Dust radiative forcing in snow of the Upper Colorado River Basin: 2. Interannual variability in radiative forcing and snowmelt rates

Here we present the radiative and snowmelt impacts of dust deposition to snow cover using a 6-year energy balance record (2005-2010) at alpine and subalpine micrometeorological towers in the Senator Beck Basin Study Area (SBBSA) in southwestern Colorado, USA. These results follow from the measurements described in part I. We simulate the evolution of snow water equivalent at each station under scenarios of observed and dust-free conditions, and +2 degrees C and +4 degrees C melt-season temperature perturbations to these scenarios. Over the 6 years of record, daily mean dust radiative forcing ranged from 0 to 214 W m(-2), with hourly peaks up to 409 W m(-2). Mean springtime dust radiative forcings across the period ranged from 31 to 49 W m(-2) at the alpine site and 45 to 75 W m(-2) at the subalpine site, in turn shortening snow cover duration by 21 to 51 days. The dust-advanced loss of snow cover (days) is linearly related to total dust concentration at the end of snow cover, despite temporal variability in dust exposure and solar irradiance. Under clean snow conditions, the temperature increases shorten snow cover by 5-18 days, whereas in the presence of dust they only shorten snow duration by 0-6 days. Dust radiative forcing also causes faster and earlier peak snowmelt outflow with daily mean snowpack outflow doubling under the heaviest dust conditions. On average, snow cover at the towers is lost 2.5 days after peak outflow in dusty conditions, and 1-2 weeks after peak outflow in clean conditions.