An extremely heavy precipitation event occurred in the mountainous Kii Peninsula in Japan, associated with Typhoon Meari in 2004. A marked characteristic of this heavy precipitation was its extreme rainfall rate, more than 100 mm h^-1. Another feature was that the area of precipitation was far from the storm center, more than 500 km. From radar and surface observations, it was found that the heavy rains were composed of a stationary precipitation system and two moving precipitation systems.
In order to evince a physical mechanism for the precipitation systems, the event was analyzed in detail using data from cloud-resolving simulations and observations. The results demonstrated that the heavy precipitation was produced in a synergistic manner from the three precipitation systems. The following are identified as the key factors for the formation and maintenance of each precipitation system: a) elimination of vertical convective instability in a low-level warm and moist easterly on the eastern slope of the mountainous region, b) moisture supply due to a low-level confluent flow along the boundary between the low-level easterly and south-easterly flow, and lifting of a slightly warmer south-easterly flow, and c) low-level convergence due to cold pool, running nearly parallel to the rainband axis and located slightly to the southwest of the band, acting as an obstacle to the low-level inflow into the system.
Precipitation efficiency revealed that precipitation was enhanced when moving precipitation systems merged with the stationary precipitation system. The enhancement was attributed to the greater rate of conversion of cloud water to rainwater via accretion of cloud water by rain, under the condition of intense water vapor flux convergence. The moving precipitation systems provided raindrops for the accretion of cloud droplets in the stationary precipitation system. Based on our findings, extremely heavy precipitation in the present case is caused by the enhancement of the accretion process due to the merger of precipitation systems in addition to precipitation in each system.