| 03 Jun 2026
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.
Competing interests: Author Nora Helbig serves as associate editor for this journal.
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Review of egusphere-2026-3049 “Seasonal evolution of suncup roughness describes broadband albedo decay on alpine snow”
General
This is a very relevant paper that addresses the difficult topic of disentangling surface roughness and albedo. Suncups and similar features do occur regularly on the snow surface during ablation, and this paper uses a combination of measurements and modeling to understand the processes and the model limitations. This work will help inform additional data collection and is a step towards model improvement.
The collect hourly lidar data for a ∼10 x ∼4 m region of interest over three winters, interpolated to a 5 mm grid. The physical evolution of the suncups is evaluated from a series of metrics. The snowpack is modeled in 1-D using SNOWPACK driven by meteorological data collected onsite. TARTES is used to “simulate the spectral albedo of a flat snowpack” to consider a several impurity scenarios. The flat snowpack surface is modified considering the Löwe and Helbig (2012) albedo parametrization for subgrid topographic shading. The modeling describes the within snowpack properties to match with the geometric properties of the snowpack. Overall, this is a solid approach.
The time series analysis of the suncups, together with the interpretation of the meteorological data provides some excellent insight into processes. This is likely a unique dataset that allows for this interpretation – well done.
Some of the interpretation, such as “impurities act as thermal insulators for snow” (lines 347-348) may not be fully justified. However, I appreciate that the authors are piecing together the story. Overall, well done. I really like this paper and enjoyed reading it. I wanted to have a lot more to say (I do have specific comments below), but I don’t need to criticize excellent work!
What to address:
How is z0 computed from Lettau (1969)? This is not trivial, yet it is not explained in the Methods section.
I not this below (lines 433 and 483) about “scavenging of impurities.” This could be true but has not directly been seen. Instead, impurities, specifically dust, tend to accumulate at the surface. Within a suncup, they can be redistributed to the bottom and concentrated at the ridges. Consider how you write this.
While it is impossible to disentangle impurities versus roughness, it should at least be discussed that some amount of impurities may not allow suncups to form (Fassnacht et al., 2009; cited in the paper) or may reduce roughness (Fassnacht et al., 2010; cited above). This could be briefly discussed in the paper.
Specifics