![Convective instability animation 12Z 21Z Jan08](https://upload.wikimedia.org/wikipedia/commons/2/2b/Convective_instability_animation_12Z_21Z_Jan08.gif)
In
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...
, convective available potential energy (commonly abbreviated as CAPE), is the integrated amount of
work
Work may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
** Manual labour, physical work done by humans
** House work, housework, or homemaking
** Working animal, an animal t ...
that the upward (positive)
buoyancy force would perform on a given mass of air (called an
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, ...
) if it rose vertically through the entire atmosphere. Positive CAPE will cause the air parcel to rise, while negative CAPE will cause the air parcel to sink.
Nonzero CAPE is an indicator of
atmospheric instability
Atmospheric instability is a condition where the Earth's atmosphere is generally considered to be unstable and as a result the weather is subjected to a high degree of variability through distance and time. Atmospheric stability is a measure of t ...
in any given
atmospheric sounding Atmospheric sounding or atmospheric profiling is a measurement of vertical distribution of physical properties of the atmospheric column such as pressure, temperature, wind speed and wind direction (thus deriving wind shear), liquid water content, ...
, a necessary condition for the development of
cumulus
Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...
and
cumulonimbus
Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful Buoyancy, buoyant air ...
clouds with attendant
severe weather
Severe weather is any dangerous meteorological phenomenon with the potential to cause damage, serious social disruption, or loss of human life. Types of severe weather phenomena vary, depending on the latitude, altitude, topography, and atmos ...
hazards.
Mechanics
![B and LCL-LFC](https://upload.wikimedia.org/wikipedia/commons/b/b1/B_and_LCL-LFC.jpg)
CAPE exists within the
conditionally unstable layer of the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, the
free convective layer
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 occ ...
(FCL), where an ascending air parcel is warmer than the ambient air. CAPE is measured in
joule
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...
s per kilogram of air (J/kg). Any value greater than 0 J/kg indicates instability and an increasing possibility of thunderstorms and hail. Generic CAPE is calculated by
integrating vertically the local buoyancy of a parcel from 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) to the
equilibrium level In meteorology, the equilibrium level (EL), or level of neutral buoyancy (LNB), or limit of convection (LOC), is the height at which a rising parcel of air is at the same temperature as its environment.
This means that unstable air is now stable ...
(EL):
Where
is the height of the level of free convection and
is the height of the equilibrium level (neutral buoyancy), where
is the
virtual temperature of the specific parcel, where
is the virtual temperature of the environment (note that temperatures must be in the Kelvin scale), and where
is the
acceleration due to gravity. This integral is the work done by the buoyant force minus the work done against gravity, hence it's the excess energy that can become kinetic energy.
CAPE for a given region is most often calculated from a
thermodynamic
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of the ...
or
sounding diagram (e.g., a
Skew-T log-P diagram
A skew-T log-P diagram is one of four thermodynamic diagrams commonly used in weather analysis and forecasting. In 1947, N. Herlofson proposed a modification to the emagram that allows straight, horizontal isobars and provides for a large angle ...
) using air
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
and
dew point
The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...
data usually measured by a
weather balloon.
CAPE is effectively positive buoyancy, expressed ''B+'' or simply ''B''; the opposite of
convective inhibition (CIN), which is expressed as ''B-'', and can be thought of as "negative CAPE". As with CIN, CAPE is usually expressed in J/kg but may also be expressed as m
2/s
2, as the values are equivalent. In fact, CAPE is sometimes referred to as ''positive buoyant energy'' (''PBE''). This type of CAPE is the maximum energy available to an ascending parcel and to moist convection. When a layer of CIN is present, the layer must be eroded by surface heating or mechanical lifting, so that
convective boundary layer
In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary condi ...
parcels may reach their
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).
On a sounding diagram, CAPE is the ''positive area'' above the LFC, the area between the parcel's virtual temperature line and the environmental virtual temperature line where the ascending parcel is warmer than the environment. Neglecting the virtual temperature correction may result in substantial relative errors in the calculated value of CAPE for small CAPE values. CAPE may also exist below the LFC, but if a layer of CIN (
subsidence) is present, it is unavailable to deep, moist convection until CIN is exhausted. When there is mechanical lift to
saturation
Saturation, saturated, unsaturation or unsaturated may refer to:
Chemistry
* Saturation, a property of organic compounds referring to carbon-carbon bonds
**Saturated and unsaturated compounds
** Degree of unsaturation
**Saturated fat or fatty aci ...
,
cloud base
A cloud base (or the base of the cloud) is the lowest altitude of the visible portion of a cloud. It is traditionally expressed either in metres or feet above mean sea level or above a planetary surface, or as the pressure level corresponding to ...
begins at the
lifted condensation level
The lifted condensation level or lifting condensation level (LCL) is formally defined as 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 ...
(LCL); absent forcing,
cloud base
A cloud base (or the base of the cloud) is the lowest altitude of the visible portion of a cloud. It is traditionally expressed either in metres or feet above mean sea level or above a planetary surface, or as the pressure level corresponding to ...
begins at the
convective condensation level The convective condensation level (CCL) represents the height (or pressure) where an air parcel becomes saturated when heated from below and lifted adiabatically due to buoyancy.
In the atmosphere, assuming a constant water vapor mixing ratio, th ...
(CCL) where heating from below causes spontaneous buoyant lifting to the point of
condensation when the
convective temperature The convective temperature (CT or Tc) 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 (CCL), whilst with mechanica ...
is reached. When CIN is absent or is overcome, saturated parcels at the LCL or CCL, which had been small
cumulus clouds, will rise to the LFC, and then spontaneously rise until hitting the stable layer of the equilibrium level. The result is deep, moist convection (DMC), or simply, a thunderstorm.
When a parcel is unstable, it will continue to move vertically, in either direction, dependent on whether it receives upward or downward forcing, until it reaches a stable layer (though momentum, gravity, and other forcing may cause the parcel to continue). There are multiple types of CAPE, ''downdraft CAPE'' (''DCAPE''), estimates the potential strength of rain and evaporatively cooled
downdraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, ...
s. Other types of CAPE may depend on the depth being considered. Other examples are ''surface based CAPE'' (''SBCAPE''), ''mixed layer'' or ''mean layer CAPE'' (''MLCAPE''), ''most unstable'' or ''maximum usable CAPE'' (''MUCAPE''), and ''normalized CAPE'' (''NCAPE'').
Fluid elements displaced upwards or downwards in such an atmosphere expand or compress
adiabatically in order to remain in pressure equilibrium with their surroundings, and in this manner become less or more dense.
If the adiabatic decrease or increase in density is ''less'' than the decrease or increase in the density of the ambient (not moved) medium, then the displaced fluid element will be subject to downwards or upwards pressure, which will function to restore it to its original position. Hence, there will be a counteracting force to the initial displacement. Such a condition is referred to as ''convective stability''.
On the other hand, if adiabatic decrease or increase in density is ''greater'' than in the ambient fluid, the upwards or downwards displacement will be met with an additional force in the same direction exerted by the ambient fluid. In these circumstances, small deviations from the initial state will become amplified. This condition is referred to as ''
convective instability
In meteorology, convective instability or stability of an air mass refers to its ability to resist vertical motion. A ''stable'' atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an ''uns ...
''.
Convective instability is also termed ''static instability'', because the instability does not depend on the existing motion of the air; this contrasts with
dynamic instability Dynamic instability may refer to any of several scientific phenomena:
* Aircraft dynamic modes, including aircraft dynamic instability
*Atmospheric instability, in meteorology
* Dynamic instability of microtubules, in biology
*Firehose instability ...
where instability is dependent on the motion of air and its associated effects such as
dynamic lifting.
Significance to thunderstorms
Thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are someti ...
s form when
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, ...
s are lifted vertically. Deep, moist convection requires a parcel to be lifted to the LFC where it then rises spontaneously until reaching a layer of non-positive buoyancy. The
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
is warm at the surface and lower levels of the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
where there is
mixing (the
planetary boundary layer (PBL)), but becomes substantially cooler with height. The temperature profile of the atmosphere, the change in temperature, the degree that it cools with height, is the
lapse rate. When the rising air parcel cools more slowly than the surrounding atmosphere, it remains warmer and less
dense
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically ...
. The parcel continues to rise freely (
convectively; without mechanical lift) through the atmosphere until it reaches an area of air less dense (warmer) than itself.
The amount, and shape, of the positive-buoyancy area modulates the speed of
updraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding regi ...
s, thus extreme CAPE can result in explosive thunderstorm development; such rapid development usually occurs when CAPE stored by a
capping inversion
A capping inversion is an elevated inversion layer that caps a convective planetary boundary layer.
The boundary layer is the part of the atmosphere which is closest to the ground. Normally, the sun heats the ground, which in turn heats th ...
is released when the "lid" is broken by heating or mechanical lift. The amount of CAPE also modulates how low-level
vorticity
In continuum mechanics, vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along wi ...
is entrained and then stretched in the
updraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding regi ...
, with importance to
tornadogenesis
Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes and these vary in methods of formation. Despite ongoing scientific study and high-profile research projects such as VORTEX, tornadogenesis is a volatile pro ...
. The most important CAPE for
tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
es is within the lowest 1 to 3 km (0.6 to 1.9 mi) of the atmosphere, whilst deep layer CAPE and the width of CAPE at mid-levels is important for
supercell
A supercell is a thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms ( ...
s.
Tornado outbreak
__NOTOC__
A tornado outbreak is the occurrence of multiple tornadoes spawned by the same synoptic scale weather system. The number of tornadoes required to qualify as an outbreak typically are at least six to ten, with at least two rotational l ...
s tend to occur within high CAPE environments. Large CAPE is required for the production of very large hail, owing to updraft strength, although a rotating updraft may be stronger with less CAPE. Large CAPE also promotes lightning activity.
Two notable days for severe weather exhibited CAPE values over 5 kJ/kg. Two hours before the
1999 Oklahoma tornado outbreak occurred on May 3, 1999, the CAPE value sounding at
Oklahoma City
Oklahoma City (), officially the City of Oklahoma City, and often shortened to OKC, is the capital and largest city of the U.S. state of Oklahoma. The county seat of Oklahoma County, it ranks 20th among United States cities in population, a ...
was at 5.89 kJ/kg. A few hours later, an
F5 tornado ripped through the southern suburbs of the city. Also on May 4, 2007 CAPE values of 5.5 kJ/kg were reached and an
EF5 tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
tore through
Greensburg, Kansas
Greensburg is a city in, and the county seat of, Kiowa County, Kansas, United States. As of the 2020 United States census, the population of the city was 740. It is home to the world's largest hand-dug well. On the evening of May 4, 2007, Greens ...
. On these days, it was apparent that conditions were ripe for tornadoes and CAPE wasn't a crucial factor. However, extreme CAPE, by modulating the updraft (and downdraft), can allow for exceptional events, such as the deadly F5 tornadoes that hit
Plainfield, Illinois
Plainfield is a village in Will and Kendall counties, Illinois, United States. The population was 44,762 at the 2020 census.
The village includes land in Will County's Plainfield and Wheatland townships, as well as Na-Au-Say and Oswego tow ...
on August 28, 1990 and
Jarrell, Texas
Jarrell is a city in Williamson County, Texas, United States. It was incorporated as a city in 2001. The total population is 1,753 according to the 2020 census.
History
Founded in 1909 by real estate developer O.D. Jarrell, along with E. C. Ha ...
on May 27, 1997 on days which weren't readily apparent as conducive to large tornadoes. CAPE was estimated to exceed 8 kJ/kg in the environment of the
Plainfield storm and was around 7 kJ/kg for the
Jarrell storm.
Severe weather and tornadoes can develop in an area of low CAPE values. The
surprise severe weather event that occurred in
Illinois
Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolita ...
and
Indiana
Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th s ...
on April 20, 2004 is a good example. Importantly in that case, was that although overall CAPE was weak, there was strong CAPE in the lowest levels of the troposphere which enabled an outbreak of minisupercells producing large, long-track, intense tornadoes.
Example from meteorology
A good example of convective instability can be found in our own atmosphere. If dry mid-level air is drawn over very warm, moist air in the lower
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, a
hydrolapse (an area of rapidly decreasing dew point temperatures with height) results in the region where the moist
boundary layer
In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary cond ...
and mid-level air meet. As daytime heating increases mixing within the moist boundary layer, some of the moist air will begin to interact with the dry mid-level air above it. Owing to thermodynamic processes, as the dry mid-level air is slowly saturated its temperature begins to drop, increasing the
adiabatic lapse rate
The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
. Under certain conditions, the lapse rate can increase significantly in a short amount of time, resulting in
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
. High convective instability can lead to severe
thunderstorms
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are somet ...
and
tornadoes
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, alth ...
as moist air which is trapped in the boundary layer eventually becomes highly negatively buoyant relative to the adiabatic lapse rate and escapes as a rapidly rising bubble of humid air triggering the development of a
cumulus
Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...
or
cumulonimbus
Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful Buoyancy, buoyant air ...
cloud.
Limitations
As with most parameters used in
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...
, there are some caveats to keep in mind. One of which is what CAPE represents physically and in what instances you can use CAPE. One example where the more common method for determining CAPE might start to break down is in the presence of
Tropical Cyclones
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dependi ...
(e.x. Tropical Depressions, Tropical Storms,
Hurricanes
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
).
The more common method of determining CAPE can break down near
Tropical Cyclones
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dependi ...
because CAPE assumes that liquid water is lost instantaneously during
condensation. This process is thus irreversible upon
adiabatic descent. This process is not realistic for
Tropical Cyclones
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dependi ...
(TC for short). To make the process more realistic for
Tropical Cyclones
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dependi ...
is to use Reversible CAPE (RCAPE for short). RCAPE assumes the opposite extreme to the standard convention of CAPE and is that no liquid water will be lost during the process. This new process gives parcels a greater
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
related to water loading.
RCAPE is calculated using the same formula as CAPE, the difference in the formula being in the
virtual temperature. In this new formulation, we replace the parcel saturation
mixing ratio (which leads to the
condensation and vanishing of liquid water) with the parcel water content. This slight change can drastically change the values we get through the integration.
RCAPE does have some limitations, one of which is that RCAPE assumes no evaporation keeping consistent for the use within a TC but should be used sparingly elsewhere.
Another limitation of both CAPE and RCAPE is that currently, both systems do not consider
entrainment
Entrainment may refer to:
* Air entrainment, the intentional creation of tiny air bubbles in concrete
* Brainwave entrainment, the practice of entraining one's brainwaves to a desired frequency
* Entrainment (biomusicology), the synchronization o ...
.
See also
*
Atmospheric thermodynamics Atmospheric thermodynamics is the study of heat-to-work transformations (and their reverse) that take place in the earth's atmosphere and manifest as weather or climate. Atmospheric thermodynamics use the laws of classical thermodynamics, to descr ...
*
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 ...
*
Maximum potential intensity The maximum potential intensity of a tropical cyclone is the theoretical limit of the strength of a tropical cyclone.
Maximum potential intensity
Due to surface friction, the inflow only partially conserves angular momentum. Thus, the sea surface ...
References
Further reading
* Barry, R.G. and Chorley, R.J. ''Atmosphere, weather and climate'' (7th ed) Routledge 1998 p. 80-81
External links
Map of current global CAPE
{{DEFAULTSORT:Convective Available Potential Energy
Severe weather and convection
Atmospheric thermodynamics
Fluid dynamics