Evaluating continuous and autonomous snow water equivalent measurements by a cosmic ray sensor on a Swiss glacier

Abstract. Snow water equivalent (SWE) measurements are crucial in many research fields. Yet accurate measurements at a high temporal resolution are difficult to obtain in high mountain regions. With a cosmic ray sensor (CRS), SWE can be directly derived from neutron counts. In this study, we present the analyses of temporally continuous SWE measurements by a CRS on a Swiss glacier (Glacier de la Plaine Morte) over two winter seasons (2016/17 and 2017/18), which were markedly different in terms of amount and timing of snow accumulation. By combining the SWE values with snow depth measurements, we calculate the daily mean density of the snowpack. The autonomous measurements overestimate SWE by +2 % ± 12 % compared to manual field observations (snow pits). Snow depth and mean density agree with manual in situ measurements with a standard deviation of ±6 % and ±8 %, respectively. In general, the cosmic ray sensor measured with high reliability during these two winter seasons and is, thus, considered an effective method to measure SWE at remote high alpine sites. We use the daily observations to break down the winter season into days either dominated by accumulation (solid precipitation, snow drift), ablation (snow drift, melt) or snow densification. The prevailing meteorological conditions of these periods are clearly distinct for each of the classified processes. Moreover, we compare daily SWE amounts to precipitation sums from three nearby weather stations located at lower elevations, and to a gridded precipitation dataset. We determine the best-possible scaling factor for these precipitation estimates in order to reproduce the measured accumulation on the glacier. Using only one scaling factor for the whole time series, we find a mean absolute error of less than 8 cm w.e. for the reproduced snow accumulation. By applying temperature-specific scaling factors, this mean absolute error can be reduced to less than 6 cm w.e. for all stations.