Object-Oriented Composite Analysis of Warm-Sector Rainfall in North China

Warm-sector rainfall (WSR), which forms in the environment with abundant moisture and strong CAPE but weak synoptic forcing, is very common during the warm season in China. While many studies have studied the WSR in South China since the pioneering work of Huang (1988), the significance and impacts of WSR were not fully recognized until the 21 July 2012 extreme rainfall event in Beijing that led to catastrophic flash flooding with many fatalities. Due at least partly to the lack of full understanding of the synoptics, dynamics and climatology of the occurrence of WSR, its forecast is still a big challenge for both the numerical models and weather forecasters.

Based on the definition of WSR, for which precipitation is located in the warm-air region with a noticeable distance away from a frontal boundary (while usually is between the cold front and warm front boundaries), an objective identification technique of WSR is proposed. Firstly, synoptic fronts and their influence area are automatically detected using ECMWF Interim data and an objective frontal detection method based on the surface temperature gradient. Secondly, rainstorms are identified by finding a contiguous rainfall area using 1-h grid rainfall observations. Finally, a WSR event is identified by excluding the rainfalls that are significantly impacted by the fronts.

Based on this automatic detection method, a total of 768 WSR rainstorm events (with precipitation larger than 10 mm/h) are identified during the warm-season period from June to September over 2012-2017 in North China. The composite analysis of these WSR events during this 6-year period shows that: The WSR rainstorms have an average area of 6200 km^2 and the mean maximum rainfall of 35mm/h. Spatially, the WSR frequency and its rainfall amount are mainly located on the plains area with two regions of relatively high frequency of occurrence, one in the northeastern part of North China and the other near the intersect region of the Shandong, Jiangsu and Henan provinces. The spatial distribution of the WSR events are similar to the warm-season total rainfall in North China. Temporally, WSR can form all months from June to September but the peak is during mid July and mid August, in which the WSR occurrence probability is about 40% among all rainstorm types. From Mid-June to mid-September, the WSR high frequency belt gradually moves northward, which is driven by the movement of the summer monsoon circulation, bringing plentiful water vapor and convective unstable energy. WSR diurnal variation shows two peaks, one in the period from late afternoon to early evening and the other from late evening to early morning.

The composite analysis of circulation for the WSR are conducted. The results indicate that the environmental flow of WSR in the area with high frequency of formation can be classified into several synoptic weather patterns. Based on the related environmental variables of WSR versus those of non-WSR under similar synoptic circulation background, we have identified the basic ingredients for WSR occurrence in North China. Further efforts will also be made to explore the initiation, development and organization of mesoscale convective systems during the WSR period in an environment featured by weak synoptic force through radar, and satellite observations as well as through cloud-resolving model simulations of events with and without WSR.