Description: Report of the Liquid Water in Snow Workshop, Davos, Switzerland, 2–4 April 2014

Description: We present an overview of current state-of-the art measurements and knowledge of dielectric properties of ice and snow. They are the fundamental property for electromagnetic applications in cryospheric sciences, like ground-penetrating radar (GPR) and satellite remote sensing. Relevance ranges from improved determination of snow-cover properties for improved avalanche risk estimation to reconstruction of past climate signals from ice cores to improved understanding of dynamics of ice masses like Greenland and Antarctica. Our results are based on several techniques: (i) laboratory observations with a coaxial cell (CC) set-up, employed to measure dielectric properties on artificial and natural snow and ice samples in the range from 1 MHz to 1.5 GHz; (ii) dielectric profiling (DEP) of firn and ice cores in the range of 100 kHz; and (iii) usage of GPR to indirectly deduce the dielectric properties of the bulk medium via variations in wave speed and reflection coefficients. In contrast to many other substances, H2O as the underlying molecule, exists close to its melting point under ordinary conditions, involving snow and ice on Earth. This is of large interest for environmental applications as the so-called snow-water equivalent (i.e. the total mass) can greatly vary depending on density and liquid-water saturation of a snow cover, without showing considerable changes in snow height. In contrast, it poses a major problem for determining dielectric properties close to the melting point, especially in the laboratory, as the medium of interest partly undergoes a phase transition while varying temperature. In addition, especially for applications on glaciers and ice sheets, the anisotropic nature of ice has to be taken into account, as ice viscosity - and thus flow behavior - varies over four orders of magnitude depending on the crystal orientation fabric.

Description: Increasing melt over the Greenland Ice Sheet (GrIS) recorded over the past several years has resulted in significant changes of the percolation regime of the ice sheet. It remains unclear whether Greenland's percolation zone will act as a meltwater buffer in the near future through gradually filling all pore space or if near-surface refreezing causes the formation of impermeable layers, which provoke lateral runoff. Homogeneous ice layers within perennial firn, as well as near-surface ice layers of several meter thickness have been observed in firn cores. Because firn coring is a destructive method, deriving stratigraphic changes in firn and allocation of summer melt events is challenging. To overcome this deficit and provide continuous data for model evaluations on snow and firn density, temporal changes in liquid water content and depths of water infiltration, we installed an upward-looking radar system (upGPR) 3.4 m below the snow surface in May 2016 close to Camp Raven (66.4779∘ N, 46.2856∘ W) at 2120 m a.s.l. The radar is capable of quasi-continuously monitoring changes in snow and firn stratigraphy, which occur above the antennas. For summer 2016, we observed four major melt events, which routed liquid water into various depths beneath the surface. The last event in mid-August resulted in the deepest percolation down to about 2.3 m beneath the surface. Comparisons with simulations from the regional climate model MAR are in very good agreement in terms of seasonal changes in accumulation and timing of onset of melt. However, neither bulk density of near-surface layers nor the amounts of liquid water and percolation depths predicted by MAR correspond with upGPR data. Radar data and records of a nearby thermistor string, in contrast, matched very well for both timing and depth of temperature changes and observed water percolations. All four melt events transferred a cumulative mass of 56 kg m−2 into firn beneath the summer surface of 2015. We find that continuous observations of liquid water content, percolation depths and rates for the seasonal mass fluxes are sufficiently accurate to provide valuable information for validation of model approaches and help to develop a better understanding of liquid water retention and percolation in perennial firn.

Description: Snow stratigraphy and water percolation are key parameters in avalanche forecasting. It is, however, difficult to model or measure stratigraphy and water flow in a sloping snowpack. Numerical modeling results depend highly on the type and availability of input data and the parameterization of the physical processes. Furthermore, the sensors themselves may influence the snowpack or be destroyed due to snow gliding and avalanches. Radar technology allows non-destructive scanning of the snowpack and deducing internal snow properties. If the radar system is buried in the ground, it cannot be destroyed by avalanche impacts or snow creep. During the winter seasons 2010-2011 and 2011-2012 we recorded continuous data with upward-looking pulsed radar systems (upGPR) at two test sites. We demonstrate that it is possible to determine the snow height with an accuracy comparable to conventional snow depth measuring devices. We determined the bulk volumetric liquid water content and tracked the position of the first stable wetting front. Wet-snow avalanche activity increased, when melt water penetrated deeper into the snowpack.

Description: The cold alpine saddle Colle Gnifetti, Monte Rosa, Swiss-Italian Alps resembles very much polar and subpolar ice masses in terms of glaciological conditions.It has been the site for several ice-core drilling campaigns over more than 20 years to determine paleoclimatological and glaciological conditions.To investigate the feasibility of geophysical methods for improved characterization of ice masses surrounding borehole and ice-core sites, a combined active reflection seismic and ground-penetrating radar pilot study has been carried out in summer 2008.Aims are the characterization of density, internal layering, seismic and radar wave speed and attenuation, identification of anisotropic features (like crystal orientation or bubble content and shape).Here we present the overall setup and first results. Seismic and GPR profiles were centered on an existing borehole location covering the full ice thickness of 62 m.Active seismics was carried out with 24-channel 3-m spacing recording, using a Seismic Impulse Source System (SISSY) along two profiles parallel and perpendicular to the ice-flow direction.The same profiles were complemented with GPR measurements utilizing 250, 500 MHz frequencies.Additionally, circular profiles with 250, 500 and 800 MHz were carried out circumferencing the borehole to detect anisotropic features.

Description: The cold alpine saddle Colle Gnifetti, Monte Rosa, Swiss-Italian Alps resembles very much polar and subpolar ice masses in terms of glaciological conditions.It has been the site for several ice-core drilling campaigns over more than 20 years to determine paleoclimatological and glaciological conditions.To investigate the feasibility of geophysical methods for improved characterization of ice masses surrounding borehole and ice-core sites, a combined active reflection seismic and ground-penetrating radar pilot study has been carried out in summer 2008.Aims are the characterization of density, internal layering, seismic and radar wave speed and attenuation, identification of anisotropic features (like crystal orientation or bubble content and shape).Here we present the overall setup and first results. Seismic and GPR profiles were centered on an existing borehole location covering the full ice thickness of 62 m.Active seismics was carried out with 24-channel 3-m spacing recording, using a Seismic Impulse Source System (SISSY) along two profiles parallel and perpendicular to the ice-flow direction.The same profiles were complemented with GPR measurements utilizing 250, 500 MHz frequencies.Additionally, circular profiles with 250, 500 and 800 MHz were carried out circumferencing the borehole to detect anisotropic features.

Description: Ground-penetrating radar systems (GPR) offer a wide field of applications.Especially in cryospheric implementations GPR proved to be an adequate toolto determine fast and non-destructively media transitions. In this study, weanalyse the feasibility of impulse radar in recording internal snowpack transitions of density or moisture content. The utilized impulse radar systems for this research purpose are commercially available and the gathered data needs no calibration measurement for interpretation, which is a distinct advantage in comparison to frequency modulated continuous wave (FMCW) systems. Currently available methods monitoring seasonal snowpacks are either destructive as snow profiling or insufficient for measuring in slope areas or to determine snow stratigraphy as ultra-sonic sensors. Additionally, the risk exposure for the profiling teams is often a limiting factor for the data acquisition, especially in avalanche paths and ridge areas. In such regions an all-season monitoring system must be secured against being destroyed by avalanches. Thus, the implemented system operates from beneath the snowpack measuring in upward direction. The GPR system was tested in several varying snow conditions as cold dry snow and wet snowpacks. Furthermore, different frequencies, polarisations and two different radar systems were analyzed on their applicability for the snowpack monitoring from beneath and the system was utilized in periods with various meteorological parameter. The results of these preliminary tests showed, that with a moved antenna it is possible to record snow layers in dry snow with adequate density steps and layer thickness, supplementary to the snow depth. A one meter-thick wet snowpack was penetrateable although the signal was very much attenuated. GPR systems with frequencies above 1GHz provided insufficient pentration depth in late season snowpacks. Analysis of reflection phases allowed interpretation of their physical origin in terms of permittivity. The system set-up used is capable of improving information of spatial and temporal snow-pack characteristics especially in stratigraphy and snow depth and has the potential to be remotely operated.