Influences of Large-scale and Meso-scale Circulation Patterns on Warm Season Precipitation in South China

Synoptic classification of weather regimes has been widely used in investigating the roles of background airflows in regional near-surface phenomena, such as heavy rainfall. However, recent studies have shown that subsidiary clustering within each major synoptic weather regime is needed to improve our understanding of regional weather variability. This is because subtle differences under each synoptic regime can lead to significant variations in the observed phenomena, suggesting the possible influences of smaller-scale circulation on precipitation. This study conducts a combined classification analysis of large-scale and mesoscale fields to investigate the relationship between rainfall in South China and circulation patterns at different scales. The Barnes filter is adopted to obtain large-scale and mesoscale fields, and the PCT (t-mode principal component analysis using oblique rotation) method is applied to the geopotential height (GH) at 900 hPa from 2000 to 2019, derived from ERA5 reanalysis data, for circulation classification. The classification results of large-scale GH reveal two primary patterns characterized by a southwest flow over South China, which is consistent with previous studies. Mesoscale GH is categorized into seven patterns, which exhibit a close relationship with rainfall patterns. Regions with positive (negative) mesoscale GH generally have relatively lower (higher) rainfall amounts. Usually, a large-scale pattern consists of two or three mesoscale patterns, which account for a large proportion (more than 75%). Furthermore, mesoscale circulations within the same large-scale circulation pattern can lead to substantial differences in precipitation by affecting water vapor transportation. To investigate the individual roles of large-scale and mesoscale circulation in precipitation forecasts in South China, quasi-ideal simulations of two cases are performed using raw and large-scale composite fields as initial and boundary conditions. The simulation results show that under the influence of southwest monsoonal flow, the mesoscale component in the initial and boundary conditions could induce stronger early-morning rainfall in the central Guangxi Province, and affect the timing and location of nocturnal precipitation near the eastern coast of Guangdong Province.