Diverse Synoptic Patterns of Warm-Season Heavy Rainfall Events in South Korea

Synoptic patterns associated with warm-season (June-September) heavy rainfall events (HREs) during 1979-2018 in South Korea are classified based on mean sea-level pressure field of ERA-interim reanalysis using the self-organizing map algorithm. The HREs are defined as rainfall events with a 12-hour accumulated precipitation of 110 mm or more without those directly related to any typhoon activities. HREs have two frequency peaks in the first and second Changma periods (late June to July and late August to September respectively) and are generally characterized by an eastward-moving depression, expanded North Pacific High and low-level jet (LLJ) coupled with the upper-level jet (ULJ). However, the synoptic patterns where rainfall systems are embedded are different among events, and it is found that the six clusters can effectively summarize them. They are 1) Continental lows (19.2 %), 2) Synoptic-scale cyclones (15.4 %), 3) Confluent system (20.4 %), 4) Local instabilities on the edge of the North Pacific high (21.3 %), 5) Mesoscale cyclones surrounded by anticyclones (13.5 %) and 6) Subtropical rainfall systems in September (10.0 %). Their synoptic features are investigated through a composite analysis. Clusters 1 and 3 are dominated by the strong southwesterly LLJ with a high frequency during the first Changma period. Clusters 5 and 6 are characterized by the strong ULJ and mainly occur during the second Changma period. Clusters 2 and 4, which are characterized by baroclinic and local instability respectively, are frequent during both the first and second Changma periods. The results suggest that various environmental conditions should be considered for diagnosing and forecasting the warm-season HREs in South Korea especially with consideration of the phase of Changma.