Convection Initiation and Growth at the Coast of South China. Part II: Effects of the Terrain, Coastline, and Cold Pools

Abstract

Through conducting dynamic and thermodynamic diagnoses as well as a series of numerical sensitivity simulations, we investigated the effects of the terrain, coastline, and cold pools on convection initiation (CI) and its subsequent upscale convective growth (UCG) during a case of heavy rainfall along the coast of South China. CI occurred at the vertex of the coastal concave mountain geometry as a combined result of coastal convergence, orographic lifting and mesoscale ascent driven by the terminus of a marine boundary-layer jet (MBLJ). In numerical simulations with the coastline or terrain of South China removed, the coastal CI does not occur or becomes weaker as the MBLJ extends farther north, suggesting that the coastline and terrain play a role in CI. In addition, local small-scale terrain can modulate the detailed location and timing of CI and UCG. When the coastal concave terrain and coastline near the CI are artificially removed or filled by additional mountains, the orographic lifting and the local convergence along the coast correspondingly change, which strongly affects the CI and UCG. From a thermodynamic perspective, the coastal concave terrain plays the role of a local moisture “catcher”, which promotes low-level moistening by blocking water vapor coming from an upstream moist tongue over the ocean. Furthermore, new convection is continuously generated by the lifting of low-level moist southerlies at the leading edges of cold pools that tend to move southeastward because of the blocking coastal mountains. Sensitivity experiments suggest that the MCS becomes weaker and moves more slowly when cold pools are weakened through a reduction of rain-evaporation cooling.

The Synoptic Impacts on the Convection Initiation of a Warm‐Sector Heavy Rainfall Event Over Coastal South China Prior to the Monsoon Onset: A Numerical Modeling Study

“…Even if with the strong southerly component, the wind speeds of BL airflows over the offshore in bad member still is smaller than that in good members, which cause in the negative correlation of the meridional winds at 950 hPa over the offshore with the coastal CI time. These findings differ from numerous studies of the heavy rainfall during the postmonsoon‐onset period of South China (e.g., Du & Chen, 2018, 2019; Du, Chen, Han, Bai, & Li, 2020; Du, Chen, Han, Mai, et al., 2020; Liu et al., 2020), which found the southerly component of the southwesterly BL airflows playing a prominent role on the amount of heavy rainfall over South China after the onset of SCS monsoon. Such difference is consistent with the shift of prevailing marine BL winds over SCS from southeasterly before the monsoon onset to southwesterly after the monsoon onset (M. Li, Luo, Zhang, et al., 2020; Z. Li, Luo, Du, & Chan, 2020).…”

“…These two CTL experiments are hereafter referred to as ERA and FNL , respectively. The two CTL simulations allow for a 6‐h spin up, which seems sufficient for simulating the coastal CI of the warm‐sector heavy‐rainfall‐producing MCSs, similar to previous studies (e.g., Du & Chen, 2018; Du, Chen, Han, Bai, & Li, 2020; Du, Chen, Han, Mai, et al., 2020).…”

“…Terrain‐induced modifications on approaching airflow may have a non‐negligible impact on CI in warm‐section heavy rainfall over SC (Du, Chen, Han, Bai, & Li, 2020). Froude number (Fr = U/NH; U is the upstream wind speed, N is the Brunt‐Väisälä frequency, and H is the mountain height) is usually used to evaluate the terrain influence on the low‐level prevailing winds in orographic precipitation (Cheng & Yu, 2019; Tsai et al., 2018).…”

“…Occurrence of CI usually depends on multi‐scale atmospheric processes (Rotunno & Houze, 2007; Trier, 2003; Weckwerth & Parsons, 2006). Numerous studies have revealed various factors contributing to CI in the warm‐sector heavy rainfall events over inland and coastal SC, including terrain effects (Du, Chen, Han, Bai, & Li, 2020; Wang et al., 2014), mesoscale cold pools (Liu et al., 2018; Wu & Luo, 2016), surface heating (Jiang et al., 2017), boundary layer (BL) flows of tropical origins (Luo et al., 2017), land‐sea breeze fronts (X. Chen et al., 2017), low‐level jets (LLJs) (Du & Chen, 2018, 2019), and urban heat island effects interacting with onshore BL airflows (Wu et al., 2019; Yin et al., 2020). Previous studies utilizing integrated observations demonstrated the importance of coastline, coastal terrain, upstream oceanic airflow in the BL, and precipitation‐generated cold outflows/pools in triggering and maintaining the extreme rain‐producing MCSs near the SC coasts (Du, Chen, Han, Mai, et al., 2020; Du, Chen, Han, Bai, & Li, 2020; Liu et al., 2018; Wang et al., 2014; Wu & Luo, 2016).…”

Tropopause‐penetrating convection in the extratropics: A satellite‐based investigation for the year 2018

“…In the Southern Hemisphere, they appeared frequently over the southern Indian Ocean (Figure S1). Multiple studies show that complex terrains such as mountain ranges and coastlines tend to trigger and strengthen the development of convection (Barthlott and Kirshbaum, 2013; Mulholland et al ., 2019; Du et al ., 2020). Thus, it is evident that the spatial and temporal distribution of penetrating convection in the extratropics is closely linked to large‐scale circulation and topographic forcing.…”