Convective quasi‐equilibrium in midlatitude continental environment and its effect on convective parameterization

The quasi‐equilibrium assumption proposed by Arakawa and Schubert assumes that convection is controlled by the large‐scale forcing in a statistical sense, in such a way that the stabilization of the atmosphere by convection is in quasi‐equilibrium with the destabilization by the large‐scale forcing. The assumption was developed largely based on observations in the tropical maritime environment and has not been evaluated in midlatitudes. This study examines the quasi‐equilibrium assumption in midlatitude continental convection environment using summertime observations from the Southern Great Plains of the United States. Two complementary approaches are taken for this purpose. The first one compares the net time rate of change of convective available potential energy to that due to the large‐scale forcing. The second one examines the contributions to the net change of CAPE from the boundary layer air and the free tropospheric air above. Results from both the approaches indicate that the quasi‐equilibrium assumption is not well suited for midlatitude continental convection. It is shown that the net change of CAPE is comparable to and largely comes from that due to thermodynamic changes of the boundary layer air, while the contribution from the free troposphere above the boundary layer is negligible. The analysis also shows that the role of convective inhibition to suppress convection is the most pronounced when the large‐scale forcing in the free troposphere is weak. On the basis of these and other observations, a modification to the quasi‐equilibrium assumption is proposed. It assumes that convective and large‐scale processes in the free troposphere above the boundary layer are in balance, so that contribution from the free troposphere to changes in CAPE is negligible. This assumption is then tested using the single column model of the NCAR CCM3 by modifying the closure in the CCM3 convection scheme. Such a modification significantly improves the single column model simulation. The applicability of this new quasi‐equilibrium assumption to tropical convection environment is also discussed.