Effects of sea-salt aerosols on precipitation in Trade-Wind Cumulus

It is assumed that stronger winds result in more wave breaking and injection of sea salt aerosols into the atmosphere. Large-eddy simulations with bin microphysics were used to assess how sea-salt nuclei from the ocean surface influence precipitation in trade cumulus. The baseline simulation using observations from RICO campaign contained a background CCN spectrum of predominantly submicron aerosol. Sensitivity experiments explored the effects of adding sea-salt nuclei in different size ranges and showed that addition of large (small) size sea-salt nuclei tends to accelerate (suppress) precipitation. However, when realistic spectra are specified as a function of surface wind, the effect of the larger nuclei to enhance precipitation predominates, and accumulated precipitation increases with wind speed. This effect, however, is strongly influenced by the choice of background CCN spectrum. The same specification of sea-salt spectra, but in an environment with a higher background aerosol load, resulted in a decrease in accumulated precipitation with increasing surface wind speed. Results also suggest that the slope of the relationship between vertical velocity and the concentration of embryonic precipitation particles at cloud base may indicate the role of sea-salt nuclei. A negative slope indicates a predominance of small sea-salt nuclei, in which larger updrafts activate a greater number of smaller cloud drops with smaller coagulation efficiencies, resulting in fewer embryonic rain drops. A positive slope indicates the presence of large sea-salt nuclei. These large sea-salt particles are the source of large cloud drops, which grow to embryonic rain drops by condensation and coagulation. The research results indicate the most essential aerosol parameters needed for parameterizing aerosol effects in meso and large scale models.