Seasonal evolution of suncup roughness describes broadband albedo decay on alpine snow

We monitored the formation and seasonal evolution of suncup roughness over three snow ablation seasons at Weissfluhjoch in the Swiss Alps using a terrestrial LiDAR scanner. From high-temporal-resolution digital surface models, we find that suncup onset required three concurrent conditions: sustained daytime surface melting, above-freezing wet-bulb temperatures, and reduced wind speeds. Suncups formed in all three years, but their planar arrangement and geometry varied substantially, controlled by whether radiative or turbulent conditions dominated ablation. Comparing measured broadband albedo to flat-surface TARTES simulations forced by SNOWPACK-modelled snow properties, we find that suncup roughness and surface impurity loading together reduce albedo by 0.03–0.1, depending on illumination geometry and impurity load, consistent with previous work. Isolating the two contributions is complicated by their co-evolution: the same melt processes that deepen suncups also drive surface enrichment and the spatial redistribution of impurities, which are uniformly distributed during early suncup formation but concentrate into the hollows as melt progresses. As a practical alternative, we identify a robust logarithmic relationship between broadband albedo and the aerodynamic roughness length z 0 that captures the combined radiative effect of roughness and impurities regardless of their relative contribution. Because broadband albedo decays fastest at the onset of suncup formation, we infer that a uniform early impurity distribution is particularly effective at accelerating albedo loss. Because z 0 evolves by one order of magnitude through ablation, yet is kept constant in most snow models, our results call for its explicit, time-varying representation in physics-based snow models, which would improve both the turbulent fluxes z 0 governs directly and, through its link to albedo, the roughness–impurity darkening.