20.9. Stability Indices#
To quickly assess the stability of the atmosphere, many different stability indices have been developed to give a quick, one value snapshot of the vertical stability. Some of the most common measures are: Lifted Index (LI) and Convectively Available Potential Energy (CAPE). We will review the basics of these two parameters, how they are calculated, and what they imply about the stability of the atmosphere.
Lifted Index#
The Lifted Index (LI) is calculated from the difference between the environmental temperature at 500 hPa and the surface parcel temperature at 500 hPa. Mathematically LI can be described as,
\(LI = T_{{ENV}_{500}} - T_{{PARCEL}_{500}}\)
Typical values for LI and likely weather associated with those values:
LI Value |
|
|---|---|
> +2 |
No convective activity |
0 – +2 |
Showers probable, isolated T-storms possible |
-2 – 0 |
Thunderstorms probable |
-4 – -2 |
Severe T-storms possible |
< -4 |
Severe T-storms probable, tornadoes possible |
The associated weather is if the atmosphere is otherwise conditioned for precipitation and convective weather. One major drawback of using LI as a forecast tool is that it is only a look at the stability of one level of the atmosphere, which may or may not play an important role in a particular forecast.
CAPE#
The Convectively Available Potential Energy (CAPE) is defined as the ‘positive’ area on a skew-t chart between the environmental temperature profile and the parcel path, and furthermore between the LFC and the Equilibrium Level. Mathematically CAPE can be described by,
Where EL is the equilibrium level, \(R_d\) is the gas constant, LFC is the level of free convection, \(T(p)\) is the parcel temperature at pressure level \(p\) in Kelvin, and \(T_a(p)\) is the environmental temperature at pressure \(p\) in Kelvin.
CAPE Value |
|
|---|---|
< 1000 J kg^{-1} |
Weak instability |
1000 – 2500 |
Moderate instability |
2500 – 5000 |
Strong instability |
> 5000 |
Extreme instability |
Once major advantage of CAPE is that it is an integrated value through the atmosphere considering the changing parcel profile and environmental profile.