The results show a significant response to varying aerosol amounts. Specifically, precipitation is found to decrease on the windward slopes and increase on the leeward slopes. This effect is most evident in the southern and western region of the San Juan Range where high moisture-laden storms are more prevalent. The aerosol-induced downwind precipitation shift over the Park Range is also present in each simulated season, but with lower amplitudes and slightly varying magnitudes among seasons. Seasons with greater overall snowfall exhibit a greater aerosol response in terms of both the magnitude of the change in total snow water equivalent as well as percentage change. The results are consistent with the findings of Saleeby et al. (2009) wherein higher CCN concentrations reduce ice particle riming rates and thus alter the seeder-feeder process. This effect requires the coexistence of a supercooled orographic cloud and snow falling from above and upwind. Snow crystals falling through a polluted cloud acquire very little rime, remain low density, have slower fall speeds, and thus have trajectories that more readily carry them toward lee slopes. Likewise, snow falling through a clean cloud may acquire substantial rime, increase in density, and transition into a graupel-like particle with higher fall speeds, reduced horizontal advection, and deposition on windward slopes. The shift in snow deposition due to aerosol loading can have serious hydrologic implications as snowpack is redistributed into neighboring river basins.
Decreasing the grid spacing from 3km to 1km in the simulations over the San Juan region leads to better resolution of the orographic cloud and topographic features. The 1km spacing simulations resulted in a 1.5% increase in resolved snowfall as well as a greater response to increases in CCN concentration. The 1km simulations experienced an increase in the areal impact and magnitude of the aerosol effect, likely due to a better resolved seeder-feeder effect along the major mountain ridges where orographic clouds are most prevalent.