Most global climate models with convective parameterization have trouble
in simulating the observed diurnal cycle of convection. Maximum
precipitation usually happens too early during summertime, especially
over land. Observational analyses indicate that deep convection over
land cannot keep pace with rapid variations in convective available
potential energy, which is largely controlled by boundary-layer forcing.
In this study, a new convective closure in which shallow and deep
convection interact strongly, out of equilibrium, is implemented in
atmosphere-only and ocean-atmosphere coupled models. The diurnal cycles
of convection in both simulations are significantly improved with small
changes to their mean states. The new closure shifts maximum
precipitation over land later by about three hours. Compared to
satellite observations, the diurnal phase biases are reduced by half.
Shallow convection to some extent equilibrates rapid changes in the
boundary layer at subdiurnal time scales. Relaxed quasi-equilibrium for
convective available potential energy holds in significant measure as a
result. Future model improvement will focus on the remaining biases in
the diurnal cycle, which may be further reduced by including stochastic
entrainment and cold pools.