Enhancement mechanism of the 30 June 2006 precipitation system observed over the northwestern slope of Mt. Halla, Jeju Island, Korea

摘要

Doppler radar analysis and a cloud-resolving storm simulator (CReSS) are used to investigate the enhancement mechanism of a localized intense precipitation system that occurred over Jeju Island, Korea, on 30 June 2006. Rain gauge data revealed intensive rainfall (>80 mm in 100 min) over northwest of Jeju Island, and relatively low rainfall (≤20 mm) over the northeast of the island. The environment of the precipitation system consisted of near-saturated air (relative humidity (RH): ～95%) from the surface to 700 hPa and a low lifting condensation level of 951 hPa. Doppler radar analysis revealed enhancement of the precipitation system on the northwestern lateral side (in this study, the term 'lateral side' refers to the sides of the mountain other than the lee-side and windward side) of Mt Halla, the central mountain on the island (1950 m). In the region of enhancement, low-level convergence (7×10~(-4)s~(-1)) induced by orographic blocking with a relatively low Froude number (0.55) was observed. These observations were well simulated by a control run (CNTL) using CReSS. The simulated precipitation system was enhanced over the northwestern part of the island due to wind passing around Jeju Island, low-level convergence (1.6×10~(-3) s~(-1)) due to orographic blocking, and high RH (～95%). In order to clarify the influence of topography and the low-level moist environment on enhancement of the precipitation system, numerical sensitivity experiments were conducted. The NOTR (no terrain) experiment produced straightforward wind without low-level convergence over the topography. Associated with these features, the CNTL rainfall amount on the northwestern lateral side of the island was 30.6% more than that produced in NOTR. A comparison of RH-controlled experiments and CNTL revealed that increasing moisture in the low-level environment resulted in greater intensity of the precipitation system. A reduction in low-level RH by as little as 2% could result in a 20.8% reduction in rainfall amount Therefore, land areas with small, narrow, but steep topographic features, such as Jeju Island, could determine the intensity and location of the precipitation system under a moist environment during the rainy seasoa