Mineral dust impact on snow radiative properties in the European Alps combining ground, UAV, and satellite observations
Journal of Geophysical Research - Atmospheres
In this paper, we evaluate the impact of mineral dust (MD) on snow radiative properties in the European Alps at ground, aerial, and satellite scale. A field survey was conducted to acquire snow spectral reflectance measurements with an Analytical Spectral Device (ASD) Field Spec Pro spectroradiometer. Surface snow samples were analyzed to determine the concentration and size distribution of MD in each sample. An overflight of a four-rotor Unmanned Aerial Vehicle (UAV) equipped with an RGB
... l camera sensor was carried out during the field operations. Finally, Landsat 8 Operational Land Imager (OLI) data covering the central European Alps were analyzed. Observed reflectance evidenced that MD strongly reduced the spectral reflectance of snow, in particular, from 350 to 600 nm. Reflectance was compared with that simulated by parameterizing the Snow, Ice, and Aerosol Radiation radiative transfer model. We defined a novel spectral index, the Snow Darkening Index (SDI), that combines different wavelengths showing nonlinear correlation with measured MD concentrations (R 2 = 0.87, root-mean-square error = 0.037). We also estimated a positive instantaneous radiative forcing that reaches values up to 153 W/m 2 for the most concentrated sampling area. SDI maps at local scale were produced using the UAV data, while regional SDI maps were generated with OLI data. These maps show the spatial distribution of MD in snow after a natural deposition from the Saharan desert. Such postdepositional experimental data are fundamental for validating radiative transfer models and global climate models that simulate the impact of MD on snow radiative properties. Light absorption due to impurity content in snow and ice is involved in complex mechanisms such as the snow albedo positive feedback [Hansen and Nazarenko, 2004; Myhre et al., 2013] , which represents a classical example of nonlinearity in the climate system, that is, the more the snow melts, the more radiation is absorbed and the more other snow melts. This process has shown to produce a significant impact on snow chemical [Rhoades et al., 2010] and radiative properties [Clarke and Noone, 1985; Flanner et al., 2007 Flanner et al., , 2009 , snow hydrology [Hadley et al., 2010; Painter et al., 2012a; Sterle et al., 2013; Kaspari et al., 2015] , timing of snowmelt [Li et al., 2013] , vegetation phenology [Steltzer et al., 2009] , and also glacier retreat [Oerlemans et al., 2009; Wientjes et al., 2011; Painter et al., 2013a] . The optical properties of snow and ice depend largely on the shape and dimension of their crystals: with an increase in the size of the crystals (often referred as grain size or "effective radius," according to Mie's theory DI MAURO ET AL.