Antecedent Midtropospheric Frontogenesis ahead of Tropical Cyclone

Abstract

In this study, we demonstrate that the antecedent midtropospheric frontogenesis (AMF) occurred across the middle part of the Korean Peninsula and contributed to the antecedent indirect precipitation (AIP) ahead of TC. We carried out 3 research topics, i.e., case study, comparison study, and statistical analysis. First, we examined AMF resulting from the interaction between Typhoon Rusa (2002) and a midlatitude trough over the Korean Peninsula. In this event, the AMF contributed to the first peak in the time series of rainfall in Gangneung (37.75°N, 128.90°E), occurring about 12 hours before the landfall of the tropical cyclone (TC). A Weather Research Forecasting (WRF) model experiment revealed that the AMF was mainly forced by the horizontal deformation frontogenetical forcing (HDF). The HDF increased positively due to the confluence of the southeasterlies from the TC and the northwesterlies emanating from the midlatitude trough. Second, we also investigated favorable synoptic environments for the AIP through comparison of two TCs, Rammasun (2002) and Maemi (2003). Although both had a remarkably similar accumulated rainfall pattern over the peninsula, the temporal evolutions of hourly rainfall were different. Only Maemi had the AIP in conjunction with a midlatitude trough to its north. The confluent flows at middle-to-upper levels were strengthened due to the increased pressure gradient between the midlatitude trough and the subtropical high, and the warm advection by the confluent flows also became stronger near the confluent zone. The highly baroclinic synoptic backgrounds in the Maemi case lead to the AMF. Finally, we found that many cases showed AIP associated with TCs that were approaching the Korean Peninsula. In order to generalize the characteristics of the AIP occurrence, we collected all the TC cases that influenced the Korean Peninsula and classified them into two groups depending on the presence of the AIP over the Korean Peninsula. It is found that the AIP cases occur more frequently (about 58% of the total 41 TCs that influenced Korea from 1993 to 2004) than the non-AIP cases and the averaged rain rate of the AIP is about 115 mm 6h-1. Composite analyses demonstrated that the AIP events tend to be coherent with AMF induced by TC-midlatitude environment interaction. Analysis of the (quasi-geostrophic) QG omega equation indicates that QG forcing, which is related to the AMF, significantly contributes to the ascending motion of the AIP. The AMF was associated with a thermally direct circulation that contributed to strong ascent and the AIP over the peninsula ahead of TC. Moreover, the AIP could intensify due to the abundant low-level moisture supply to the frontal zone by the southerly wind on the east side of the TC.