Precipitation system associated with the Asian summer monsoon, called the Baiu in Japan, is one of the most remarkable weather phenomena in East Asia. During the Baiu season, a belt-shaped rainfall zone extends eastward along the northern boundary of the tropical maritime airmass from the southern part of the China Continent to the Japanese Islands. This rainfall zone climatologically corresponds to the summer monsoon front, called the Baiu front. The Baiu frontal zone often accompanies a huge and intense precipitation over the Japanese Islands and causes severe damages to human life and the society (Ninomiya, 2000, Biao et al. 2004, Kato 2006). Changes in the characteristics of the Baiu as well as associated precipitation, are of great concern to those living in East Asia.

Therefore, future changes in precipitation and vertical structures of the frontal zone during the Baiu season, in the vicinity of the Japanese Islands are investigated from regional climate experiments with a 5km-mesh non-hydrostatic model, driven by the results of global warming experiments using a 20km-mesh atmospheric global circulation model under the SRES-A1B scenario for the present-day (1979-2003) and the future (2075-2099) climates. In the future climate, significant increments of daily precipitation amounts around western Japan are projected in the late Baiu season. Remarkable increments with statistical confidence above the 98% confidence level are found in the ratio of intense daily precipitation amounts over 100 mm day^-1 to total daily amounts. In the present-day climate, 9% of 25yr-mean precipitation amounts in early-July in the region of 30N–35N and 127E–137E is estimated to be dominated by intense precipitation above 100 mm day^-1, meanwhile the value rises to 15% in the future climate.

The changes of the Baiu front and the environmental conditions around western Japan are investigated, focusing on the relationship to the increases of intense precipitation. Its northward march is detected to delay in the future climate. In the late Baiu season, mean location of the front is respectively found around 33.5N and 33.0N in the present-day and future climates, and mean amount of water vapor on the southern side of the front at a 500 m level increases to 19.7 g kg^-1, 2.8 g kg^-1 larger than in the present-day climate. Mean vertical cross sections reveal that tall structures of intense vertical vorticity above 1.0 X 10^-5 s^-1 are detected along the intensified frontal zone with intense mean updrafts and large amounts of rainwater in the future climate. Two of characteristic jets associated with the Baiu front, located at a 700 hPa level on the southern side and a 200 hPa level on the northern side, are intensified in the future climate. Abundant supply of water vapor and the intensified convective activities could affect the increment of intense daily precipitation amounts.

References Biao, G., H. Yamada, K. K. Reddy, H. Uyeda, and Y. Fudeyoshi, 2004: An observational study of the development of a rainband on a Meiyu front causing heavy rainfall in the downstream region of the Yangtze River. J. Meteor. Soc. Japan, 82, 1095-1115.

Kato, T., 2006: Structure of the band-shaped precipitation system inducing the heavy rainfall observed over northern Kyushu, Japan on 29 June 1999. J. Meteor. Soc. Japan, 84, 129-153.

Ninomiya, K., 2000: Large- and Meso-a-scale characteristics of Meiyu/Baiu front associated with intense rainfall in 1-10 July 1991. J. Meteor. Soc. Japan, 78, 141-157.