Meltwater Layer Research Articles

Biophysical characterization of summer Arctic sea-ice habitats using a remotely operated vehicle-mounted underwater hyperspectral imager

The impact of a rapidly shifting sea-ice cover on climate, ecosystem processes and biophysical habitat properties is not yet fully understood, particularly in the central Arctic Ocean, due to a lack of spatially representative observations. From June to July 2020 during the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC, leg 4) in the Transpolar Drift, we deployed an underwater hyperspectral imager (UHI) mounted on a remotely operated vehicle (ROV) to characterize the biophysical properties of different sea-ice habitats. We conducted UHI surveys along two transects: i) under level first-year sea ice (FYI), which had a mean sea-ice draft of 1.4 m and was composed of primarily level FYI but also had a relatively shallow ridge (keel depth ∼2.6 m); and ii) under the flank of a ridge, named Jaridge, with a mean ice draft of 1.7 m, which was composed of a mix of level ice and thicker ridge blocks with over 3 m draft. We present a new unsupervised bio-optical quantification algorithm for hyperspectral surveys, the relative ice algal biomass index (RBI), using spectral mixture analysis (SMA). We compare this method to the supervised machine learning habitat classification algorithm, Support Vector Machine (SVM). The RBI showed good agreement to literature-based normalized difference indices (NDI) and PCA analyses, which confirm the RBI as a reliable unsupervised index for ice algal biomass. Our biophysical characterization of the two surveyed regions showed an association of sea-ice algal biomass with sea-ice ridge features. Our surveys also indicate that ice algal spatial distribution may be influenced by ice melt rates, and the formation of under ice meltwater layers and false bottoms. With high spatial coverage (>100 m) at microscale resolution (∼cm) we documented large spatial variability of summer Arctic sea-ice algal biomass and different patterns between adjacent ice habitats. We further demonstrate the need for improved understanding of sea-ice algal spatial variability as a complementary tool for sea-ice biogeochemical sampling using destructive ice core sampling.

Stepwise Subduction Observed at a Front in the Marginal Ice Zone in Fram Strait

AbstractAt high latitudes, submesoscale dynamics act on scales of (100 m–1 km) and are associated with the breakdown of geostrophic balance, vertical velocities, and energy cascading to small scales. Submesoscale features such as fronts, filaments, and eddies are ubiquitous in marginal ice zones forced by the large horizontal density gradients. In July 2020, we identified multiple fronts and filaments using a towed undulating vehicle near the sea ice edge in central Fram Strait, the oceanic gateway to the Arctic Ocean between Greenland and Svalbard. Sea ice covered the entire study region 1–2 weeks earlier, and a stratified meltwater layer was present. We observed a front between warm and saline Atlantic Water (AW) and cold and fresh Polar Water (PW) at 30–85 m depth, where we identified a subsurface maximum in chlorophyll fluorescence and other biogeochemical properties extending along the tilted isopycnals down to 75 m, indicating subduction of AW (mixed with meltwater) that had previously occurred. The meltwater layer also featured multiple shallow fronts, one of which exhibited high velocities and a subsurface maximum in chlorophyll fluorescence, possibly indicating subduction of PW below the meltwater layer. The fronts at different depth levels suggest a stepwise subduction process near the ice edge, where water subducts from the surface below the meltwater and then further down along subsurface fronts. The submesoscale features were part of a larger‐scale mesoscale pattern in the marginal ice zone. As sea ice continuously retreats, such features may become more common in the Arctic Ocean.

Thin and transient meltwater layers and false bottoms in the Arctic sea ice pack—Recent insights on these historically overlooked features

The rapid melt of snow and sea ice during the Arctic summer provides a significant source of low-salinity meltwater to the surface ocean on the local scale. The accumulation of this meltwater on, under, and around sea ice floes can result in relatively thin meltwater layers in the upper ocean. Due to the small-scale nature of these upper-ocean features, typically on the order of 1 m thick or less, they are rarely detected by standard methods, but are nevertheless pervasive and critically important in Arctic summer. Observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in summer 2020 focused on the evolution of such layers and made significant advancements in understanding their role in the coupled Arctic system. Here we provide a review of thin meltwater layers in the Arctic, with emphasis on the new findings from MOSAiC. Both prior and recent observational datasets indicate an intermittent yet long-lasting (weeks to months) meltwater layer in the upper ocean on the order of 0.1 m to 1.0 m in thickness, with a large spatial range. The presence of meltwater layers impacts the physical system by reducing bottom ice melt and allowing new ice formation via false bottom growth. Collectively, the meltwater layer and false bottoms reduce atmosphere-ocean exchanges of momentum, energy, and material. The impacts on the coupled Arctic system are far-reaching, including acting as a barrier for nutrient and gas exchange and impacting ecosystem diversity and productivity.

Study on hydraulic cracking mechanism and propagation characteristics of ice hole based on Peridynamics

Drilling of polar deep ice cores is an important means to study the evolution of the earth's climate. When drilling into complex formations such as brittle ice layer, warm ice layer, basement meltwater layer, or ice-rock interlayer, drilling problems frequently occur, especially the hydraulic cracking of ice holes. This paper studies the dynamic propagation process and the influence mechanism by theory and simulation to solve the problem of cracks initiation and propagation around the ice hole. Based on continuum mechanics, a stress model is established to obtain the stress state around the borehole well. In local continuum mechanics, the stress state is represented by spatial partial differential equations, and its spatial partial derivatives cannot describe the discontinuous situation. Peridynamics (PD) use integral equations to solve this problem effectively. The PD model of the ice hole is established to realize the unified description of the continuous and discontinuous space around the ice hole. The dynamic cracks propagation process, the influence mechanism of hydraulic pressure and fracture toughness are analyzed. The loading rate of ice in the paper and experiment is 0.008 mm/s, the elastic modulus is 9 GPa, and the basic fracture toughness is 175 kPa·m0.5. Case study shows that hydraulic pressure promotes cracks propagation, the hydraulic pressure increases by 1 MPa, the volume ratio of cracks increases from 7.67% to 14.93%. The volume ratio of cracks is defined: the ratio of the cracks volume to the total model volume. Fracture toughness hinders cracks propagation, which increases from 150 kPa·m0.5 to 175 kPa·m0.5, the proportion of cracks volume decreases from 13.85% to 7.67%. When drilling into the formation with prominent ductile-brittle transition, the mud density shall be adjusted in time to control the hydraulic pressure in the hole to reduce cracks propagation and ensure the safety and efficiency of ice drilling.

Snowmelt contribution to Arctic first-year ice ridge mass balance and rapid consolidation during summer melt

Sea ice ridges are one of the most under-sampled and poorly understood components of the Arctic sea ice system. Yet, ridges play a crucial role in the sea ice mass balance and have been identified as ecological hotspots for ice-associated flora and fauna in the Arctic. To better understand the mass balance of sea ice ridges, we drilled and sampled two different first-year ice (FYI) ridges in June–July 2020 during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC). Ice cores were cut into 5 cm sections, melted, then analyzed for salinity and oxygen (δ18O) isotope composition. Combined with isotope data of snow samples, we used a mixing model to quantify the contribution of snow to the consolidated sea ice ridge mass. Our results demonstrate that snow meltwater is important for summer consolidation and overall ice mass balance of FYI ridges during the melt season, representing 6%–11% of total ridged ice mass or an ice thickness equivalent of 0.37–0.53 m. These findings demonstrate that snowmelt contributes to consolidation of FYI ridges and is a mechanism resulting in a relative increase of sea ice volume in summer. This mechanism can also affect the mechanical strength and survivability of ridges, but also contribute to reduction of the habitable space and light levels within FYI ridges. We proposed a combination of two pathways for the transport of snow meltwater and incorporation into ridge keels: percolation downward through the ridge and/or lateral transport from the under-ice meltwater layer. Whether only one pathway or a combination of both pathways is most likely remains unclear based on our observations, warranting further research on ridge morphology.

An evaluation of a physics-based firn model and a semi-empirical firn model across the Greenland Ice Sheet (1980–2020)

Abstract. The Greenland Ice Sheet's (GrIS) firn layer buffers the ice sheet's contribution to sea level rise by storing meltwater in its pore space. However, available pore space and meltwater retention capability is lost due to ablation of the firn layer and refreezing of meltwater as near-surface ice slabs in the firn. Understanding how firn properties respond to climate is important for constraining the GrIS's future contribution to sea level rise in a warming climate. Observations of firn density provide detailed information about firn properties, but they are spatially and temporally limited. Here we use two firn models, the physics-based SNOWPACK model and the Community Firn Model configured with a semi-empirical densification equation (CFM-GSFC), to quantify firn properties across the GrIS from 1980 through 2020. We use an identical forcing (Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) atmospheric reanalysis) for SNOWPACK and the CFM-GSFC in order to isolate firn model differences. To evaluate the models, we compare simulated firn properties, including firn air content (FAC), to measurements from the Surface Mass Balance and Snow on Sea Ice Working Group (SUMup) dataset of snow and firn density. Both models perform well (mean absolute percentage errors of 14 % in SNOWPACK and 16 % in the CFM-GSFC), though their performance is hindered by the spatial resolution of the atmospheric forcing. In the ice-sheet-wide simulations, the 1980–1995 average spatially integrated FAC (i.e., air volume in the firn) for the upper 100 m is 34 645 km3 from SNOWPACK and 28 581 km3 from the CFM-GSFC. The discrepancy in the magnitude of the modeled FAC stems from differences in densification with depth and variations in the sensitivity of the models to atmospheric forcing. In more recent years (2005–2020), both models simulate substantial depletion of pore space. During this period, the spatially integrated FAC across the entire GrIS decreases by 3.2 % (−66.6 km3 yr−1) in SNOWPACK and 1.5 % (−17.4 km3 yr−1) in the CFM-GSFC. These differing magnitudes demonstrate how model differences propagate throughout the FAC record. Over the full modeled record (1980–2020), SNOWPACK simulates a loss of pore space equivalent to 3 mm of sea level rise buffering, while the CFM-GSFC simulates a loss of 1 mm. The greatest depletion in FAC is along the margins and especially along the western margin where observations and models show the formation of near-surface, low-permeability ice slabs that may inhibit meltwater storage.

Meltwater layer dynamics in a central Arctic lead: Effects of lead width, re-freezing, and mixing during late summer

Leads play an important role in the exchange of heat, gases, vapour, and particles between seawater and the atmosphere in ice-covered polar oceans. In summer, these processes can be modified significantly by the formation of a meltwater layer at the surface, yet we know little about the dynamics of meltwater layer formation and persistence. During the drift campaign of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), we examined how variation in lead width, re-freezing, and mixing events affected the vertical structure of lead waters during late summer in the central Arctic. At the beginning of the 4-week survey period, a meltwater layer occupied the surface 0.8 m of the lead, and temperature and salinity showed strong vertical gradients. Stable oxygen isotopes indicate that the meltwater consisted mainly of sea ice meltwater rather than snow meltwater. During the first half of the survey period (before freezing), the meltwater layer thickness decreased rapidly as lead width increased and stretched the layer horizontally. During the latter half of the survey period (after freezing of the lead surface), stratification weakened and the meltwater layer became thinner before disappearing completely due to surface ice formation and mixing processes. Removal of meltwater during surface ice formation explained about 43% of the reduction in thickness of the meltwater layer. The remaining approximate 57% could be explained by mixing within the water column initiated by disturbance of the lower boundary of the meltwater layer through wind-induced ice floe drift. These results indicate that rapid, dynamic changes to lead water structure can have potentially significant effects on the exchange of physical and biogeochemical components throughout the atmosphere–lead–underlying seawater system.

Temporal evolution of under-ice meltwater layers and false bottoms and their impact on summer Arctic sea ice mass balance

Low-salinity meltwater from Arctic sea ice and its snow cover accumulates and creates under-ice meltwater layers below sea ice. These meltwater layers can result in the formation of new ice layers, or false bottoms, at the interface of this low-salinity meltwater and colder seawater. As part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), we used a combination of sea ice coring, temperature profiles from thermistor strings and underwater multibeam sonar surveys with a remotely operated vehicle (ROV) to study the areal coverage and temporal evolution of under-ice meltwater layers and false bottoms during the summer melt season from mid-June until late July. ROV surveys indicated that the areal coverage of false bottoms for a part of the MOSAiC Central Observatory (350 by 200 m2) was 21%. Presence of false bottoms reduced bottom ice melt by 7–8% due to the local decrease in the ocean heat flux, which can be described by a thermodynamic model. Under-ice meltwater layer thickness was larger below first-year ice and thinner below thicker second-year ice. We also found that thick ice and ridge keels confined the areas in which under-ice meltwater accumulated, preventing its mixing with underlying seawater. While a thermodynamic model could reproduce false bottom growth and melt, it could not describe the observed bottom melt rates of the ice above false bottoms. We also show that the evolution of under-ice meltwater-layer salinity below first-year ice is linked to brine flushing from the above sea ice and accumulating in the meltwater layer above the false bottom. The results of this study aid in estimating the contribution of under-ice meltwater layers and false bottoms to the mass balance and salt budget for Arctic summer sea ice.

How ploughing and frictional melting regulate ice-skating friction

The slipperiness of ice is well known while, for ice skating, its mechanism still needs further investigation, where the complex interactions including the thermal conduction of the skate—meltwater—ice system, the ploughing and the frictional melting of ice to the friction force are still unclear. This study presents a theoretical framework and a simplified analytical solution to unveil the friction mechanism when a curved skate sliding on ice. The theory is validated by experiments and the effects of these various factors, including the sliding velocity, the ice temperature, the supporting weight, and the geometry of the skate blade to the friction are revealed in detail. This study finds that the contribution of friction force from the ploughing deformation through skate indentation and that from the fluid friction through the shear motion of the meltwater layer is comparable with each other, which thus clarifies how the ploughing deformation of the ice substrate together with its frictional melting regulates the friction during skating.

Quantifying false bottoms and under-ice meltwater layers beneath Arctic summer sea ice with fine-scale observations

During the Arctic melt season, relatively fresh meltwater layers can accumulate under sea ice as a result of snow and ice melt, far from terrestrial freshwater inputs. Such under-ice meltwater layers, sometimes referred to as under-ice melt ponds, have been suggested to play a role in the summer sea ice mass balance both by isolating the sea ice from saltier water below, and by driving formation of ‘false bottoms’ below the sea ice. Such layers form at the interface of the fresher under-ice layer and the colder, saltier seawater below. During the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition in the Central Arctic, we observed the presence of under-ice meltwater layers and false bottoms throughout July 2020 at primarily first-year ice locations. Here, we examine the distribution, prevalence, and drivers of under-ice ponds and the resulting false bottoms during this period. The average thickness of observed false bottoms and freshwater equivalent of under-ice meltwater layers was 0.08 m, with false bottom ice comprised of 74–87% FYI melt and 13–26% snow melt. Additionally, we explore these results using a 1D model to understand the role of dynamic influences on decoupling the ice from the seawater below. The model comparison suggests that the ice-ocean friction velocity was likely exceptionally low, with implications for air-ice-ocean momentum transfer. Overall, the prevalence of false bottoms was similar to or higher than noted during other observational campaigns, indicating that these features may in fact be common in the Arctic during the melt season. These results have implications for the broader ice-ocean system, as under-ice meltwater layers and false bottoms provide a source of ice growth during the melt season, potentially reduce fluxes between the ice and the ocean, isolate sea ice primary producers from pelagic nutrient sources, and may alter light transmission to the ocean below.

Quantifying the Water‐to‐Melt Mass Ratio and Its Impact on Eruption Plumes During Explosive Hydromagmatic Eruptions

AbstractThe interaction of magma with external water commonly enhances magma fragmentation through the conversion of thermal to mechanical energy and results in an increased production of fine‐grained volcanic tephra. Magma‐water interaction is thus of importance for hazard mitigation on both a local and a regional scales. The relative proportion of water that interacts with magma, quantified as the water‐to‐melt mass ratio, is thought to determine the efficiency of thermal to mechanical energy conversion, termed the fragmentation efficiency. Here, we analyze the pyroclast size distributions from the 10th century Eldgjá fissure eruption in Iceland, where parts of the fissure erupted subglacially and other erupted subaerially. The subglacially erupted magma passed through a column of glacial meltwater, resulting in a larger proportion of finer pyroclast sizes relative to the subaerially erupted, purely magmatic tephra. This finer grain size distribution has been attributed to quench‐granulation induced by enhanced cooling upon interaction with external water. We hypothesize that the additional fragmentation (surface) energy required to produce the finer grained hydromagmatic deposits is due to the conversion of thermal to mechanical energy associated with the entrainment of water into the volcanic jet, as it passed through a column of subglacial melt water. Based on field and granulometry data, we estimate that the interaction of the volcanic jet with the meltwater provided an additional fragmentation energy of approximately 3–14 kJ per kg of pyroclasts. We numerically model the hydrofragmentation energy within a jet that passes through a layer of meltwater. We find that the water‐to‐melt mass ratio of entrained water required to produce the additional fragmentation energy is in the range of 1–2, which requires a minimum ice melting rate of 104 m3 s−1. Our simulation results show that the water‐to‐melt ratio is an important parameter that controls the ascent of plume in the atmosphere.

Overview of the MOSAiC expedition: Physical oceanography

Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present along-drift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean.

Direct Observations of the Role of Lateral Advection of Sea Ice Meltwater in the Onset of Autumn Freeze Up

AbstractIn seasonally ice‐free parts of the Arctic Ocean, autumn is characterized by heat loss from the upper ocean to the atmosphere and the onset of freeze up, in which first year sea ice begins to grow in open water areas. The timing of freeze up can be highly spatially variable, complicating efforts to provide accurate sea ice forecasting for marine operations. While melt season anomalies can be used to predict freeze up anomalies in some parts of the Arctic, this one‐dimensional view merits further examination in light of recent work demonstrating the importance of three‐dimensional flows in setting mixed layer properties in marginal ice zones. In this study, we show that horizontal advection of sea ice meltwater hastens freeze up in areas distant from the ice edge. We use nearly 800 temperature and salinity profiles along with satellite imagery collected in the central Beaufort Sea in autumn 2018 to document the roughly 100 km advection of a cold and fresh surface meltwater layer over several weeks. After the meltwater arrived, the mixed layer was cooler and shallower than the mixed layer in adjacent areas unaffected by the meltwater. The cooler and shallower meltwater‐influenced mixed layer promoted earlier ice formation. Within the meltwater‐affected area, advection was nearly as important as heat loss to the atmosphere for seasonally integrated mixed layer heat loss.

A cost-efficient low-weight autonomous profiler for measurements in polar coastal waters and other regions with strong density gradients

The rapid warming of our planet has resulted in accelerated melting of ice in polar regions. Currently we have limited knowledge on how, where and when the surface meltwater layer is mixed with the underlying ocean due to lack of observations in these remote areas. We present a lightweight (17 kg) and low-cost (6000€) instrument for autonomous profiling across the strongly stratified upper layer in Arctic coastal waters, freshened by the riverine input and meltwater from glaciers, icebergs, and sea ice. The profiler uses a specially designed plunger buoyancy engine to displace up to 700 cm3 of water and allows for autonomous dives to 200 m depth. It can carry different sensor packages and convey its location by satellite communication. Two modes are available: (a) a free-floating mode and (b) a moored mode, where the instrument is anchored to the seafloor. In both modes, the profiler controls its velocity of 12 ± 0.3 cm/s resulting in 510 ± 22 data points per 100 m depth. Equipped with several sensors, e.g. conductivity, temperature, oxygen, and pressure, the autonomous profiler was successfully tested in a remote Northeast Greenlandic fjord. Data has been compared to traditional CTD instrument casts performed nearby.

Ice speed skating: Onset of lubrication by frictional heating

In ice speed skating, the friction between the steel skate blade and the ice stems from boundary friction where the temperature of the interface is below zero and ice surface molecules exhibit unconventional mobility, and hydrodynamic friction where the ice melts and a thin water layer between the blade and the ice forms. An analytical solution of the steady-state lubrication equation shows that the boundary friction only plays a role at the tip of the skate blade over an extremely short contact length between the skate blade and the ice. Albeit its negligible contribution to total friction, boundary friction generates enough heat to melt the ice which allows the skater to slide smoothly on a thin layer of melt water. It is also shown that the precise value of the boundary friction coefficient is not important for speed skate friction because it works only over an extremely short contact length.

Vertical processes and resolution impact ice shelf basal melting: A multi-model study

Understanding ice shelf–ocean interaction is fundamental to projecting the Antarctic ice sheet response to a warming climate. Numerical ice shelf–ocean models are a powerful tool for simulating this interaction, yet are limited by inherent model weaknesses and scarce observations, leading to parameterisations that are unverified and unvalidated below ice shelves. We explore how different models simulate ice shelf–ocean interaction using the 2nd Ice Shelf–Ocean Model Intercomparison Project (ISOMIP+) framework. Vertical discretisation and resolution of the ocean model are shown to have a significant effect on ice shelf basal melt rate, through differences in the distribution of meltwater fluxes and the calculation of thermal driving. Z-coordinate models, which generally have coarser vertical resolution in ice shelf cavities, may simulate higher melt rates compared to terrain-following coordinate models. This is due to the typically higher resolution of the ice–ocean boundary layer region in terrain following models, which allows better representation of a thin meltwater layer, increased stratification, and as a result, better insulation of the ice from water below. We show that a terrain-following model, a z-level coordinate model and a hybrid approach give similar results when the effective vertical resolution adjacent to the ice shelf base is similar, despite each model employing different paradigms for distributing meltwater fluxes and sampling tracers for melting. We provide a benchmark for thermodynamic ice shelf–ocean interaction with different model vertical coordinates and vertical resolutions, and suggest a framework for any future ice shelf–ocean thermodynamic parameterisations.

C-Band Simulations of Melting Icebergs Using GRECOSAR and an EM Model: Varying Wind Conditions at Lower Beam Mode

This article presents an electromagnetic backscatter model of iceberg and compares the modeled scattering behavior with C-band RADARSAT-2 synthetic aperture radar (SAR) images. It also explores iceberg SAR signature variability over various ocean parameters. Three-dimensional (3-D) profiles of icebergs were captured in a field study off the coast of Bonavista, NL, Canada, in June 2017 at the time of an SAR satellite overpass. The 3-D profiles were captured from a vessel, using a LiDAR and multibeam sonar. The SAR image and 3-D profiles were captured within hours of one another. Simulated SAR images of the icebergs were generated in a simulator called GRECOSAR with the satellite, target orientation, and ocean parameters that closely mimic the real SAR scene. A new ocean model was introduced to model an ocean backscatter at satellites' lower incidence angle beam mode. Comparison between real and simulated SAR images of the icebergs shows good agreement in terms of SAR signature, total radar cross section, and polarimetric decomposition. Wind direction was varied over 90° extent to observe icebergs' backscatter variability in the simulator. Furthermore, simulated SAR images were generated for low and high wind conditions. Our study finds that the macrostructure of the melt iceberg dominates its polarimetric behavior of its backscatter. Large variability of iceberg SAR signature over varying ocean parameters was also observed. A mathematical model that considers the melting condition of iceberg suggested that significant backscattering can reflect from top surface when the melt water layer could be as little as 0.1 mm.

Pulse interfacial defrosting

Frost formation and ice accretion are major problems for a plethora of industries. Common defrosting and deicing techniques utilize energy-intensive mechanical actuation to dislodge ice/frost or heating methods to melt ice/frost from surfaces. Here, we develop an ultraefficient method to defrost/deice surfaces by spatially and temporally localizing thermal energy at the substrate-ice/frost interface. To remove ice/frost efficiently, it is beneficial only to melt the interfacial layer adhering the ice/frost to the solid surface by using a localized “pulse” of heat, allowing gravity or gas shear in conjunction with the ultrathin lubricating melt water layer to remove the ice/frost. To probe the physics of pulse defrosting, we first developed a transient numerical heat transfer model. Experimental validation of the model was achieved via pulse (≈100 ms) joule heating of indium tin oxide on glass samples. Utilizing transient heat fluxes ranging from 10 to 100 W/cm2, spontaneous melting of the interfacial ice/frost layer was achieved, leading to rapid ice removal. We employed our validated model to outline design guidelines for pulse defrosting applications, showing <1% of the energy and <0.01% of the defrosting time needed when compared to conventional thermal-based defrosting methods.

Investigation of steam explosion duration in stratified configuration

A steam explosion is an energetic fuel-coolant interaction process, which may occur during a severe reactor accident when the molten core comes into contact with the coolant water. In the stratified melt-coolant configurations, where the corium melt is released in a shallow water pool or in an initially dry cavity later flooded with coolant water, the melt-water interaction may occur on the interface between the bottom melt layer and the upper water layer. It was long believed that stratified melt-water conditions cannot generate strong explosive interactions, because it was assumed that there is no significant premixing of melt and coolant water prior to the shock wave propagation. However, the recently performed experiments in the PULiMS and SES (KTH, Sweden) facilities with corium simulant materials revealed that strong steam explosions may spontaneously develop also in a stratified melt-water configuration, where a formation of a considerable melt-water premixed layer was observed. Understanding the premixture formation mechanisms is crucial for predicting the stratified steam explosion energetics. One of the external indicators of the way the melt and water are mixed during the stratified steam explosion process is the explosion duration.In the article, the duration of the stratified steam explosion is studied by the analysis of experimental observations in the PULiMS and SES tests using simulation results obtained with the MC3D code (IRSN, France). The explosion characteristics are analysed by varying the available melt mass in the premixed layer, the triggering position, the melt droplet diameter, the void fraction in the premixed layer, the void fraction in the water above the premixed layer and the size of the fine fragments. The analysis showed that longer explosion duration for the experiments with strong explosions cannot be predicted solely by varying the premixed layer conditions. A significant increase of the explosion duration could be obtained only by increasing the size of the fine fragments, which depends on the nature of the explosion. Thus the observed discrepancies between the experimental results and simulations can be explained by the fact that in strong stratified steam explosions there is also significant melt-water layer mixing during the explosion propagation.

New late Pleistocene species of Acharax from Arctic methane seeps off Svalbard

We report, for the first time, the solemyid Acharax svalbardensis sp. nov., from deep-sea methane seep sites on the western Svalbard margin, 79°N. This species is rather small and so far the northernmost representative of its genus. It is identified based on the following combination of diagnostic characters: umbo 27–30% valve length from posterior margin; H/L ratio ∼0.35; broadly rounded to truncated anterior margin; 15 moderately developed, flat double ribs with middle ribs about as strong as posterior ribs. The shells from Acharax svalbardensis sp. nov. were found in sediment cores from two pockmarks at Vestnesa Ridge at ∼1200 m water depth in the Fram Strait off NW Spitsbergen, Svalbard archipelago. Previously, the vesicomyid bivalves Archivesica arctica and Isorropodon nyeggaensis have been documented from the same pockmarks. Here, we describe the new solemyid species and report its stratigraphical occurrence and co-occurrence with the previously described methane seep-associated vesicomyids. All findings of the vesicomyids and the new solemyid species are restricted to the time interval ∼19,000–15,600 cal. years BP, correlating with Heinrich Stadial HS1. This period was characterized by cold surface conditions and extensive ice rafting from sea ice and icebergs in the North Atlantic and Arctic region. Inflow of a warm subsurface current of Atlantic water below the melt water layer led to higher bottom water temperatures at the Svalbard margin than at present. This increase in bottom water temperature probably allowed several methane seep-associated bivalve species to settle for a short period of time, namely the vesicomyids A. arctica and I. nyeggaensis and the new species of the solemyid bivalve genus Acharax described here.http://zoobank.org/urn:lsid:zoobank.org:pub:0AAE625F-2BDB-4ACE-8328-25274D30CDF