Interannual variability in radiative forcing by desert dust in snowcover in the Colorado River Basin

Dust in snow accelerates snowmelt through its direct reduction of snow albedo and its further indirect reduction of albedo by accelerating the growth of snow effective grain size. Since the Anglo expansion into the western United States that began in the mid 19th century, the mountain snow cover of the Colorado River Basin has been subject to five-fold greater dust loading, largely from the Colorado Plateau and Great Basin. Previous work showed that in 2005 and 2006, dust loading in the San Juan Mountains (east central CRB) caused springtime surface radiative forcing of 25 to 50 W/m2 and shortened snow cover by 25 to 35 days. This research will expand on prior work by assessing the interannual variability in radiative forcing, melt rates, and shortening of snow cover duration from 2005 to 2009, and the relative response of melt rates to simulated increases in air temperature. The number of recorded dust deposition events has increased monotonically since 2005, but the greatest dust loadings by mass occurred in 2006 and 2009. The dust loading in 2009 was a factor of 5 greater than 2006, and an order of magnitude greater than the other years in this time series. In 2009, snow albedo was less than 0.35 for the last 3 weeks of snow cover whereas in previous years albedo was rarely less than 0.50. With continued soil disturbance and projected drought in the southwestern US under global warming scenarios, the 2009 season may represent a new regime of dust forcing of radiative and hydrologic systems in the intermountain west.