Dual-Doppler radar analysis of the enhancement of a precipitation system on the northern side of Mt. Halla, Jeju Island, Korea on 6 July 2007

An elongated intense precipitation system associated with a stationary front delivered enhanced rainfall to the northern lateral and lee sides of an isolated mountain on Jeju Island on 6 July 2007. The convective region of the system passed around the mountain (Mt. Halla; height 1950 m, width 35 km, length 78 km) under moist synoptic conditions (relative humidity ~ 95% below 3 km) during the rainy season. In this study, the detailed 3-dimensional structure and enhancement mechanisms of this precipitation system are examined using dual-Doppler radar observations. Analysis of the Doppler radar indicates that the precipitation system was organized in an elongated shape that extended southwest–northeast about 20 km off the northwestern shore of Jeju Island. The development of this precipitation system with respect to Mt. Halla can be divided into three stages: approaching stage, lateral-side stage, and lee-side stage. During the approaching stage, variational wind analysis indicates that the updraft region was aligned with the convergence zone of relatively strong westerlies and weak southwesterlies on the western side of the system. The southwesterlies accelerated gradually, generating local updrafts between the system and the northwestern slope of the mountain that were crucial to the enhancement of the southern part of the system. The enhanced convective region intensified further as it reached the northwestern shore of the island. The elliptical shape and isolated terrain of Jeju Island played an important role in this intensification by modifying the low-level winds, with a relatively low Froude number of 0.2. This modification of the low-level wind by the terrain provided a plentiful supply of moist air to the convective region of the system, maintaining the enhanced convection. During the lee-side stage, the system was characterized by relatively weak westerlies at low altitude as it approached the eastern slope of the mountain. The convergence of these weak westerlies and relatively strong southwesterlies over the southeastern slope of the mountain induced a stationary updraft region and re-enhancement of convection on the lee-side of the mountain. The regional enhancement of this precipitation system on the lateral and lee sides of Mt. Halla resulted from localized terrain-induced low-level convergence under moist environmental conditions with predominant west-southwesterly winds. These environmental conditions are well organized when a stationary front is located off the northern shore of Jeju Island.