Case study of a heavy rainfall event in Amami Island on 20 October 2010

On 20 October 2010, heavy rainfall was observed in Amami Island, located in southern Japan. The daily accumulated rainfall of 622 mm was observed at the Naze meteorological observatory in Amami Island. This heavy rainfall caused serious disasters in Amami Island. In this study, the characteristics of environmental fields and the development and maintenance mechanisms of precipitation systems inducing the heavy rainfall are examined from observations and numerical simulations results.

According to objective analysis data, low-level humid air (specific humidity larger than 16 g kg-1) was transported to Amami Island by strong easterly winds during the heavy rainfall event. Easterly winds were also observed by the boundary layer radar at Naze. These results indicate that low-level humid air from the east continuously flowed into Amami Island to develop and maintain the precipitation systems.

The numerical model simulations using a nonhydrostatic model with a horizontal resolution of 1 km (1 km-NHM) were performed to examine the development and maintenance mechanisms of the precipitation systems inducing the heavy rainfall. The 1 km-NHM succeeded in producing regions of heavy rainfall, the location and distribution of which were similar to observations. To examine the terrain effect of Amami Island, a sensitivity experiment in which the topography of Amami Island was replaced with sea areas was performed. The daily accumulated rainfall amount near Amami Island in the sensitivity experiment was less than that in the control experiment. This result suggests that the terrain of Amami Island affected the development and maintenance of the precipitation systems inducing the heavy rainfall.