Analysis of causes of
localized heavy rains
in Korean and countermeasures
Prediction Method
Focused on heavy rains in the central district of the Korea
High frequency of heavy rain in areas adjacent to the coast where water vapor supply is favorable
Average Days of heavy rain of more than 80mm per hour in the central region during the summer(6~9) of 2010-2020
in areas bordering the west coast: 15.7 days
(metropolitan area, Daejeon Chungcheongnam-do, and Jeonbuk)
in inland areas: 3.5 days
(Gangwon Yeongseo, Chungbuk)
Lower jets & meso-scale low pressure system account for more than 70% in types of localized heavy rain over 80mm per hour
-> both cases at North Pacific high pressure edges
Heavy rains occurring on the edge of the North Pacific frequently occur in High-south & Low-North barometers
[Classify 3 levels according to the flow of the barometer]
<STEP1> <STEP2> <STEP3>
- STEP 1: The precipitation accompanying the pressure trough and the precipitation at the edge of the high pressure appear separately from the front of the pressure trough, making it difficult to produce strong precipitation. However, if the northern pressure trough moves south to the edge of the high pressure, low pressure of the ground may develop and meso-scale low pressure system type heavy rain may occur.
- STEP 2: As the northwest flow of the back of the barometric valley moves south, the lower jet of the high pressure edge is strengthened, resulting in the occurrence of lower jet-type heavy rain, the most frequent pattern
- STEP 3: The north-west or northeast flow of the front of the pressure capability moves southward, strengthening the lower jet, causing lower jet type heavy rain & The development of a trough between the high pressure in the south and the pressure in the north causes meso-scale low pressure system type heavy rain
-> Strong precipitation in all three stages occurs at the edge of the North Pacific high pressure
Why the edge of the North Pacific high pressure?
Strong heavy rain of more than 80mm per hour is caused by deep convection, and the amount of water vapor and convergence in the lower layer affect convection strength. The closer the convection is to the ground, the higher the amount of water vapor and the larger the area of the CAPE resulting in deep convection. Since high dew point temperature or low convergence elevation results in deeper convection, North Pacific high pressure edge meets these conditions.
Why North Pacific high pressure is important?
North Pacific high pressure is located in the ocean. Airflow flowing along the edge of the North Pacific high pressure is high-temperature and humid because it passes through the low-level warm ocean, and a large amount of water vapor flows into the lower atmosphere, increasing instability of the upper and lower floors thermally. Because Large amounts of water vapor supplied and it leads to very high dew point temperatures on the ground and under 925 hpa, leading to strong convection. So accurate positioning of high pressure edges is important
Lower jet type precipitation
- usually occurs at dew point temperatures above 20 degrees Celsius at 925 hpa,
- most precipitation ranges: 80~110 mm/h
Meso-scale Low System precipitation
- mainly occurs above the ground dew point temperature of 23 degrees Celsius
- 110 to 140 mm/h over 24 degrees Celsius (very strong heavy rain)
* The most dangerous type in terms of the strength of precipitation because convection starts from the ground can become very strong
How to find North Pacific high Pressure?
Interpreting changes in 500 hpa status elevation is mainly used. Because it reflects the state of high subtropical high pressure and shows the change in position of the rainy season front accordingly.
-STEP1: Analyze the position of the contour line 50 gpm lower at 500 hpa high-pressure central position elevation.
* Difference between the reference line of the positional elevation at the developed position of the strong convective precipitation zone and the value of the positional elevation at the center of high pressure is 50 gpm
<50gpm: Determine the location of the convective precipitation zone as the edge
>50 gpm: Precipitation associated with north pressure trough
<50gpm & precipitation zone sporadic: sonic precipitation (occur within high pressure area)
if two or more organized convective precipitation zones exist, most likely the southernmost precipitation zone developed on the edge of the North Pacific high pressure and since the precipitation zone located south of the edge occurs inside the high pressure, it is difficult to develop as organized into convective precipitation.
-STEP2: Compare if convective clouds or convective precipitation zones are located around the analyzed contours
- STEP3: If the above steps match, analyze the contour to the North Pacific High Pressure Edge
Lower Jet
STEP1.
Location of center of High Pressure
mostly southeast of the Korean Peninsula
- north latitude: 25-35'
- east longitude: 125-145°
STEP2.
Path of the high pressure edge 925 hPa streamline formed from East China Sea to the west sea - forming favorable conditions for receiving water vapor from the sea
* if water vapor from a typhoon is released, it is possible to pass through inland China
STEP3.
200 hPa jet & the 500 hPa trough pass north 40° and not directly affected by the upper trough
STEP4.
Determine whether the North Pacific high pressure edge matches the temperature Streak area equivalent to 500 hPa
STEP5.
If wind direction of 270-360° or 0-90° is expected at an altitude of 500-700 hPa within 150 km north of the North Pacific High Pressure Edge, dry air is expected to move south.
STEP6.
- wind speed of 925 hPa >= 25 ktS
- dew point temperature >= 20°C
*precipitation zone stagnates when the wind speed of the 700 hPa directional flow at the edge of the high pressure 25 kts and the wind direction is around 270°
RESULT
satisfy STEP 1~5:
forecast of more than 80 mm/h
satisfy STEP 1~6:
forecast of more than 200 mm/h
Key Factors of Low Jet Type Heavy Rain: Southward Flow of Dry Air
As dry air pushes down to the edge of the stagnant high pressure, the iso-wind line of the lower jet is compressed, strengthening the wind speed into a narrow area -> Convection caused by convergence of the lower jet is concentrated in a narrow area, leading to heavy rain
Dry air in the north must be located southward near the edge of the North Pacific high pressure - a point located 100-150 km north of the edge of the high pressure must be identified southward of the dry air at a height of 500-700 hpa.
Low jet type heavy rain: High pressure in the south and dry air in the north are at odds
Frequent congestion at the edge of high pressure -> Concentration of precipitation into a narrow area -> Strong precipitation strength, high total cumulative precipitation
Meso-scale
low pressure system
STEP1.
location of the center of high pressure
- north latitude: 25-35°
- east longitude: 125-145°
STEP2.
Path of the high pressure edge 925 hPa streamline formed from East China Sea to the west sea - forming favorable conditions for receiving water vapor from the sea
* if water vapor from a typhoon is released, it is possible to pass through inland China
STEP3.
700 hPa pressure trough must pass through the West Sea
STEP4.
dew point temperature on the ground should be at least 23°C.
STEP5.
A wind speed of 925 hPa or more and a wind direction of 180-230° should be expected at the west coast point.
RESULT
satisfy STEP1~5: forecast over 80 mm/h
Lower Jet vs Meso-scale Low Pressure System
- Common: High pressure forces in the North Pacific extend to the southern part of Korea and occur with the high pressure river in the central position
- Difference (Feature of Meso-scale low pressure system)
High pressure edges directly affect the mid-level trough, developing medium-scale low pressure on the ground, resulting in strong convective precipitation accompanied by low pressure.
Less frequent occurrence than the lower jet type, but more often than not, strong precipitation strength, and appears more strongly in the west coast area, which is less affected by friction.
Key factor of forecasting meso-scale low pressure system type heavy Rain
: Analysis of the development of meso-scale low pressure
Analysising pressure trough is important because medium-scale low pressure can develop in the process of passing through a mid-level short wave valley from the West Sea or the process of developing and passing a jointed low pressure.
However, since the pressure trough developed in these processes are a small short fracture or joint low pressure, it is difficult to analyze the pressure trough because it does not appear clearly on the 500hPa weather chart, so it is recommended to refer to the water vapor image together.
When the short wave valley directly passes through the West Sea 40° south of the north latitude, the lower jet develops due to the increase in hardness caused by the pressure trough. Since the mobile low pressure passes, convection strength at the time of passage determines precipitation strength rather than the time when the precipitation zone is stagnant. So strength determining factor is dew point temperature on the ground and the higher the dew point temperature, the higher the precipitation per hour.
Determinants of heavy rain area: Degree of Low Pressure Development
- If the low pressure is weak, the heavy rain area is precipitated by the warm air located east of the low pressure.
- If the low pressure is strong, the cold circulation is strengthened, and the cold front precipitation zone is pulled down. So cold front precipitation zone is formed south of the low pressure, and the colder the cold front precipitation zone is sagging south
