INVITED ABSTRACT
Efficacy of Sea Spray Geoengineering: Influence of Emission Rate, Microphysics and Transport

HANNELE KORHONEN (1,2), Antti-Ilari Partanen (2), Kenneth S. Carslaw (3), Sami Romakkaniemi (2)

(1) Finnish Meteorological Institute, Kuopio Unit (2) University of Eastern Finland, Kuopio (3) University of Leeds, United Kingdom

     Abstract Number: 219
     Last modified: April 30, 2010

Abstract
Previous global studies of sea spray geoengineering have imposed high, homogeneous cloud drop concentration in low level clouds to explore the radiative and climatic effects. We use the global CTM GLOMAP and the climate model ECHAM5-HAM to simulate explicitly the flux and atmospheric microphysics of the emitted sea spray particles and to quantify how they affect the natural aerosol processes, the particle size distribution, and concentrations of cloud drops. The model results show that fractional changes in cloud drop number concentration (CDNC) vary substantially between different regions because of differences in wind speed (which affects the spray efficiency of the vessels), cloud altitude, transport and particle deposition rates, and because of variations in aerosols from anthropogenic sources. Using spray emission rates comparable to those implied by previous studies we find that the predicted CDNC changes are very small (~10-20%) and in some regions even negative. This is because only a fraction of the emitted particles are transported to cloud altitude and because the added particles suppress the in-cloud supersaturation and prevent existing aerosol particles from forming cloud drops. A scenario with five times higher emissions (considerably higher than previously assumed) doubles the CDNC in some regions but median concentrations are still below the 375 cm-3 assumed in previous studies. Seeding ~13% of the Earth’s surface, we predict global radiative forcings of -0.4 W/m2 for the baseline and -3.1 W/m2 for the 5 times higher scenario, of which direct forcing accounts for 73% and 46%, respectively. Next we will use large eddy models with two-moment aerosol description to investigate the small scale processes crucial to the efficacy of cloud seeding (e.g., dispersion, transport to clouds) and the sensitivity to atmospheric conditions (boundary layer height, shear at cloud top, inversion strength, subsidence rate).