Spatial and temporal characteristics of abrupt heavy rainfall events over Southwest China during 1981-2017

摘要

The spatial and temporal characteristics of abrupt heavy rainfall events (AHRE: defined as 3-hr rainfall amount >= 50 mm and at least one 1-hr rainfall amount >= 20 mm during the 3 hr) over southwest China (SWC) between 1981 and 2017, including their occurrence frequency, intensity, and duration, were investigated based on hourly rainfall data collected from 468 rain gauge stations. The occurrence frequency of these AHREs exhibited large spatial variability among different regions. We distinguished three regions with high occurrence rates of AHREs: the Sichuan Basin (subregion A, SR-A), South Guizhou-North Guangxi (subregion B, SR-B), and West Hunan (subregion C, SR-C). Overall, the amount of rainfall generated by the AHREs was more closely related to event duration than event intensity. Analysis of the monthly variations showed that the frequency of AHREs over SWC mostly (ca., 86.8%) appeared in the warm season (May-August) and peaked in July. But the peak month exhibited distinct regional features, with it occurring in midsummer over SR-A, late spring or early summer over SR-B, and early summer over SR-C. With regards to diurnal variations, AHREs over areas with higher (lower) topography were initiated mostly in the afternoon (around midnight). In addition, short-duration (1-3 hr) AHREs began most frequently in the late afternoon, whereas the long-duration events (>6 hr) tended to begin around midnight. Composite analysis of the anomalies of moisture, equivalent potential temperature, and wind during extreme AHRE days revealed that AHRE occurrence coincided with an abnormal low-level cyclonic circulation, which enhanced wind convergence and the transport of moist and warm air, providing favourable thermodynamic conditions for AHRE formation. These results advance our understanding of rainfall characteristics over SWC and provide observation-based metrics for the evaluation of numerical simulations.