Convective Available Potential Energy

CAPE is the vertically integrated buoyancy available to a lifted air parcel between its level of free convection and its equilibrium level. It has units of energy per unit mass (J kg-1), and it answers one specific question: if a parcel gets to the level of free convection, how much energy is available to accelerate it upward from there.

The conversion to a velocity is the part worth remembering, because it is where intuition usually breaks. Converting all of the CAPE to kinetic energy gives a maximum updraft speed of sqrt(2 x CAPE) — about 45 m/s for 1000 J/kg. Observed updrafts reach roughly half that. The energy goes into entrainment of drier environmental air, the weight of condensate the updraft carries, and the pressure perturbations that develop as the updraft interacts with the shear it rises through. Treating CAPE as a prediction of updraft strength rather than a bound on it is a reliable way to be wrong.

Why the number alone says little

CAPE is necessary for deep convection and nowhere near sufficient. Three things break the relationship:

• Convective inhibition. CAPE says what happens after the parcel reaches the level of free convection; CIN says whether it gets there at all. High CAPE under a strong cap produces nothing, right up until the cap breaks and produces a great deal at once.
• Shape, not just area. The same integrated CAPE distributed as a tall thin sliver or as a wide bulge in the lower troposphere gives very different storms. Low-level CAPE accelerates parcels where entrainment does least damage, so it is disproportionately effective.
• Vertical wind shear. Shear determines whether convection organises. Modest CAPE with strong shear produces longer-lived, better-organised storms than large CAPE with none.

Which parcel is lifted also matters, and the choice is a convention rather than a fact. Surface-based, mixed-layer, and most-unstable CAPE can differ by a factor of several in the same sounding. A CAPE value without its parcel definition is not fully specified.

In models

CAPE is a diagnostic, not a prognostic variable — it is computed from the temperature and humidity profile, so it inherits every error in them. That makes it useful for evaluation in a particular way: comparing modelled and observed CAPE tests the thermodynamic profile, which is often a more informative target than comparing rainfall directly.

In WRF work I find the CAPE-CIN pair more diagnostic than either alone, especially for a cumulus scheme that fires too early. A scheme consuming CAPE as fast as the model generates it leaves a suspiciously smooth field with no diurnal build-up, which shows up in the CAPE time series long before it becomes obvious in the precipitation.

See also: convective inhibition for the barrier term, convective rain for what happens when convection does fire, and cumulus convection schemes for how models handle the whole process.