Evaluating the Impact of Saltwater Spray Strategies and Evaporative Cooling on Marine Cloud Brightening Effectiveness

Marine cloud brightening (MCB) is a suggested climate intervention technique that involves intentionally introducing aerosol particles into shallow liquid clouds to enhance their reflectivity (albedo) and prolong their lifespan. To evaluate its feasibility, the cloud-resolving Dutch Atmospheric Large Eddy Simulation (DALES) model is adapted to assess the impact of saltwater spraying on moistening and cooling of the atmosphere, in addition to enhancing the concentration of cloud condensation nuclei. The EUROCS model intercomparison case, which was based on measurements collected as part of the FIRE measurement campaign in a marine subtropical stratocumulus region off the coast of California, an area considered to be ideal for MCB efforts, serves as a case study. We focus on the effects of meteorological conditions (wind speed, humidity, and temperature), different spray strategies and evaporative cooling on horizontal plume dispersion and lofting time. Informed by a Lagrangian cloud model, the effect of salt crystal activation and background cloud droplet number concentration on enhancing reflectivity are investigated. Key findings include the formation of Gaussian humid rising plumes and kilometer-scale evaporative cooling. The evaporative cooling slows down the transport of salt to cloud base and gives rise to lower mean salt mass concentrations in comparison to simulations in which the evaporative cooling effect is not considered. Simulations show an increase in cloud reflectivity under different spraying strategies and background aerosol concentrations. Furthermore, to capture cloud feedback effects at time scales of multiple days, a "Lagrangian" modeling approach is applied, in which a cloudy air mass is followed as it is advected by the mean wind.