View source for Free convective layer

[[File:Meteorology_thermodynamic_en.svg|thumb|Diagram showing an air parcel path when raised along B-C-E compared to the surrounding air mass Temperature (T) and humidity (Tw)]]
In [[atmospheric sciences]], the '''free convective layer''' ('''FCL''') is the layer of conditional or [[potential instability]] in the [[troposphere]]. It is a layer in which rising air can experience positive [[buoyancy]] (PBE) so that 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.

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

: FCL = Z<sub>EL</sub> - Z<sub>LFC</sub> 
or 
: FCL = P<sub>EL</sub> - P<sub>LFC</sub>

Deep, moist convection is essentially a [[thunderstorm]] or thundercloud, although some such convection does not produce lightning and thus not thunder. It is [[cumulus congestus cloud]]s or [[cumulonimbus cloud]]s. An air parcel ascending from the near surface layer ([[mixed layer]] (ML) or [[planetary boundary layer|boundary layer]] (PBL)) must [[work (thermodynamics)|work]] through the stable layer of [[convective inhibition]] (CIN) when present. This work comes from sufficiently 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<sub>c</sub>) before moist convection ([[cloud]]) begins near the [[convective condensation level]] (CCL), whereas with dynamic lift, [[cloud base]] begins near the [[lifted condensation level]] (LCL). When such a [[capping inversion]] is present, this will remain as shallow, moist convection (small [[cumulus cloud]]s) 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 (and sufficient moisture is available in the [[Inflow (meteorology)|inflow]] layer). At the level of neutral buoyancy (the EL), a parcel is cooler than the environment and is thermodynamically stable, continuing to rise via [[momentum]] and thus it slows down until eventually ceasing ascent at the [[maximum parcel level]] (MPL) --which may visually manifest itself as an [[overshooting top]]. Ignoring other influences, higher amount of total CAPE in the FCL, and especially greater thickness of this positive area, which can be measured as [[lifted index]] (LI) at a respective altitude, results in more vigorous [[updraft]]s and faster air parcel ascent.

== See also ==
* [[Atmospheric thermodynamics]]

== References ==
* Blanchard, David O. (Sep 1998). Assessing the Vertical Distribution of Convective Available Potential Energy. ''[[Weather and Forecasting]]'', 13 (3): 870–877.

== External links ==
* [http://www.crh.noaa.gov/lsx/science/pdfppt/convective_indices.pdf Atmospheric indices] ([[National Weather Service]] St. Louis)

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[[Category:Severe weather and convection]]
[[Category:Atmospheric thermodynamics]]


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