Statistical Analysis of Regional Rainfall Distributions around Jeju Island by Low-level Winds during the Winter Season

In Korea, precipitation systems passing over an isolated mountain tend to develop by orographic effects. Statistical studies of isolated terrain effects on precipitation distributions were relatively fewer. In this study, we statistically analyzed regional rainfall distributions around Jeju Island which is elliptically shaped and oriented from the west-southeast to east-northeast direction with a height of 2 km, width of 35 km, and length of 78 km according to wind characteristics at low levels (925–850 hPa) during the winter season in 2006 – 2016.

 Long-term rain gauge data at 21 sites, two operational S-band Doppler radar data, and mesoscale analysis model data were used. Precipitation times were extracted from rain gauge data in the winter season of December, January, and February. For radar data analyses, 3-Dimensional CAPPI radar data corresponding to the precipitation times were used. To determine a representative environmental wind on Jeju Island, the MSM-GPV (Mesoscale Model - Grid Point Value) data between 850 hPa and 925 hPa levels were used. By wind direction and wind speed, CAPPI radar data were classified into 32 wind categories. The CAPPI radar reflectivities were averaged in each category.

 We investigated characteristics of regional rainfall distributions that were classified in the 32 wind categories by wind direction and speed. First, the most of high rainfall frequencies occurred when the north-westerlies (51.6 %) and northerly winds (16.0 %) prevailed at low levels. Rainfall frequencies at westerlies (8.6 %) and south-westerlies (7.5 %) were relatively low. At relatively strong north-westerlies (SPD > 10 m s^-1), precipitation system that moved from the offshore region developed on inland. When the relatively weak south-westerlies (5 m s^-1 <  10 m s^-1) and strong south-westerlies (SPD 20 m s^-1) were dominant, there were high rainfall distribution on the top of the mountain and the southern upslope, respectively. Similarly in the environment of the relatively weak southerly wind (5 m s^-1 < SPD  10 m s^-1) and strong southerly wind (SPD  20 m s^-1), it was found that high rainfall frequencies were concentrated on the top of the mountain and southern upslope, respectively.

 Therefore, developing precipitation systems passing over the mountain around the Jeju Island are closely related to characteristics of wind direction and wind speed at low levels.

Acknowledgements

 This work was funded by the Korea Meteorological Industry Promotion Agency under Grant KMIPA 2015-5060 and the BK21 plus Project of the Graduate School of Earth Environmental Hazard System.