Heavy Rainfall Associated with Double Low-Level Jets over Southern China. Part II: Convection Initiation

Abstract

Heavy rainfall that occurred at the south coast of China on 10–11 May 2014 was associated with a synoptic-system-related low-level jet (SLLJ) and a boundary layer jet (BLJ). To clarify the role of the double low-level jets in convection initiation (CI), we perform convective-permitting simulations using a nonhydrostatic mesoscale model. The simulations reproduce the occurrence location and mesoscale evolution of new convective cells as well as their small-scale wavelike structures at the elevated layers, which are generally consistent with radar observations despite some differences in their orientation. The nighttime BLJ over the northern South China Sea strengthens the convergence at ~950 hPa near the coast where the BLJ’s northern terminus reaches the coastal terrain. Meanwhile, the SLLJ to the south of the inland cold front provides divergence at ~700 hPa near the SLLJ’s entrance region. Such low-level convergence and midlevel divergence collectively produce strong mesoscale lifting for CI at the coast. In addition to the enhanced mesoscale lifting, the double low-level jets also provide favorable conditions for the superimposed small-scale disturbances that can serve as effective moistening mechanisms of the lower troposphere during CI. In a sensitivity experiment with coastal terrain removed, CI still occurs near the coast but is delayed and weaker compared to the control run. This latter experiment suggests that double low-level jets and their coupling indeed exert key effects on CI, while the BLJ colliding with terrain may enhance coastal convergence for amplifying CI. These findings provide new insights into the occurrence of coastal heavy rainfall in the warm sector far ahead of the fronts.

Influence of Synoptic Pattern and Low‐Level Wind Speed on Intensity and Diurnal Variations of Orographic Convection in Summer Over Pearl River Delta, South China

“…The close relationship between the nighttime low‐level southwesterly jet in the boundary layer and convection over South China has been well recognized in previous studies (X. Chen et al, ; Du & Chen, , ), in which the nocturnal boundary layer jet can be mainly attributed to the inertial oscillation (Blackadar, ). According to the inertial oscillation theory, the amplitude of nighttime acceleration is proportional to the diurnally varying turbulent mixing and frictional effect in the boundary layer (Blackadar, ; Du & Rotunno, ).…”

Ensemble Sensitivity Analysis of Heavy Rainfall Associated With Three MCSs Coexisting Over Southern China

“…In the present study, we confirmed there were differences in the key factors of three coexisting MCSs, and we further found that the complex interaction of SLLJ and BLJ increased the forecast uncertainty of MCS‐B, resulting in the worse quantitative forecast skill of MCS‐B than of MCS‐A and MCS‐C. These results are different from the results in previous studies that also showed LLJs were essential in rainfall centers, but without distinguishing between the two types of LLJs (Huang & Luo, ; Zhang & Meng, ), and that a SLLJ (BLJ) was associated with frontal rainfall (the coastal warm‐sector rainfall; Du & Chen, , ). In addition, we also discussed the differences in the predictability of the three MCSs.…”

Diurnal variation of the duration and environment for heavy rainfall during the warm season in South China

“…Over South China regions, the localized rainfall maxima coincide well with the specific topography along the coastlines (Lin et al ., ; Ge et al , ; Wang et al ., ; Wu and Luo, ; Chen et al ., ; Chen et al ., ; Zhong et al ., ). It has been speculated that the localized rainfall maxima may be associated with the local topographic effects (Wang et al ., ; Gao et al ., ; Chen et al ., ), such as the topographic blocking effects (Chen et al ., a; Zhong et al ., ), surface heating (Wu et al ., ) and topographic lifting in the pre‐flood season (Jiang et al ., ), as well as the enhanced nocturnal low‐level southwesterly winds over north of Guangdong Province (GD, Chen et al ., ) and strong mesoscale lifting for convection initiation associated with double low‐level jets (LLJs) in the southwesterly moist flow (Du and Chen, , ).…”