Using the Weak‐Temperature Gradient Approximation to Evaluate Parameterizations: An Example of the Transition From Suppressed to Active Convection

Abstract

AbstractTwo single‐column models are fully coupled via the weak‐temperature gradient approach. The coupled‐SCM is used to simulate the transition from suppressed to active convection under the influence of an interactive large‐scale circulation. The sensitivity of this transition to the value of mixing entrainment within the convective parameterization is explored. The results from these simulations are compared with those from equivalent simulations using coupled cloud‐resolving models. Coupled‐column simulations over nonuniform surface forcing are used to initialize the simulations of the transition, in which the column with suppressed convection is forced to undergo a transition to active convection by changing the local and/or remote surface forcings. The direct contributions from the changes in surface forcing are to induce a weakening of the large‐scale circulation which systematically modulates the transition. In the SCM, the contributions from the large‐scale circulation are dominated by the heating effects, while in the CRM the heating and moistening effects are about equally divided. A transition time is defined as the time when the rain rate in the dry column is halfway to the value at equilibrium after the transition. For the control value of entrainment, the order of the transition times is identical to that obtained in the CRM, but the transition times are markedly faster. The locally forced transition is strongly delayed by a higher entrainment. A consequence is that for a 50% higher entrainment the transition times are reordered. The remotely forced transition remains fast while the locally forced transition becomes slow, compared to the CRM.