Analyses On Cold-wet Spells In China And Its Correspondingly Anomalous Large Scale Circulation

Two types of persistent extreme weather events, the cold-wet spell (CWS) and the persistent wet-freezing event (PWFE) are defined by considering the persistence of both extreme low temperature and precipitation. Regional CWS and PWFE are identified based on temporal persistence and spatial contiguity of single station based CWS and PWFE events respectively. Relevant factors are further discussed to reveal the features of spatial distribution and temporal variability of such events. The results indicate that:Station-based CWSs are mainly found in southern China, especially in eastern part of Southwest China and South China. PWFEs are mostly observed in the region immediate south of middle reach of the Yangtze River. Due to the strict condition for the concurrence of persistent precipitation, CWS and PWFE are much less found in northern China. Both CWS and PWFE with relative more frequency and longer duration are found in the region north to the Yunnan-Guizhou Plateau, where temperature is relatively lower due to higher elevation and precipitation is more abundant. A total of48regional CWSs and21PWFEs are detected during1951-2011.The spatial distribution of regional events is corresponding to that of station-based ones Multiday persistent precipitation is the key factor in determining the spatial distribution of CWSs or PWFEs and is a limitation condition in northern and western China. However, persistent low temperature condition is the dominate factor for the occurrence of CWSs in southern China since persistent precipitation condition is easier to satisfy.Using a composite analysis for’CWS event-days’,’PWEF event-days’and’non-event days’, the anomalous circulation characteristics and the persistence of key elements of that are analyzed. Main conclusions are summarized as follows:For the event days, at500hPa, the blocking high in the west of the Baikal was robust, and the low trough (vortex) in central and western Asia was vigorous but less versatile in its position. At850hPa, there are active shears and vortex activities. In the upper troposphere (200hPa), the displacement the jets favors robust divergences for circulation patterns. The northward transport of warm and moist airflows from the Bay of Bengal and the South China Sea finally warranted the formation of the CWS and PWFE and the prolonged dominance in the southern part of China. The anomalous circulation pattern of PWFE is almost the same with that of CWS. However, PWFE contains higher potential for causing severe disaster for the elements of PWFE anomalous circulation patterns are stronger. Based on the analysis on cases of2008and1955, it indicates that the anomalous centers moves a little,while,a dipole circulation pattern, with the blocking high over the Ural and the low trough (vortex) in central and western Asia, keep on strengthening generally. The anomalous regions of the geopotential height transient at500hPa are corresponding to that of the geopotential height. The maintaining of the transient anomalous center is responsible for the occurrence of the CWS and the PWFE. The transient anomalous circulation pattern also maintain when the event happen.Finally, climate change characteristics of CWS and PWFE are analyzed. Results indicate that station-based or regional CWSs and PWFEs occurred frequently during1960s-1980s, with higher intensity, longer duration and larger affected areas, but have rarely been seen since mid-1980s. The persistent extreme low temperature, rainy (snowy) and freezing event in2008is the most severe one in the last five decades.