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
Received: 05 November 2009 – Published in Atmos. Chem. Phys. Discuss.: 02 December 2009 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.
Revised: 16 April 2010 – Accepted: 17 April 2010 – Published: 03 May 2010
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.
Citation: Uchida, J., Bretherton, C. S., and Blossey, P. N.: The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree?, Atmos. Chem. Phys., 10, 4097-4109, doi:10.5194/acp-10-4097-2010, 2010.