Role of Baroclinic Trough in Triggering Vertical Motion during Summertime Heavy Rainfall Events in Korea

Abstract

The nature of the vertical motion responsible for the summertime heavy rainfall events (HREs) in South Korea is quantitatively examined. By compositing 318 HREs from June to September in 1979-2018, it is found that the synoptic conditions of the HREs are typically characterized by a developing surface cyclone with a southwesterly low-level jet on its southeastern flank and an upper-level trough to the west of the HREs. This baroclinic environment allows for well-organized vertical motion over South Korea at the equatorward side of the upper-level jet entrance. The relative importance of dynamic and diabatic forcings in driving the vertical motion is further quantified by solving the quasigeostrophic omega equation. It turns out that the dynamic forcing, defined as Q-vector convergence, is comparable to the diabatic forcing in the developing stage of the HREs. The diabatic forcing, however, becomes more important in the mature stage as latent heating rapidly increases. The decomposition of the Q vector into the transverse (cross-isentropic) and shearwise (along-isentropic) components reveals that the dynamic uplift is largely caused by the shearwise Q-vector convergence, which is closely related to the developing trough in the upper to middle troposphere on the west of the HREs. This result indicates that the HREs in South Korea are organized by the baroclinic trough coupled to moist processes, with a minor contribution of the thermally direct secondary circulation at the entrance region of the upper-level jet.