Time‐scales of adjustment to radiative‐convective equilibrium in the tropical atmosphere

In the tropics the assumed existence of a balanced atmospheric state of radiative‐convective equilibrium is a useful and widely utilized concept. Given an atmospheric state of radiative‐convective equilibrium which is perturbed, this paper attempts to identify the mechanisms that determine the time‐scale for the restoration of the balanced state. The perturbation could arise from large‐scale atmospheric wave motions, local‐scale convective downdraughts or sea surface temperature perturbations for example. The resulting state immediately after the perturbation is applied can be one of suppressed or convective conditions, and it is the complex response under convective conditions that is explored in this paper. A three‐dimensional cloud resolving model is operated to a radiative‐convective equilibrium state to which sea surface temperature perturbations are then applied. It is found that the variability of the model state variables, such as temperature and total water vapour amount, can be divided by processes into an exponential adjustment to the new balanced state on a long time‐scale (15 days), superimposed by short time‐scale variability (<4 days) that is governed almost solely by the convective mass‐flux. The determination of the long time‐scale trend to equilibrium is then investigated with further numerical experiments, which demonstrate that radiation determines the adjustment time‐scale via its control of the subsidence velocities in the clear‐sky regions surrounding convection. Some implications of the results for cumulus parametrization are discussed.