Journal article

Droplet number uncertainties associated with CCN: An assessment using observations and a global model adjoint

Moore R, Karydis V, Capps S, Lathem T, Nenes A ...see all

Atmospheric Chemistry and Physics, vol. 13, issue 8 (2013) pp. 4235-4251

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We use the Global Modelling Initiative (GMI) chemical transport model
with a cloud droplet parameterisation adjoint to quantify the
sensitivity of cloud droplet number concentration to uncertainties in
predicting CCN concentrations. Published CCN closure uncertainties for
six different sets of simplifying compositional and mixing state
assumptions are used as proxies for modelled CCN uncertainty arising
from application of those scenarios. It is found that cloud droplet
number concentrations (N-d) are fairly insensitive to the number
concentration (N-a) of aerosol which act as CCN over the continents
(partial derivative lnN(d)/partial derivative lnN(a) similar to 10-30
%), but the sensitivities exceed 70% in pristine regions such as the
Alaskan Arctic and remote oceans. This means that CCN concentration
uncertainties of 4-71% translate into only 1-23% uncertainty in cloud
droplet number, on average. Since most of the anthropogenic indirect
forcing is concentrated over the continents, this work shows that the
application of Kohler theory and attendant simplifying assumptions in
models is not a major source of uncertainty in predicting cloud droplet
number or anthropogenic aerosol indirect forcing for the liquid,
stratiform clouds simulated in these models. However, it does highlight
the sensitivity of some remote areas to pollution brought into the
region via long-range transport (e. g., biomass burning) or from
seasonal biogenic sources (e. g., phytoplankton as a source of
dimethylsulfide in the southern oceans). Since these transient processes
are not captured well by the climatological emissions inventories
employed by current large-scale models, the uncertainties in
aerosol-cloud interactions during these events could be much larger than
those uncovered here. This finding motivates additional measurements in
these pristine regions, for which few observations exist, to quantify
the impact (and associated uncertainty) of transient aerosol processes
on cloud properties.

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