Article ID: 2023-017
The precipitation system and environment causing the heavy rainfall event in July 2020 over Kyushu Island, Japan are analyzed focusing on a hierarchical structure. The moisture budget analysis over Kyushu reveals 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 indicate that the moisture-advection controlled the moistening. Contributions of both the boundary-layer and free-tropospheric wind-convergence terms increase after the moistening. Wide areas with weak precipitation characterize the moistening phase, whereas concentrated intense precipitation areas develop after the moistening. 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 sub-synoptic scale cloud system in front of the trough, providing a moist environment favorable for the precipitation systems bringing a large amount of precipitation. 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 sub-synoptic scale cloud system enhance the baroclinicity around Kyushu. An active convective area develops to a mesoscale convective system covering Kyushu under such conditions. 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 suggest that the organized precipitation system develops under the moist environment prepared by the free-tropospheric moisture flux convergence associated with the hierarchical structure.
