Susceptibility of marine clouds to emission increases - observations and model
simulations
Kari Alterskjær1 and Jón Egill Kristjánsson1
1
: Department of Geosciences, Meteorology and Oceanography Section, University of Oslo,
Oslo, Norway
The aerosol indirect effect and the treatment of this effect in climate models are much
debated. The objective of this study is to find a measure of the radiative susceptibility of
clouds to changes in the concentration of cloud condensation nuclei (CNN). We investigate
the susceptibility of marine clouds to changes in the CCN concentration, using observational
data of cloud droplet number concentration (CDNC), cloud optical depth, and liquid cloud
fraction from the MODIS (Moderate Resolution Imaging Spectroradiometer) instruments. We
then compare this derived susceptibility to a corresponding estimate from the Norwegian
Earth System Model (NorESM). Results are somewhat dependent on the time resolution, but
compare well between simulations and observations. Stratocumulus regions off the west
coasts of the major continents along with large regions in the Pacific and the Indian oceans
are sensitive.
We then carried out sensitivity experiments to validate the derived function. Uniformly
increasing emissions of 0.13 µm radius sea salt over ocean by 1.0e-09 kg m-2 s-1 led to
changes in marine cloud properties due to aerosol-cloud interactions and a globally averaged
forcing of -4.8 Wm-2 at the top of the atmosphere. The forcing is large in areas found to be
sensitive by using the susceptibility function, confirming its usefulness an indicator of
radiative susceptibility to changes in CCN concentration.
We propose an AeroCom intercomparison effort on cloud susceptibility as a useful proxy for
the aerosol indirect effect.