Geoengineering marine stratocumulus clouds

Marine cloud brightening (MCB) geoengineering has been proposed as a means of ameliorating anthropogenic climate change. High concentrations of nanometre-sized aerosols would be emitted from seagoing vessels, with the intention of increasing the albedo of low-lying marine stratocumulus clouds (MSc) via indirect aerosol effects. Realistic estimates of the potential effectiveness of MCB are needed to inform policy-making on climate change. However, in spite of increasing model complexity and developments in representing MCB, the relatively coarse resolution of global-scale models prevents implementation details from being captured. This work identified three previously unrepresented implementation details, and examined their importance in achieving realistic estimates of MCB effectiveness. For this, the Weather Research and Forecasting model incorporating aerosol processes (WRF/Chem) was used, allowing clouds to be resolved over a kilometre-scale domain. Firstly, for a weakly precipitating cloud regime, cloud brightening was found to be sensitive to the timing of MCB aerosol emissions. The largest cloud albedo increase occurred for early morning emissions, with little change occurring for daytime emissions. Timing was less important for the non-precipitating regime where cloud albedo perturbations were generally smaller owing to the absence of a large second indirect (or ‘cloud lifetime’) effect. Secondly, near-surface evaporative cooling resulting from the more realistic simulation of MCB emissions as wet droplets rather than the previously assumed dry aerosols reduced aerosol plume heights by up to 30% (40 m), reducing cloud albedo increases by up to one-third. Finally, aerosol coagulation within the MCB aerosol plume (simulated at sub-metre resolution) resulted in number concentration decreases of up to 50%, consequentially reducing the cloud albedo increases by approximately half for the non-precipitating regime. These findings suggest that in omitting these details, global-scale model estimates of MCB effectiveness may be exaggerated. The inclusion of these details in global-scale MCB modelling could therefore materially improve the realism of future effectiveness estimates.