Aerosol sources and marine stratocumulus during VOCALS-REx in a WRF/Chem Large Eddy Simulation

We present simulations and observations investigating the role of sea salt emissions and nucleation from the gas phase as sources of CCN in the marine boundary layer, and their contributions to the maintenance of stratocumulus clouds. The measurements and model inputs are from VOCALS-REx (VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment).
Collision-coalescence and drizzle in stratocumulus clouds reduce the number of cloud condensation nuclei (CCN). In the absence of aerosol sources, the cloud drop number concentration will therefore decline and the cloud thin, resulting in a reduction of cloud top cooling and a possible decoupling of the boundary layer, accompanied by a distinct change in cloud microphysical and radiative properties. Aerosol sources, such as surface emissions or nucleation from the gas phase, however, replenish the aerosol population and provide CCN for maintenance of the cloud.
We have coupled, in detail, aerosol processes, cloud microphysics, and gas and aqueous chemistry in the WRF/Chem model, and operate it in Large Eddy Simulation mode. Aerosol formation from the gas phase is described with a sulfuric acid/water scheme based on laboratory measurements of the nucleation process, and sea salt emissions with a parametrization of field measurements. We highlight the importance of aerosol sources for the maintenance of the cloudy marine boundary layer and infer the relative contributions of nucleation and surface emissions.