Convective available potential energy
This article may be too technical for most readers to understand.(November 2014) |
In
Mechanics
CAPE exists within the conditionally unstable layer of the troposphere, the free convective layer (FCL), where an ascending air parcel is warmer than the ambient air. CAPE is measured in joules 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 (LFC) to the equilibrium level (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 or sounding diagram (e.g., a Skew-T log-P diagram) using air temperature and dew point data usually measured by a weather balloon.
CAPE is effectively positive buoyancy, expressed B+ or simply B; the opposite of
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.
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
Fluid elements displaced upwards or downwards in such an atmosphere expand or compress
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.[4]
Convective instability is also termed static instability, because the instability does not depend on the existing motion of the air; this contrasts with
Significance to thunderstorms
The amount, and shape, of the positive-buoyancy area modulates the speed of
Two notable days for severe weather exhibited CAPE values over 5 kJ/kg. Two hours before the
Severe weather and tornadoes can develop in an area of low CAPE values. The
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
Limitations
As with most parameters used in
The more common method of determining CAPE can break down near
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.
See also
References
- .
- .
- ^ Thompson, Rich (2006). "Explanation of SPC Severe Weather Parameters". Storm Prediction Center. Retrieved 2007-05-30.
- ISBN 978-0-935702-65-1.
- ^ Craven, Jeffrey P.; H.E. Brooks (December 2004). "Baseline climatology of sounding derived parameters associated with deep moist convection" (PDF). National Weather Digest. 28: 13–24.
- ^ Pietrycha, Albert E.; J.M. Davies; M. Ratzer; P. Merzlock (October 2004). "Tornadoes in a Deceptively Small CAPE Environment: The 4/20/04 Outbreak in Illinois and Indiana". Preprints of the 22nd Conference on Severe Local Storms. Hyannis, Massachusetts: American Meteorological Society.
- .
- NOAA. Retrieved December 27, 2021.
Further reading
- Barry, R.G. and Chorley, R.J. Atmosphere, weather and climate (7th ed) Routledge 1998 p. 80-81 ISBN 0-415-16020-0