Abstract
Abstract. The sensitivity of a stratocumulus-capped mixed layer to a change in cloud droplet concentration is evaluated with a large-eddy simulation (LES) and a mixed layer model (MLM). The strength of the second aerosol indirect effect simulated by the two model types agrees within 50% for cases in which the LES-simulated boundary layer remains well mixed, if the MLM entrainment closure includes the effects of cloud droplet sedimentation. To achieve this agreement, parameters in the MLM entrainment closure and the drizzle parameterization must be retuned to match the LES. This is because the LES advection scheme and microphysical parameterization significantly bias the entrainment rate and precipitation profile compared to observational best guesses. Before this modification, the MLM simulates more liquid water path and much more drizzle at a given droplet concentration than the LES and is more sensitive to droplet concentration, even undergoing a drizzle-induced boundary layer collapse at low droplet concentrations. After this modification, both models predict a comparable decrease of cloud liquid water path as droplet concentration increases, cancelling 30–50% of the Twomey effect for our case. The agreement breaks down at the lowest simulated droplet concentrations, for which the boundary layer in the LES is not well mixed. Our results highlight issues with both types of model. Potential LES biases due to inadequate resolution, subgrid mixing and parameterized microphysics must be carefully considered when trying to make a quantitative inference of the second indirect effect from an LES of a stratocumulus-topped boundary layer. On the other hand, even slight internal decoupling of the boundary layer invalidates the central assumption of an MLM, substantially limiting the range of conditions that MLM-predicted sensitivities to droplet concentration are meaningful.
Highlights
The indirect effect of anthropogenic aerosol on clouds and thereby on the global radiation balance remains a key uncertainty in climate modeling and prediction
We compare the mixed layer model (MLM)-predicted response of the liquid water path (LWP) to the droplet concentration to that of the large-eddy simulation (LES), discarding cases for which the MLM internally predicts decoupling based on the criterion BIR>0.2
We find that for a well-mixed stratocumulus-topped boundary layer, an LES and a suitably configured mixed layer model predict a similar decrease of cloud liquid water path as droplet concentration increases
Summary
The indirect effect of anthropogenic aerosol on clouds and thereby on the global radiation balance remains a key uncertainty in climate modeling and prediction. The cloud droplet number, cloud fraction and liquid water path (LWP) of subtropical marine stratocumulus (Sc) cloud decks respond to changes in aerosol in ways global models struggle to represent, because they involve a subtle interplay between turbulence, vertical structure, entrainment and evaporating precipitation. Intermediate aerosol concentrations can support fully cloud-covered but drizzling boundary layers in which subcloud cooling from evaporating drizzle inhibits turbulent mixing and reduces entrainment of dry air, supporting stratocumulus with a high liquid water path, e.g. Lu and Seinfeld (2005). High aerosol concentrations produce small cloud droplets that do not sediment out of the entrainment zone, promoting mixing-induced evaporative cooling that enhances cloud top entrainment and can thin the Sc layer (Ackerman et al, 2004; Bretherton et al, 2007). Ship or volcano tracks of enhanced aerosol concentration and reduced droplet effective radius in nonprecipitating Sc often appear to have lower LWP than the surrounding
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
