The Meteorological Research Institute of the Japan Meteorological Agency has developed a cloudresolving nonhydrostatic 4-dimensional variational assimilation system (NHM-4DVAR), based on the Japan Meteorological Agency Nonhydrostatic Model (JMA-NHM), in order to investigate the mechanism of heavy rainfall events induced by mesoscale convective systems (MCSs). A horizontal resolution of the NHM-4DVAR is set to 2 km to resolve MCSs, and the length of the assimilation window is 1-hour. The control variables of the NHM-4DVAR are horizontal wind, vertical wind, nonhydrostatic pressure, potential temperature, surface pressure and pseudo relative humidity. Perturbations to the dynamical processes, and the advection of water vapor are considered, but these to the other physical processes are not taken into account.The NHM-4DVAR is applied to the heavy rainfall event observed at Nerima, central part of Tokyo metropolitan area, on 21 July 1999. Doppler radar's radial wind data, Global Positioning System's precipitable water vapor data, and surface temperature and wind data are assimilated as high temporal and spatial resolution data. The Nerima heavy rainfall is well reproduced in the assimilation and subseCorresponding author: Takuya Kawabata, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan. E-mail: tkawabat@mri-jma.go.jp 1 Present affiliation: Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan.( 2007, Meteorological Society of Japan quent forecast, with respect to time sequence of 10-minute rainfall amount. The formation mechanism of the Nerima heavy rainfall is clarified from this study. A surface convergence line of horizontal winds was made of a southerly sea breeze and north-easterly winds over the Kanto plain around Nerima. Since the rise of temperature over the northern part of the Kanto plain was suppressed, due to a shield of clouds against sunshine, the difference of temperature between the convergence line and its northern side became large. Consequently, the wind convergence was enhanced around Nerima. An air with high equivalent potential temperature was lifted over this enhanced convergence line to generate cumulonimbi that caused the Nerima heavy rainfall.

An extreme precipitation event happened at Hiroshima in 2014. Over 200 mm of total rainfall was observed on the night of August 19th, which caused floods and many landslides. The rainfall event was estimated to be a rare event happening once in approximately 30 years. The physical response of this event to the change of the future atmospheric condition, which includes a temperature increase on average and convective stability change, is investigated in the present study using a 27-member ensemble experiment and pseudo global warming downscaling method. The experiment is integrated using the Japan Meteorological Research Institute non-hydrostatic regional climate model. A very high-resolution horizontal grid, 500 m, is used to reproduce dense cumulonimbus cloud formation causing heavy rainfall in the model. The future climate condition determined by a higher greenhouse gas concentration is prescribed to the model, in which the surface air temperature globally averaged is 4 K warmer than that in the preindustrial era. The total amounts of precipitation around the Hiroshima area in the future experiments are closer to or slightly lower than in the current experiments in spite of the increase in water vapor due to the atmospheric warming. The effect of the water vapor increase on extreme precipitation is found to be canceled out by the suppression of convection due to the thermal stability enhancement. The fact that future extreme precipitation like the Hiroshima event is not intensified is in contrast to the well-known result that extreme rainfall tends to be intensified in the future. The results in the present study imply that the response of extreme precipitation to global warming differs for each rainfall phenomenon.

Abstract:To evaluate the impact of climate change on snowfall in Japan, a hydrological simulation was made in the Agano River basin by using a regional climate model's output. A hindcast experiment was carried out for the two decades from 1980 to 1999. The average correlation coefficient of 0.79 for the monthly mean discharge in the winter season showed that the interannual variation of the river discharge could be reproduced and that the method can be used for climate change study. The future hydrological response to global warming in the 2070s was investigated using a pseudo-global-warming method. In comparison to data from the 1990s, the monthly mean discharge for the 2070s was projected to increase by approximately 43% in January and 55% in February, but to decrease by approximately 38% in April and 32% in May. The flood peak in the hydrograph was moved forward by approximately one month, changing from April in the 1990s to March in the 2070s. Furthermore, the projection for the 10-year average snowfall amount was projected to be approximately 49.5% lower in the 2070s than in the 1990s.

The Baiu (Mei-yu) front over East Asia in the global warming climate as well as that in the present one, is studied using outputs of a non-hydrostatic regional model with a horizontal grid size of 5 km (NHM). The NHM was run in June and July for ten years, applying a spectral boundary coupling method to reduce the horizontal phase differences of large-scale disturbances using the outputs of a global climate model with a grid size of 20 km. In the global warming climate, the Baiu front is likely to stay over the southern Japan Islands around the latitudes of 30 N 32 N and will not move northward. Therefore, the activity of the Baiu front maintains longer than that in the present climate, and the precipitation increases. On the other hand, the precipitation decreases over the northern Japan Islands and the northern Korean Peninsula. Years with no end of the Baiu season are often seen, and the frequency of occurrence of heavy rainfall greater than 30 mm h 1 increases over the Japan Islands.

Changes in the Baiu frontal activity in the future climate are examined, making use of super-highresolution 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 anticyclone 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 km 2 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.