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| Titel |
Sensitivity to deliberate injections of sea salt into marine clouds - observations and model simulations |
| VerfasserIn |
Kari Alterskjær, Jón Egill Kristjánsson |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054826
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| Zusammenfassung |
| Sea salt seeding of marine clouds to increase their albedo is a proposed technique to
counteract or slow global warming. The objectives of this study are (i) to find areas where the
global albedo is influenced the most by changes in the amount of sea salt or cloud
condensation nuclei, (ii) to study whether these areas are reproduced by a leading climate
model, and (iii) to study the magnitude and the location of forcing resulting from increased
sea salt concentrations through simulated sensitivity experiments. The study was based both
on results from a global climate model (NorESM) and on satellite observations
of cloud droplet number concentration (CDNC), cloud optical depth, and liquid
cloud fraction from the MODIS (Moderate Resolution Imaging Spectroradiometer)
instruments.
We created a susceptibility function which is based on cloud albedo, CDNC and the solar
zenith angle to map areas that are sensitive to particle number increase. The susceptibility is
large for intermediate values of cloud albedo, small CDNC and low solar zenith
angles. Combining this function with data on cloud fraction, we find areas where an
increase in particle number should influence the global albedo the most. Results
compare well between simulations and observations and show that stratocumulus
regions off the west coasts of the major continents along with large regions in the
Pacific and the Indian oceans are sensitive. However, the results vary significantly
depending on what cloud fraction is used in combination with the susceptibility
function.
The sensitivity experiments included a uniform increase of 1.0e-09 kg m-2 s-1 in the
emissions of 0.13 μm radius sea salt over ocean, an increased emission rate considered
plausible in recent publications. The increased sea salt concentrations and the resulting
change in marine cloud properties due to aerosol-cloud interactions led to a globally averaged
forcing of -5.9 Wm-2 at the top of the atmosphere. By comparison, a doubling of
atmospheric CO2 concentrations has a forcing of approximately 3.8 Wm-2. The magnitude
of the forcing from increased cloud reflectivity may therefore be sufficient to counteract
global warming, depending on future greenhouse gas concentrations. The sensitivity
experiments also show that the forcing is large in areas found to be sensitive by using
the susceptibility function, especially in the region between 30Ë south and 30Ë
north. The susceptibility function therefore seems to be a good proxy for the areas
where seeding of marine clouds would have the largest impact on the global albedo. |
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