Numerical Simulations of Stratocumulus Cloud Response to Aerosol Perturbations

Mirek Andrejczuk (1), Laura Stevens (1), Ben Parkes (1), ALAN GADIAN (1), John Latham (2) and Alan Blyth (1)

(1) Environment, University of Leeds, UK (2) NCAR, MMM Division, Boulder

     Abstract Number: 333
     Last modified: May 7, 2010

Abstract
Geoengineering of maritime stratocumulus clouds to increase their albedo
is proposed as a method to offset global warming. Idealized climate model
simulations indicate that this approach may delay global warming by up to
50 years. However cloud-aerosol interactions are not fully understood,
and their representation in climate models is over-simplified, leading to
significant uncertainty in climate model predictions.

Problems exist with quantifying aerosol
distribution/composition/concentration leading to a cloud droplet number /
albedo relation. This applies even to higher resolution models with more
sophisticated microphysics. Stratocumulus clouds are especially difficult
to model because they are often long-lived and affected, both locally and
globally, by aerosol. Although there are indications that cloud seeding
may affect cloud albedo based on results from 0D models, assumptions made
in this type of model about homogeneity and the effect of dynamics are
neglected and may affect the solution.

Herein, a new approach to cloud microphysics, represented in a Lagrangian
framework with two-way coupling between Lagrangian parcels and Large Eddy
Simulations, model dynamics and thermodynamics will be discussed. Results
from this model will be presented and compared with observations from the
VOCALS field campaign. Model response to aerosol perturbations and its
effect on cloud albedo will be shown for cases with high and low cloud
droplet concentration. The results show that cloud albedo response depends on the initial cloud droplet concentrations in the cloud. Adding aerosol may increase albedo for the MED (120/cm^3) and LOW (65/cm^3) cases up to 10% depending on additional aerosol concentration; for the HIGH (250/cm^3) case, adding aerosol has little effect on albedo.

In addition, field results will be considered in relation to climate model
simulations, using the UK Meteorological Office HadGEM1 climate model: and
results will be shown from a more standard Meteorological Office Large
Eddy Simulation model, initialised with VOCALS data.