Relative Impacts of Orographic Forcing and Pollution Aerosols on Mountain Snowfall

The mountain ranges of Colorado receive a substantial portion of their yearly rainfall 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) 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 is run with maximum cloud condensation nuclei (CCN) concentrations varying from 100 – 2000 /cm3, so as to examine the impact of pollution on the riming process that contributes significantly to accumulated snowfall. RAMS is run at 750m grid spacing centered over Storm Peak Lab, and covers the Park Range in northwest Colorado.

These sensitivity studies demonstrate the impact of aerosol concentrations for different airmass characteristics, and the interplay between updraft dynamics, supersaturation, temperature structure across the mountain barrier, and the relative concentrations of cloud droplets and ice crystal types. Given a relatively moist environment, as the strength and depth of the orographic forcing increases so does the liquid water content and spatial extent of the supercooled orographic cloud. Furthermore, the greater the cloud water and cloud size, the greater the impact of pollution aerosols upon cloud droplet concentration, the riming process, and accumulated snow water equivalent. Case study analysis and verification is accomplished using in-cloud microphysical measurements and snowfall observations that were obtained during February 2005 at SPL and surrounding area of the Park Range. Furthermore, new cases are being examined following the Inhibition of Snowfall by Pollution Aerosols (ISPA-II) field project at SPL during January and February of 2007.