Convective Quasi-Equilibrium Reconsidered

The hypothesis of convective quasi-equilibrium states that moist convection reacts almost instantly to drive the atmospheric temperature profile throughout the troposphere to one of a special subset of possible profiles. In a simple case this might consist of a moist adiabat with the saturated moist entropy of the adiabat equaling the moist entropy of the boundary layer. In more complex cases the target temperature profile might depend on the moisture profile as well as boundary layer conditions. We present evidence from analytical and numerical calculations which challenge the validity of this hypothesis. These calculations involve both a simple, physically based parameterization of the effects of convection on its environment and a complex cloud-resolving model. In both cases we show that imposed temperature perturbations in the lower troposphere are rapidly eliminated, while those in the upper troposphere are not. The rapid temperature relaxation in the lower troposphere is due to the near-instantaneous change in the precipitation rate and latent heat release provoked by the temperature anomaly. Water vapor is so limited in the upper troposphere that this process cannot act there.