A Hierarchical Structure of the Heavy Rainfall Event over Kyushu in July 2020

Abstract

The precipitation system and environment that caused the heavy rainfall event in July 2020 over Kyushu Island, Japan, were analyzed, with a focus on a hierarchical structure. The moisture budget analysis over Kyushu revealed the contribution of the free-tropospheric moisture flux convergence moistening the atmosphere before the rainfall event. Further analyses by dividing the flux convergence into moisture advection and wind-convergence terms revealed that the moisture advection controlled the moistening. The contributions of both the boundary-layer and free-tropospheric wind-convergence terms increased after the moistening. Wide areas with weak precipitation characterized the moistening phase, whereas concentrated intense precipitation areas developed after the moisten-ing. A synoptic-scale upper-tropospheric trough transports free-tropospheric moisture from the South China Sea to Kyushu via southern China. The free-tropospheric moisture converges in a subsynoptic scale cloud system in front of the trough, providing a moist environment favorable for the precipitation systems bringing a large precipitation amount. A mesoscale depression below the trough developed with active convection over central China enhances the free-tropospheric moisture transport. Cyclonic circulations associated with the mesoscale depression and the subsynoptic scale cloud system enhance the baroclinicity around Kyushu. Under such conditions, an active convective area develops to a mesoscale convective system covering Kyushu. A line-shaped convective area is generated along the southern edge of the convective system, causing the heavy rainfall event. Two intense precipitation areas are embedded in the convective area along the inflow direction. At the same time, weak precipitation areas spread downstream of the intense precipitation areas. The vertical cross sections of the intense precipitation areas show structures consistent with the organized precipitation systems with deep inflow layers and the moist absolutely unstable layers. These results indicate that a hierarchical structure characterizes the rainfall event, in which the organized precipitation system develops under the environment prepared by the large-scale features.