Aerosol Impacts on the Microphysical Processes of Orographic Snow Growth

Cloud nucleating aerosols have been found to modify the amount and spatial distribution of snowfall in mountainous areas where riming growth of snow crystals contributes substantially to the total snow water equivalent precipitation. In the Park Range of Colorado, a 2km deep supercooled liquid water orographic cloud frequently enshrouds the mountaintop during snowfall events. This leads to a seeder-feeder growth regime in which snow falls through the orographic cloud and collects cloud water prior to surface deposition. The addition of higher concentrations of cloud condensation nuclei (CCN) modifies the cloud droplet spectrum toward smaller size droplets and suppresses riming growth. Without rime growth, the density of snow crystals remains low and horizontal trajectories carry them further downwind due to slower vertical fall speeds. This leads to a downwind shift in snowfall accumulation known as the Spillover Effect.

Cloud resolving model simulations were performed (at 600m horizontal grid spacing) for four snowfall events over the Park Range. The chosen events were well simulated and occurred during winter field campaigns in 2007 and 2010 based at Storm Peak Laboratory in Steamboat Springs, CO. For each event, sensitivity simulations were run with various initial aerosol concentration vertical profiles that represent clean to polluted environments. Microphysical budget analyses were performed for these simulations in order to determine the relative importance of the various cloud properties and growth processes that contribute to precipitation production.

Observations and modeling results indicate that initial vapor depositional growth of snow tends to be maximized within about 1km of mountaintop above the windward slope while the majority of riming growth occurs within 500m of mountaintop. This suggests that precipitation production is predominantly driven by locally enhanced orography. The addition of cloud nucleating aerosols to this scenario tends to reduce the amount of riming and leads to greater snow vapor growth. Increased vapor growth leads to larger snow crystals but does not necessarily increase their density or fall speed.

There is frequently a zone on the periphery of the orographic cloud where water saturation is low and ice saturation remains high. Here the Bergeron process allows for snow to continue growing at the expense of the cloud water. Furthermore, since less cloud water is removed by riming, and droplets are smaller in polluted conditions, there is an increase in cloud water evaporation along the lee slope. This enhanced droplet evaporation in polluted conditions allows for more saturated air to persist to the lee of the ridge. Relative humidity within the lowest 1km has been found to increase upwards of 20% in the more polluted regimes. Higher relative humidity reduces the amount of snow crystal sublimation prior to surface deposition. In very moist winter events, the lee slope evaporation relative to the primary mountain barrier can saturate the air relative to a downstream ridge and allow the orographic cloud to persist. The combination of reduced riming, the Bergeron process, and reduced lee-side sublimation leads to the snowfall spillover effect under polluted conditions.