Convective available potential energy
convective available potential energy[edit | edit source]
(Abbreviated CAPE.) The potential energy of an air parcel is due to positive buoyancy, which is a useful tool for forecasting, parameterizing, and estimating the potential updraft strength of convective clouds.
On a thermodynamic diagram, this is called the positive area and can be seen as the region between the lifted parcel process curve and the environmental sounding, from the parcel's level of free convection to its level of neutral buoyancy. CAPE may be expressed by the following vertical integral:
where B is the buoyancy of an undiluted air parcel lifted from the level of free convection to the level of neutral buoyancy (zLFC and zLNB, respectively). The value depends on whether the parcel is lifted adiabatically (no water loss, condensates retained) or pseudo-adiabatically (all condensates fall out immediately upon forming). It is assumed that the pressure of the parcel is the same as that of the environment.
To determine B, the virtual temperature is commonly used, which allows CAPE to be computed as follows:
where Tυp and Tυe are the virtual temperatures of the lifted parcel and the environment, respectively.
As a parcel of air is lifted above its level of free convection and to the level of neutral buoyancy, work is performed due to buoyancy. This work results in the potential energy of buoyancy being realized as kinetic energy, hence the common relation to updraft strength. In reality, not all of the work done by the buoyancy force is converted to kinetic energy due to several factors, e.g., dissipation to the environment. Therefore, CAPE can be viewed as the maximum kinetic energy that could be gained by a rising air parcel due to work done by buoyancy.
Compare convective inhibition (CIN), downdraft convective available potential energy (DCAPE).
Bohren, C. F., and B. A. Albrecht, 2023. Atmospheric Thermodynamics. 2nd ed. Oxford University Press, 560 pp.
Term updated 26 February 2024.
