Laboratory study of frazil ice accumulation under wave conditions

Ice growth in turbulent seawater is often accompanied by the accumulation of frazil ice crystals at its surface, forming a grease ice layer. The thickness and volume fraction of this ice layer play an important role in shaping the gradual transition from a loose to a solid ice cover, however, observations are very sparse. Here we analyse an extensive set of observations of frazil ice, grown in two parallel tanks with controlled wave conditions and thermal forcing, focusing on the first one to two days of grease ice accumulation. The following unresolved issues are addressed: (i) at which volume fraction the frazil crystals' rising process starts and how densely they accumulate at the surface, (ii) how the grease ice solid fraction and salinity evolve with time until solid ice starts to form and (iii) how do these conditions affect, and are affected by, waves and heat loss from the ice. We obtained estimates of the minimum initial grease ice solid fraction (0.03–0.05) and the maximum solid fraction to which it accumulates before freezing into pancakes (0.23–0.31). The equivalent thickness of solid ice that needs to be accumulated until grease ice packs close to maximum (95% of the compaction accomplished), was estimated as 0.4 to 1.2 cm. Comparison of grease ice thickness and wave observations indicates that a grease ice layer first begins to affect the wave field significantly when its thickness exceeds the initial wave amplitude. These results are relevant for modelling frazil ice accumulation and freeze-up of leads, polynyas and along the seasonal ice zone.publishedVersio

Microstructure evolution of young sea ice from a Svalbard fjord using micro-CT analysis

We analysed the three-dimensional microstructure of sea ice by means of X-ray-micro computed tomography. Microscopic (brine- and air- pore sizes, numbers and connectivity) and macroscopic (salinity, density, porosity) properties of young Arctic sea ice were analysed. The analysis is based on ice cores obtained during spring 2016. Centrifuging of brine prior to CT imaging has allowed us to derive confident relationships between the open, vertically connected and total porosity of young sea ice at relatively high temperatures. We analysed the dependence of the microscopic properties on vertical position and total brine porosity. Most bulk properties (salinity,density) and pore space properties (pore sizes and their distribution) show a strong dependence on total brine porosity, but did not change significantly over the course of the field work.However, significant changes were observed for pore numbers decreasing over time) and pore connectivity (increasing over time). CT-based salinity determinations are subject to larger than standard uncertainties (from conductivity), while the CT method yields important information about the salinity contributions from closed and open pores. We also performed a comparison of CT-based air porosity with calculations based on density from hydrostatic weighing. The consistency is encouraging and gives confidence to our CT-based results.publishedVersio

The plate spacing of sea ice

Columnar sea ice grows with an interface of tiny parallel ice plates, the distance of which is known as plate spacing. While it has been proposed as a fundamental microstructure scale of sea ice, the physics behind its formation has not been fully understood. Here the problem is analysed on the basis of morphological stability theory to propose a model that results in a physically consistent prediction of the relationship between the plate spacing $a_0$ and growth velocity $V$. The relationship may be divided into two regimes. In the diffusive regime, for $V$ above $\approx 2 \times 10^{-4}$ cm/s one finds $a_0 \sim V^{-2/3}$ to first order. In the convective regime the extent of diffusive boundary layer is controlled by solutal convection near the interface, which leads to the proportionality $a_0 \sim V^{-1/3}$. From a comparison to observations it is evident that the plate spacing is predictable over 5 orders of magnitude in the growth velocity, covering the range from fast laboratory ice growth to slow accretion at the bottom of marine ice shelves. The predictability opens new paths towards concise modelling of marine and sea ice microstructure and physical properties

Laboratory study of frazil ice accumulation under wave conditions

Ice growth in turbulent seawater is often accompanied by the accumulation of frazil ice crystals at its surface, forming a grease ice layer. The thickness and volume fraction of this ice layer play an important role in shaping the gradual transition from a loose to a solid ice cover, however, observations are very sparse. Here we analyse an extensive set of observations of frazil ice, grown in two parallel tanks with controlled wave conditions and thermal forcing, focusing on the first one to two days of grease ice accumulation. The following unresolved issues are addressed: (i) at which volume fraction the frazil crystals' rising process starts and how densely they accumulate at the surface, (ii) how the grease ice solid fraction and salinity evolve with time until solid ice starts to form and (iii) how do these conditions affect, and are affected by, waves and heat loss from the ice. We obtained estimates of the minimum initial grease ice solid fraction (0.03–0.05) and the maximum solid fraction to which it accumulates before freezing into pancakes (0.23–0.31). The equivalent thickness of solid ice that needs to be accumulated until grease ice packs close to maximum (95% of the compaction accomplished), was estimated as 0.4 to 1.2 cm. Comparison of grease ice thickness and wave observations indicates that a grease ice layer first begins to affect the wave field significantly when its thickness exceeds the initial wave amplitude. These results are relevant for modelling frazil ice accumulation and freeze-up of leads, polynyas and along the seasonal ice zone

Microstructure evolution of young sea ice from a Svalbard fjord using micro-CT analysis

We analysed the three-dimensional microstructure of sea ice by means of X-ray-micro computed tomography. Microscopic (brine- and air- pore sizes, numbers and connectivity) and macroscopic (salinity, density, porosity) properties of young Arctic sea ice were analysed. The analysis is based on ice cores obtained during spring 2016. Centrifuging of brine prior to CT imaging has allowed us to derive confident relationships between the open, vertically connected and total porosity of young sea ice at relatively high temperatures. We analysed the dependence of the microscopic properties on vertical position and total brine porosity. Most bulk properties (salinity,density) and pore space properties (pore sizes and their distribution) show a strong dependence on total brine porosity, but did not change significantly over the course of the field work.However, significant changes were observed for pore numbers decreasing over time) and pore connectivity (increasing over time). CT-based salinity determinations are subject to larger than standard uncertainties (from conductivity), while the CT method yields important information about the salinity contributions from closed and open pores. We also performed a comparison of CT-based air porosity with calculations based on density from hydrostatic weighing. The consistency is encouraging and gives confidence to our CT-based results

Consolidation of fresh ice ridges for different scales

This study characterizes the refreezing process of deformed ice. Twenty laboratory experiments in ice ridge consolidation were conducted to study the influence of ridge blocks size, initial temperature, and top surface roughness on the consolidation rate. Experiments covered a ridge block thickness range of 2–6 cm, initial block temperatures from −1 °C to −23 °C, ridge sail height up to 3 cm, and consolidated layer thickness up to 14 cm. Experiments were conducted with the average value of the convectional heat transfer coefficient of 20 W/m2K. The presented analytical model for ridge solidification was able to predict the observed ice growth rates and differences between level ice and consolidated layer thicknesses at different stages of the experiments. For the provided experiments, the consolidated layer was as much as 2.2–2.8 times thicker than the surrounding ice level. The consolidation rate was lower than in the analytical solution at the start of the experiment and approached the analytical solution only when the thickness of the surrounding level ice was larger than the ridge void width. The developed numerical model confirmed the observed experimental effects from the block size, initial temperature and surface roughness. Both numerical and analytical models can predict solidification rates for previous studies at the large range of scales for both fresh and saline ice. The advantages of the simplified experimental ridge geometry include high accuracy of the main parameters governing the process, including the ridge macroporosity

Microstructure evolution of young sea ice from a Svalbard fjord using micro-CT analysis

We analysed the three-dimensional microstructure of sea ice by means of X-ray-micro computed tomography. Microscopic (brine- and air- pore sizes, numbers and connectivity) and macroscopic (salinity, density, porosity) properties of young Arctic sea ice were analysed. The analysis is based on ice cores obtained during spring 2016. Centrifuging of brine prior to CT imaging has allowed us to derive confident relationships between the open, vertically connected and total porosity of young sea ice at relatively high temperatures. We analysed the dependence of the microscopic properties on vertical position and total brine porosity. Most bulk properties (salinity,density) and pore space properties (pore sizes and their distribution) show a strong dependence on total brine porosity, but did not change significantly over the course of the field work.However, significant changes were observed for pore numbers decreasing over time) and pore connectivity (increasing over time). CT-based salinity determinations are subject to larger than standard uncertainties (from conductivity), while the CT method yields important information about the salinity contributions from closed and open pores. We also performed a comparison of CT-based air porosity with calculations based on density from hydrostatic weighing. The consistency is encouraging and gives confidence to our CT-based results

Infiltration Response of Adsorbent Amended Filters for Stormwater Management under Freezing/Thawing Conditions

Coastal cold climates experience frequent intermittent melting and freezing periods over the cold period. This intermittent freezing in stormwater systems affects the infiltration capacity and hence the performance. This paper investigates the infiltration capacity of engineered filter media (composed of sand mixed with charcoal, pine bark, or olivine) under freezing temperatures in a column-based laboratory setup. Infiltration into partially frozen filter media was replicated using a climate room. The filter media in the columns were brought to −2.5 °C, and water at +2 °C was percolated through the columns with a constant head of 5 cm. Infiltration performance was assessed by observing the time until breakthrough, and the infiltration rate 24 h after breakthrough. The results were compared to the observed hydraulic conductivity for the unfrozen filter media. A novel approach combining the unfrozen water content curves with X-ray tomographic (XRT) images of the materials was adopted to better understand the thermal and infiltration processes. Breakthrough was observed between ca. 21 and 56 h in all columns. The column with homogeneously mixed filter media with sand yielded the quickest breakthrough. The infiltration rates were higher than recommendations for infiltration-based systems in cold climates, making them a suitable option in cold climates

Oil saturation of the sea ice pore space

Observations have shown that oil spilled under sea ice collects in patches within the under-ice relief and is encapsulated in the growing ice sheet. Due to its lower density, compared to seawater brine, oil will migrate upwards into the sea ice pore space. During winter migration is limited to lower levels where pore sizes and porosity are larger than near the colder ice surface. Upon warming the oil may continue migration through the widening pore network. To predict when oil may reach the surface, good information about oil saturation, the fraction of the sea ice pore space that may be occupied by oil, is needed. However, at present little is known about the influence on oil saturation and its dependence on sea ice porosity and microstructure. Here we analyse a recently obtained dataset of 3-d X-ray micro-tomographic images of young sea ice to determine oil saturation. We use these images to perform numerical simulations of the immiscible displacement of brine (wetting fluid) by oil (non-wetting) and thereby determine the dependence of oil saturation on porosity and capillary pressure/oil patch thickness. Comparing the results to published observations of laboratory-grown seawater ice of similar age highlights the importance of internal convection for oil entrainment. We perform the analysis for a limited set of 3-d images of older first-year ice of similar porosity. The comparison suggests one to two order higher oil saturation and oil storage capacity in the old ice. To determine the seasonal evolution in the saturation-porosity relationship of oil in sea ice during aging, more observations are needed. This relationship is crucial to estimate the porosity-dependent storage capacity of oil in sea ice, predict oil surfacing, and will be important for planning of response scenarios to oil spills under sea ice

Experimental and Micro-CT study on the Oil Distribution in laboratory grown Sea Ice

Increasing activities in Arctic waters bare a risk of oil spills under ice-covered conditions and afford sound understanding of the interplay between sea ice and oil. Towards better knowledge, this study focuses on X-ray- micro computed-tomography (μ-CT) investigations of a laboratory oil- in ice experiment. The 3-dimensional distribution of oil in the porous space of 11- 13 cm columnar ice grown in a laboratory was investigated. Two different oil content measurement methods are discussed. (i) The first method quantifies the oil volume fraction based on μ -CT-scan investigations, allowing spatial oil distribution analysis in the porous space of sea ice. Oil inclusions were mapped manually over the acquired CT-scans with a resolution of 18 μm and 25 μm, respectively. Results give higher oil contents for smaller resolutions. Oil migration of 4 cm was observed. (ii) The second method quantifies the present oil concentration with fluorescent measurements. CT- scans give in comparison to fluorescent measurements a root mean square error of 1.27 % (18 μm) 0.76 % (25 μm), respectively. Bulk salinity determined from melted samples is compared with salinity estimated from μ –CT data.publishedVersio