Inatmospheric sciences, thefree convective layer (FCL) is the layer of conditional orpotential instability in thetroposphere. It is a layer in which rising air can experience positivebuoyancy (PBE) so that deep, moist convection (DMC) can occur. On anatmospheric sounding, it is the layer between thelevel of free convection (LFC) and theequilibrium 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" onthermodynamic diagrams where an ascendingair parcel is warmer than its environment.Integrating buoyant energy from the LFC to the EL gives the amount ofconvective available potential energy (CAPE), an estimate of the maximum energy available toconvection. The depth of the FCL is expressed by the formula:
or
Deep, moist convection is essentially athunderstorm or thundercloud, although some such convection does not produce lightning and thus not thunder. It iscumulus congestus clouds orcumulonimbus clouds. An air parcel ascending from the near surface layer (mixed layer (ML) orboundary layer (PBL)) mustwork through the stable layer ofconvective inhibition (CIN) when present. This work comes from sufficiently increasing instability in the low levels by raising thetemperature ordew point, or by mechanical lift. Without the aid of mechanical forcing, a parcel must reach itsconvective temperature (Tc) before moist convection (cloud) begins near theconvective condensation level (CCL), whereas with dynamic lift,cloud base begins near thelifted condensation level (LCL). When such acapping inversion is present, this will remain as shallow, moist convection (smallcumulus 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 (and sufficient moisture is available in theinflow layer). At the level of neutral buoyancy (the EL), a parcel is cooler than the environment and is thermodynamically stable, continuing to rise viamomentum and thus it slows down until eventually ceasing ascent at themaximum parcel level (MPL) --which may visually manifest itself as anovershooting top. Ignoring other influences, higher amount of total CAPE in the FCL, and especially greater thickness of this positive area, which can be measured aslifted index (LI) at a respective altitude, results in more vigorousupdrafts and faster air parcel ascent.
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