Reduced efficacy of marine cloud brightening geoengineering due to in-plume aerosol coagulation: parameterization and global implications 1Physics and Atmospheric Science, Dalhousie University, Halifax, NS, Canada
25 Oct 2013
2School of Earth and Environment, University of Leeds, Leeds, UK
3Kuopio Unit, Finnish Meteorological Institute, Kuopio, Finland
4Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA
Received: 07 June 2013 – Published in Atmos. Chem. Phys. Discuss.: 12 July 2013 Abstract. The intentional enhancement of cloud albedo via controlled sea-spray
injection from ships (marine cloud brightening) has been proposed as a
possible method to control anthropogenic global warming; however, there
remains significant uncertainty in the efficacy of this method due to,
amongst other factors, uncertainties in aerosol and cloud microphysics. A
major assumption used in recent cloud- and climate-modeling studies is that
all sea spray was emitted uniformly into some oceanic grid boxes, and thus
these studies did not account for subgrid aerosol coagulation within the
sea-spray plumes. We explore the evolution of these sea-salt plumes using a
multi-shelled Gaussian plume model with size-resolved aerosol coagulation.
We determine how the final number of particles depends on meteorological
conditions, including wind speed and boundary-layer stability, as well as
the emission rate and size distribution of aerosol emitted. Under previously
proposed injection rates and typical marine conditions, we find that the
number of aerosol particles is reduced by over 50%, but this reduction
varies from under 10% to over 90% depending on the conditions. We
provide a computationally efficient parameterization for cloud-resolving and
global-scale models to account for subgrid-scale coagulation, and we
implement this parameterization in a global-scale aerosol-climate model.
While designed to address subgrid-scale coagulation of sea-salt particles,
the parameterization is generally applicable for coagulation of
subgrid-scale aerosol from point sources. We find that accounting for this
subgrid-scale coagulation reduces cloud droplet number concentrations by
46% over emission regions, and reduces the global mean radiative flux
perturbation from −1.5 W m−2 to −0.8 W m−2.
Revised: 25 September 2013 – Accepted: 26 September 2013 – Published: 25 October 2013
Citation: Stuart, G. S., Stevens, R. G., Partanen, A.-I., Jenkins, A. K. L., Korhonen, H., Forster, P. M., Spracklen, D. V., and Pierce, J. R.: Reduced efficacy of marine cloud brightening geoengineering due to in-plume aerosol coagulation: parameterization and global implications, Atmos. Chem. Phys., 13, 10385-10396, doi:10.5194/acp-13-10385-2013, 2013.