The search for and location of inhomogeneities in seasonal snowpacks utilizing ground-penetrating radar

The location of singular objects or layered transitions below the surface and properties thereof in the ground are a pivotal topic in geosciences. In mountainous regions is the investigation of objects and layer transitions specifically of interest for the seasonal snowpack, primarily to reduce the threat to humans and infrastructures by natural hazards. Snow avalanches are a major natural hazard causing numerous fatalities throughout the world and they are a direct consequence of snowpack conditions. The annual fatality numbers of avalanches are fairly constant for the last 30 years, while in other fields such as e.g. road traffic these numbers decreased significantly. It can be assumed that the permanent enhancements in active and passive safety systems in road traffic are the reason for the decrease in victim numbers. In the field of professional search and rescue operations or accident prevention in avalanches such as hazard forecast, enhancements of instrumentations are marginal for the last three decades. The present study describes two different assessments for the use of ground-penetrating radar (GPR) systems to improve the instrumentation for the location of buried avalanche victims and the prediction of avalanches. Consequently, it demonstrates the feasibility of radar systems for the detection of inhomogeneities in seasonal snowpacks. With regard to the improvement of current methods to search and locate buried avalanche victims, which are not equipped with a location device (e.g. avalanche beacon), the main objective is to shorten search time. The assessment of this thesis was therefore to use helicopter-borne non-invasive location methods. To simulate helicopter flights, test arrangements were designed to perform field tests from above the surface. I developed methods to measure from 6--12 m above the snow cover. To measure non-invasively, the arrangement is based on pulsed radar technology. To shorten search time and to minimize the influence of man-made error possibilities, an automatic location software was developed. The results of the field tests present the answers of the fundamental questions for an airborne location operation and enabled the development of a location algorithm. Measurements showed, that the sidewise detectable range of 3--5 m of an antenna set-up with one transmitter -- receiver pair is rather small for the given flight height of 6 to 12 m. Furthermore, the reflection amplitude of the snow surface decreases almost linearly with the flight height. Unfortunately, in wet snow avalanches a buried object in the snowpack does not appear as typical reflection pattern and is therefore not explicitly locatable. The developed software algorithm proved to be sufficient for all applied test arrangements in dry snow conditions. The algorithm is able to distinguish between buried victims in the snowpack and reflections caused by only air holes within the snow cover. Further implementations on helicopters can be achieved, based on these results, but more field tests are necessary to adapt the software to the rougher flight conditions in helicopters. Concerning the observation of stratigraphic inhomogeneities within a snowpack, this thesis showed that a record of specific snowpack conditions from beneath the snow cover is feasible with GPR. The assessment of the present work is to provide snowpack information in avalanche endangered slopes and to follow the temporal evolution of the snowpack over a whole season. Two different kinds of field measurements in dry and wet snow conditions were performed to ascertain the GPR set-up, which provides the best trade-off between penetration depth and layer resolution. On the one hand, temporally singular measurements at different locations, concerning altitude, snowpack conditions and climatic regions in the European Alps, enabled the determination of capable test arrangements. On the other hand, a temporal monitoring of the snow cover at a fixed position over several months, facilitated the record of the change of specific parameters in the snowpack. In terms of system parameters, antennas with a center frequency of about 800--900 MHz are able to penetrate and adequately record stratigraphic transitions in dry and wet snow conditions. The radar-measured snow height in dry snow using a mean wave speed value for the conversion of the two-way travel time was in a good agreement to the probed snow depth and arose in an uncertainty slightly higher than of ultrasonic sensors. In terms of snowpack parameters, the recorded signals of the various snow covers were in good agreement with the measured snow properties. For dry snow conditions, the appearance and the manner of reflections recorded in the snow cover corresponded to the size and the algebraic sign of the gradient in snow density. Moisture in the snowpack attenuates the radar signal significantly. This thesis presents encouraging results of the use of impulse radar technology for the location of inhomogeneities in seasonal snowpacks. Parts of the presented results and methodologies (e.g. the automatic location algorithm) are possibly easily adaptable in related areas of geoscientific research and could also provide advances in other, non-snow related fields

Isotopic Evidence for Lateral Flow and Diffusive Transport, but Not Sublimation, in a Sloped Seasonal Snowpack, Idaho, USA

Oxygen and hydrogen isotopes in snow were measured in weekly profiles during the growth and decline of a sloped subalpine snowpack, southern Idaho, 2011–2012. Isotopic steps (10‰, δ18O; 80‰, δD) were preserved relative to physical markers throughout the season, albeit with some diffusive smoothing. Melting stripped off upper layers without shifting isotopes within the snowpack. Meltwater is in isotopic equilibrium with snow at the top but not with snow at each respective collection height. Transport of meltwater occurred primarily along pipes and lateral flow paths allowing the snowpack to melt initially in reverse stratigraphic order. Isotope diffusivities are ~2 orders of magnitude faster than estimated from experiments but can be explained by higher temperature and porosity. A better understanding of how snowmelt isotopes change during meltout improves hydrograph separation methods, whereas constraints on isotope diffusivities under warm conditions improve models of ice core records in low-latitude settings

Differences in Seasonal Melt in Greenland for Summer 2016 and 2017 - upGPR to determine liquid water percolation, retention and accumulation over the last two melt seasons

While summer 2016 air temperatures were above long term average over the entire Greenland ice Sheet (GrIS), melt in summer 2017 was considered as significantly below average, which may lead to an even positive surface mass balance in 2017 for the GrIS. However, apart from surficial extent of melt, only very little is known about effects of melt induced changes for snow and firn such as liquid water content, percolation depth and mass fluxes. To overcome this deficit, we installed an upward-looking radar systems (upGPR) 3.5 m below the snow surface in May 2016 close to Camp Raven (66.4779N/ 46.2856W) at 2120 m a.s.l. within the deep percolation zone of the GrIS. The radar is capable to monitor quasi-continuously 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. The last event in mid-August resulted in the deepest percolation down to about 2.5 m beneath the surface. For the subsequent summer season in 2017, liquid water percolation barely reached the previous summer horizon until 15 August. In consequence, seasonal mass flux into underlying firn was strongly different for summer 2016 and 2017 at the site. While until mid-August 2016, melt events transferred a cumulative mass of almost 60 kg m−2 from the surface into firn, in 2017, for the same time period, no mass flux beneath the previous summer horizon has been observed. Comparisons with results predicted by the regional climate model MAR are in very good agreement in terms of specific surface accumulation, while neither the temporal evolution of density, nor bulk liquid water contents nor percolation depths agree with upGPR data. Such inaccuracies bias simulations of changes in snow and firn and limit our understanding of effects of water percolation as well as water retention in firn. A multi-yearsummer monitoring with upGPR may lead to a valuable data base for melt effects in perennial firn. At the current stage, we have continuous observations for a very strong melt season and a below average melt in 2017. We are looking forward to monitor even more extreme events to provide temporally continuous in-situ data for a large variety of melt years in perennial firn within the percolation zone of the GrIS

Seasonal monitoring of melt and accumulation within the deep percolation zone of the Greenland Ice Sheet and comparison with simulations of regional climate modeling

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 degrees N, 46.2856 degrees 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

Continuous monitoring of the temporal evolution of the snowpack using upward-looking ground penetrating radar technology

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

Firn data compilation reveals widespread decrease of firn air content in western Greenland

The perennial snow, or firn, on the Greenland ice sheet each summer stores part of the meltwater formed at the surface, buffering the ice sheet’s contribution to sea level. We gathered observations of firn air content, indicative of the space available in the firn to retain meltwater, and find that this air content remained stable in cold regions of the firn over the last 65 years but recently decreased significantly in western Greenland

Recent warming trends of the Greenland ice sheet documented by historical firn and ice temperature observations and machine learning

Surface melt on the Greenland ice sheet has been increasing in intensity and extent over the last decades due to Arctic atmospheric warming. Surface melt depends on the surface energy balance, which includes the atmospheric forcing but also the thermal budget of the snow, firn and ice near the ice sheet surface. The temperature of the ice sheet subsurface has been used as an indicator of the thermal state of the ice sheet's surface. Here, we present a compilation of 4612 measurements of firn and ice temperature at 10m below the surface (T10m) across the ice sheet, spanning from 1912 to 2022. The measurements are either instantaneous or monthly averages. We train an artificial neural network model (ANN) on 4597 of these point observations, weighted by their relative representativity, and use it to reconstruct T10m over the entire Greenland ice sheet for the period 1950-2022 at a monthly timescale. We use 10-year averages and mean annual values of air temperature and snowfall from the ERA5 reanalysis dataset as model input. The ANN indicates a Greenland-wide positive trend of T10m at 0.2°C per decade during the 1950-2022 period, with a cooling during 1950-1985 (-0.4°C per decade) followed by a warming during 1985-2022 (+0.7° per decade). Regional climate models HIRHAM5, RACMO2.3p2 and MARv3.12 show mixed results compared to the observational T10m dataset, with mean differences ranging from -0.4°C (HIRHAM) to 1.2°C (MAR) and root mean squared differences ranging from 2.8°C (HIRHAM) to 4.7°C (MAR). The observation-based ANN also reveals an underestimation of the subsurface warming trends in climate models for the bare-ice and dry-snow areas. The subsurface warming brings the Greenland ice sheet surface closer to the melting point, reducing the amount of energy input required for melting. Our compilation documents the response of the ice sheet subsurface to atmospheric warming and will enable further improvements of models used for ice sheet mass loss assessment and reduce the uncertainty in projections

The search for and location of inhomogeneities in seasonal snowpacks utilizing ground-penetrating radar technology

The location of singular objects or layered transitions below the surface and properties thereof in the ground are a pivotal topic in geosciences. In mountainous regions is the investigation of objects and layer transitions specifically of interest for the seasonal snowpack, primarily to reduce the threat to humans and infrastructures by natural hazards. Snow avalanches are a major natural hazard causing numerous fatalities throughout the world and they are a direct consequence of snowpack conditions. The annual fatality numbers of avalanches are fairly constant for the last 30 years, while in other fields such as e.g. road traffic these numbers decreased significantly. It can be assumed that the permanent enhancements in active and passive safety systems in road traffic are the reason for the decrease in victim numbers. In the field of professional search and rescue operations or accident prevention in avalanches such as hazard forecast, enhancements of instrumentations are marginal for the last three decades. The present study describes two different assessments for the use of ground-penetrating radar (GPR) systems to improve the instrumentation for the location of buried avalanche victims and the prediction of avalanches. Consequently, it demonstrates the feasibility of radar systems for the detection of inhomogeneities in seasonal snowpacks. With regard to the improvement of current methods to search and locate buried avalanche victims, which are not equipped with a location device (e.g. avalanche beacon), the main objective is to shorten search time. The assessment of this thesis was therefore to use helicopter-borne non-invasive location methods. To simulate helicopter flights, test arrangements were designed to perform field tests from above the surface. I developed methods to measure from 6--12 m above the snow cover. To measure non-invasively, the arrangement is based on pulsed radar technology. To shorten search time and to minimize the influence of man-made error possibilities, an automatic location software was developed. The results of the field tests present the answers of the fundamental questions for an airborne location operation and enabled the development of a location algorithm. Measurements showed, that the sidewise detectable range of 3--5 m of an antenna set-up with one transmitter -- receiver pair is rather small for the given flight height of 6 to 12 m. Furthermore, the reflection amplitude of the snow surface decreases almost linearly with the flight height. Unfortunately, in wet snow avalanches a buried object in the snowpack does not appear as typical reflection pattern and is therefore not explicitly locatable. The developed software algorithm proved to be sufficient for all applied test arrangements in dry snow conditions. The algorithm is able to distinguish between buried victims in the snowpack and reflections caused by only air holes within the snow cover. Further implementations on helicopters can be achieved, based on these results, but more field tests are necessary to adapt the software to the rougher flight conditions in helicopters. Concerning the observation of stratigraphic inhomogeneities within a snowpack, this thesis showed that a record of specific snowpack conditions from beneath the snow cover is feasible with GPR. The assessment of the present work is to provide snowpack information in avalanche endangered slopes and to follow the temporal evolution of the snowpack over a whole season. Two different kinds of field measurements in dry and wet snow conditions were performed to ascertain the GPR set-up, which provides the best trade-off between penetration depth and layer resolution. On the one hand, temporally singular measurements at different locations, concerning altitude, snowpack conditions and climatic regions in the European Alps, enabled the determination of capable test arrangements. On the other hand, a temporal monitoring of the snow cover at a fixed position over several months, facilitated the record of the change of specific parameters in the snowpack. In terms of system parameters, antennas with a center frequency of about 800--900 MHz are able to penetrate and adequately record stratigraphic transitions in dry and wet snow conditions. The radar-measured snow height in dry snow using a mean wave speed value for the conversion of the two-way travel time was in a good agreement to the probed snow depth and arose in an uncertainty slightly higher than of ultrasonic sensors. In terms of snowpack parameters, the recorded signals of the various snow covers were in good agreement with the measured snow properties. For dry snow conditions, the appearance and the manner of reflections recorded in the snow cover corresponded to the size and the algebraic sign of the gradient in snow density. Moisture in the snowpack attenuates the radar signal significantly. This thesis presents encouraging results of the use of impulse radar technology for the location of inhomogeneities in seasonal snowpacks. Parts of the presented results and methodologies (e.g. the automatic location algorithm) are possibly easily adaptable in related areas of geoscientific research and could also provide advances in other, non-snow related fields