Objective Classification of Controlling Factors for the Occurrence of the Wide-spread Extreme Precipitation Events during the Baiu Season over Western Japan

Abstract

Atmospheric patterns associated with wide-spread extreme precipitation events during the Baiu season over western Japan have a diversity in the record. Using an objective approach based on the empirical orthogonal function (EOF) analysis, this study introduces a classification of atmospheric parameters related to the wide-spread extreme precipitation events which are not directly caused by tropical cyclones. The number of a rain gauge observation stations that record extreme precipitation during the Baiu season over western Japan is equivalently proportional to the scores of the first two Principal Components, implying that there are two orthogonal controlling factors for the occurrence of wide-spread extreme precipitation. The first Principal Component is well correlated with a typical frontal dynamical structure as the enhanced westerly jet, the large gradient of the equivalent poten-tial temperature, and the upper-level Rossby wave train injecting into a cyclonic anomaly at the north of the precipitation area. On the other hand, the second Principal Component is dominated by moisture fields with a low-level cyclone and no upper-level signal. This finding could provide a physical understanding of the diversity of atmospheric patterns causing wide-spread extreme precipitation over western Japan and physical insight into how it will change in the future climate