Structural and Environmental Characteristics of Western Baiu Frontal Depressions


Baiu frontal depressions (BFDs) are the major cyclones that form and develop along Baiu fronts. The structure and environment of BFDs are examined using the Japanese 55-year Reanalysis data set. BFDs detected automatically were categorized into four groups based on the region where the BFDs peaked: the East China Sea (ES); off the south coast of the Japanese Islands (SC); the Yellow Sea (YS); and the Sea of Japan (SJ). A BFD-centered composite analysis demonstrates that, for YS-BFDs and SJ-BFDs, compared to ES-BFDs and SC-BFDs, the strength of upper-level troughs is greater, and the upper-level troughs are closer, suggesting that the effect of upper-level troughs on BFD development is greater. BFDs for all categories are located in the region of cyclonic horizontal shear and have a horizontal trough running from southwest to northeast, favoring barotropic energy conversion. The BFD-centered composite analysis of the energy conversion terms demonstrates that the barotropic energy conversion of mean kinetic energy due to the environmental cyclonic horizontal shear to eddy kinetic energy contributes to BFD development, and the conversion of eddy available potential energy, which is generated mainly by the diabatic heating, is the major contributor to eddy kinetic energy.