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| Titel |
A multi-model assessment of the impact of sea spray geoengineering on cloud droplet number |
| VerfasserIn |
K. J. Pringle, K. S. Carslaw, T. Fan, G. W. Mann, A. Hill, P. Stier, K. Zhang, H. Tost |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 23 ; Nr. 12, no. 23 (2012-12-06), S.11647-11663 |
| Datensatznummer |
250011652
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| Publikation (Nr.) |
 copernicus.org/acp-12-11647-2012.pdf |
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| Zusammenfassung |
| Artificially increasing the albedo of marine boundary layer clouds by the
mechanical emission of sea spray aerosol has been proposed as a
geoengineering technique to slow the warming caused by anthropogenic
greenhouse gases. A previous global model study (Korhonen et al., 2010) found that
only modest increases (< 20%) and sometimes even decreases in cloud
drop number (CDN) concentrations would result from
emission scenarios calculated using a windspeed dependent geoengineering flux parameterisation. Here we extend that work to examine the
conditions under which decreases in CDN can occur, and use three independent
global models to quantify maximum achievable CDN changes. We find that
decreases in CDN can occur when at least three of the following conditions
are met: the injected particle number is < 100 cm−3, the injected
diameter is > 250–300 nm, the background aerosol loading is large
(≥ 150 cm−3) and the in-cloud updraught velocity is low
(< 0.2 m s−1). With lower background loadings and/or increased
updraught velocity, significant increases in CDN can be achieved. None of the
global models predict a decrease in CDN as a result of geoengineering,
although there is considerable diversity in the calculated efficiency of
geoengineering, which arises from the diversity in the simulated marine
aerosol distributions. All three models show a small dependence of
geoengineering efficiency on the injected particle size and the geometric
standard deviation of the injected mode. However, the achievability of
significant cloud drop enhancements is strongly dependent on the cloud
updraught speed. With an updraught speed of 0.1 m s−1 a global mean
CDN of 375 cm−3 (previously estimated to cancel the forcing caused by
CO2 doubling) is achievable in only about 50% of grid boxes which
have > 50% cloud cover, irrespective of the amount of aerosol
injected. But at stronger updraft speeds (0.2 m s−1), higher values of
CDN are achievable due to the elevated in-cloud supersaturations. Achieving a
value of 375 cm−3 in regions dominated by stratocumulus clouds with
relatively weak updrafts cannot be attained regardless of the number of
injected particles, thereby limiting the efficacy of sea spray
geoengineering. |
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