Ice-snow Interface Research Articles

The Impact of Diffusive Water Vapor Transport on Snow Profiles in Deep and Shallow Snow Covers and on Sea Ice

Water vapor transport has been highlighted as a critical process in Arctic snowpacks, shaping the snow cover structure in terms of density, thermal conductivity, and temperature profile among others. Here, we present an attempt to describe the thermally-induced vertical diffusion of water vapor in the snow cover and its effects of the snowpack structure using the SNOWPACK model. Convection, that may also constitute a significant part of vapor transport, is not addressed. Assuming saturated conditions at the upper boundary of the snowpack and as initial condition, the vapor flux between snow layers is expressed by a 1-dimensional transient diffusion equation, which is solved with a finite difference routine. The implications on the snowpack of this vertical diffusive flux, are analyzed using metrics such as the cumulative density change due to diffusive vapor transport, the degree of over- or undersaturation, the instantaneous snow density change rate, and the percentage of snow density change. We present results for four different regions sampling the space of natural snow cover variability: Alpine, Subarctic, Arctic, and Antarctic sea ice. The largest impact of diffusive water vapor transport is observed in snow on sea ice in the Weddell Sea and the shallow Arctic snowpack. The simulations show significant density reductions upon inclusion of diffusive water vapor transport: cumulative density changes from diffusive vapor transport can reach �62 and �66 kg m�3 for the bottom layer in the shallow Arctic snowpack and snow on sea ice, respectively. For comparison, in deeper snow covers, they rarely exceed �40 kg m�3. This leads to changes in density for shallow snowpacks at the soil-snow interface in the range of �5 to �21%. Mirroring the density decease at depth is a thicker deposition layer above it with increase in density around 7.5%. Similarly, for the sea ice, the density decreased at the sea ice-snow interface by �20%. We acknowledge that vapor transport by diffusion may in some snow covers�such as in thin tundra snow�be small compared to convective transport, which will have to be addressed in future work.

Brief communication: Conventional assumptions involving the speed of radar waves in snow introduce systematic underestimates to sea ice thickness and seasonal growth rate estimates

Abstract. Pan-Arctic sea ice thickness has been monitored over recent decades by satellite radar altimeters such as CryoSat-2, which emits Ku-band radar waves that are assumed in publicly available sea ice thickness products to penetrate overlying snow and scatter from the ice–snow interface. Here we examine two expressions for the time delay caused by slower radar wave propagation through the snow layer and related assumptions concerning the time evolution of overlying snow density. Two conventional treatments introduce systematic underestimates of up to 15 cm into ice thickness estimates and up to 10 cm into thermodynamic growth rate estimates over multi-year ice in winter. Correcting these biases would impact a wide variety of model projections, calibrations, validations and reanalyses.

Change of structure and some physical properties of level fast ice during the spring and summer period of 2014 in the vicinity the research station “Ice base Cape Baranov”

The determination of the main physical properties of the fast ice around the research station “Ice base Cape of Baranov” was carried out from May 23 to August 7, 2014. At the beginning of the observations the ice was a system of three main layers reflecting the conditions of fast ice formation. The top 70 cm of the ice was formed under the conditions of dynamic variability. The ice formation to the level of 110 cm was more stable, and the lowerst layer below 110 cm grew without hummocking and thawing. In the second decade of June the ice accretion began on top due to recrystallization on snow and ice boundary. In the third decade of June the accretion process at the ice-snow interface was replaced by surface melting and destruction layer development. These changes are assumed to begin as maximum air temperatures keep above zero. By the second decade of July all the ice core had undergone serious internal changes. In the third decade of July the process of inter-crystalline-binding degradation affected the whole ice core.From the end of the third decade of June the formation of plate-like crystals was registered at the lower ice border, as well as the accretion of new ice in the form of layesr of various thickness and density. The process had reached the maximum by mid July when new ice up to 30 cm thick was formed. In the third decade of July this ice begun to melt. New ice accretion is primarily due to the presence of a layer of desalinated water under the ice.

Estimating snow depth over Arctic sea ice from calibrated dual-frequency radar freeboards

Abstract. Snow depth on sea ice remains one of the largest uncertainties in sea ice thickness retrievals from satellite altimetry. Here we outline an approach for deriving snow depth that can be applied to any coincident freeboard measurements after calibration with independent observations of snow and ice freeboard. Freeboard estimates from CryoSat-2 (Ku band) and AltiKa (Ka band) are calibrated against data from NASA's Operation IceBridge (OIB) to align AltiKa with the snow surface and CryoSat-2 with the ice–snow interface. Snow depth is found as the difference between the two calibrated freeboards, with a correction added for the slower speed of light propagation through snow. We perform an initial evaluation of our derived snow depth product against OIB snow depth data by excluding successive years of OIB data from the analysis. We find a root-mean-square deviation of 7.7, 5.3, 5.9, and 6.7 cm between our snow thickness product and OIB data from the springs of 2013, 2014, 2015, and 2016 respectively. We further demonstrate the applicability of the method to ICESat and Envisat, offering promising potential for the application to CryoSat-2 and ICESat-2, which launched in September 2018.

Methane-oxidizing seawater microbial communities from an Arctic shelf

Abstract. Marine microbial communities can consume dissolved methane before it can escape to the atmosphere and contribute to global warming. Seawater over the shallow Arctic shelf is characterized by excess methane compared to atmospheric equilibrium. This methane originates in sediment, permafrost, and hydrate. Particularly high concentrations are found beneath sea ice. We studied the structure and methane oxidation potential of the microbial communities from seawater collected close to Utqiagvik, Alaska, in April 2016. The in situ methane concentrations were 16.3 ± 7.2 nmol L−1, approximately 4.8 times oversaturated relative to atmospheric equilibrium. The group of methane-oxidizing bacteria (MOB) in the natural seawater and incubated seawater was > 97 % dominated by Methylococcales (γ-Proteobacteria). Incubations of seawater under a range of methane concentrations led to loss of diversity in the bacterial community. The abundance of MOB was low with maximal fractions of 2.5 % at 200 times elevated methane concentration, while sequence reads of non-MOB methylotrophs were 4 times more abundant than MOB in most incubations. The abundances of MOB as well as non-MOB methylotroph sequences correlated tightly with the rate constant (kox) for methane oxidation, indicating that non-MOB methylotrophs might be coupled to MOB and involved in community methane oxidation. In sea ice, where methane concentrations of 82 ± 35.8 nmol kg−1 were found, Methylobacterium (α-Proteobacteria) was the dominant MOB with a relative abundance of 80 %. Total MOB abundances were very low in sea ice, with maximal fractions found at the ice–snow interface (0.1 %), while non-MOB methylotrophs were present in abundances similar to natural seawater communities. The dissimilarities in MOB taxa, methane concentrations, and stable isotope ratios between the sea ice and water column point toward different methane dynamics in the two environments.

Investigating the thermophysical properties of the ice–snow interface under a controlled temperature gradient Part II: Analysis

In order to develop a more intuitive understanding of the physical mechanisms and processes responsible for enhanced kinetic snow metamorphism at the ice–snow interface, we have performed a detailed and quantitative analysis of the in situ micro-thermocouple data originally presented in Part I of this study. In our detailed analysis, we have focused primarily on the observed temperature gradients from within one millimeter above and below the ice–snow interface, as measured in our laboratory prepared specimen. Our findings show via a simple one-dimensional model for energy balance that thermal contact resistance followed by decreases in the effective thermal conductivity are the primary contributors to the dramatic increases in the local temperature gradient near the ice–snow interface. Additional mechanisms for heat and mass transfer are also reviewed in our analysis, including the water vapor flux and latent heat flux.

DMSP and DMS cycling within Antarctic sea ice during the winter–spring transition

This study describes within-ice concentrations of dimethylsulfoniopropionate (DMSP), its degradation product dimethylsulphide (DMS), as well as nutrients and chlorophyll a, that were sampled during the Sea Ice Physics and Ecosystems eXperiment-2 (SIPEX-2) in 2012. DMSP is a methylated substrate produced in large amounts annually by ice-associated microalgae, while DMS plays a significant role in carbon and sulphur cycling in the Southern Ocean. In the East Antarctic study area between 115–125°E and 64–66°S, ice and slush cores, brine, under-ice seawater and zooplankton (Antarctic krill) samples were collected at 6 ice stations. The pack-ice was characterised by high snow loading which initiated flooding events and triggered nutrient supply to the sea-ice surface, while variation in ice conditions influenced sea-ice permeability. This ranged from impermeable surface and middle sections of the sea ice, to completely permeable ice cores at some stations. Chlorophyll a maxima shifted from the sea-ice surface horizon at the first station to the sea ice bottom layer at the last station. Highest DMSP concentrations were detected in brine samples at the sea-ice surface, reflecting a mismatch with respect to the distribution of chlorophyll a. Our data suggest enhanced DMSP production by sea-ice surface algal communities and its release into brine during freezing and melting, which in turn is coupled to flooding events early in the season. A time-cycle of DMS production by DMSP degradation and DMS efflux is evident at the sea ice-snow interface when slush is formed during melt. Seawater under the ice contained only low concentrations of DMSP and DMS, even when brine drainage was evident and the sea ice became permeable. We postulate that in situ grazing by zooplankton may act as sink for the DMSP produced early in the season.

Investigating the thermophysical properties of the ice–snow interface under a controlled temperature gradient: Part I: Experiments & Observations

Of critical importance for avalanche forecasting, is the ability to draw meaningful conclusions from only a handful of field observations. To that end, it is common for avalanche forecasters to not only have to rely on sparse data, but also on their own intuitive understanding of how their field-based observations may be correlated to complex physical processes responsible for structural instability within a snowpack. One such well-documented basis for mechanical instability to increase within a snowpack is that caused by the presence of a buried ice lens or ice crust. Although such icy layers are naturally formed and frequently encountered in seasonal snowpacks, very little is known about the microstructural evolution of these layers and how they contribute toward weak layer development. Furthermore, in terms of assessing the structural integrity of the snowpack, there is at the present time no consistent treatment for identifying these layers a priori as problematic or benign. To address this issue, we have created an idealized laboratory scenario in which we can study how an artificially created ice lens may affect the thermophysical and microstructural state of the interface between the ice lens and adjacent layers of snow while under a controlled temperature gradient of primarily −100Km−1. Utilizing in situ micro-thermocouple measurements, our findings show that a super-temperature gradient exists within only a millimeter of the ice lens surface that is many times greater than the imposed bulk temperature gradient. Such large temperature gradients on such a small scale would not be measurable by most field-based instrumentation and to our knowledge these laboratory-based in situ measurements are the first of their kind. Additionally, we have also investigated and characterized the microstructural evolution of the ice–snow interface with X-ray Micro-computed Tomography and Scanning Electron Microscopy. In our analysis, we have been able to identify distinct regions of simultaneous ice crystal growth, sublimation, and kinetic snow metamorphism. We hold that these observations are both consistent with previous laboratory studies and observations made in the natural environment.

Frost flowers growing in the Arctic ocean‐atmosphere–sea ice–snow interface: 1. Chemical composition

Frost flowers, intricate featherlike crystals that grow on refreezing sea ice leads, have been implicated in lower atmospheric chemical reactions. Few studies have presented chemical composition information for frost flowers over time and many of the chemical species commonly associated with Polar tropospheric reactions have never been reported for frost flowers. We undertook this study on the sea ice north of Barrow, Alaska to quantify the major ion, stable oxygen and hydrogen isotope, alkalinity, light absorbance by soluble species, organochlorine, and aldehyde composition of seawater, brine, and frost flowers. For many of these chemical species we present the first measurements from brine or frost flowers. Results show that major ion and alkalinity concentrations, stable isotope values, and major chromophore (NO3− and H2O2) concentrations are controlled by fractionation from seawater and brine. The presence of these chemical species in present and future sea ice scenarios is somewhat predictable. However, aldehydes, organochlorine compounds, light absorbing species, and mercury (part 2 of this research and Sherman et al. (2012)) are deposited to frost flowers through less predictable processes that probably involve the atmosphere as a source. The present and future concentrations of these constituents in frost flowers may not be easily incorporated into future sea ice or lower atmospheric chemistry scenarios. Thinning of Arctic sea ice will likely present more open sea ice leads where young ice, brine, and frost flowers form. How these changing ice conditions will affect the interactions between ice, brine, frost flowers and the lower atmosphere is unknown.

Frost flowers growing in the Arctic ocean‐atmosphere–sea ice–snow interface: 2. Mercury exchange between the atmosphere, snow, and frost flowers

Frost flowers are ice crystals that grow on refreezing sea ice leads in Polar Regions by wicking brine from the sea ice surface and accumulating vapor phase condensate. These crystals contain high concentrations of mercury (Hg) and are believed to be a source of reactive halogens, but their role in Hg cycling and impact on the fate of Hg deposited during atmospheric mercury depletion events (AMDEs) are not well understood. We collected frost flowers growing on refreezing sea ice near Barrow, Alaska (U.S.A.) during an AMDE in March 2009 and measured Hg concentrations and Hg stable isotope ratios in these samples to determine the origin of Hg associated with the crystals. We observed decreasing Δ199Hg values in the crystals as they grew from new wet frost flowers (mean Δ199Hg = 0.77 ± 0.13‰, 1 s.d.) to older dry frost flowers (mean Δ199Hg = 0.10 ± 0.05‰, 1 s.d.). Over the same time period, mean Hg concentrations in these samples increased from 131 ± 6 ng/L (1 s.d.) to 180 ± 28 ng/L (1 s.d.). Coupled with a previous study of Hg isotopic fractionation during AMDEs, these results suggest that Hg initially deposited to the local snowpack was subsequently reemitted during photochemical reduction reactions and ultimately accumulated on the frost flowers. As a result of this process, frost flowers may lead to enhanced local retention of Hg deposited during AMDEs and may increase Hg loading to the Arctic Ocean.

Ultraviolet attenuation by dissolved and particulate constituents of first‐year ice during late spring in an Arctic polynya

Vertical profiles of ultraviolet radiation (UVR) and photosynthetically available radiation (PAR) were measured under five large ice floes located in the North Water Polynya, northern Baffin Bay, in June 1998. Together with incident irradiance measurements, these profiles were used to assess the irradiance attenuation by the ice and its constituents. We also measured vertical distribution of absorption by colored dissolved organic matter (CDOM) and particulate organic matter (POM) in three melted ice cores. The ice thickness and snow depth varied from 0.5 to 1.3 m and from 1 to 9 cm, respectively. The ice–snow interface was infiltrated by meltwater. About 2–13% of incident UV‐B irradiance was transmitted through the snow, ice, and ice algae biomass; transmittance increased to 5–19% for UV‐A and to 5–12% for PAR. CDOM and POM contributed significantly to the attenuation of irradiance within the ice. The relatively high UVR transparency found in this study coincided with the seasonal maximum of incident UV irradiance. Hence, the resulting very high UVR:PAR ratio could affect the communities in the sea ice, at the ice–water interface, and in the surface waters underneath the ice cover. In addition, the strong absorption by CDOM found in this high‐UVR environment indicates that significant photochemical reactions could occur.

On the use of electromagnetic induction sounding to determine winter and spring sea ice thickness in the Antarctic

The suitability of using a portable electromagnetic induction (EMI) sounding device for determining sea ice thickness under winter and spring conditions in the Antarctic is evaluated. Ice thickness estimates from the instrument are compared with drilled measurements along a series of transects on East Antarctic pack ice and fast ice. The data indicate that the instrument estimates the undeformed sea ice thickness to within 10% of drilled thickness measurements; however, thickness estimates over deformed ice show much larger deviations. Sea ice conductivity profiles along each transect were calculated using ice core salinity data and ice-snow interface temperatures. The analysis indicates that first-year Antarctic sea ice generally has a higher conductivity (35–75 mS m −1) than reported for Arctic sea ice. However, this does not adversely affect the EMI sounding technique for determining sea ice thickness as this conductivity is still significantly less than for seawater, and the ice effectively remains transparent to the EM field. The EMI instrument used in this experiment was fitted with an ice thickness processing module (PM) which provided accurate estimates of sea ice thickness once correctly calibrated. The results from these preliminary experiments over winter and spring Antarctic sea ice indicate that EMI sounding has great potential to provide thickness data over the undeformed component of the Antarctic pack ice at this time of year.

Near-Surface Temperatures in the Superimposed Ice Zone and Lower Part of the Soaked Zone of Polar Ice Sheets

Near-Surface Temperatures in the Superimposed Ice Zone and Lower Part of the Soaked Zone of Polar Ice Sheets

Near-Surface Temperatures in the Superimposed Ice Zone and Lower Part of the Soaked Zone of Polar Ice Sheets

Near-Surface Temperatures in the Superimposed Ice Zone and Lower Part of the Soaked Zone of Polar Ice Sheets