Downdraft convective available potential energy
From AMS Glossary
downdraft convective available potential energy
(Abbreviated DCAPE.) The maximum energy available to a descending parcel, according to parcel theory.
On a thermodynamic diagram this is called negative area, and can be seen as the region between the descending parcel process curve and the environmental sounding, from the parcel's level of free sink to some lower level, usually the surface. Its quantitative definition is
where αp and αe are the specific volumes of the parcel and its environment, respectively, and ps and pf are the surface pressure and the pressure of the level of free sink, respectively.