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

Abstract. We monitored the formation and seasonal evolution of suncup roughness over three snow ablation seasons at Weissfluhjoch, Swiss Alps, using a terrestrial LiDAR scanner. Suncup onset required two concurrent conditions, identified from high-temporal-resolution digital surface models of surface roughness: sustained surface melting through most of the day and reduced wind speeds. Suncups formed in all three years, but their planar arrangement and geometric properties varied substantially across seasons, controlled by whether radiative or turbulent heat exchange dominated ablation. Comparing measured broadband albedo to flat-surface simulations from TARTES forced by SNOWPACK-modelled snow properties, we find that the combined effect of suncup roughness and surface impurity loading reduces albedo by 0.02–0.15, depending on illumination geometry and impurity load, consistent with previous literature. Isolating the two contributions is complicated by their co-evolution during the ablation season: the same melt processes that progressively deepen suncups also drive surface enrichment and spatial redistribution of impurities. Resolving the two effects independently would in principle require equally fine-scale measurements of both roughness and impurity distribution. We identify a robust logarithmic correlation between broadband albedo and aerodynamic roughness length that simultaneously captures the radiative effects of roughness and impurities, regardless of their relative contributions. During early suncup formation, impurities remained uniformly distributed. At later stages, meltwater scavenging concentrated impurities to the suncup hollows. As the rate of broadband albedo decay is strongest at the beginning of suncup formation and relaxes thereafter, we infer that the interaction between the multiple-reflection mechanism and the more uniform distribution of impurities is particularly effective in accelerating albedo decay, beyond the effect of either factor alone. Given that C-band SAR backscatter is sensitive to the early development of surface roughness on wet snow, these findings encourage future work on the assimilation of surface roughness into snow energy balance models.