Abstract
In view of the limited predictability of heavy rainfall (HR) events and the limited understanding of the physical mechanisms governing the initiation and organization of the associated mesoscale convective systems (MCSs), a composite analysis of 58 HR events over the warm sector (i.e., far ahead of the surface cold front), referred to as WSHR events, over South China during the months of April to June 2008~2014 is performed in terms of precipitation, large-scale circulations, pre-storm environmental conditions, and MCS types. Results show that the large-scale circulations of the WSHR events can be categorized into pre-frontal, southwesterly warm and moist ascending airflow, and low-level vortex types, with higher frequency occurrences of the former two types. Their pre-storm environments are characterized by a deep moist layer with >50 mm column-integrated precipitable water, high convective available potential energy with the equivalent potential temperature of ≥340 K at 850 hPa, weak vertical wind shear below 400 hPa, and a low-level jet near 925 hPa with weak warm advection, based on atmospheric parameter composite. Three classes of the corresponding MCSs, exhibiting peak convective activity in the afternoon and the early morning hours, can be identified as linear-shaped, a leading convective line adjoined with trailing stratiform rainfall, and comma-shaped, respectively. It is found that many linear-shaped MCSs in coastal regions are triggered by local topography, enhanced by sea breezes, whereas the latter two classes of MCSs experience isentropic lifting in the southwesterly warm and moist flows. They all develop in large-scale environments with favorable quasi-geostrophic forcing, albeit weak. Conceptual models are finally developed to facilitate our understanding and prediction of the WSHR events over South China.
Highlights
Previous studies have shown that more than 40% of annual rainfall in South China occurs during the pre-summer months (April to June) [1,2,3,4]
In contrast to relatively weak and broad rainfall occurring behind slowly moving surface cold fronts, many of the growing-season rainfall events over the region are heavy and highly localized, where heavy rainfall is defined as the daily rainfall amount of greater than 50 mm, following that defined by the China Meteorological Administration. They are typically generated in mesoscale convective systems (MCSs) that develop in the warm sector, i.e., far ahead of surface fronts, where southwesterly warm and moist flows prevail with weak thermal gradients
We focus on the area ranging within 20–27◦ N and 105–120◦ E, which includes almost all of South China, except for Hainan Province
Summary
Previous studies have shown that more than 40% of annual rainfall in South China occurs during the pre-summer months (April to June) [1,2,3,4]. After analyzing two high-impact WSHR events, Sun and Zhao [14,15,16], revealed the importance of surface heating, topography and sea breeze circulations in the associated HR-producing MCSs, which differ from those associated with dynamical forcing such as frontal lifting and upper-level troughs Both observational and numerical modeling studies have shown the effects of topographical lifting on the development of HR-producing MCSs [17,18,19,20]. Given our limited understanding of the largescale conditions associated with WSHR and the morphologies of HR-producing MCSs during the pre-summer months over South China, the objectives of this study are to (i) document the spatiotemporal characteristics of WSHR and classify a few major types of WSHR events during the years of 2008–2014; (ii) identify the large-scale environmental. A summary and concluding remarks are given in the final section
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