Heavy Precipitation over Southwestern Japan during the Baiu Season due to Abundant Moisture Transport from Synoptic-Scale Atmospheric Conditions

Abstract

Heavy precipitation frequently occurs over Kyushu, southwestern Japan, during the Baiu season, and abundant moisture transport is a key driving factor. To statistically understand the intensification of moisture transport to Kyushu during the Baiu season, synoptic-scale atmospheric conditions are examined using a composite analysis of reanalysis data. A heavy precipitation day is defined as a day with area-averaged daily precipitation over Kyushu that is larger than 1.0 mm and ranked in top 10% during May 31 to July 19 from 1981 to 2015. During such heavy precipitation days, the precipitation observed over Kyushu exceeds 100 mm day⁻¹. For several days before the occurrence of heavy precipitation over Kyushu, a plateau-scale disturbance develops over the Tibetan Plateau associated with daytime surface heating, and is characterized by cloud convection formation and eastward extension. During the eastward extension, latent heating from the cloud and upper-level high potential vorticity maintains the disturbance. The disturbance reaches northwest Kyushu on the heavy precipitation day, and a pair of positive and negative anomalies of relative vorticity over northwestern and southeastern Kyushu intensify the anomalous moisture transport.