The Influence of Mixing Processes on the Climate Forcing of Marine Stratocumulus

Recent studies have shown that mixing processes may influence the estimation of the indirect effect through two processes: 1)Increases in cloud condensation nuclei (CCN) concentration can enhance mixing between cloudy and clear air, which modifies the cloud liquid water content (LWC), thereby impacting the change in cloud optical depth and hence the indirect effect. 2)Mixing influences the cloud drop distribution, with the mixing scenario, i.e. rate of mixing relative to the rate of evaporation, governing the change in the distribution. The influence of the mixing scenario on the indirect effect is very uncertain, while the influence of both the mixing scenario and the change in LWC due to changes in CCN concentration has not been addressed. In this work, we investigate the role of both of these processes in the estimation of the indirect effect. This investigation is undertaken with a 3-D large eddy simulation model with fully integrated size bin resolved cloud microphysics (BR-LEM). We present results from BR-LEM simulations of non-precipitating marine Sc in which the CCN concentration is varied between 100 and 1000 cm-3. We simulate the homogeneous and inhomogeneous mixing scenarios for each CCN concentration using a newly developed bin microphysical mixing scheme. We demonstrate that mixing processes do influence the estimation of the indirect effect. We discuss the importance of this influence relative to the impact of increasing CCN concentration alone