Global simulations of aerosol processing in clouds C. Hoose1,*, U. Lohmann1, R. Bennartz2, B. Croft3, and G. Lesins3 1ETH Zurich, Institute for Atmospheric and Climate Science, Zurich, Switzerland 2Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI, USA 3Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada *now at: Department of Geosciences, University of Oslo, Norway
Abstract. An explicit and detailed representation of in-droplet and in-crystal
aerosol particles in stratiform clouds has been introduced in the global
aerosol-climate model ECHAM5-HAM. The new scheme allows an evaluation of the cloud
cycling of aerosols and an estimation of the relative contributions of nucleation and
collision scavenging, as opposed to evaporation of hydrometeors in the global aerosol
processing by clouds. On average an aerosol particle is cycled through stratiform clouds 0.5 times.
The new scheme leads to important changes in the simulated fraction of aerosol scavenged in clouds,
and consequently in the aerosol wet deposition. In general, less aerosol is scavenged into clouds with the
new prognostic treatment than what is prescribed in standard ECHAM5-HAM. Aerosol concentrations, size distributions,
scavenged fractions and cloud droplet concentrations are evaluated and compared to different observations.
While the scavenged fraction and the aerosol number concentrations in the marine boundary layer are well
represented in the new model, aerosol optical thickness, cloud droplet number concentrations in the
marine boundary layer and the aerosol volume in the accumulation and coarse modes over the oceans are
overestimated. Sensitivity studies suggest that a better representation of below-cloud scavenging,
higher in-cloud collision coefficients, or a reduced water uptake by seasalt aerosols could
reduce these biases.
Citation: Hoose, C., Lohmann, U., Bennartz, R., Croft, B., and Lesins, G.: Global simulations of aerosol processing in clouds, Atmos. Chem. Phys., 8, 6939-6963, doi:10.5194/acp-8-6939-2008, 2008.