Impact of Seawater Biological Activity on Sea Spray Aerosols (SSA) Number Concentration and Cloud Properties in WRF-Chem—Case Studies on the Southern Ocean

Abstract

Sea spray is a large source of cloud condensation nuclei (CCN) over remote oceans, still the dependence of its emission number flux on marine biology is not often represented in models and is subject to high uncertainty. We implemented a recently developed parameterization which relates the number flux of sea spray aerosols to nanophytoplankton cell abundances in the mesoscale model WRF-Chem and evaluated the impact on the prediction of sea spray aerosols (SSA) number concentrations and cloud properties over the Southern Ocean. Compared to SSA concentrations simulated using the whitecap function available in the model, the use of a function derived from recent observations in the Southern Ocean leads to a decrease in predicted SSA number concentrations (by ∼46%–47% over the domain); this further translates into a decrease in the cloud droplet number concentrations and a decrease (respectively an increase) of the liquid cloud water path (CWP) and rain water path (RWP). A strong increase in SSA number concentrations is predicted when the influence of marine biota is taken into account (by 172%–343%), with a concurrent clear increase of the number of cloud droplets and a subsequent increase (decrease) of the CWP (RWP). Detailed comparison of model outputs and measurements further demonstrates that accounting for the role of marine biological activity is essential to predict more realistic SSA number concentrations and spatio-temporal variability. Comparisons of modeled vertical profiles of CWP and RWP with remote sensing measurements are also ameliorated with the account of marine biology.