Vertical profiles of broadband albedo in snow-covered complex terrain

The surface radiation budget, controlled by the surface albedo, is a key driver of the Earth’s energy balance. Snow-covered surfaces, with their high reflectivity, scatter a large fraction of the incoming shortwave radiation back to the atmosphere and thereby modulate both local and global energy exchange. In complex topography, multiple and anisotropic forward scattering redistributes solar radiation within valleys and across slopes. To investigate terrain-scattered radiation, we combined drone-based albedo measurements with simulations using the surface radiation model Groundeye in the Meierhofer Tälli near Davos, Switzerland. Albedo was measured at different heights above ground, which serves as a proxy for different footprints for remote sensing products or different grid resolutions in models. Our objective is to understand the impact of the complex terrain on albedo, and specifically how multiple reflections and anisotropic scattering affect local reflectivity at different heights above the ground. We discuss our results primarily in the context of the representation of albedo in complex terrain and the potential biases in energy balance models and remote sensing retrievals at different resolutions. We show that 1. The ground albedo at the chosen locations in this valley varies between 0.54 and 0.98, depending on local time, slope and aspect, and on the surrounding topography, if multiple reflections are considered in our model, and 2. That the changes in albedo with height are location-specific and can increase or decrease depending on the footprint of the upward radiation flux. The albedo decreases if more shaded areas enter the footprint and increases if more sunlit surfaces become visible. However, changes of albedo with height are not only a function of simplified terrain parameters, such as the skyview factor. These results highlight the complexity inherent in interpreting albedo measurements across mountainous terrain. Such a complex change in albedo with elevation above ground is demonstrated in connection with topographic effects and has implications for improving the parametrisation of surface reflectivity in models and the derivation of surface properties from remote sensing observations.