Variations and Circulation Patterns of Warm-sector Heavy Rainfall in Southern China

Warm-sector heavy rainfall (WSHR) is a type of significant heavy precipitation that frequently occurs in the southern China. Based on daily observation precipitation data of China Meteorological Administration, spatiotemporal variation characteristics of WSHR, its contribution to extreme precipitation, the differences among WSHR, tropical cyclone heavy rainfall (TCHR) and frontal heavy rainfall (FHR), as well as the main circulation patterns and potential formation mechanisms of WSHR from April to June during 1980-2023 are analyzed. Results indicate that among all the heavy precipitation of pre-summer flood season in southern China, the multi-year mean proportions of WSHR station frequency and wet days account for 26.21% and 9.06%, respectively. The number of WSHR days is higher than that of TCHR, although WSHR station frequency is lower than that of both TCHR and FHR. Proportions of WSHR's multi-year mean regional daily precipitation and single-station daily precipitation account for 26.73% and 30.34% of the total heave precipitation during pre-summer flood season. Furthermore, the mean and the maximum of annual single-station daily precipitation associated with WSHR are higher than those of TCHR and FHR. Annual cumulative days and days exceeding extreme precipitation threshold for WSHR both show an ascending trend that contribute positively to the increase in extreme precipitation days, but annual cumulative precipitation of WSHR shows a descending trend. The main area of high WSHR frequency area is located along the southeast coast of Guangdong, which is roughly similar to the spatial distribution of daily precipitation proportion. The number of stations with maximum daily precipitation from WSHR exceeding 450 mm is higher than that from TCHR, and these stations primarily concentrate along the southwest and southeast coasts of Guangdong. Stations where WSHR exceeding the 95th percentile extreme precipitation threshold are mainly distributed in most of Guangdong and the central-southern part of Guangxi. Stations with maximum daily precipitation from WSHR exceeding 450 mm, which also surpass the 95th extreme precipitation threshold, are predominantly distributed in the southeast and southwest of Guangdong. This pattern is basically consistent with the spatial distribution of maximum daily precipitation contribution from WSHR. WSHR events are mainly classified into 3 types. Shear-line warm-sector heavy rainfall (SL-WSHR) exhibits the strongest convergence of water vapor flux, which is influenced by the combined effect of dynamic uplift and low-level jets that transport water vapor. Precipitation of SL-WSHR is mainly distributed on the south side of the shear line. Low-vortex warm-sector heavy rainfall (LV-WSHR) exhibits the smallest precipitation magnitude and spatial extent. In this type, vertical upward motion is widely distributed with rainfall occurring to the southeast of the cyclonic circulation center. In contrast, southerly warm-sector heavy rainfall (S-WSHR) shows the highest frequency of occurrence and the widest precipitation range, while also the largest total precipitation amount.