Practical and Intrinsic Predictability of a Warm‐Sector Torrential Rainfall Event in the South China Monsoon Region

Abstract

Warm‐sector torrential rainfall (WR) in the South China monsoon region has long been a forecasting challenge because of the limited capability of numerical models in heavy rainfall without strong synoptic forcing. Through convection‐allowing ensemble forecasts, this study explores both the intrinsic and practical predictability of a coastal WR event on 19–20 May 2015 during SCMREX (the Southern China Monsoon Rainfall Experiment). The results show a large variability in forecast performance among different members, indicating the practical limit of predictability. In general, GOOD members tend to have a stronger low‐level southerly wind over the sea (monsoon flow) and a considerable surface cooling over the northern mountains (associated with land/mountain breeze). Further investigation via ensemble‐based sensitivity analysis shows that the occurrence of WR is closely related to the nighttime strengthened (cooling) southerly wind (temperatures) over the sea (mountains), 1–3 hr prior to the convection initiation. In contrast, spatial scaling of the initial perturbations has little impact on the forecast after 3 hr and the meso‐γ‐scale rainfall is fully decorrelated after 12 hr, suggesting an intrinsic predictability limit for lead times as short as 6–12 hr. Sensitivity experiments are conducted with the initial‐condition differences reduced by almost an order of magnitude smaller than typical ensemble perturbations, with the results demonstrating that the rainstorm might be near the point of bifurcation, where predictability is intrinsically limited. The limits of both intrinsic and practical predictability highlight the need for rapidly updated and probabilistic convection‐allowing ensemble forecasts for events of this type.

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

“…Therefore, the FNL_WSM6 experiment initialized at 2000 LST 7 May is selected as one of the CTL experiments, as it outperforms the other FNL‐forced experiments in terms of the simulation of rainfall amount and distribution. Note that the WSM6 single‐moment microphysics scheme was used in previous modeling studies on squall lines and heavy rainfall events over South China (e.g., D. Wu et al., 2013; N. Wu et al., 2020). Also of note is that FNL_WSM6 show similar performance in the heavy rainfall simulation from D3 and D4 (Figures and ).…”

A Preliminary Study on the Organizational Modes of Mesoscale Convective Systems Associated With Warm‐Sector Heavy Rainfall in South China

“…Warm‐sector heavy rainfall is mostly produced by mesoscale convective systems (MCSs; He et al., 2016) and thus, is generally characterized by localized areas, concentrated timespans, and high rainfall intensities (Lin, 2006), which may cause severe flooding and enormous losses of life and property. Relative to the generally low quantitative precipitation forecast (QPF) skill of rainfall where MCSs occur, as revealed in real‐time statistics (https://www.wpc.ncep.noaa.gov/html/hpcverif.shtml) and in many studies (e.g., Squitieri & Gallus, 2016), the QPF skill is even lower for warm‐sector heavy rainfall in South China, which is influenced by weakly synoptic‐scale baroclinic forcing, land‐sea contrasts, and complex terrain and has been the region representing the biggest challenge in heavy rainfall forecasting (Luo, 2017; Luo et al., 2017; Wu, Ding et al., 2020; Wu, Zhuang et al., 2020; Zhang and Meng, 2019). The organizational modes of MCSs associated with warm‐sector heavy rainfall are key factors in determining the distribution and evolution of warm‐sector heavy rainfall in South China, which however has not received adequate attention.…”

Roles of Double Low‐Level Jets in the Generation of Coexisting Inland and Coastal Heavy Rainfall Over South China During the Presummer Rainy Season

“…Its formation is closely associated with oceanic southerly airflows in the boundary layer (BL) (Du & Chen, 2018; Du & Rotunno, 2018; Luo et al, 2017), convectively generated cold pools (Liu et al, 2018; Wu & Luo, 2016), and surface forcing such as land‐sea contrast (Chen et al, 2016) and orographic lifting (Wang et al, 2014; Yu & Cheng, 2013). Formation of the warm‐sector heavy rainfall over inland south China is usually related to the tropical‐originating low‐level jets (LLJs) (Chen et al, 2017; Du & Chen, 2019b; Huang & Luo, 2017; Wu et al, 2020), local topographic lifting (Jiang et al, 2017), surface heating, and urban heat island effect interacting with onshore flows/sea breezes (Wu et al, 2019; Yin et al, 2020).…”

“…Recently, the importance of southerly airflows in the BL from the northern South China Sea (SCS), not necessarily reaching the BLJ's minimum intensity (peak wind speed >10 m s −1 ), is emphasized by several observational analyses of extremely heavy warm‐sector rainfall over the south China coasts (Liu et al, 2018; Wang et al, 2014; Wu & Luo, 2016; Yin et al, 2020). When the southerly flows in the BL from the SCS encounters terrain, cold pool, or the coastline of south China, it could produce lifting that favors the generation of deep convection and helps maintain the rainstorms (Chen et al, 2014; Liu et al, 2018; Wu, Zhuang, et al, 2020). Du and Chen (2018, 2019a) pointed out that, for coexisting inland synoptic and coastal warm‐sector heavy rainfall over south China, an SLLJ and a BLJ are more closely related to the former and latter, respectively.…”