Quantifying the elusive sublimation flux in the Swiss Alps

Between 1 March and 30 April 2025, Isabella Anglin (MSc student, Wageningen/SLF), supervised by Dr. Harsh Beria (Research Scientist, ETH/SLF), led 17 field trips to the Weissfluhjoch research site in eastern Switzerland, with the support of Leah Gaillard Festa (Research Assistant, SLF). The primary objective was to determine whether sublimation, the direct conversion of snow into vapor, leaves a traceable isotopic fingerprint in the remaining snowpack. 

The campaign involved detailed snow stratigraphy profiling at five snow pits, all within the footprint of a newly installed eddy-covariance flux tower (set up by SLF’s Snow Processes group in 2024). Three sampling strategies were carried out: 

1. Intensive campaign – 5 days of high-frequency sampling (4 to 5 times per day) to capture short-term isotopic changes in the top 11 cm of snowpack. 

2. Biweekly sampling – focusing on the top 20 cm of the snowpack 

3. Fortnightly full snow profiling– aligned with SLF’s official snow microstructure measurements of the entire snowpack. 

In total, the team collected over 900 snow samples to understand snow-atmosphere exchanges from the lens of stable water isotopes in the Swiss Alps. 

Early results 

While the snow samples awaited laboratory analysis for stable water isotopes, Isabella processed 20 Hz eddy-covariance measurements to estimate latent heat fluxes throughout 2024-25 winter. Early results show:  

1. Sublimation losses accounted for an estimated 5.4% of winter precipitation at Weissfluhjoch 

2. Solar radiation emerged as the dominant driver of sublimation, followed by wind speed and vapor pressure deficit (VPD). These findings were supported by traditional statistical analyses and a new machine-learning technique known as SHAP analysis 

3. Surprisingly, air temperature itself had little direct effect on sublimation. Its apparent influence was mostly explained by solar radiation and VPD 

These results were presented at the International Mountain Conference in September 2025, held in Innsbruck. 

Early isotope results 

After analyzing over 500 snow samples, early results indicate that isotope based sublimation estimates are challenging. Sublimation effects are confined to the top 2 cm of the snowpack, which is often reset by new snowfall. However, during snowfall-free periods when snow does not melt, isotope signatures can effectively track sublimation fluxes. 

These early findings shed light on atmosphere-cryosphere interactions and demonstrate the promise (and complexity) of using isotopic tracers to quantify snow sublimation. Isabella continues her research as an intern within SLF’s Snow Processes group, refining these analyses further.