Hail
Analysis
DW-D
What is Hail?
- Hail is an ice particle whose diameter is larger than 5mm, and it is formed when the cold water droplet freezes around the snow crystals.
- Under the diameter of 2~4mm, it is called ice pellets.
- It usually occurs at the spring and the late fall, because the temperature difference between upper and lower layer is huge at this time of year.
What causes hail?
-The heated lower part of the atmosphere with the cold air mass at the upper middle layer will cause instable atmosphere condition when the water vapor is provided.
-Strong wind shear between the upper and lower layer causes active convection, which causes hail.
Hail Growth
1.During a storm, rain drops can be picked up by a warm updraft forcing the droplets of water higher in the clouds into freezing temperature where they freeze. The frozen droplet will soon begin to fall and as they do they will star to thaw.
2. As is falls, it is possible for it to be picked up again by the updraft and thrown back into freezing emperature where it will re-freeze. This can happen many times and as it does the droplet adds another later of ice making the size larger each time.
3. The droplet, now considered hail because of it's many layers of ice will become too heavy to be picked up anymore by the updraft and it will fall to the ground.
-Saturation vapor pressure : supercooling water droplet > ice crystal
-Due to the difference between saturation vapor pressure, the supercooling water droplet evaporates.
The evaporated water vapor sticks to the ice crystal, so that the ice crystal can grow up.
Hail Forecast Application
"Hail Judgement Guidance Condition"
1. If there occurs precipitation at the field where model forecasted, the hail judgement graph is only applied at the precipitation atmosphere.
2. Calculation of 500hPa temperature and 500hPa ~ 400hPa temperature difference
- Understanding the degree of the cold advection at the upper layer
- Precipitation possibility – If the difference between dew point temperature and the temperature is smaller than 6.
3. It is important to analyze the weather chart when the hail is created at the cold front.
- Rapid wind change engages with orographic effect
Type of Hail
Frontal Type
Frontal type hail occurs a lot during the spring, fall, and winter season at the west coast area. This type of hail has lower melting altitude, and relatively smaller size of hail.
At the lower layer above the west coast area of the Korean peninsula, the moist air flow is advected at the ground following the general wind and local wind. At this condition, when the cold front passes, hail occurs at the squall line of the frontal surface.
Tropopause Jointed Type
Tropopause jointed type is a type that creates hail due to increased instability due to the tropopause folding. Tropopause folding is a phenomenon in which the tropopause is bent, resulting in the presence of a troposphere layer at a much lower altitude (400 to 500 hPa).
When this happens, the cold at the top generally subsides, which strengthens the upper and lower temperature gradient and increases instability.
Cold Air Advection Type
The hail occurs at the instable atmospheric layer is usually happening during the summer at the inland area. It has higher melting altitude, and bigger size of hail. It is occurred by topographic convergence and local heating. This occurs at the condition where the K-Index is bigger than 30.
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