Changes in the Baiu Frontal Activity in the Future Climate Simulated by Super-High-Resolution Global and Cloud-Resolving Regional Climate Models

摘要

Changes in the Baiu frontal activity in the future climate are examined, making use of super-high-resolution global and cloud-resolving regional climate models (20-km-mesh AGCM and 5-km-mesh NHM). In the present study, the focus is on the lengthened duration of the Baiu, and the characteristics of the precipitation during the Baiu season in the future climate.First, 10-year global-scale simulations of the present, and future climates are conducted by the 20-km-mesh AGCM. The present climate simulation accurately reproduces the northward shift of the Baiu front with time, and the end of the Baiu season around Japan. In the future climate, the Pacific anti-cyclone remains at the south of the Japan islands even late in July, resulting in the obscure migration of the Baiu front to the north and the lengthened Baiu season.Second, regional climate simulations are conducted by the 5-km-mesh NHM covering East Asia, in order to investigate the small-scale response to large-scale conditions, simulated by the 20-km-mesh AGCM. While the rainfall does not vary in June between the present and future climates, there is more rainfall in July in the future climate. Moreover, the frequency of the precipitation greatly increases with the intensity of the precipitation in July in the future climate simulation.In order to investigate the typical size of the precipitation systems that bring rainfall during the Baiu season, precipitation systems are classified according to the area coverage of the systems. Precipitation systems with an area larger than 90,000 km2 are more frequently seen in July in the future climate, than in the present climate, which corresponds to more rainfall. The increase of the large system in July is most remarkable in the vicinity of Kyushu Island, and the baroclinicity in that area is stronger in the future climate.