The potential impacts of pollution on a nondrizzling stratus deck: Does aerosol number matter more than type?

In this paper results from a cloud-resolving model that can efficiently examine the impact of aerosol on nondrizzling stratus clouds will be shown. Because the model tracks aerosol and cloud droplets in a Lagrangian framework, it does not suffer from numerical errors associated with advection, and unlike most Eulerian approaches, the method can track cloud boundaries as they move across a grid cell. After illustrating the capability of the model to reproduce various observed cloud statistics such as the cloud water mixing ratio and the mean cloud droplet radius from the DYCOMS-II field program, the ability of the model to assess the impact of changes in aerosol number and composition on a stratus deck will be highlighted. Specifically, by using activation curves appropriate for soluble, insoluble, or a mixture of both types of aerosol and for certain extreme aerosol regimes, i.e., a majority of the aerosol are hydrophobic carbon aerosol, limiting situations were examined to bound their impact on clouds. However, though these situations may be somewhat extreme, they could occasionally occur in the atmosphere, e. g., an oceanic stratus field downwind of a large ship or an urban area. Not unexpectedly, results from these simulations support previous ship track observations that for increasing aerosol numbers, cloud droplet number concentrations increase, whereas cloud droplet radii decrease. However, these simulations also suggest that the correlation between cloud droplet number concentration and aerosol number concentration may be not only a function of aerosol number concentration but also aerosol types and/or cloud dynamics.