The response time of a convective cloud ensemble to a change in forcing

In this paper. the timescale of the convective mass-flux response to step-function external forcing conditions is measured directly using idealized cloud-resolving, model simulations of unorganized convection in radiative-convective equilibrium. Many parametrization schemes require an estimate of this response time in the form of a fixed adjustment timescale. but the physical mechanism which sets its value is currently Unknown. These experiments confirm the existence of a unique timescale. for given initial forcing conditions. of the order of one hour. The hypothesized role of gravity waves in effecting the convective adjustment is tested by changing the mean cloud spacing of the initial state. This reveals a linear dependence of the response time on the cloud spacing, in agreement with the theory. Meanwhile. similar experiments to investigate the sensitivity of the timescale to changes in gravity-wave speed reveal a remarkable robustness of the average wave speed over a range of stratifications and tropospheric depths.