Accurate snow depth observations are critical to assess water resources. More than a billion people rely on water from snow, most of which originates in the Northern Hemisphere mountain ranges. Yet, remote sensing observations of mountain snow depth are still lacking at the large scale. Here, we show the ability of Sentinel-1 to map snow depth in the Northern Hemisphere mountains at 1 km² resolution using an empirical change detection approach. An evaluation with measurements from ~4000 sites and reanalysis data demonstrates that the Sentinel-1 retrievals capture the spatial variability between and within mountain ranges, as well as their inter-annual differences. This is showcased with the contrasting snow depths between 2017 and 2018 in the US Sierra Nevada and European Alps. With Sentinel-1 continuity ensured until 2030 and likely beyond, these findings lay a foundation for quantifying the long-term vulnerability of mountain snow-water resources to climate change.

Our current knowledge on multi‐decadal to centennial changes of snow in different parts of the world is based largely on observations of snow depth and depth of snowfall from national weather and hydrographic services. Studies analysing these snow observations in the European Alps are predominantly based on national data and are therefore limited by their respective borders in the detection of robust, spatiotemporal snow trends. In order to overcome this limitation, data from Austria and Switzerland, which cover a substantial fraction of the Alps when taken together, are merged for this study (196 station‐records). Additionally, it is the first time that such an analysis is based on homogenized data. Our homogenization study shows that, although the detection of breaks in snow depth series works quite well with the existing methods, further research is needed to adequately correct snow depth series at a daily resolution. Roughly, 70% (139 station‐records) of the snow depth series could be homogenized and are used for further trend analysis. The findings concern seven climatologically different areas that are identified by a regionalization (using empirical orthogonal functions) using station records from 1961 to 2012. These regions share a high degree of inner similarity and outer separation, and the temporal trends detected are rather different across the Swiss‐Austrian domain. Regions in the south show a clear decrease in the snow depth of up to −12 cm/decade on average, while those in the northeast are characterized by almost no change. The declining trend in the southern regions intensifies as altitude increases. Comparisons of these variations in depth changes with concurrent changes in air temperature and precipitation totals reveal a clear dichotomy with respect to elevation. Snow depths in low elevated areas are highly sensitive to air temperature changes, whereas those at high elevations strongly depend on alterations in precipitation totals.

[1] Measurement and modeling of downward longwave irradiance are a special challenge in arctic winter due to its low water vapor content and the extreme meteorological conditions. There are questions about the representativeness of the instrument calibration, the consistency and uncertainty of measurements and models in these environments. The Second International Pyrgeometer and Absolute Sky-scanning Radiometer Comparison (IPASRC-II), which was conducted at Atmospheric Radiation Measurement (ARM) program's North Slope of Alaska (NSA) site in Barrow provided a unique opportunity to compare high accuracy downward longwave irradiance measurements and radiative transfer model computations during arctic winter. Participants from 11 international institutions deployed 14 pyrgeometers, which were field-calibrated against the Absolute Sky-scanning Radiometer (ASR). Continuous measurements over a 10-day period in early

Ablation climate studies were made at two locations in northern Greenland in the summers of 1993 and 1994, respectively. Daily ablation was measured at ten stakes within a small area, and the data were compared with each other to detect gross errors. For example, high standard deviations for data taken on the same day, or low correlations between data series at different stakes, indicate erroneous data. After discarding data for one stake in 1993 and two stakes in 1994, random errors in daily ablation data for individual stakes are ± 5 kg m 2 d−1, which is further reduced to only about db 2 kg m−2d−1 by averaging over eight or nine stakes. Random errors in calculated energy balances using the present ablation data are much lower than found in earlier stuthes in West Greenland where ablation was only measured on three stakes without any attempt to detect gross errors. Aside from day-to-day errors, there are ±10% differences in mean ablation at different stakes, which are probably caused by small-scale variations in surface albedo. Such interstate differences give a ± 10% uncertainty in positive degree-day factors, which are 9.8 ± 0.9 and 5.9 ± 0.6 kg m 2 d −1 deg −1 for the two sites.

Abstract. The mountain cryosphere is recognized to have important impacts on a range of environmental processes. This 30 paper reviews current knowledge on snow, glacier, and permafrost processes, as well as their past, current and future The Cryosphere Discuss., doi:10.5194/tc-2016Discuss., doi:10.5194/tc- -290, 2017 Manuscript under review for journal The Cryosphere Published: 9 January 2017 c Author(s) 2017. CC-BY 3.0 License. 2 evolution in mountain regions in mainland Europe. We provide a comprehensive assessment of the current state of cryosphere research in Europe and point to the different domains requiring further research to improve our understanding of climate-cryosphere interactions, cryosphere controls on physical and biological mountain systems, as well as related impacts.We highlight advances in the modeling of the cryosphere, and identify inherent uncertainties in our capability of projecting changes in the context of a warming global climate. 5