontal surface.