Free convective layer

The free convective layer (FCL) is the layer of conditional or potential instability in the troposphere. It is a layer of positive buoyancy (PBE) and is the layer where deep, moist convection (DMC) can occur. On an atmospheric sounding, it is the layer between the level of free convection (LFC) and the equilibrium level (EL). The FCL is important to a variety of convective processes and to severe thunderstorm forecasting.

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It is the layer of layer of instability, the "positive area" on thermodynamic diagrams where an ascending air parcel is warmer than its environment. Integrating buoyant energy from the LFC to the EL gives the convective available potential energy (CAPE), an estimate of the maximum energy available to convection. The depth of the FCL is expressed by the formula:

FCL = Z_EL - Z_LFC

or

FCL = P_EL - P_LFC

Deep, moist convection is essentially a thunderstorm, it is cumulus congestus clouds or cumulonimbus clouds. An air parcel ascending from the near surface layer (mixed layer or boundary layer) must work through the stable layer of convective inhibition (CIN) when present. This work comes from increasing instability in the low levels by raising the temperature or dew point, or by mechanical lift. Without the aid of mechanical forcing, a parcel must reach its convective temperature (T_c) before moist convection (cloud) begins near the convective condensation level (CCL}, whereas with dynamic lift, cloud base begins near the lifted condensation level (LCL). This will remain as shallow, moist convection (small cumulus clouds) until breaking through the convective inhibition layer, after which DMC ensues as a parcel hits the LFC and enters the FCL, if thermal or mechanical forcing continues. At the level of neutral buoyancy (the EL), a parcel is cooler than the environment and is stable, so it slow down, eventually ceasing at the maximum parcel level (MPL).

References

• Blanchard, David O. (Sep 1998). Assessing the Vertical Distribution of Convective Available Potential Energy. Weather and Forecasting, 13 (3): 870–877.

See also

• Atmospheric thermodynamics
• Mixed layer