Characteristics of springtime cyclones with heavy rainfall in Japan are examined. Heavy rain episodes are obtained from the upper five percent of an area-averaged daily rainfall in the mainland Japan for the period 1982-2009. For heavy rain episodes, the cyclogenesis area shifts westward. The primary path of cyclones, locates along the south coast of Japan, displaces northward from the climatological path and that of the moderate rain episodes. Maximum developments of cyclones are frequently observed over the East China Sea and western Japan near the oceanic frontal zone. A composite analysis of heavy rain episodes exhibits that a lower tropospheric ridge covers Japan two days before a heavy rain date (Day -2). The ridge, which locates to the east of the trough, amplifies and extends meridionally with eastward progress until Day 0. As the eastward migration of the trough and ridge, poleward moisture flux is enhanced between the trough and ridge systems. In moderate rain episodes, amplification of trough and ridge systems and associated poleward moisture flux are rather weak for the period around Day 0. Enhanced poleward moisture flux due to the amplification of both trough and ridge may cause early development and northward shift of cyclones.
2012 the Meteorological Society of Japan