Abstract: Convective available potential energy
(CAPE), frequently regarded as an indicator of the potential intensity
of deep convection, is strongly controlled by the properties of the
planetary boundary layer (BL). Variations in CAPE observed during field
experiments in midcontinent North America, the tropical east Atlantic,
and the tropical west Pacific, can be accounted for mostly by changes in
the temperature and humidity in the BL. The coupling between CAPE and
the BL holds for both convective and nonconvective conditions. The
coupling under conditions of deep convection implies a constraint on the
intensity of deep convection which can be used as a closure for cumulus
parameterization. This constraint requires equilibrium in the
environment of the parcel used as a basis for calculating CAPE. Over
many cases, parcel-environment equilibrium is observed to hold more
robustly than equilibrium of CAPE itself. When observational
uncertainties are considered, it is uncertain whether quasi-equilibrium,
in which the rate of change of CAPE is substantially less than the rate
at which mean advection and BL fluxes change CAPE, holds at subdiurnal
timescales in the eastern Atlantic and the western Pacific.
Quasi-equilibrium is a poor approximation at subdiurnal timescales in
midcontinent North America. At timescales approaching diurnal,
quasi-equilibrium holds in all cases. Cumulus parameterizations based on
quasi-equilibrium may be limited in their ability to model diurnal
cycles as a result. CAPE fluctuations related to large, subdiurnal
variations in surface fluxes are much sharper than CAPE fluctuations
related to changes in mean advection above the BL, especially over land.
The strong BL control on CAPE indicates that deep convection does not
equilibrate rapid, high-amplitude variations in CAPE originating there. |