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

Warm-sector heavy rainfall in South China refers to the heavy rainfall that occurs from April to June within the weakly forced synoptic environment far from (or without) front systems. It has long been a forecast headache due to the limited capability of numerical models in predicting heavy rainfall without strong synoptic forcing. Previous studies have shown that warm-sector heavy rainfall is frequently associated with low-level jet (LLJ). To further understand the poor predictability of warm-sector heavy rainfall in South China and its relationship with LLJ, this work aims at 1) presenting the characteristics of warm-sector heavy rainfall in association with LLJ; 2) evaluating WRF forecast of warm sector heavy rainfall and LLJ; 3) revealing the relation between precipitation forecast skills and LLJ forecast accuracy.

A total of 45 warm-sector heavy rainfall cases are selected[1] in Guangdong Province in South China in 2013 and 2014. In the present study, a grid point is considered as LLJ-involved when its horizontal wind speed is over 12 m/s on either 850 or 925 hPa. A case is classified as a LLJ type if there is a LLJ-grid-point within 200 km from the rainfall center at either beginning or ending time of the case. Results showed that most warm sector rainfall cases are associated with LLJ. In particular, 32 cases (79%) are attributed to LLJ type and 13 cases (21%) to no-LLJ type. Compared with no-LLJ type, LLJ type has more sufficient moisture and stronger coastal rainfall with significant rainfall in the early morning. In the LLJ type, there are 22 cases where LLJ exists on both 850 hPa and 925 hPa (coupled LLJ, CLLJ), 8 cases where LLJ only exists on 850 hPa (synoptic LLJ, SLLJ) and 2 cases where LLJ only exists on 925 hPa (boundary layer jet, BLJ). Composite analysis revealed that 850-hPa LLJ core is located to the southwest of Guangdong Province, while the 925-hPa LLJ cores are located at Beibu Gulf and South China Sea. CLLJ type is characterized with prominent coastal rainfall which is likely due to the coastal convergence associated with BLJ.

The performance of convection-allowing WRF simulations (with a 3-km horizontal grid spacing) of all the 45 warm-sector heavy rainfall cases are evaluated. Generally, LLJ type has lower ETS of rainfall forecasts than no-LLJ type. Correlation analyses show that more accurate LLJ forecast is associated with higher warm-sector heavy rainfall forecast skill. The poor forecast skills of LLJ type can be attributed to the missing of coastal rainfall in CLLJ type simulation, mainly due to the significant overestimation in 925-hPa v-component. The overestimation of 925-hPa v component inland weakens the convergence near coastal area, leading to the missing of coastal rainfall.

[1] The warm-sector heavy rainfall cases are selected based on criteria as follows: 1) the accumulated precipitation should be greater than 50 mm in 6 h within a contiguous area extending at least 50 km in any orientation; 2) the rainfall center is at least 200 km from the synoptic front (or without front); 3) the rainfall is not influenced by tropical cyclones or northern cold high.