Depth Hoar Formation Research Articles

Metamorphism observation and model of snow from summit, Greenland under both positive and negative temperature gradients in a micro computed tomography

AbstractThree sets of snow specimens with different initial densities ranging from 311 to 486 kg m−3, which were collected from depths of 7, 40 and 57 cm at Summit, Greenland (72°35′ N, 38°25′ W) in June 2017, were subjected to the temperature gradient (TG) of ~160 K m−1 in opposing directions. The snow metamorphism was characterized using both X‐ray micro computed tomography and optical microscopy. The formation of depth hoar is evident as the snow grains transformed from the initial approximately rounded particles to needles, broad‐plates, columns and cup‐shapes. The de‐densification process of snow is also reflected in various microstructural parameters, viz., the density and the structure thickness (the feature size of ice particle) generally decreased with time, while the area‐equivalent circle diameter (the feature size of pore), the total porosity, the specific surface area and the structure model index (a measure of convexity/concavity of ice surface) increased. Based on an ice‐sphere pair of equal size, a vapour transport model, which depends on the initial particle radius, , and the ratio of the initial particle‐bond radius to the initial particle radius, , was constructed to simulate the main characteristics of change in the density with time under the TG metamorphism. We found critical values of = 0.1 mm and = 0.1 that determine how fast the geometric coefficient, (a tuning parameter used in this model at the cone angle between the two ice spheres), changes. We propose a conceptual model in which an ice‐sphere pair can be regarded as a volume element of morphogenesis of depth hoar. This model can be used to explain the formation of the needle‐like, broad‐plate and column depth hoar by combining particle‐by‐particle with inter‐layer vapour transport.

Surface Mass Balance Controlled by Local Surface Slope in Inland Antarctica: Implications for Ice‐Sheet Mass Balance and Oldest Ice Delineation in Dome Fuji

AbstractThe limited number of surface mass balance (SMB) observations in the Antarctic inland hampers estimates of ice‐sheet contribution to global sea level and locations with million‐year‐old ice. We present finely resolved SMB over the past three centuries in a low‐accumulation region with significant depth hoar formation on Dome Fuji derived from ∼1,100 km of microwave radar stratigraphy dated with a firn core. The regional‐mean SMB over the past 264 years is estimated to ∼22.5 ± 3.3 kg m−2 a−1, but with large local variability of up to 30%. We found that local SMB is negatively correlated with surface slope at scales of a few hundred meters, resulting in anomalous zones of low SMB which represent as much as 8–10% of the total SMB on the inland plateau if the SMB‐slope relationship is more widely valid. This impact should be investigated further to improve estimates of Antarctic mass balance and sea‐level contribution.

Soil moisture, wind speed and depth hoar formation in the Arctic snowpack

ABSTRACTBasal depth hoar that forms in Arctic snowpacks often has a low thermal conductivity, strongly contributing to the snowpack thermal insulance and impacting the permafrost thermal regime. At Ward Hunt Island (Canadian high Arctic, 83°05′N, 74°07′W) almost no depth hoar was observed in spring 2016 despite favorable thermal conditions. We hypothesize that depth hoar formation was impeded by the combination of two factors (1) strong winds in fall that formed hard dense wind slabs where water vapor transport was slow and (2) low soil moisture that led to rapid ground cooling with no zero-curtain period, which reduced soil temperature and the temperature gradient in the snowpack. Comparisons with detailed data from the subsequent winter at Ward Hunt and from Bylot Island (73°09′N, 80°00′W) and with data from Barrow and Alert indicate that both high wind speeds after snow onset and low soil moisture are necessary to prevent Arctic depth hoar formation. The role of convection to form depth hoar is discussed. A simple preliminary strategy to parameterize depth hoar thermal conductivity in snow schemes is proposed based on wind speed and soil moisture. Finally, warming-induced vegetation growth and soil moisture increase should reduce depth hoar thermal conductivity, potentially affecting permafrost temperature.

Comparison of effects of cold‐region soil/snow processes and the uncertainties from model forcing data on permafrost physical characteristics

AbstractWe used a land surface model to (1) evaluate the influence of recent improvements in modeling cold‐region soil/snow physics on near‐surface permafrost physical characteristics (within 0–3 m soil column) in the northern high latitudes (NHL) and (2) compare them with uncertainties from climate and land‐cover data sets. Specifically, four soil/snow processes are investigated: deep soil energetics, soil organic carbon (SOC) effects on soil properties, wind compaction of snow, and depth hoar formation. In the model, together they increased the contemporary NHL permafrost area by 9.2 × 106 km2 (from 2.9 to 12.3—without and with these processes, respectively) and reduced historical degradation rates. In comparison, permafrost area using different climate data sets (with annual air temperature difference of ∼0.5°C) differed by up to 2.3 × 106 km2, with minimal contribution of up to 0.7 × 106 km2 from substantial land‐cover differences. Individually, the strongest role in permafrost increase was from deep soil energetics, followed by contributions from SOC and wind compaction, while depth hoar decreased permafrost. The respective contribution on 0–3 m permafrost stability also followed a similar pattern. However, soil temperature and moisture within vegetation root zone (∼0–1 m), which strongly influence soil biogeochemistry, were only affected by the latter three processes. The ecosystem energy and water fluxes were impacted the least due to these soil/snow processes. While it is evident that simulated permafrost physical characteristics benefit from detailed treatment of cold‐region biogeophysical processes, we argue that these should also lead to integrated improvements in modeling of biogeochemistry.

Indicative properties on snow cover based on the results of experimental studies in the winter 2011/12 in the central part of the East European Plain

Local and regional differences in the snow formation were studied in different landscapes of the central part of the East European Plain – within reserves in the Moscow and Tver’ regions (south-north direction); the study period is the winter 2011/12. The observed increase of snow storage in 1.3–1.5 times in the direction south-north is connected, apparently. The difference in the five-day appearance of snow cover maximum is related to differences in regional winter air temperature. Throughout the snow depth and snow storage in spruce are smaller than in deciduous forest – in the ratio of 0.81 in south area and 0.93 in north area; in spruce the large part of solid precipitation is intercepted by the crowns pine trees. Snow stratigraphy at south areas has four layers, six layers at the north area are more variable in snow density and snow storage. Perhaps, gravitational conversion is more noticeable due to larger snow depth. Snow density and snow storage at the open areas are more heterogeneous than in the forest. This is due to sharp fluctuations in air temperature, wind transport and compaction of snow, evaporation from the snow surface. The stratigraphy of snow also reflects the history of winter changes of air temperature and snow accumulation. Common feature for reserves at south and north is the availability of layers with maximum snow storage in the middle of the snow thickness, which were formed during the air temperature drops to the lowest seasonal values in period with increase of snow depth to maximum. Formation of depth hoar in snow thickness are touched everywhere the bottom and middle layers, respectively, it was formed both before and during the period with minimal air temperature. Thus, the results of experimental studies confirm the significance of the differences of individual components of the landscape setting. Analytical conclusions are largely qualitative in nature due to the lack to date of initial information, and therefore to continue experimental studies is expected.

Nonequilibrium thermodynamics of kinetic metamorphism in snow

Entropy production rates derived from nonequilibrium thermodynamics are used to show that depth hoar develops from the natural progression of snow toward thermal equilibrium. Laboratory experiments were undertaken to examine the evolution of snow microstructure under nonequilibrium thermal conditions. Snow samples with similar initial microstructure were subjected to either a fixed temperature gradient or fixed heat input. The metamorphism for both sets of boundary conditions produced similar depth hoar chains with comparable increases in effective thermal conductivity. Examination of the entropy production rates showed that the microstructural changes resulted from the snow or the surroundings moving toward a stationary state under the given non-equilibrium constraints imposed by the boundary conditions. This behavior is dictated by the second law of thermodynamics.A numerical model applied nonequilibrium thermodynamics to depth hoar formation at the grain scale. Entropy production rate relations were developed for an open system of ice and water vapor subjected to heat and mass flow. Heat conduction in the bonds had the highest specific entropy production rate, indicating it was the most inefficient part of the snow system at transferring heat. As the metamorphism advanced, the bond sizes grew to enhance the conduction pathways through the snow and increase the heat transfer. This spontaneous microstructural evolution moved the system and the surroundings toward equilibrium by reducing the local temperature gradients across the bonds and increasing the entropy production rate density.

Toward a robust retrieval of snow accumulation over the Antarctic ice sheet using satellite radar

The sensitivity of radar measurements to snow accumulation rate is determined by the firn volume characteristics of the ice sheets. Here we present a new approach for calculating the volume backscattering of dry firn, which is combined with recently developed empirical parameterizations of firn grain size and density as functions of depth, surface temperature, and accumulation rate. To this end, dense medium radiative transfer theory is applied to calculate the volume scattering and absorption coefficients. The coefficients for the density transition between snow and dense firn are evaluated using polynomial interpolation. For testing the method, we used measured accumulation rates, Envisat ASAR C‐band wide‐swath mode images, and QuikSCAT Ku‐band backscattering data from Dronning Maud Land, Antarctica. The comparison between measured and simulated backscattering coefficients shows that no tuning parameter is necessary to obtain the correct absolute level of scattering intensity. The robustness of accumulation rate retrievals depends on the consideration of technical and environmental factors. Due to the presence of sastrugi on the ice sheet surface the measured intensities are sensitive to the radar look direction. Wind compaction of snow and depth hoar formation change the depth‐dependent snow density and grain size profiles. Theoretical simulations revealed that the backscattering coefficient at C‐band is more sensitive to changes of accumulation rates than at Ku‐band. Penetration depths can vary significantly, dependent on radar frequency and firn characteristics. This has to be taken into account when comparing accumulation rates from different locations.

Dielectric permittivity of snow measured along the route traversed in the Japanese–Swedish Antarctic Expedition 2007/08

AbstractAs a joint contribution of Japan and Sweden to the International Polar Year 2007–09, a field expedition between Syowa and Wasa stations in East Antarctica was carried out in the 2007/08 austral summer season. Along the 2800 km long expedition route, the dielectric permittivity of the upper 1 m snow layer was measured at intervals of approximately 50 km using a snow fork, a parallel-wire transmission-line resonator. More than 2000 measurements were performed under carefully calibrated conditions, mostly in the interior of Antarctica. The permittivity ε′ was a function of snow density as in previous studies on dry snow, but the values were significantly smaller than those reported before. In the light of the dielectric mixture theory, the relatively smaller ε′ obtained in this study can be attributed to the snow structures characteristic in the studied region. Our data suggest that the permittivity of snow in the Antarctic interior is significantly affected by weak bonding between snow grains, which is due to depth-hoar formation in the extremely low-temperature conditions.

The Characteristics and Classification of Australian Snow Cover: an Ecological Perspective

We provide a description of the structural and thermal characteristics of snow cover in the Snowy Mountains of southeast Australia. Using the snow classification system developed by Sturm et al. (1995), the snow cover in the Snowy Mountains is classified primarily as maritime in areas where there is sufficient accumulation, and as ephemeral at lower elevations and on ablating aspects. Maritime snow is generally deep (>100 cm), with a density >0.30 g cm−3. The snow-ground interface is maintained within 1°C of freezing and relatively high air temperatures promote equitemperature metamorphism throughout the winter. The formation of depth hoar (temperature gradient snow), which is considered to be important in facilitating the development of the subnivean space, does not occur under these conditions. Ephemeral snow is characterized by warm shallow snow that often melts before new snow is deposited. Basal melt is a common feature of snow cover in the Snowy Mountains throughout most of the winter. We reappraise the processes responsible for the formation of the subnivean space under Australian snow conditions and discuss the importance of these processes for understanding the ecology of fauna in the subnivean space. Our findings highlight the value of an explicit description of snow conditions to ecological research in snow-covered areas. As a minimum, researchers should provide a description of basic structural and thermal properties of the snow pack that would allow other researchers to view the work in an appropriate context.

Problems with the retrieval of glacier net surface balance from SAR imagery

AbstractThere are relatively few comparisons between synthetic aperture radar (SAR) observations and glacier mass-balance measurements. More typically, SAR has been deployed to identify changes in the end-of-summer snowline and other facies boundaries. In this paper, we analyze the geophysical processes affecting SAR amplitude data over two Arctic glacier systems in northern Scandinavia to assess the potential of SAR observations for the retrieval of surface balance parameters. Using a backscatter model and in situ data, we identify the controls on SAR imagery in terms of mass-balance measurement and discuss the glaciological parameters which can reasonably be derived from multi-temporal SAR data. Our results show that amplitude SAR imagery, in the absence of in situ measurements, is not capable of providing meaningful mass-balance data. We show that backscatter from temperate glaciers is affected primarily by snow grain-size and density, and therefore processes such as firnification or depth-hoar formation can complicate the analysis of imagery. We conclude that SAR imagery over temperate glaciers can provide valuable proxy information but not direct mass-balance terms.

SNOWPACK model simulations for snow in Hokkaido, Japan

AbstractNumerical simulations using SNOWPACK, a snow-cover model, were carried out to confirm the model’s applicability to conditions in Hokkaido, Japan, where temperatures are fairly low and for 3 months the snow surface is usually dry except during occasional periods of rain or above-freezing temperatures. The simulations were conducted for Sapporo, Kitami and Niseko using meteorological data, and the results were compared with the observed snow profiles. In Sapporo, snow-profile observations were carried out every day for two winters. In Niseko, one of the most popular ski resorts in Japan, an avalanche accident occurred on 28 January 1998 and a snow pit was dug through the fracture line the next morning. The simulated snow profiles agreed fairly well with the observed ones. However, near the surface we observed depth hoar, which can be an important factor in avalanche release after successive snowfalls, that the model did not reproduce distinctly. Extending the model’s metamorphism laws with an expression of depth-hoar formation under a large temperature gradient, as formulated from an experiment by Fukuzawa and Akitaya (1993), the model reproduced the depth hoar adequately.

Snow megadunes in Antarctica: Sedimentary structure and genesis

Megadune fields occupy large areas in the interior of the East Antarctic ice sheet and are the result of unusual snow accumulation and redistribution processes. They therefore are important to surface mass balance and ice core interpretation. Field observations (GPS, GPR, and surface measurements) have provided a detailed description of megadune sedimentation and morphology over a 70 km2 area, located 200 km east of Dome C. A combination of remote sensing analysis (using Landsat and satellite radar altimetry) and field measurements indicate that slope in the prevailing wind direction (SPWD) and climatic conditions play a crucial role in megadune genesis. The megadune areas tend to be characterized by slightly steeper regional slope and the presence of highly persistent katabatic winds. The megadunes represent 2 to 4 m amplitude waves of 2 to 5 km wavelength formed by variable net accumulation, ranging between 25% (leeward faces) to 120% (windward faces) of the accumulation in adjacent nonmegadune areas. Leeward faces are characterized by glazed, sastrugi‐free surfaces and extensive depth hoar formation. Windward faces are covered by large rough sastrugi up to 1.5 m in height.

Snowpack model calculations for avalanche warning based upon a new network of weather and snow stations

The Swiss Federal Institute for Snow and Avalanche Research (SLF) began to construct a network of high Alpine automated weather and snow measurement stations in the Summer of 1996. Presently, more than 50 stations are in operation. The stations measure wind, air temperature, relative humidity, snow depth, surface temperature, ground (soil) temperature, reflected short wave radiation and three temperatures within the snowpack. The measurements are transferred hourly to the SLF in Davos and the data are used to drive a finite-element based physical snowpack model. The model runs every hour and provides supplementary information regarding the state of the snowpack at the sites of the automatic stations. New snow amounts, settling rates, possible surface hoar formation, temperature and density profiles as well as the metamorphic development (grain types) of the snowpack are all predicted by the model. The model is based on a Lagrangian finite element implementation but solves the instationary heat transfer and settlement equations. It includes phase changes and transport of water vapor and liquid water. Special attention is given to the metamorphism of snow and its connection with the mechanical properties such as thermal conductivity and viscosity. The model is connected to a relational database that stores the measurements as well as the model results. New visualization tools are available which allow a fast, easy and comprehensive access to the stored data. The model has been tested in operational mode during the Winter of 1998/1999. The calculations is reliable in terms of the energy budget and the mass balance. The implemented snow metamorphism formulations yield reasonable grain types and are able to reproduce important processes such as formation of depth hoar. The results of the simulations are used by local, regional and national avalanche forecasters and provide valuable information on the snow conditions in the vicinity of avalanche starting zones during the catastrophic avalanche situation in February 1999.

Winter snow cover variability on East Antarctic sea ice

Analysis of the first detailed data set of snow characteristics collected over East Antarctic sea ice in winter confirms that on small scales, snow on Antarctic sea ice is highly variable in both thickness and properties. High‐amplitude cyclical variability in atmospheric forcing related to the passage of storms is responsible for the high degree of textural heterogeneity observed. Changes in snow properties were examined over a 3‐week period, during which a largely icy snow cover, formed at near‐freezing temperatures, metamorphosed to snow in which facetted crystals and depth hoar dominated, as the air temperature plummeted. Even on flat ice, significant localized thickening of snow occurs in the form of barchan dunes. Although we observed great variability in snow thickness and properties on local scales, overall snow thickness distribution and the complex textural assemblage of snow types are similar from region to region. Similar observations were made by Sturm et al. [1998] in West Antarctica. Large‐scale similarities are also apparent in mean snow density, grain size, and bulk snow salinity, although high variability is again found across individual floes. Rapid depth hoar formation is a ubiquitous process that greatly affects the density, texture, grain size, and effective thermal conductivity of the snow cover. The observed heterogeneity results in varying snow effective thermal conductivities. The mean bulk effective thermal conductivity, computed from the proportion of observed snow types, is 0.164 W m−1 K−1, significantly lower than values typically used in large‐scale sea ice modeling but similar to that derived by Sturm et al. [1998] in a near‐simultaneous experiment in the Bellingshausen and Ross Seas. It varies from 0.097 to 0.383 W m−1 K−1 in different snow pits. The findings support those of Sturm et al. [1998] that periodic flooding and subsequent snow ice formation, which are also ubiquitous processes, effectively diminish the degree to which basal snow processes create inhomogeneities in the snow pack.

A Model for Deuterium and Oxygen 18 Isotope Changes During Evergreen Interception of Snowfall

A one‐dimensional, physically based numerical model was constructed to describe the isotopic enrichment observed in throughfall of snow intercepted on evergreens. The process of enrichment is similar to that which results in formation of depth hoar in snowpack. On‐site data were obtained at a high‐altitude (3500 m) watershed in the Colorado Rocky Mountains. The model includes the ambient atmospheric variables of temperature, relative humidity, and water vapor isotopic composition and the intercepted snow variables of temperature profile, permeability for viscous flux, and isotopic composition. Model simulations yield results similar to those observed on site and suggest that the process is dominated by diffusive flux despite the very high permeability of freshly fallen snow. Median enrichments were observed to be 2.1‰ in oxygen 18 and 13‰ in deuterium.

Characterization of a hoar-development episode using SSM/I brightness temperatures in the vicinity of the GISP2 site, Greenland

Formation of a surface-hoar/depth-hoar complex at the GISP2 site in central Greenland was correlated with large changes in Special Sensor Microwave/Imager (SSM/I) brightness-temperature data. Pass-averaged SSM/I brightness-temperature data over a 1/2° latitude by 1° longitude cell for the 19 and 37 GHz, vertically (V) and horizontally (Η) polarized bands were manipulated to yield differential (V-Η) trends which clearly show a gradual decline as the hoar formation caused a progressively rougher surface with progressively lower density. The hoar episode ended as snowfall, and high winds buried and destroyed the surface-hoar layer and caused rapid V-Η increases in ≈ 1 day. Comparison of the different trends with changes in the field-monitored variables and theoretical values suggest that the V-Η trends are sensitive primarily to changes in surface roughness, and secondarily to near-surface density changes. Consistent expression of trends in microwave brightness temperature over 35 adjacent study cells indicates that this technique may provide a remote-sensing signature capable of defining the timing and spatial extent of surface- and depth-hoar formation in central Greenland.

Characterization of a hoar-development episode using SSM/I brightness temperatures in the vicinity of the GISP2 site, Greenland

Formation of a surface-hoar/depth-hoar complex at the GISP2 site in central Greenland was correlated with large changes in Special Sensor Microwave/Imager (SSM/I) brightness-temperature data. Pass-averaged SSM/I brightness-temperature data over a 1/2° latitude by 1° longitude cell for the 19 and 37 GHz, vertically (V) and horizontally (Η) polarized bands were manipulated to yield differential (V-Η) trends which clearly show a gradual decline as the hoar formation caused a progressively rougher surface with progressively lower density. The hoar episode ended as snowfall, and high winds buried and destroyed the surface-hoar layer and caused rapid V-Η increases in ≈ 1 day. Comparison of the different trends with changes in the field-monitored variables and theoretical values suggest that the V-Η trends are sensitive primarily to changes in surface roughness, and secondarily to near-surface density changes. Consistent expression of trends in microwave brightness temperature over 35 adjacent study cells indicates that this technique may provide a remote-sensing signature capable of defining the timing and spatial extent of surface- and depth-hoar formation in central Greenland.

Acquisition of Natural Remanent Magnetization for Dirt-Snow Containing Rock Dust.

Magnetization process of dirt-snow containing fine-grained magnetic minerals was investigated in order to understand the acquisition mechanism of natural remanent magnetization (NRM) by dirt-ice layers in Antarctica. Dirt-snow acquired NRM from the direction of geomagnetic field (GMF) direction at rooms -10°C and -20°C, and inclination becomes shallower than the GMF after 50 days.The following two types of NRM acquisition mechanisms in the GMF are proposed; (1) magnetic grains align along the GMF when they drop onto the surfaces of underlying snow due to vaporization of snow; (2) the magnetic grains are rotated to align in the GMF direction on the snow surfaces by the torque with the GMF and the NRM. The compaction of the natural snow at formation of depth hoar works to flatten the NRM direction.Probably the Antarctic dirt-ice has acquired NRM during the periods of accumulation of snow and magnetic grains and formation of the depth hoar. The NRM of dirt-snow and -ice are usuful for understanding the glacial dynamics and paleomagnetism of the Antarctic ice sheet, although many problems still remain.

Summertime formation of Depth Hoar in central Greenland

Summertime solar heating of near‐surface snow in central Greenland causes mass loss and grain growth. These depth hoar layers become seasonal markers which are observed in ice cores and snow pits. Mass redistribution associated with depth‐hoar formation can change concentrations of immobile chemicals by as much as a factor of two in the depth hoar, altering atmospheric signals prior to archival in ice. For methanesulfonic acid (MSA) this effect is not significant because the summer maximum does not coincide with the density minimum, and the amplitude of the annual (MSA) signal is more than a factor of ten.

Snowfall and Oxygen-Isotope Variations off the North Coast of Ellesmere Island, N.W.T., Canada

AbstractSnow-pack along the land-fast ice fringe off the north coast of Ellesmere Island was generally characterized by depth-hoar overlain by dense snow and wind slab. Mean snow depth in the study area was 0.54 m (1982-85) and the mean δ18O value of the snow-pack was -31.3˚/00. Isotope data were not obtained previously for this geographic region and, therefore, complement a previous study of δ18O variations in High Arctic snow (Koerner, 1979). The data are consistent with an Arctic Ocean moisture source. The δ18O profiles show seasonal variations, with winter snow being more depleted in18O than fall and spring snow. However, the δ18O profiles are dominated by a trend to higher δ18O values with increasing depth. This is attributed to a decrease in δ18O values as condensation temperatures fall during the autumn-winter accumulation period. During this time, there is also a change from relatively open to almost complete ice cover in the Arctic Ocean. The change in evaporation conditions and consequent effect on δ values gives rise to a sharp discontinuity in the δ18O profiles and a bi-modal δ18O frequency distribution. The bi-modal distribution is reinforced by a secondary isotope fractionation that occurs during depth-hoar formation. This isotope effect leads to a wider δ18O range but does not significantly alter the mean δ18O value.