Meso-&beta scale precipitation systems evolved at Tsushima Straits during late Baiu season in 2009

The 2009 Baiu season in the west Japan has characteristics of the anomalously low precipitation (less than 50% of normal year) during the first half and the record-breaking rainfall in late season. Some area of the west Japan observed heavy precipitation exceeding 100 mm per hour or 600 mm per 72 hours. This study aims to investigate the structure of the precipitation systems continuously evolved at the Tsushima Straits, using JMA reanalysis dataset and numerical model.

The closeness of midlatitude trough caused subtropical jets wound to south. Mesoscale Cloud Cluster (MCC) and tapering clouds were intensively evolved in the south east of trough, in which the lower moist warm air with the southwest flew together. The 700hPa moisture distribution showed that the dry air, with the relative humidity lower than 20%, was trapped in the middle of East China Sea (30-30šN, 120-125šE) and the moist tongue at the same level extended from south of the dry region. Such dry zone in 500 hPa extended toward the southwest of Japan Sea thorough Tsushima Strait with the south boundary corresponding with the edge of mid latitude trough. The CAPE was estimated at 200-1000 J/kg just the south of the dry area.