Marine Cloud Brightening

Abstract

Due to the insufficient worldwide efforts to reduce greenhouse gas emissions and related global warming, marine cloud brightening (MCB) is gaining interest as an instrument to artificially lower Earth's temperature. MCB is based on exploiting the aerosol-cloud effects to enhance the albedo of stratocumulus clouds and prolong their lifetime by injecting clouds with sea salt aerosols. As there are still many uncertainties in the expected output of MCB due to the unresolved questions related to cloud processes and feedback mechanisms, this thesis aims to assess the efficacy of MCB to enhance solar radiation reflection of stratocumulus clouds using the turbulence-resolving Dutch Atmospheric Large-Eddy Simulation model. This assessment is made by investigating which cloud properties and processes that determine the cloud's radiative forcing are affected by the aerosol injection for different meteorological conditions and injection strategies in 30-hour simulations. The simulation domain has a horizontal size of 25.2 $ imes$ 25.2 km$^2$ and 2 km in the vertical, with a mesh size of 100 m horizontally and 20 m vertically. The two studied cloud cases are based on the measurements of the first and second research flights of the DYCOMS-II field experiment and are characteristic for a shallow stratocumulus-topped boundary layer (STBL). The investigated surface aerosol injection strategies are a horizontally uniform source and a point source that is restricted to a single grid cell. In addition, an assessment is made of the effects of differences in aerosol number concentrations between the STBL and the free troposphere on the cloud's aerosol number concentration and radiative properties. This is done in 6-hour simulations with and without a uniform aerosol source, based on the second research flight of DYCOMS-II. The simulated horizontal domain is 3.2 $ imes$ 3.2 km$^2$ and 2 km in the vertical, with a mesh size of 25 m horizontally and 20 m vertically. Our simulations showed that boundary layers with relatively low background aerosol concentrations were most effective in generating a negative radiative forcing, compensating slightly less than half of the forcing related to the CO$_2$ doubling in the atmosphere. For boundary layers with an average background aerosol concentration, the radiative forcing approaches a quarter of this value, which diminishes to negligible effects for boundary layers with relatively high background aerosol concentrations. For precipitating boundary layers, the enhanced radiative forcing is mainly caused by its suppressing effect on precipitation. For non-precipitating boundary layers, the reduction in cloud droplet size showed to be the primary source of the enhancement. No pronounced differences in efficacy were found when using a homogeneously distributed aerosol source or a point source. For the two studied cloud cases, investigation of the contributions to changes in the liquid water path showed that to exploit the effect of aerosol injection on the liquid water path, it is most effective to suppress precipitation and enhance cloud cover. The differences in aerosol number concentration between the STBL and the free troposphere significantly and consistently altered the aerosol number concentration in the cloud layer due to the entrainment of air from just above the cloud-top into the cloud layer. This indicates that these differences in aerosol number concentration need to be considered when modelling MCB, as they change the intended aerosol number concentration enhancement and thereby affect their cloud-modifying effects.