New Indices for the Classification of Cold Surges Over South Korea

Abstract

Cold surges over South Korea are among the most extreme winter weather events, typically triggered by wave-train propagation or subarctic blocking in the upper troposphere. A widely used classification framework categorizes cold surges based on the upper-tropospheric circulation pattern on the day of onset. To overcome the limitation of this framework, cold surge indices are newly developed by considering the spatiotemporal evolution of circulation features characteristic of each cold surge type. During the pre-onset period (Day − 3 to 0), the daily geopotential height anomalies at 300 hPa are projected onto wave-train and blocking features in the dynamically defined domains, and new wave-train index (WI_n) and blocking index (BI_n) are developed based on this projection. Compared to the original scheme, the newly developed indices more accurately represent the dynamical evolution of upper-tropospheric circulation patterns, significantly reducing the number of Mixed-type cold surges (MT) from 115 to 77 and Unclassified cold surges from 64 to 18. Furthermore, thermodynamic analysis reveals that blocking-type cold surges (BT) exhibit longer duration, colder temperature anomalies, and greater intensity than wave-train-type cold surges (WT), consistent with their quasi-stationary blocking structure. By incorporating the evolution of circulation features before onset, the new indices offer clearer dynamical distinctions between cold surge types and provide a robust, physically interpretable foundation for their improved diagnosis and prediction.