Comparison of two orographically-enhanced precipitation systems occurred around Mt. Halla, Jeju Island, Korea during rainy season

1. Introduction

In East Asia, most intense precipitation systems occur during the rainy season (June to mid–July), when low–level warm and humid air passing around the Pacific high–pressure flows into the Changma/Baiu/Meiyu front region. Under the moist environment, orographically–enhanced localized rainfalls frequently occurred in the vicinity of Mt. Halla (height 1950 m, width 35 km, length 78 km), in Jeju Island, southern part of Korea (Lee et al., 2010; Atmos. Res., 97, 343–358). Comprehensive understanding of such a regional severe rainfall has been required for developing the quantitative precipitation estimation and mitigating damages arising from flood and landslide around this mountainous area. On 30 June 2006 and 6 July 2007, intense precipitation systems passed in Doppler radar observation area showing the system enhancement at the northwestern slope of the island. In this article, the comparison analyses of two intense precipitation systems revealed the vital ingredients for the system enhancement around the narrow but steep topographic feature, such as Jeju Island.

2. Data and methods

Two operational S–band Doppler radars by Korean Meteorological Administration are located at Gosan (GSN, west) and Seongsan (SSN, east) across of Jeju Island at around 65 km distance. Each Doppler radar, covering a radius of 250 km around Jeju Island, records sets of volume scans of reflectivity and Doppler velocity every 10 min. The sampling resolution of the radar data is 500 m in the radial direction and 1.0° in the azimuthal direction. The Doppler radar data was interpolated in a Cartesian coordinate system with vertical and horizontal grid intervals of 0.25 and 1.0 km, respectively. To determine the horizontal wind field for 30 June 2006 precipitation system, simplified VVP (velocity volume processing) method (Tatehira and Suzuki, 1994; Tenki, 41, 761–764) directly calculated the horizontal component of wind within a volume based on the spatial wind–velocity distribution in the radial direction using GSN data. Since SSN radar began to operate in 2007, variational method (Gao et al., 1999; Mon. Wea. Rev., 127, 2128–2142) using dual–Doppler radar data was employed to calculate 3–dimensional wind field for 6 July 2007 precipitation system.

3. Results

In case of 30 June 2006 precipitation system (hereafter ‘06P'), the eastward–moving convective region (area with reflectivity exceed for 45 dBZ) within the system passed from the western to northeastern island under the predominant westerly wind. When the convective region approached to close to the island from the offshore, the substantially–enhanced convective region was observed at the northwestern slope of Jeju Island with the convergence of westerly and southwesterly winds. Subsequently, the enhanced convective region passed to the northern slope and it began to dissipate gradually showing the downdraft region at the northeastern slope of the island (see Lee et al. 2010 for the details). In case of 6 July 2007 precipitation system (hereafter ‘07P'), the system passed from the western to eastern side of the island with predominant westerly wind. As the convective region approached to the northwestern slope of the island, it enhanced showing the convergence of westerly and accelerated southwesterly winds. Subsequently, the convective region moved to the northern island showing the persistent intensity. When the convective region moved to the eastern slope, the reflectivity was re–enhanced in horizontal and vertical. Associated to the re–enhancement, the substantial updraft which was induced by strong southwesterly was found at the eastern slope. As the system moved out of the island, the intensity was gradually weakened. The enhancement mechanisms of two intense precipitation systems occurred around Jeju Island were investigated and compared. The common phenomenon of two precipitation systems is that the enhancement of the convective region within the system was occurred at the northwestern slope of Jeju Island under the predominant westerly winds. Corresponding to the enhancement, substantial low–level wind convergence which was induced by westerly and mountain–modified southwesterly winds was concentrated at the northwestern slope with the concentrated low–level moist air. The different point between two precipitation systems is that the re–enhancement at lee–side of the mountain was shown at 07P in contrast, 06P was gradually dissipated as passing around the northern slope of the island. Related to the re–enhancement at the lee–side, the east–southeastward moving convective region of 07P passed to the substantial updraft region at the eastern slope which was generated by the strong southwesterly winds over the southeastern slope. In case of 06P, the convective region moved east–northeastward with the relatively strong low–level southwesterly wind, and its associated downdraft was found at the downwind side at the northeastern slope. Subsequently, the enhanced convective region at the northwestern slope moved to the downdraft region, and it gradually dissipated.

4. Discussion and conclusion

The evolution of convective region (area with reflectivity > 45 dBZ) of intense precipitations occurred on 30 June 2006 (06P) and 6 July 2007 (07P) and the enhancement mechanism were investigated mainly using Doppler radar data. Under the low–level moist environment, the convective regions within both precipitation systems were orographically–enhanced at the northwestern slope of the island as the systems approached to the island from the western offshore. It is inferred that the low–level moist air and predominant westerly wind play an important role enhancing the convection. After the enhanced system passed to the northern slope of the island, the evolution of two systems showed the distinctive phenomena. In east–northeastward passage, the 06P moved to the northeastern downwind slope with relatively–strong low–level wind (Fr, 0.5) and subsequently it dissipated. In contrast, the southeastward–moving 07P retained the intensity at the northern slope with relatively weak low–level wind (Fr, 0.2). When the 07P moved to the lee–side, it was re–enhanced by substantial updraft. It seems that the different system passage and ambient wind distribution are strongly associated to the re–enhancement of the precipitation system occurred around narrow but steep mountainous area, such as Jeju Island.