Convection Initiation at a Coastal Rainfall Hotspot in South China: Synoptic Patterns and Orographic Effects

Abstract

Deep moist convection and the associated rainfall frequently occur on global coasts, especially on the windward sides of coastal orography in monsoon regions, playing a vital role in energy and hydrologic cycles Xie et al., 2006). They threaten the public safety and property with flash floods, high winds, lightning, hail, and occasionally tornadoes. The genesis of convection, namely convection initiation (CI), is one of the most important issues in quantitative precipitation forecast (QPF) because rainfall strongly depends on the timing and location of CI. The improved understanding and prediction of CI in coastal areas are thus integral parts of the disaster prevention and mitigation for severe weather.CI has long been known to be a result of the interaction of multiscale atmospheric processes which contributes to the locally enhanced lifting and moisture pooling. Here, the moisture pooling indicates the development of an area in the planetary boundary layer (PBL) where moisture values become higher than in the surrounding region (American Meteorological Society, 2021). The synoptic-scale circulation often primes the mesoscale environment for CI by large-scale mean ascent, moistening the atmosphere and thus increasing the convective instability (Markowsk & Richardson, 2010). Local convergent zones in the PBL usually play a crucial role in triggering convection primarily by the forced lifting. The common enhanced PBL lifting is often located on mesoscale boundaries, such as gust fronts, drylines, surface wind shift lines, sea-breeze fronts, gravity waves, and horizontal convective rolls (e.g., Saito et al., 2018; Wilson & Roberts, 2006, and references therein). These boundaries can be identified by a variety of observational platforms and are carefully assessed by forecasters to help the CI nowcasting.In addition to the PBL convergent boundary, orography is also a key factor for triggering convection. The resultant convection often repeatedly initiates in the region with some particular topographical features rather than more or less random as in flat areas (Banta & Schaaf, 1987). The orography-related dynamics such as

Roles of Multi‐Scale Orography in Triggering Nocturnal Convection at a Summer Rainfall Hotspot Over the South China Coast: A Case Study

“…The triggering of convection in the PRD is significantly influenced by the winds in the boundary layer (Bai et al., 2021; Chen et al., 2017; Rao et al., 2019; Wang et al., 2021), and thus the horizontal winds below 5 km of VDRAS were compared with those of ERA5 (Figure 4). VDRAS results show that from 00:00 LST to 05:00 LST, southerly winds prevailed below 2 km while the upper layer was dominated by southwesterly winds.…”

“…The initiation of deep moist convection, which we usually refer to as convection initiation (CI), dramatically affects the subsequent convective storm development and occurrence of severe weather (Markowski & Richardson, 2010). It has long been recognized that convection results from the multi‐scale coupling effects of synoptic and local circulations and underlying surfaces, such as coastlines, mountains, or urban agglomeration (Bai et al., 2021; Chen, Zhang, et al., 2016; Chen et al., 2014; Du & Chen, 2019; Rao et al., 2019; Wang et al., 2014; Wilson & Roberts, 2006). Due to the complex interactions between local surface forcings and multiple atmospheric processes, it is difficult to accurately forecast where and when convection is likely to be initiated and severe weather will occur.…”

“…The most straightforward mechanism of orographic convection initiation is the forced ascent of air on the windward slope. Incoming flow with a low Froude number (i.e., weak or statically stable flow) may become blocked and get diverted around the mountain barrier, leading to the upward motion in the windward slope and lee of the mountain range (Bai et al., 2021; Cheng & Yu, 2019; Wang et al., 2014; Yu & Tsai, 2017; Yu et al., 2006). Solar heating of elevated terrain is responsible for the triggering of afternoon thunderstorms.…”

Synoptic Impacts on the Occurrence of Mesoscale Boundaries and Their Associated Convection Over an Area of Sharp Vegetation Contrast

“…Huth et al (2008) suggested that the T-PCA method outperforms other objective classification methods in reproducing predefined dominant weather patterns embedded in the data set and deriving classification results that are less sensitive to preset parameters with good temporal and spatial stability. Previous studies have demonstrated successful applications of T-PCA in different regions of China (e.g., Bai et al, 2021;He et al, 2017;Wang et al, 2021;Xu et al, 2020). Following these studies, T-PCA was conducted in this study via the open-source software "cost733class," which was developed within the framework of COST Action 733 "Harmonisation and Applications of Weather Type Classifications for European Regions" (Philipp et al, 2016).…”

A Case Study of the Effects of Aerosols on South China Convective Precipitation Forecast

“…It is found that a large fraction of the pre-summer rainfall is produced by convection in the warm sector region hundreds of kilometers ahead of a cold or quasi-stationary front. However, the convection initiation (CI), which depends on the multiscale interaction of atmospheric dynamics, is notoriously known for its relatively low predictability in the warm sector regions (Luo et al, 2017;Bai et al, 2021;Zhang et al, 2022). Therefore, the warm-sector CI is a major contributor to the errors in quantitative precipitation forecast (QPF) for the region.…”