Radiative Forcing by Dust in Snowmelt-dominated Hydrologic Systems Using Coupled Satellite and In Situ Measurements

Snow is the primary source of fresh water in the western United States. With the highest albedo of any naturally occurring surface on earth, pure snow reflects roughly 90% of incident solar radiation. The addition of dust to snow decreases the albedo, increases the melt rate of snow, and spurs a decline in total snow extent, which in turn leads to increased heating of the troposphere (the snow-albedo feedback). Although dust has had an enduring presence in snow, land-use change in semi-arid regions around the world has increased dust deposition to mountain regions in the past 200 years. This paper describes the MOD-DRFS model, which leverages the MODIS surface reflectance for snow grain size and radiative forcing from dust in snow. The model has been initialized using the past 10 years of MODIS data nadir to the Colorado River Basin. MODIS data coupled with in situ data, provide insight into the relationship between dust concentration in the optically-relevant portion of the snowpack and a spatially comprehensive dataset of basin scale (and larger) dust-induced energy inputs to the snowpack. Further, this paper shows there is heterogeneity of dust deposition throughout the study region and that the 2008/2009-snow year had significantly more dust radiative forcing than other years in the record. The products of this research will inform hydrologic modeling in the western US and lay the foundation for understanding how radiative forcing by dust on snow and the snow-albedo feedback can affect the global hydrologic cycle and climate.