The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree?
1Department of Applied Mathematics, University of Washington, Seattle, Washington, USA
2Department of Atmospheric Science, University of Washington, Seattle, Washington, USA
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), to see if the two model types agree on the strength of the second aerosol indirect effect. Good agreement can be obtained if the MLM entrainment closure explicitly reduces entrainment efficiency proportional to the rate of cloud droplet sedimentation at cloud top for cases in which the LES-simulated boundary layer remains well mixed, with a single peak in the vertical profile of vertical velocity variance.
To achieve this agreement, the MLM entrainment closure and the drizzle parameterization must be modified from their observationally-based defaults. 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 similar 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 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 MLM-predicted sensitivity to droplet concentrations.