Analysis of causes of
localized heavy rains
in Korean and countermeasures
Heavy Rainfall prediction using convective instability index
What is the convective instability index?
Convective Available Potential Energy(CAPE)
Bulk Richardson Number(BRN)
The main elements of the sever storm that cause heavy rain
1. A large amount of water vapor present or diverted in a certain place
2. Strong vertical upward flow
3. Static instability
As a tool for predicting malicious storms, an instability index indicating the degree of instability in the atmosphere has been developed and used.
ex ) Showalter Stability Index(SSI). Kindex(KI), Lifted index(LI)
As Korea's air instability in summer increases, strong convective heavy rains are increasing.
Therefore, since SSI, KI, and LI, which have been mainly used so far, do not reflect the changed atmospheric instability well, it is necessary to introduce an instability index that can well reflect the gradually changing atmospheric instability. Convective Available Potential Energy (CAPE) and Bulk Richardson Number (BRN) are introduced as convective instability indices that reflect instability well to analyze the effectiveness of the index during intensive heavy rains.
Good conditions for storm occurrence include strong convective instability and strong vertical wind shear in the lower atmosphere.
Convection instability provides information on the possibility of a strong storm occurring, and the lower vertical wind shear is mainly used to determine the shape of the storm and the duration of the storm.
CAPE is converted into energy by calculating the area formed by temperature lines and wet insulation lines from Level of Free Convection (LFC) to Equilibrium Level (EL) where thunderstorm activity stops due to air rising on the SkewT-logP line.
In other words, CAPE is defined as the amount of energy required for storm growth.
It suggests the possibility that the air mass is warmer than the surrounding environment, creating the amount of buoyancy needed for the rise.
In malignant diaries, CAPE values appear in a wide range, but there is less possibility of strong convection at values below 1000 m2s-2, and when malignant storms develop, they are more than 2000 m2s-2. In this way, CAPE is a method that suggests the possibility of strong upward movement that can cause thunderstorms and torrential rains.
The BRN calculation formula is a form of Richardson Number calculated in relation to CAPE and vertical wind terms in the atmosphere, and is used as follows
Here, Uz is a vertical wind shear and is an average value in the layer of the lower troposphere from approximately 3km to 6km.
BRN is useful in distinguishing between types of storms when the surrounding environment is unstable and thunderstorms are likely to occur. It is known that instrument images rarely appear at values below 10, supercell storms occur at values 15 to 35, and multicell storms occur at values above 50.
