Analysis of causes of
localized heavy rains
in Korean and countermeasures
1. Low Level Jet
Heavy rains occur under the influence of the low level jet stream on the stationary front such as the rainy season front, accounting for 56% of the total heavy rains. The low level jet stream occurs in the lower atmosphere (altitude around 1,500m above the ground) along the edge of North Pacific high due to the strong pressure gardient force between low pressure of East Sea and Nortg Pacific high (high pressure difference). During the low level jet flows, warm and wet southwest wind air flows into the Korea through the low level jet stream, and meets cold air near the rainy season front and converges (the air stream gathers to one place). If high level jet stream raise this air higher, the heavy rain occurs. In some cases, it is caused by a mountain effect (rise of the low leve jet).
2. Direct Effect of the Typhoon
Heavy rain due to direct effects of typhoons are heavy rains that occur mainly in the southern region and on the east coast (23% of the total torrential rains). It is caused by the direct impact of the typhoon, but it is less related to the intensity of the typhoon. Heavy rain is inversely proportional to the typhoon's moving speed, so the slower the typhoon moves, the longer the precipitation lasting time is. In general, typhoons that land in Korea and exit the East Sea show the most precipitation in the Yeongdong area, and the frequency of heavy rain is the highest near Jirisan Mountain.
3. Convergence of the typhoon
It is a case of heavy rain in the convergence zone (where air currents converge) located in the north of the typhoon heading north, accounting for 17% of the total torrential rain. There is no clear low pressure on the weather map, but the vertical structure of the lower convergence and upper divergence of the airflow is distinct, and heavy rain occurs near the boundary where hot air current from the right side of the typhoon meets the cold air current. In the water vapor channel of satellite image, you can see a clear range of dark area in the west and northwest of the heavy rain zone.
4. Tropical Cyclone
It is a factor that accounts for 3% of the total cases of heavy rain, and is caused by a tropical cyclone in which a low leve jet stream does not exist. The tropical cyclone moving north meets the low temperature air mass and occurs rain, and heavy rains occur on the north and right sides of the tropical cyclone. The center of the tropical cyclone is not clearly visible on the ground weather map, but satellite and radar images show the shape of a vortex system of about 100km.
5. East sea coast heavy rain
It is a form of heavy rain on the east coast caused by topographical influence around the Yeongdong region. This is a case in which a large amount of hot and humid water vapor located on the East Sea meets and converges with the cold air of the upper layer descending from the northern Manchuria region, and causes heavy rain in the Yeongdong region by the Föhn phenomenon. The causes can be summarized as strong convergence of hot and humid air, forced rise due to topographical effects, and the occurrence of a combination of relatively high water temperature in the East Sea and atmospheric instability due to cold air in the upper atmosphere.
