Key uncertainties to model CCN
Climate models have not been able to calculate cloud condensation nuclei concentrations explicitly. We identify key parameters in the GLOMAP model that contribute to uncertainty in modelled CCN concentrations. This will help us understand the important processes that need to be treated in climate models to correctly calculate CCN and the aerosol indirect effect.
The figure below shows the sensitivity of model CCN to different parameters. Predicted global mean CCN concentrations change by 10–60% when several microphysical processes are changed within reasonable uncertainty ranges. CCN concentrations are sensitive to realistic uncertainties in the in-cloud activation diameter, primary anthropogenic emissions, sea-spray flux and wet scavenging. Uncertainties in the binary H2SO4-H2O nucleation rate have a relatively insignificant effect on CCN concentrations. Read the paper here
Percentage change in air-mass weighted monthly mean CCN (at 0.2% supersaturation) concentrations during December 1995 for changes in model parameters. Thick bars show global means and thin bars show means over the oceans only. The model parameters were varied (left to right) as follows: activation diameter for in-cloud oxidation – 0.04, 0.06, 0.08μm (baseline model run 0.05 μm); accommodation coefficient – 0.65, 0.3, 0.02 (baseline 1.0); H2SO4 nucleation threshold 2 and 0.5 times the baseline; nucleation critical cluster size – 50 and 10 molecules (baseline 100 molecules H2SO4); particulate emissions – 1, 3 and 5% of anthropogenic SO2 (baseline 0%); cloud scavenging activation diameter – 0.1μm (baseline 0.206μm); sulfur emissions
– 75% and 125% of the baseline; sea spray emissions – 10 and 0.1
times the baseline flux.