资源环境科技发展态势分析平台

Persistent wet-freezing events (PWFEs), a typical type of compound extremes, combine adverse effects from both its precipitation and temperature components, thereby posing great threats to human society and ecosystem. Atmospheric circulation patterns that govern the occurrence and persistence of PWFEs over southern China are identified and classified, based on cases during 1980-2010. There are two dominating types of large-scale atmospheric circulation patterns responsible for PWFEs: a single blocking high type and a double blocking high type. The single blocking high type tends to occur under positive Arctic Oscillation (AO) phase, characteristics of a long-lived blocking high over the Lake Baikal in the middle troposphere favouring lower-level cold air masses penetrate southwards along a northeast-southwest route. Likewise, abundant water vapour advances northwards from the Bay of Bengal and the South China Sea, meeting cold air masses over southern China under the upper-level divergence caused by the intensified upper-level subtropical jet which prevails on top of the western Qinghai-Tibet Plateau. Locally, a stable temperature inversion structure is anchored in the lower troposphere. The double blocking high type features blocking highs over the Ural Mountains and the Sea of Okhotsk, along with an intensified trough between them. The upper-level subtropical jet over mid-latitude East Asia strengthens, providing divergence fields at upper troposphere. Low-level cold air masses move southwards from the Lake Baikal along a north route, converging with the water vapour transported from the Bay of Bengal to southern China by a low-level southwesterly jet. By contrast, the inversion layer is absent atop the region experiencing double blocking PWFEs. Regardless of event types, the duration of PWFEs is dedicated by Rossby wave energy dispersion through its role in maintaining blocking patterns during the PWFEs.