Role of Typhoon Prapiroon (Typhoon No. 7) on the Formation Process of the Baiu Front Inducing Heavy Rain in July 2018 in Western Japan

摘要

Heavy rain in western Japan was broadly induced by the stagnation of the Baiu front during 5-7 July 2018. This study hypothesizes that cold air advection over the Sea of Japan intensified by Typhoon Prapiroon (Typhoon No. 7) was one of the triggering factors for the formation process of the Baiu front over western Japan. Typhoon Prapiroon passed over the Sea of Japan on 4 July and became extratropical at approximately 40°N on 5 July. During its passage, the strong southward pressure gradient force to the north of Typhoon Prapiroon broke down the convergence line of the Baiu front that remained at approximately 45°N before 4 July and thick cold air from the Okhotsk High flowed over the Sea of Japan. The Okhotsk High expanded toward the Sea of Japan and enhanced cold air advection to the north of western Japan. As a result, the Baiu front was stationary at approximately 35°N after 5 July. In addition, the westerly jet in the east of an upper-level trough deepened along the typhoon track was associated with the adiabatic component of the ascending motion over the isentropic upslope and was suggested to contribute to the maintenance of Baiu frontal convection.