Atmos. Chem. Phys. Discuss., 12, 24205-24241, 2012
www.atmos-chem-phys-discuss.net/12/24205/2012/
doi:10.5194/acpd-12-24205-2012
© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
The effects of timing and rate of marine cloud brightening aerosol injection on albedo changes during the diurnal cycle of marine stratocumulus clouds
A. K. L. Jenkins, P. M. Forster, and L. S. Jackson
School of Earth and Environment, University of Leeds, Leeds, UK

Abstract. The marine-cloud brightening geoengineering technique has been suggested as a~possible means of counteracting the positive radiative forcing associated with anthropogenic atmospheric CO2 increases. The focus of this study is to quantify the albedo response to aerosols injected into marine stratocumulus cloud from a point source at different times of day. We use a cloud-resolving model to investigate both weakly precipitating and non-precipitating regimes. Injection into both regimes induces a first indirect aerosol effect. Additionally, the weakly precipitating regime shows evidence of the second indirect aerosol effect and leads to cloud changes indicative of a regime change to more persistent cloud. This results in a cloud albedo increase up to six times larger than in the non-precipitating case. These indirect effects show considerable variation with injection at different times in the diurnal cycle. For the weakly precipitating case, aerosol injection results in average increases in cloud albedo of 0.28 and 0.17 in the early and mid morning (03:00:00 local time (LT) and 08:00:00 LT, respectively) and 0.01 in the evening (18:00:00 LT). No cloud develops when injecting into the cloud-free day (13:00:00 LT). However, the all-sky albedo increases (which include both the indirect and direct aerosol effects) are highest for early morning injection (0.11). Mid-morning and daytime injections produce increases of 0.06, with the direct aerosol effect compensating for the lack of cloud albedo perturbation during the cloud-free day. Evening injection results in an increase of 0.04. Penetration and accumulation of aerosols above the cloud top may lead to a reduction of all-sky albedo that tempers the cloud albedo increases. The apparent direct aerosol tempering effect increases with injection rate, although not enough to overcome the increase in all-sky planetary albedo resulting from increases in cloud albedo. For the weakly precipitating case considered, the optimal injection time for planetary albedo response is the early morning. Here, the cloud has more opportunity develop into a more persistent non-precipitating regime prior to the dissipative effects of solar heating. The effectiveness of the sea-spray injection method is highly sensitive to diurnal injection time and the direct aerosol effect of an intense aerosol point source. Studies which ignore these factors could overstate the effectiveness of the marine cloud brightening technique.

Citation: Jenkins, A. K. L., Forster, P. M., and Jackson, L. S.: The effects of timing and rate of marine cloud brightening aerosol injection on albedo changes during the diurnal cycle of marine stratocumulus clouds, Atmos. Chem. Phys. Discuss., 12, 24205-24241, doi:10.5194/acpd-12-24205-2012, 2012.
 
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