13.3: Atmospheric Instability and Thunderstorms New Page

Because the potential for thunderstorms to develop depends on atmospheric stability and layering, atmospheric soundings (e.g., Skew-T log-P) are used by meteorologists to help forecast storms. The soundings receive their data from the rawinsonde balloon launches, aircraft observations, dropsondes, satellites, or other meteorological data sources.

In atmospheric soundings, there are several labels indicating atmospheric stability. The labels are lifted condensation level (LCL), level of free convection (LFC), equilibrium level (EL), convective available potential energy (CAPE), and convective inhibition (CIN).

Lifted Condensation Level (LCL) was discussed in previous chapters. It is the altitude where the temperature cools to the dew point temperature, resulting in saturation and condensation. The LCL is the location where the cloud base of a thunderstorm develops.

Level of Free Convection (LFC) is the height at which the environmental temperature rate decreases faster than the air parcel’s moist adiabatic lapse rate. This leads to atmospheric instability. In an atmospheric sounding, the LFC can be found where the moist adiabatic lapse rate of the rising parcel returns to the buoyant or warmer side of the environmental temperature.

Equilibrium Level (EL) is the height in the atmosphere where the temperature of the rising air parcel is the same as the temperature of its surroundings. The EL caps the atmospheric instability. This is where the moist adiabatic lapse rate returns to the negatively buoyant colder side of the environmental temperature. This is also where the anvil top of the thunderstorm is typically located.

Convective Inhibition (CIN) is the amount of energy that prevents a rising air parcel from reaching the level of free convection. On a thermodynamic diagram (Skew-T Log-P), it is the negative area between the environmental lapse rate and the air parcel lapse rate. CIN must be overcome for CAPE to be realized.

Convective Available Potential Energy (CAPE) is the amount of energy an air parcel would have if lifted vertically through the atmosphere over a particular distance. It is an indicator of atmospheric instability. On a thermodynamic diagram (Skew-T Log-P), CAPE can be found by the positive area between the environmental lapse rate and the air parcel lapse rate. It is an integrated measure of the total amount of buoyancy available to a rising air parcel.

CAPE can be used to estimate the maximum updraft velocity in thunderstorms.

\[\text { Maximum Updraft Velocity }=\sqrt{2 \cdot CAPE} \nonumber \]

While the above equation gives a good estimate, the updraft velocity equation typically gives an unrealistically high value as it ignores many important processes. For example, dry air entrainment, liquid-water loading, and frictional drag are ignored. Observational studies find that the typical updraft velocity is roughly half of the maximum updraft velocity value.

\[\text { Likely Updraft Velocity }=\frac{\text { Maximum Updraft Velocity }}{2} \nonumber \]

Thunderstorms are important features of Earth’s atmospheric system. It is safe to say that there is always a thunderstorm occurring somewhere on Earth at all times. In many places, thunderstorms provide needed rain and are an important piece of the hydrological cycle. However, thunderstorms can also be hazardous and impacts will be discussed in the following chapter.

Chapter 14: Questions to Consider

1. Label the typical lifecycle of a single cell thunderstorm:

Query \(\PageIndex{1}\)

2. Describe the importance of updraft separation from the downdraft and precipitation portion of a thunderstorm.

3. Query \(\PageIndex{2}\)

4. Query \(\PageIndex{3}\)

5. With a CAPE of 1280 J/kg, calculate the likely updraft velocity.
6. Describe the importance of wind shear for thunderstorms.

Selected Practice Question Answers:

Query \(\PageIndex{4}\)