| 11 Mar 2026
Reduced Surface Hoar in a Warming World
Abstract. Surface hoar formation is a critical snow metamorphism process that influences surface roughness, radar-scattering properties, albedo, and avalanche risk of snowpacks. Despite its importance, surface hoar mechanisms and climate sensitivity remain poorly constrained, creating uncertainties in remote sensing of snow properties and infrastructure hazard forecasting. To address these gaps, we use observations from the Surface Atmosphere Integrated Field Lab (SAIL) alongside the Structure for Understanding Multiple Modeling Alternatives (SUMMA) physics-based model to investigate contemporary and future surface hoar dynamics in a representative mid-latitude snow environment in the Colorado Rockies. Modeling and observations are centered around seven high-quality manual measurements of surface hoar mass during February 2023. We confirm that surface hoar is favored on clear nights with snow surfaces that are 10 °C colder than the near-surface air, leading to a favorable humidity gradient for water vapor deposition from the atmosphere onto the snowpack. Nocturnal clouds exert a 30–40 W m-2 radiative forcing that inhibits the snowpack from cooling, thereby limiting deposition. We evaluate stability correction parameterizations and surface roughness parameters for the SUMMA model using colocated vertical gradients and eddy-covariance observed fluxes, finding that models must use an appropriate stability correction for the highly stable surface layers characteristic of surface hoar (Ri > 0.2) to model deposition rates sufficient to explain observed surface hoar mass. After taking these factors into account, both SUMMA and observations agree that deposition fluxes are favored when overnight air temperatures are less than -8 to -10 °C and ωspd is less than 3 m s-1. Sensitivity experiments demonstrate that surface hoar is favored for low-density snowpacks via a thermal conductivity mechanism. Using SUMMA forced by an ensemble of 9 downscaled GCMs shows that, at the annual timescale, the total wintertime nocturnal water vapor flux onto the snowpack decreases at a rate of 6.1 gm2 per degree of warming, yielding an 81 % decrease by the end-of-century under the SSP3-7.0 emission scenario.This decline is driven by a 14 % decrease in nightly surface hoar events per winter and an overall increase in the size and frequency of nocturnal sublimation events. Additional work reconciling observed and modeled amounts of surface hoar mass, turbulent exchanges of water vapor during high stability, and relationships to katabatic winds in complex terrain is warranted in order to improve the understanding of this fundamental snow metamorphosis process.
This manuscript is the first report mentioned climate change effects on surface hoar events. Stability correction is a critical factor in discussing surface hoar formation, and this paper focuses on its parameterization. As for the observation results of the surface heat balance (Fig. 2, Fig. 3), the findings are essentially consistent with those of Hachikubo and Akitaya (1997) and Stossel et al. (2010). Although I would assess the novelty as limited, the observed data are valuable, and I do not see any issues with the content. This is likely the first instance of upper-air meteorological observations during surface hoar formation, and it is interesting that latent heat flux was demonstrated to be downward from the upper troposphere to the snow surface. I feel like I've read an impressive paper in a while. I can recommend its publication, because the manuscript will be of interest to the readers of EGUsphere. Minor revision will be needed, my comments are as follows:
Because I am an field researcher rather than a modeler, I will focus exclusively on observation results.
1) I have a question regarding the Stossel box. When surface hoar forms, the relative humidity profile reaches its minimum at the snow surface, as shown in Fig. 4. In addition to surface hoar, another "frost", which generates near-surface faceted crystals, should also form under the box. Removing this frost is considerably difficult. I understand this should have been addressed in this study, but I would like to confirm that the frost on the underside of the box is properly removed before each measurement. Based on this point, I believe that the automatic measurements described in the section of Discussion are practically impossible.
2) An infrared hygrometer is required to measure latent heat flux using the eddy covariance method. When surface hoar forms, it is likely that frost will also form on the window of the infrared hygrometer. How was this phenomenon overcome?
3) I would like to confirm whether the specific heat of air (cp) is a function of temperature and pressure in the turbulent flux calculations. In Fig. 2, the atmospheric pressure is approximately 700 hPa. Compared to zero meters above sea level, sensible heat flux is relatively smaller at high altitudes such as mountainous regions.
4) There is no explanation for Fig. 1e in the figure caption, there are two instances of (d). The relationship between (a) and (e) is unclear. Latitude and longitude information might be required for Fig. 1a.
Small points:
L285: Fig. 3c is wrong, 3d is correct?
L309: (Fig.9))?
L373-: It is incorrect that the explanation for Fig. 8 appears before that for Fig. 7 in the text. The figure cited in Section 3.4.1 is listed as Fig. 7, but Fig. 9 is correct. The original Fig. 7 is not cited in the text. The figure citations throughout the manuscript should be checked again.
L481-485:I do not understand the meaning of "apparent paradox". I do not find it obvious that surface hoar accumulation increases with increasing wind speed. As Hachikubo (2001) Ann. Glaciol. demonstrates, an increase in wind speed promotes sensible heat, raises the snow surface temperature, and reduces delta q. On the other hand, under calm conditions, only molecular diffusion of water vapor occurs, which is insufficient to maintain the surface hoar growth. Therefore, it is clear that an optimal wind speed range exists.
L526-531: This is a comment. The snow surface is "layer", not "plane". Even when sublimation dominates in the energy budget, surface hoar may still be maintained on the snow surface (Hachikubo and Akitaya, 1998, Ann. Glaciol.; Stossel et al., 2010).