Understanding the physical processes in the evolution of a cold air outbreak over China in late November 2022 from a Lagrangian perspective

From 26 November to 1 December 2022, intense cold air masses swept across China from northwest to south, resulting in a nationwide cold air outbreak (CAO) case characterised by drastic and sudden temperature drops with rain, snow and strong winds. The physical processes that dominate the intensification of the cold air masses during this CAO event remain unclear. In this study, the evolution of the CAO case, which is indicated by the dry static energy (DSE), is investigated using a novel approach in the framework of Lagrangian backtracking. The dominant processes can be identified by decomposing the DSE change into four diabatic heating terms due to shortwave radiation, longwave radiation, latent heat and turbulent processes. Overall, in this case, most of the cold air parcels originated from the east of Novaya Zemlya and crossed Central Siberia before reaching China. Thus, these air parcels mainly manifested on the northwest-southeast path. The duration of the cold air intensification differed between subregions. The cold air parcels experienced long cooling periods (approximately 9 d) before reaching northern China (i.e. Northwest, North and Northeast China), whilst the southern parts (i.e. Central, East and South China) underwent relatively short cooling periods (6-8 d). Accordingly, the cold air affecting northern China is more intense than that affecting the southern parts, especially for East and South China. For all six subregions, longwave radiative cooling is identified as the dominant contributor to the cold air intensification, and the latent heat processes as the secondary contributor. The weakening of cold air parcels as they approach and pass over these regions is driven by turbulent processes and shortwave heating. Central Siberia and Lake Baikal are identified as key areas for the intensification of cold air passing over both regions. In addition, air parcels affecting Northwest China are intensely cooled as they pass over the Junggar Basin, while the North China Plain is a key area for cooling air parcels reaching Central, East and South China. From a Lagrangian perspective, these findings provide insights into the physical processes driving the behaviour of cold air parcels, which would help understand the mechanisms involved in the past changes and future projections in CAOs.