Post Date: 22 March 2019

Warm-Sector Heavy Rainfall in South China and its Predictability: A Low-Level-Jet Perspective

Abstract

Warm-sector heavy rainfall in southern China refers to the heavy rainfall that occurs within the warm sector hundreds of kilometers to the south of the front system during the presummer.

Based on 45 warm-sector heavy rainfall episodes in 2013 and 2014 in Guangdong Province, we examined the general characteristics of warm-sector heavy rainfall and their convection-permitting WRF model performance. The results showed that 71 % (29%) of the warm-sector heavy rainfall episodes are (not) associated with LLJ (LLJ type/no-LLJ type). The rainfall distribution in LLJ type is distinct from no-LLJ type with large rainfall accumulation along the southern coast. The coastal convergence associated with the decreasing of 925-hPa southerly winds is conducive to the coastal heavy rainfall. The ETSs of LLJ type are significantly correlated with the forecast accuracy of LLJ, especially the accuracy of 925-hPa southerly winds. WRF tends to underestimate coastal precipitation due to a north bias of the 925-hPa LLJ over South China Sea.

Ensemble-based sensitivity analysis showed that the rainfall forecast skill could be determined by the forecast accuracy in the location of the LLJ, which could be possibly related to the different juxtapositions between the direction of the movement of the low-level vortex and the orientation of the LLJ. The impact of LLJ on the elevated initiation of a warm-sector quasi-linear mesoscale convective system in a weak-lifting environment was also examined. An elevated moist absolutely unstable layer (MAUL) was found to be essential to the convection initiation. The elevated MAUL was mainly attributed to significant adiabatic cooling by weak vertical ascent and sufficient horizontal moisture transport near the terminus of a LLJ.

Speaker bio:

Prof. MENG, Zhiyong is currently a full professor in the Department of Atmospheric Sciences of Peking University (PKU). She her Ph.D. in atmospheric science in Texas A&M University in 2007, and joined PKU in 2008. In 2014, she was awarded "The National Science Fund for Distinguished Young Scholars” in China and the "Editor’s Award of Monthly Weather Review”. She is a member of the Expert Team on Severe Monsoon Weather of the WGTMR/Monsoon Panel of WMO. Her research interests involve data assimilation and numerical weather prediction, mesoscale predictability and ensemble forecast, mesoscale convective systems, tornados, and tropical cyclones. More information about her research and experience can be found at http://www.phy.pku.edu.cn/~zymeng/.