In a nutshell, the Downward Convective Available Convective Energy (DCAPE) is the total amount of negative buoyancy available to an air parcel traveling in a downdraft. On a skew-T, estimate the altitude where the downdraft will begin (600 mb, for example; see image below). At this pressure level, determine the wet-bulb potential temperature for a sample parcel (moist adiabat corresponding to the LCL). Then take the air parcel down moist adiabatically to the ground. The DCAPE is simply the total area (negative area) between the path of the parcel and the environmental temperature (virtual temperature, to be more accurate).
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