Environments of quasi-stationary linear precipitation system associated with Baiu frontal depressions

Abstract Heavy rainfall in Japan is often caused by quasi-stationary linear precipitation systems (QSLPSs), which frequently occur in association with Baiu frontal depressions (BFDs). While the importance of the roles of BFDs in QSLPS occurrence has been suggested by previous case studies, there is no systematic analysis to clarify the differences between the environments of BFDs that cause QSLPSs (QSL-BFDs) and those that do not (NQSL-BFDs). Using ERA5 reanalysis data, this study statistically compares the characteristics of QSL-BFDs and NQSL-BFDs, based on composite analyses of 36 QSL-BFDs and 101 NQSL-BFDs, respectively, detected over and west of the Kyushu Islands in June and July between 1989 and 2020. The results show that QSL-BFDs are characterized by significantly stronger low-level moisture flux and convergence in the eastern part of mainland China and over the East China Sea, leading to enhanced large-scale lifting and mid-troposphere moistening prior to QSLPS occurrence. Between 6 and 12 h before QSLPS occurrence, significant differences in low-level moisture, convergence, and conditional instability (especially in terms of Entraining CAPE) between QSL-BFDs and NQSL-BFDs emerge, suggesting that these quantities can be useful precursors for prediction of the potential of QSLPSs. In contrast, storm-relative environmental helicity (SREH) does not differ significantly between the two types of the BFDs, indicating that it is not an effective discriminator.