Unique Environmental and Cyclogenesis Conditions of Boreal Midsummer Extratropical Cyclones Accompanied by Torrential Rain in Central and Eastern China

In recent years, torrential rain events caused by extratropical cyclones (ETCs) during the boreal midsummer (July–August) in Central and Eastern China have shown an increasing trend. For instence, in August 2024, two ETCs brought large-scale heavy rainfall to North China, with daily precipitation exceeding 100 mm. Using reanalysis datasets and gridded precipitation data, the ETCs that affected Central and Eastern China during the boreal midsummer from 1981 to 2020 were objectively identified and tracked. ETCs causing precipitation were classified based on maximum daily precipitation, resulting in datasets for ETCs with torrential rain (daily precipitation exceeding 100 mm, referred to as ETC_R100) and heavy rain (daily precipitation exceeding 25 mm, referred to as ETC_R25). Comparative analysis can help highlight the characteristics of ETC_R100. This study compares the spatial distribution, movement paths, weather impacts, large-scale atmospheric circulation, and environmental conditions of these two types of precipitation-related ETCs. The following findings emerged: (1) ETC_R100 is driven by the combined forcing of upper-level troughs and warm-moist airflows at lower levels, exhibiting stronger thermal forcing than ETC_R25. (2) The moisture source for ETC_R100 are the Bay of Bengal and the Northwest Pacific, with moisture transported via the South China Sea. Compared to ETCs with non-extreme rainfall, ETC_R100 is characterized by greater atmospheric instability and better moisture conditions, resulting in higher precipitation intensity. (3) Regardless of the precipitation level, ETCs affected different regions but contributed significantly to precipitation in northern China, accounting for approximately 50% of the total precipitation. The results indicate that ETC_R100 differs significantly from ETCs with varying levels of precipitation in terms of statistical char-acteristics, weather impact, environmental conditions, and cyclogenesis conditions.