Statistical Study on Baiu Frontal Depressions Extracted from a Regional Reanalysis

抄録

This study investigates the statistical characteristics of Baiu frontal depressions (BFDs), which occasionally accompany heavy rainfall in Japan. Using a high resolution (5 km) regional reanalysis dataset in June and July from 1991 to 2020, we objectively detected about 7000 BFDs (more than 1,000 distinct events) by combining methods to identify isolated depressions and the Baiu front. Our analysis shows that BFDs occur widely around Japan, particularly in western areas, and BFDs with smaller radii are more prone to intense precipitation. Although smaller BFDs are more common in the western areas, both small and large BFDs also occur in the eastern areas. Composite analyses indicate that small BFDs exhibit heavy rainfall near their centers and have an upright or slightly eastward tilted vertical structure, suggesting the essential role of diabatic heating. In contrast, BFDs with larger radii exhibit maximum precipitation east of their centers and have a westward tilted structure, suggesting a greater contribution from baroclinic processes.