Fiber-optic distributed temperature sensing of alpine snowpacks

Small-scale surface topography and variations in snow density and in the snowpack matrix influence the snow temperature, a key variable for various heat flux components of the surface energy balance. Thus detailed knowledge on the spatial distribution and temporal evolution is crucial to quantify horizontal heterogeneity in the heat fluxes at the snow surface. We present measurements of small-scale temperature variations in alpine snow packs using fiber-optic distributed temperature sensing (DTS) together with traditional sensors such as thermocouples and thermistors. Almost a kilometer of fiber-optic cable was installed in the snow at Plaine Morte and Jungfraufirn glacier in the Swiss Alps, to obtain distributed information on snow temperature. The sensor cables were deployed in various configurations such as a fence-like structure anchored in the snowpack providing 2D snow temperature slices as snow accumulated and covered the fence, or a sensor tube with 4mm vertical resolution for high-resolution profiles. Measurements were taken at intervals of 5 minutes, with a spatial resolution of 1m and accuracy better than 0.1C. Besides the diurnal cycle, temperature data show some spatial variability along the transect. Subsurface heat fluxes were computed based on the Fourier heat equation using snow temperature and snow depth data, and an effective thermal conductivity of the snow derived from density measurements. Cable exposure to shortwave radiation near the surface and snow accumulation/compaction caused problems such as temperature overestimation and sagging of the cable between fence poles, respectively.