Warm-Sector Heavy Rainfall in Southern China and its WRF Forecast Evaluation: A Low-Level-Jet Perspective

Warm-sector heavy rainfall in southern China refers to the heavy rainfall that occurs within the warm sector hundreds of kilometers to the south of a front system or without a front system during the presummer rainy season (April–June), characterized by poor predictability and a close relationship with low-level jets (LLJs). Based on 45 warm-sector heavy rainfall episodes in 2013 and 2014 in southern China, this study examined their general characteristics and evaluated the performance of the convection-permitting WRF model from an LLJ perspective. The results showed that 64% of the warm-sector heavy rainfall episodes were associated with an LLJ (LLJ type) and 36% were not (no-LLJ type). The LLJ type was distinct from the no-LLJ type, with large rainfall accumulation along the coastal area. It was more common for LLJs to occur at both 800 hPa and 925 hPa in the LLJ type, where there was a wide 800-hPa LLJ to the west of Guangdong Province and two 925-hPa LLJs over Beibu Gulf and the South China Sea (SCS). The coastal convergence associated with the weakening of 925-hPa southerly winds was conducive to the coastal rainfall. WRF generally presented lower QPF skill in the LLJ type than in the no-LLJ type, due to the severe underestimation of coastal rainfall. The QPF skill of the LLJ type were significantly correlated with the forecast accuracy of LLJs, especially at 925 hPa. The north bias of the LLJ over the SCS and the overestimation of northerly wind at 925 hPa weakened the coastal convergence and eventually led to the underestimation in coastal precipitation.