The Cumulative Impact of Cloud Droplet Nucleating Aerosols on Orographic Snowfall in Colorado

Hygroscopic pollution aerosols have the potential to alter winter orographic snowfall totals and spatial distributions by modification of high-elevation supercooled orographic clouds and the riming process. The authors investigate the cumulative effect of varying the concentrations of hygroscopic aerosols during January-February for four recent winter snowfall seasons over the high terrain of Colorado. Version 6.0 of the Regional Atmospheric Modeling System (RAMS) is used to determine the particular mountain ranges and seasonal conditions that are most susceptible. Multiple winter seasonal simulations are run at both 3- and 1-km horizontal grid spacing with varying aerosol vertical profiles. Model-predicted snowfall accumulation trends are compared with automated snow water equivalent observations at high-elevation sites. An increase in aerosol concentration leads to reduced riming of cloud water by ice particles within supercooled, liquid orographic clouds, thus leading to lighter rimed hydrometers with slower fall speeds and longer horizontal trajectories. This effect results in a spillover of snowfall from the windward slope to the leeward slope. A snowfall spillover effect is most evident in the southern and western regions of the San Juan Range where high-moisture-laden storms are more prevalent. The effect 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 response in magnitude and percentage change. The smallest spillover effect occurred downwind of the primary western slope mountain barriers. Although the aerosol effect on snowfall can be locally significant in particularly wet winter seasons, the interseasonal variability in synoptic conditions can impose much larger widespread changes in snowfall accumulation.