Can Anthropogenic Aerosols Decrease the Snowfall Rate?

Observations by Borys, Lowenthal, Cohn, and Brown in midlatitude orographic clouds show that for a given supercooled liquid water content, both the riming and the snowfall rates are smaller if the supercooled cloud has more cloud droplets as, for example, caused by anthropogenic aerosols. The climatic implication of this effect was studied in global climate model simulations by replacing the constant riming efficiency with a size-dependent one appropriate for planar crystals and aggregates, respectively. In the model simulations that use a size-dependent riming collection efficiency, the pollution-induced decrease in cloud droplet size causes a decrease in the riming rate in stratiform clouds despite larger liquid water contents in polluted clouds. Contrary to the above-mentioned observations, in all model simulations the snowfall rate increases because of feedbacks in the climate system. Anthropogenic aerosol particles increase the aerosol and cloud optical thickness, which reduces the solar radiation at the top of the atmosphere and the surface. This in turn causes a cooling in Northern Hemisphere midlatitudes that favors precipitation formation via the ice phase.