Free convective layer
Encyclopedia
In atmospheric sciences
, 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.
It is the 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:
or
Deep, moist convection is essentially a thunderstorm
, it is cumulus congestus clouds or cumulonimbus cloud
s. 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
(Tc) 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 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. 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).
Atmospheric sciences
Atmospheric sciences is an umbrella term for the study of the atmosphere, its processes, the effects other systems have on the atmosphere, and the effects of the atmosphere on these other systems. Meteorology includes atmospheric chemistry and atmospheric physics with a major focus on weather...
, the free convective layer (FCL) is the layer of conditional or potential instability in the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
. It is a layer of positive buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
(PBE) and is the layer where deep, moist convection (DMC) can occur. On an atmospheric sounding
Atmospheric sounding
An atmospheric sounding is a measurement of vertical distribution of physical properties of the atmospheric column such as pressure, temperature, wind speed and wind direction , liquid water content, ozone concentration, pollution, and other properties...
, it is the layer between the level of free convection
Level of free convection
The level of free convection is the altitude in the atmosphere where the temperature of the environment decreases faster than the moist adiabatic lapse rate of a saturated air parcel at the same level....
(LFC) and the equilibrium level
Equilibrium level
In meteorology, the equilibrium level , or level of neutral buoyancy , or limit of convection , is the height at which a rising parcel of air is at a temperature of equal warmth to it....
(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
Thermodynamic diagrams are diagrams used by scientists and engineers to represent the thermodynamic states of a material and the consequences of manipulating this material...
where an ascending air parcel
Air parcel
In fluid dynamics, within the framework of continuum mechanics, a fluid parcel is a very small amount of fluid, identifiable throughout its dynamic history while moving with the fluid flow. As it moves, the mass of a fluid parcel remains constant, while—in a compressible flow—its volume may...
is warmer than its environment. Integrating
Integral
Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...
buoyant energy from the LFC to the EL gives the convective available potential energy
Convective available potential energy
In meteorology, convective available potential energy , sometimes, simply, available potential energy , is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere...
(CAPE), an estimate of the maximum energy available to convection. The depth of the FCL is expressed by the formula:
- FCL = ZEL - ZLFC
or
- FCL = PEL - PLFC
Deep, moist convection is essentially a thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
, it is cumulus congestus clouds or cumulonimbus cloud
Cumulonimbus cloud
Cumulonimbus is a towering vertical cloud that is very tall, dense, and involved in thunderstorms and other inclement weather. Cumulonimbus originates from Latin: Cumulus "Heap" and nimbus "rain". It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold...
s. An air parcel ascending from the near surface layer (mixed layer
Mixed layer
The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, cooling, or processes such as evaporation or sea ice formation which result in an increase in salinity...
or boundary layer
Planetary boundary layer
The planetary boundary layer , also known as the atmospheric boundary layer , is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface forcing in an hour or less...
) must work
Work (thermodynamics)
In thermodynamics, work performed by a system is the energy transferred to another system that is measured by the external generalized mechanical constraints on the system. As such, thermodynamic work is a generalization of the concept of mechanical work in mechanics. Thermodynamic work encompasses...
through the stable layer of convective inhibition
Convective inhibition
Convective inhibition is a numerical measure in meteorology that indicates the amount of energy that will prevent an air parcel from rising from the surface to the level of free convection....
(CIN) when present. This work comes from increasing instability in the low levels by raising the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
or dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
, or by mechanical lift. Without the aid of mechanical forcing, a parcel must reach its convective temperature
Convective temperature
The convective temperature is the approximate temperature that air near the surface must reach for cloud formation without mechanical lift. In such case, cloud base begins at the convective condensation level , whilst with mechanical lifting, condensation begins at the lifted condensation level...
(Tc) before moist convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
(cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
) begins near the convective condensation level
Convective condensation level
The convective condensation level represents the height where an air parcel becomes saturated when lifted adiabatically to achieve buoyant ascent. It marks where cloud base begins when air is heated from below to the convective temperature, without mechanical lift...
(CCL}, whereas with dynamic lift, cloud base
Cloud base
This article refers to meteorology. For the airborne base of the TV series Captain Scarlet and the Mysterons, see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...
begins near the lifted condensation level
Lifted condensation level
The lifted condensation level or lifting condensation level is formally defined as the height at which the relative humidity of an air parcel will reach 100% when it is cooled by dry adiabatic lifting...
(LCL). This will remain as shallow, moist convection (small cumulus cloud
Cumulus cloud
Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...
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. 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
Maximum parcel level
The maximum parcel level is the highest level in the atmosphere that a moist convectively rising air parcel will reach after ascending through the free convective layer and reaching the equilibrium level , near the tropopause...
(MPL).
External links
- Atmospheric indices (National Weather ServiceNational Weather ServiceThe National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
St. Louis)