Hallasan Mountain is located at the center of Jeju Island, Korea. Even though the height of the mountain is just 1,950 m, the orographic effect is strong enough to cause heavy rainfall. In this study, a rainfall event, due to Typhoon Nakri in 2014, observed in Jeju Island was analyzed fully using the radar and rain gauge data. First, the
The rain rate in a mountain area is generally higher than that in a flatland, which is mostly caused by the ascending air current, called the orographic effect [
However, in general, it is not easy to capture the orographic effect. It is mainly because the number of rain gauges available in a mountain area is very limited. Similar situation is also found in Jeju Island. A total of 24 rain gauges are being operated in Jeju Island, of which just four are located in the mountain area with altitudes of 600 m or higher [
In that sense, the radar can be a good tool to provide valuable information about the rainfall in a mountain area. Fortunately, two radars are being operated in Jeju Island. These were introduced mainly to monitor the typhoons that approach the Korean Peninsula [
A radar measures the rain rate within the atmosphere in the form of radar reflectivity. The relationship between the radar rain rate
As the rainfall at Hallasan Mountain is strongly affected by the orographic effect, the relationship between the radar rain rate and the radar reflectivity may not remain unchanged over the entire altitudinal range; that is, the
This manuscript is composed of six sections as follows. In addition to Introduction and Conclusion, an explanation of Hallasan Mountain and Typhoon Nakri is covered in Section
Jeju Island is the biggest island in Korea and is located in the southernmost region of the Korean Peninsula. Jeju Island is composed of a main island, eight inhabited islands, and 55 uninhabited islands. The shape of the island is elliptical, comprising a major axis length of 73 km along the east-west direction and a minor axis length of 31 km along the north-south direction. The total area of Jeju Island is 1,848 km2, and the coastal area with an altitude less than 200 m above the sea level covers 55.3% of the island’s total area. Figure
Location of the Jeju Island, Korea (the contour lines over the Jeju Island represent the 250 m altitude interval).
Hallasan Mountain is located at the center of Jeju Island. The height of the mountain is 1,950 m. Hallasan Mountain has a gentle slope of approximately 3° along the east-west direction and a steeper slope of approximately 5° along the north-south direction (Jeju Special Self-Governing Province,
The rainfall event considered in this study is Typhoon Nakri, the 12th typhoon of 2014. Typhoon Nakri was generated in the Pacific Ocean near the Philippines on July 30, and it arrived at Jeju Island on August 1 [
Synthetic radar image around the Korean Peninsula during the Typhoon Nakri in 2014.
For the purpose of typhoon tracking, the KMA operates two radars on Jeju Island, the Gosan and the Seongsan radars. The Gosan radar, originally a C-band radar, was introduced in 1991 but was replaced by an S-band radar in 2006. The Seongsan radar, also an S-band radar, was introduced in 2006 to supplement the Gosan radar with the particular purpose of the removal of the blind spot caused by Hallasan Mountain. Both radars have been designed with an observation radius of 240 km × 240 km and a resolution of 1 km × 1 km. The major specifications of the Gosan and Seongsan radars are summarized in Table
Major specification of the Gosan and Seongsan radars.
Radar type | Gosan radar | Seongsan radar |
---|---|---|
S-band | S-band | |
Transmitter | ||
Transmitting tube | Klystron | Klystron |
Frequency | 2,825 MHz | 2,755 MHz |
Peak power | 750 kW | 750 kW |
|
||
Receiver | ||
Pulse width | ||
Short | 1.0 |
1.0 |
Long | 4.5 |
4.5 |
PRF | ||
Short pulse | 250~1,200 Hz | 250~1,200 Hz |
Long pulse | 250~350 Hz | 250~350 Hz |
Occupied bandwidth | 8 MHz | 8 MHz |
|
||
Antenna | ||
Dynamic range | 95 dB | 95 dB |
Intermediate frequency | 10 MHz | 10 MHz |
Antenna diameter | 8.5 m | 8.5 m |
Beam width | 1.0° | 1.0° |
Antenna gain | 45 dB | 45 dB |
The locations of the Gosan and Seongsan radars are shown in Figure
Location of the Gosan and Seongsan radars and their radar beam blockages for the 1.5 km CAPPI data (during the Typhoon Nakri, 07:00 Aug. 2, 2014).
In 1990, the KMA started to introduce the AWSs onto Jeju Island [
Rain gauges over the Jeju Island (184 (ID number), Jeju (place name), 185, Gosan, 188, Seongsan, 189, Seogwipo, 328, Jungmun, 329, Ara, 330, Hawon, 725, Udo, 727, Yusuam, 751, Seonheul, 752, Seogwang, 753, Eorimok, 779, Hallim, 780, Namwon, 781, Gujwa, 782, Seongpanak, 792, Pyoseon-myeon, 793, Moseulpo, 855, Gapado, 870, Jindallebat, 871, Witse Oreum, 885, Typhoon Center).
This study used the composite radar reflectivity data arising from both the Gosan and Seongsan radars. A total of eight composite radar reflectivity fields were prepared from an altitude of 250 m to 2,000 m at intervals of 250 m. From each radar, 0.25 km CAPPI, 0.50 km CAPPI, 0.75 km CAPPI, 1.00 km CAPPI, 1.50 km CAPPI, 1.75 km CAPPI, and 2.00 km CAPPI data were prepared and utilized to create the composite field at each altitude. When the data from both radar systems were available, their arithmetic mean was calculated to compute the representative reflectivity. In case that the radar reflectivity from only one radar is available, this reflectivity was assumed to be the representative value in this study.
Every 10 min, eight composite radar reflectivity fields were prepared and used to generate the composite field for the entirety of Jeju Island, and Figure
Construction of radar reflectivity field over the Jeju Island using the radar CAPPI data (radar data observed at 2014/08/02/05:00).
The
In this study, the
Scatter plots of rain gauge rain rate and radar reflectivity at AWS locations within the 0–250 m altitude zone.
As can be seen in Figure
As can be seen in Figure
Just four rain gauges were available at the altitude zone of 250 m to 500 m. The linear relation between the radar reflectivity and the rain gauge rain rate was strong at two rain gauge stations (#727 and #751); however, at two other rain gauge stations (#329 and #330), no obvious trend could be detected. In this altitude zone, only two rain gauges, #727 and #751, were therefore considered for the determination of the
Only two rain gauges (#753 and #782) were available at the altitude zone of 750 m to 1,000 m. A linear trend was obvious at these two rain gauge stations, both of which could be used for the determination of the
At the altitude zone of 1,250 m to 1,500 m, only one rain gauge (#870) was available, for which a linear trend could be found between the radar reflectivity and the rain gauge rain rate. The resulting
At the altitude zone of 1,500 m to 1,750 m, only one rain gauge (#871) was available, for which a linear trend could be found between the radar reflectivity and the rain gauge rain rate. The resulting
As rain gauges were not available at some altitude zones, it was impossible to determine the
As can be seen in Figure
It was also found that the parameter
As the change of the parameter
The above results are also well supported by the comparison result of the drop size distribution (DSD) [
Additionally, authors tried to derive and compare the
Comparison of the
250 m (North)
250 m (South)
1,000 m (North)
1,000 m (South)
Before all, it should be mentioned that the newly derived
The above results are very interesting and well correspondent to the other studies [
In Korea, the 1.5 km CAPPI data is generally used to generate the rain rate data. The altitude of 1.5 km was determined to minimize the beam blockage by considering the radar network and the topography over the Korean Peninsula. The effects of the bright band and ground echo could also be avoided by adopting this altitude as the standard for the use of the radar data [
The
Comparison of (a) areal-average rain rates estimated by rain gauge,
Areal-average rain rates
Accumulated
As can be found in Figure
Finally, Table
Comparison of maximum areal-average rainfall depth recorded by rain gauge and radar for several rainfall durations.
10 minutes | 1 hour | Total depth | |||
---|---|---|---|---|---|
Depth |
Time of occurrence | Depth |
Time of occurrence | ||
Rain gauge | 5.1 | 2014/08/02/06:30 | 170.8 | 2014/08/02/07:00 | 1896.4 |
|
3.7 | 2014/08/02/06:40 | 125.8 | 2014/08/02/07:00 | 2582.5 |
1.5 km CAPPI |
7.9 | 2014/08/02/06:30 | 251.9 | 2014/08/02/07:00 | 2633.9 |
Marshall-Palmer equation | 0.7 | 2014/08/02/06:30 | 23.3 | 2014/08/02/07:00 | 250.5 |
This study analyzed the radar data for the entirety of Jeju Island which were measured when Typhoon Nakri passed the island in 2014. First, the
First, the
Second, the areal-average rain rate estimated using the
In this study, it was confirmed that the orographic effect at a mountain can be effectively observed by a radar. At Hallasan Mountain, the orographic effect seems to be highest around the altitude of 1,000 m. While generating the radar rain rate field, it was also confirmed that the proper
Chulsang Yoo and Jung Mo Ku declare that there are no conflicts of interest regarding the publication of this paper.
This research was supported by a grant from Water Management Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government (17AWMP-B079625-04).