Preliminary analyses on the climatological features of precipitation characteristics and large-scale atmospheric fields on the heavy rainfall days in the eastern part of Japan during the mature stage of the Baiu season

Rainfall characteristics and large-scale atmospheric fields on the “heavy rainfall days” (with more than 50 mm/day) in the mature stage of the Baiu season (16 June ~ 15 July) at Tokyo in the eastern part of the Japan Islands were examined, based on the daily and the hourly precipitation data from 1971 to 2010. Appearance frequency of the “heavy rainfall days” at Tokyo attained only about 1/3 of that at Nagasaki in the western Japan. Furthermore, it is noted that about half of the “heavy rainfall days” at Tokyo were related to the typhoon. In detail, about half of the typhoon cases were associated with the direct approach of a typhoon (referred to as Pattern A, hereafter), the other half corresponded to the situation when the Baiu front also stagnated around Kanto District with a typhoon to the southwest of Kanto (Pattern B). Although the contribution of the intense rainfall with more than 10 mm/h to the total precipitation was large in Pattern A, that with less than 10 mm/h was dominant in Patterns B and C (Pattern C: meso-α-scale cyclone on the Baiu front approaching to the Kanto District). It is noted that about half of the “heavy rainfall days” corresponded to these pattern. In other words, unlike the localized torrential rain in western Japan, the “heavy rainfall days” due to the duration of “not-so-intense-rain” appeared rather frequently in the eastern part of Japan even in the Baiu season. In both Patterns B and C, relatively strong low-level southerly wind associated with the disturbance (a typhoon or a meso-α-scale cyclone) invades into the baroclinic zone in the basic field sustained as the sowthwestern edge of the cool Okhotsk air mass. It is interesting that the “heavy rainfall days” there due to the contribution of the persistent “not-so-intense-rain” occur just in such situation.