Combined Disaster Events of Extensive and Persistent Low Temperatures, Rain/Snow, and Freezing in Southern China: Objective Identification and Key Features

Combined disaster events refer to the combination of several simultaneously occurring weather disasters. In this paper, the daily mean temperature, precipitation, and glaze data of 206 stations over southern China in winter from 1961 to 2013 are integrated to establish an objective method for identifying combined disaster events of extensive and persistent low temperatures, rain/snow, and freezing weather in winter, and the key features of these kinds of combined disaster events are discussed. First, the identification methods for extensive and persistent low temperatures and rain/snow and freezing events are proposed according to the thresholds of their intensities and impact areas. The three most prominent combined disaster events, namely cold-rain/snow (C-RS), cold-freezing (C-F), and C-RS-freezing (C-RS-F) disaster events, are identified. These combined disaster events often occur from early January to mid-February. Although they have similar low temperatures and precipitation levels, their formation conditions are significantly different. Abundant water vapor supply and large-scale strong water vapor convergence are key conditions for the occurrence of C-RS and C-RS-F disaster events, while an inversion layer and cold pad are necessary conditions for the occurrence of C-F and C-RS-F disaster events. The large-scale tilted ridge in mid- and high-latitude Asia is the key circulation feature of C-F and C-RS-F disaster events. It provides favorable circulation conditions for strong cold air activities. During C-RS disaster events, wavy circulation prevails in mid- and high-latitude Asia, which is conducive to moderate cold air activities. The water vapor supply and inversion layer formation associated with the three kinds of combined disaster events are controlled by different subtropical anomalous circulation systems. The southern branch trough over the Bay of Bengal and the anomalous anticyclone over the South China Sea are key subtropical circulation systems for the formation of C-RS and C-F disaster events, respectively, while the combination of the southern branch trough over the Bay of Bengal and anomalous anticyclone over Northwestern Pacific is the key circulation system for the formation of C-RS-F disaster events.