The Relationship Scenarios between Boundary Layer Jet and Coastal Heavy Rainfall during the Pre-Summer Rainy Season of South China

In the pre-summer rainy season of South China, the occurrence of heavy rainfall (HR) near coastal regions, particularly in Yangjiang, is closely related to an upstream boundary layer jet (BLJ) over the northern South China Sea. The emergence of this BLJ is considered a crucial forecasting factor for coastal HR. However, the specific extent of the relationship between BLJ and HR remains unclear. In this study, utilizing 22-yr TRMM rainfall records and ERA5 data, we categorize four scenarios: HR with BLJ, HR without BLJ, no HR with BLJ, and no HR without BLJ in binary classification. It is found that during NoBLJ-HR events, even when the threshold of BLJ is not met, the low-level onshore wind fosters coastal convergence and facilitates moisture transport. Conversely, BLJ-NoHR events witness the BLJ penetrating further north, inducing inland convergence, accompanied by a dry environment. The attributes of the BLJ, encompassing its intensity, direction, and location, alongside linked moisture conditions, exhibit differences between HR and non-HR events. Based on a comprehensive indicator of correlation, we suggest a relatively optimal criterion to identify BLJ: maximum wind speed exceeding 8–10 m s⁻¹, wind direction falling within 180°–270°, the jet’s core located south of 20°–21°N, and humidity near the core surpassing 0.016–0.017 kg kg⁻¹. This study provides a benchmark for predicting the occurrence of HR through presence of the BLJ.