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 ω_spd 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.