Simulated impacts of enhanced CCN and giant-CCN concentrations on orographic snowfall

The mountain ranges of Colorado receive a substantial portion of their yearly precipitation during the winter months, and this stored snowpack is crucial to the water reserves for the state during the dry season. The orographic enhancement of the Park Range in particular makes the Yampa Region, near Steamboat Springs, one of the heaviest precipitation areas of the state. Furthermore, two coal-fired power plants operate just upstream of Steamboat Springs in the towns of Hayden and Craig. Both of these facilities have been shown to contribute to the aerosol population at Storm Peak Lab (SPL), which sits at the top of the Steamboat Springs ski resort at the crest of the Park Range. While the orographic effect provides the greatest variance in precipitation in this region, the impact of pollution aerosols can modify the magnitude and spatial distribution of the orographic snowfall.

Mesoscale simulations of winter orographic cloud structure and precipitation using the CSU Regional Atmospheric Modeling System (RAMS) at 750m grid spacing have been performed to examine the local influence of orography and pollution on the snowpack of the Park Range of Colorado. An ensemble of RAMS simulations was run with various initial concentrations of cloud condensation nuclei (CCN) and giant-CCN (GCCN). Simulations were run with “clean” and “dirty” aerosol profiles, respectively, with maximum CCN concentrations of 100 & 1900 /cc, GCCN of 0.00001 and 0.5 /cc. Each of the hygroscopic aerosol types is introduced as a vertical profile with maximum values near the surface that decrease with height to a clean background value at 4km AGL. This effectively represents surface sources of aerosols.

Results indicate that the impact of cloud nucleating aerosols (CCN and GCCN) is maximized when a substantial amount of supercooled liquid water exists in the form of a low level orographic cloud. This setup is ideal for an active seeder-feeder process, whereby, snow crystals aloft fall through the supercooled cloud and collect rime before depositing at the surface. Given that warm rain processes are minimized under such sub-freezing winter conditions, it seems that any additional cloud nucleating aerosols (CCN or GCCN) tend to divide up the available liquid water into more numerous, smaller droplets with lower riming efficiencies. This then reduces the riming growth and overall liquid water equivalent that reaches the surface. The impact difference between CCN and GCCN is the following; an increase in CCN alone tends to reduce snowfall on the windward slopes and increase snowfall on the leeward slopes, with only modest overall decreases in total snowfall over the whole domain. Furthermore, the CCN impact is greatest when GCCN are few. There is effectively a downwind snowpack shift due to a blowover effect of less-rimed ice crystals. The addition of higher concentrations of GCCN (with large riming efficiencies) results in enhanced riming and a shift in primary surface accumulated hydrometeor type from aggregates to graupel. Along the windward slopes of the Park Range the total water equivalent accumulation is weakly increased due to GCCN, while, there is a large systematic decrease along the ridge crest and lee slopes.