In atmospheric sciences, the free convective layer (FCL) is the layer of conditional or potential instability in the

troposphere The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the Atmosphere, planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the ...

. It is a layer in which rising air can experience positive

buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

(PBE) so that deep, moist convection (DMC) can occur. On an atmospheric sounding, it is the layer between the

level of free convection The level of free convection (LFC) is the altitude in the atmosphere where an air parcel lifted adiabatically until saturation becomes warmer than the environment at the same level, so that positive buoyancy can initiate self-sustained 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 Thermodynamic diagrams are diagrams used to represent the thermodynamic states of a material (typically fluid) and the consequences of manipulating this material. For instance, a temperature–entropy diagram (Temperature–entropy diagram, T–s ...

where an ascending air parcel is warmer than its environment. 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

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

. 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 or thundercloud, although some such convection does not produce lightning and thus not thunder. It is cumulus congestus clouds or

cumulonimbus cloud Cumulonimbus () is a dense, towering, vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. Above the lower portions of the cumulonimbus the water ...

s. An air parcel ascending from the near surface layer ( mixed layer (ML) or

boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a Boundary (thermodynamic), bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces ...

(PBL)) must 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 Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

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 The lifting condensation level or lifted condensation level (LCL) is the height at which the relative humidity (RH) of an air parcel will reach 100% with respect to liquid water when it is cooled by dry adiabatic lifting. The RH of air increases ...

(LCL). When such a capping inversion is present, this will remain as shallow, moist convection (small

cumulus cloud Cumulus clouds are clouds that have flat cloud base, bases and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin , meaning "heap" or "pile". Cumulus clouds are low-level clouds, generally less ...

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 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 The lifted index (LI) is the temperature difference between the environment Te(p) and an air parcel lifted adiabatically Tp(p) at a given pressure height in the troposphere (lowest layer where most weather occurs) of the atmosphere, usually 500 ...

(LI) at a respective altitude, results in more vigorous

updraft In meteorology, an updraft (British English: ''up-draught'') is a small-scale air current, current of rising air, often within a cloud. Overview Vertical drafts, known as updrafts or downdrafts, are localized regions of warm or cool air that mov ...

s and faster air parcel ascent.

References

* Blanchard, David O. (Sep 1998). Assessing the Vertical Distribution of Convective Available Potential Energy. ''

Weather and Forecasting ''Weather and Forecasting'' is a scientific journal published by the American Meteorological Society. Articles on forecasting and analysis techniques, forecast verification studies, and case studies useful to forecasters. In addition, submissio ...

'', 13 (3): 870–877.

External links

Atmospheric indices
(

National Weather Service The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...

St. Louis) Severe weather and convection Atmospheric thermodynamics {{climate-stub fr:Niveau d'équilibre convectif#Notions dérivées

See also

References

External links