Ice Roughness Research Articles

Ice damage zone around the Molikpaq: Implications for evacuation systems

Effective design of an emergency evacuation system from an offshore structure in sea ice conditions must take into account all aspects of the anticipated ice conditions at the site. Evacuation systems are typically comprised of several different components or subsystems. These subsystems allow evacuees a choice in response to the emergency incident onboard the installation, as well as to environmental conditions off the installation. This paper discusses four possible evacuation means—preferred, primary, secondary and tertiary. Of these four techniques, only the preferred means of evacuation is not strongly influenced by the ice regime around the structure. This paper examines these ice regimes and quantifies the size of the broken ice region around a structure for different ice conditions. A large number of parameters affect the size of this broken ice zone including the general ice regime, the existence of a grounded rubble field, the failure mode of the ice, the ice thickness, and the ice roughness. Typical damage zone extent is extremely variable and can be in the order of 25 m from the structure for ice thickness of 1.5 m. This has significant consequences for evacuation systems deployed from the structure.

UARS/MLS Cloud Ice Measurements: Implications for H2O Transport near the Tropopause

Abstract A technique for detecting large hydrometeors at high altitudes is described here and applied to the Upper Atmosphere Research Satellite/Microwave Limb Sounder (UARS/MLS) 203-GHz radiance measurements at tangent pressures between 200 and 46 hPa. At these tangent pressures the radiances remain optically thin and cloudy-sky radiances are brighter than normal clear-sky cases. Unlike infrared/visible cloud observations, the 203-GHz radiances can penetrate most ice clouds and are sensitive to ice crystals of convective origin. Rough ice water content (IWC) retrievals are made near the tropopause using estimated size distributions from in situ convective studies. The seasonal mean IWC observed at 100 hPa reaches vapor-equivalent 20 ppmv or more over convective centers, dominating the total water content. Convectively lofted ice, therefore, appears to be hydrologically significant at the tropical cold point. IWC is well correlated spatially with relative humidity with respect to ice (RHi) at 100 hPa during both the dry (January–March) and moist (July–September) periods.

ICESat measurement of Greenland ice sheet surface slope and roughness

AbstractThe Ice, Cloud and land Elevation Satellite (ICESat) in its 8 day repeat orbit mode provided data not only on the along-track surface slope, but also on the cross-track surface slope from adjacent repeat ground tracks. During the first 36 days of operation, four to five such repeat orbits occurred within 1 km in the cross-track direction. This provided an opportunity to use ICESat data to measure surface slope in the cross-track direction at 1 km scale. An algorithm was developed to calculate the cross-track surface slope. Combining the slopes in the cross-track and along-track directions gives a three-dimensional surface slope at 1 km scale. The along-track surface slope and surface roughness at 10km scale are also calculated. A comparison between ICESat surface elevation and a European Remote-sensing Satellite (ERS-1) 5 km digital elevation model shows a difference of 1–2 m in central Greenland where the surface slope is small, and >20m at the edge of Greenland where the surface slope is large. The large elevation difference at the edge is most likely due to the slope-induced error in radar altimeter measurement. Accurate surface slope data from ICESat will help to correct the slope-induced error of radar altimeter missions such as Geosat, ERS-1 and ERS-2.

Comparison of sea ice signatures in OKEAN and RADARSAT radar images for the northeastern Barents Sea

Sea ice signatures in near-simultaneous OKEAN real aperture radar (RAR) and RADARSAT synthetic aperture radar (ScanSAR) images have been analyzed and compared for the northeastern part of the Barents Sea in April 1998. Signatures of major sea ice types and features in these images have been identified and verified by in situ observations on board the nuclear icebreaker Sovetsky Soyuz. It is shown that both RAR and ScanSAR images can be used for detection of new, young, first year, and multiyear sea ice types, multiyear and fast ice boundaries, flaw polynyas, wide fractures, giant and vast ice floes, and large areas of rough ice. Backscatter signatures of the major sea ice types are analyzed and compared, and the backscattering differences between the ice types are estimated for both types of data. High-resolution ScanSAR images are found to be particularly suited for discrimination of narrow leads and level areas in compact sea ice. A study of the radar signatures of a flaw polynya to the north of Cape Zhelaniya was conducted using ERS SAR, RADARSAT ScanSAR, and OKEAN RAR over a period of 33 h. The backscatter from the polynya varied significantly between VV and HH polarization and as a consequence of changing physical properties due to freezing during the 33 h period. Access to satellite radar data for use in sea ice research will increase in the near future. Wide-swath SAR data are available from both RADARSAT and ENVISAT from 2003, and a new RAR satellite, SICH-M1, is planned for launch in 2004.

Design and simulations of indirect drive ignition targets for NIF

Studies on simulation and design of ignition targets for the National Ignition Facility (NIF) are described. Recent effort has emphasized the systematic exploration of the parameter space of possible ignition targets, providing comparisons as specific as possible between the various targets. This study aims at providing guidance for target fabrication R&D, and for other elements of the ignition program. Targets are being considered that span 250–350 eV drive temperatures, capsule energies from 150 to 600 kJ, cocktail and gold hohlraum spectra, and three ablator materials (Be[Cu], CH[Ge] and polyimide). Capsules with graded doped beryllium ablators are found to be very stable with respect to short-wavelength Rayleigh–Taylor growth. Sensitivity to ablator roughness, ice roughness and asymmetry is being explored, as it depends on ablator material, drive temperature and absorbed energy. Three-dimensional simulations are being used to ensure adequate radiation symmetry in three dimensions (3D), and to ensure that coupling of 3D asymmetry and 3D Rayleigh–Taylor does not adversely affect planned performance. Integrated 3D hohlraum simulations indicate that 3D features in the laser illumination pattern affect the hohlraums' performance, and the hohlraum has been redesigned to accommodate these effects.

Friction and abrasive wear of UHWMPE sliding on ice

The authors present an experimental approach of abrasion resistance of material currently used in ski: ultra high molecular weight polyethylene (UHMWPE) is used as the main element of ski sole: cross country skiing, speed skiing, alpine skiing, …. The aim of this work is to characterise abrasive wear and tribological performance of ski sole when one slides on ice in different conditions of loads, ice roughness and temperature. In order to better understand the abrasive action of ice, experiments using ice with different roughness (characterised by different morphological parameters) are performed under different kinematical and thermal conditions. For this purpose, we employed especially designed tribometer in controlled atmosphere enclosure (defined in relation with the use of in situ conditions). In this study, both normal and tangential forces are measured during the friction experiments. To complete the characterisation of the friction process, 3D surfaces topography is measured in order to evaluate abrasive process and consequences on its tribological performance. Furthermore, it improves understanding of abrasion mechanisms of UHMWPE on ice.

Hydraulic controls of summer Arctic pack ice albedo

Linkages between albedo, surface morphology, melt pond distribution, and properties of first‐year and multiyear sea ice have been studied at two field sites in the North American Arctic between 1998 and 2001. It is shown that summer sea‐ice albedo depends critically on surface melt‐pond hydrology, controlled by melt rate, ice permeability, and topography. Remarkable short‐term and interannual variability in pond fraction varying by more than a factor of 2 and hence area‐averaged albedo (varying between 0.28 and 0.49 over the period of a few days) were observed to be forced by millimeter to centimeter changes in pond water level. Tracer studies show that the depth of the snow cover, by controlling the amount of superimposed ice formation in early summer, critically affects the retention of meltwater at the ice surface and hence affects pond coverage. Ice roughness as determined by deformation and aging processes explains a significant portion of the contrasts in pond coverage and albedo between ice of different ages, suggesting that a reduction in multiyear ice area and sea‐ice residence time in the Arctic Ocean is accompanied by large‐scale ice albedo decreases. Our work indicates that ice‐albedo prediction in large‐scale models with conventional methods is inherently difficult, if not impossible. However, a hydrological model, incorporating measured statistics of ice topography, reproduces observed pond features and variability, pointing toward an alternative approach in predicting ice albedo in numerical simulations.

A novel method for automated grid generation of ice shapes for local‐flow analysis

AbstractModelling a complex geometry, such as ice roughness, plays a key role for the computational flow analysis over rough surfaces. This paper presents two enhancement ideas in modelling roughness geometry for local flow analysis over an aerodynamic surface. The first enhancement is use of the leading‐edge region of an airfoil as a perturbation to the parabola surface. The reasons for using a parabola as the base geometry are: it resembles the airfoil leading edge in the vicinity of its apex and it allows the use of a lower apparent Reynolds number. The second enhancement makes use of the Fourier analysis for modelling complex ice roughness on the leading edge of airfoils. This method of modelling provides an analytical expression, which describes the roughness geometry and the corresponding derivatives. The factors affecting the performance of the Fourier analysis were also investigated. It was shown that the number of sine–cosine terms and the number of control points are of importance. Finally, these enhancements are incorporated into an automated grid generation method over the airfoil ice accretion surface. The validations for both enhancements demonstrate that they can improve the current capability of grid generation and computational flow field analysis around airfoils with ice roughness. Copyright © 2004 John Wiley & Sons, Ltd.

Experimental Frossling Numbers for Ice-Roughened NACA 0012 Airfoils

Experimental Frossling numbers are presented for two aluminum castings of ice-roughened NACA 0012 airfoil surfaces at 0-deg angle of attack for chord Reynolds number ranging from 4.00 £ £10 5 to 1.54£10 6 . The castings were meticulously obtained from actual ice accretions representing mildly rough glaze and rough glaze ice with horns. A modie ed investment casting technique wasused to capture all of the surface roughness details. The rough glaze ice with horns produced higher heat-transfer rates than those for the mildly rough glaze ice, especially at the horns. Immediately downstream of the horns, stagnation air gaps resulted and caused lower heat-transfer coefe cients. For both types of ice, higher Reynolds numbers in general produced higher heat-transfer coefe cients. For the samechord Reynoldsnumberand at thesameposition on the airfoil, theFrossling numbersweregenerally higher than those for the smooth case and those for the hemispherical roughness elements of previous studies. A maximumincreaseofapproximately306%overthesmoothcaseand192%overthedensehemisphericalroughness case was recorded at one rough glaze ice horn. This work provides some directly measured values of the Frossling number needed to improve the prediction of some icing codes. Such icing codes help in the effective design of some deicing systems of aircraft.

Modeling the drift of ridged sea ice: A view from below

The influence of long parallel pressure‐ridge keels on the ice‐water drag and the fluid velocity under ice in a rotating, non‐stratified ocean is studied using a numerical model. Turbulence in the water is modeled by a two‐equation eddy‐viscosity model, designed for adverse pressure‐gradient flow along both smooth and rough ice without assuming the logarithmic law‐of‐the‐wall to be valid. The simulations show the violation of this law in flow beneath the ice keels. The numerical results demonstrate that the orientation of the ice keels relative to the ice‐drift direction and the ice keel depth have a significant influence on the ice‐water drag coefficient, the drag‐turning angle, the near‐ice velocity and the turbulence fields. Application to free‐drifting, wind‐driven sea ice yields changes in the deflection angle between the wind stress and ice‐drift directions of about 40°, depending on the pressure‐ridge orientation relative to the drift direction. In addition, the results define a region where the ice‐drift direction is sensitive to changes in the wind stress direction.

Direct-drive cryogenic target implosion performance on OMEGA

Layered and characterized cryogenic D2 capsules have been imploded using both low- and high-adiabat (α, the ratio of the electron pressure to the Fermi-degenerate pressure) pulse shapes on the 60-beam OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the Laboratory for Laser Energetics (LLE). These experiments measure the sensitivity of the direct-drive implosion performance to parameters such as the inner-ice-surface roughness, the adiabat of the cryogenic fuel during the implosion, the laser power balance, and the single-beam nonuniformity. The goal of the direct-drive program at LLE is to demonstrate a high neutron-averaged fuel ρR at a significant fraction of the predicted one-dimensional (1-D) neutron yield using an energy-scaled, low-adiabat (α∼3) ignition pulse shape driving a hydrodynamically scaled deuterium–tritium ignition capsule. New results are reported from implosions of ∼920-μm-diam, thin (∼5 μm) polymer shells containing 100 μm D2-ice layers with characterized inner-surface ice roughness of 3–12 μm rms. These capsules have been imploded using ∼17–23 kJ of 351 nm laser light with a beam-to-beam rms energy imbalance of less than 5% and full beam smoothing [1 THz bandwidth, two-dimensional (2-D) smoothing by spectral dispersion and polarization smoothing]. Near-1-D performance has been measured for a high-adiabat (α∼25) drive pulse, and the implosion performance with a low-adiabat (α∼4) pulse is in agreement with 2-D hydrocode predictions.

Source bearing estimation using ice-mounted geophones

This paper presents the results of Arctic field trials carried out to estimate the bearing to acoustic sources in the water column using seismo-acoustic particle motion measured at a tri-axial geophone mounted on the sea ice surface. Source bearings are estimated by applying polarization filters to suppress seismic wave types with transverse particle motion, and computing the incident power rotated into radial look angles from 0–360 deg. The 180-deg ambiguity inherent in this rotational analysis is resolved by requiring prograde particle motion in the radial-vertical plane. Results are presented for smooth and rough annual ice and multiyear ice for source ranges from 200 m to 50 km. The results indicate good bearing estimation to long ranges with little dependence on ice type.

Meltwater circulation and permeability of Arctic summer sea ice derived from hydrological field experiments

AbstractPermeability and meltwater flow have been studied in sea ice in the Siberian and central Arctic during the summers of 1995 and 1996. A bail-test technique has been adapted to allow for measurements of in situ permeability, found to range between 10−11 and 10−8 m2. Permeability varied by about a factor of 2 between 1995 (above-normal melt rates) and 1996 (below-normal melt rates). Release of fluorescent tracers (fluorescein, rhodamine) furthermore allowed the derivation of flow velocities and assessment of the relevant driving forces. Hydraulic gradients in rough ice and wind stress in ponded ice were found to be particularly important, driving meltwater over distances of several meters per day. The mid- to late-summer ice was found to be permeable enough to completely divert meltwater from the surface into the ice interior. It is shown, however, that lower permeabilities of the upper ice layers as well as refreezing of meltwater, particularly during the early melt season, foster the development of surface melt ponds.

Effects of Surface Ice Roughness on Dynamic Stall

A two-dimensional Navier-Stokes algorithm is used to investigate unsteady, incompressible viscous flow past an airfoil leading edge with surface roughness that is characteristic of early-growth ice accretion. The roughness is added to the surface through the use of a Prandtl transposition and can generate both small-scale and large-scale roughness geometries. The algorithm is used to simulate steady or unsteady flow at constant angle of attack or pitch up corresponding to dynamic-stall conditions. Investigations of the dynamic stall show that some types of surface roughness can significantly alter the unsteady flow separation pattern and the formation of the dynamic-stall vortex. This includes both small-scale and large-scale roughness

ICESat's laser measurements of polar ice, atmosphere, ocean, and land

The Ice, Cloud and Land Elevation Satellite (ICESat) mission will measure changes in elevation of the Greenland and Antarctic ice sheets as part of NASA's Earth Observing System (EOS) of satellites. Time-series of elevation changes will enable determination of the present-day mass balance of the ice sheets, study of associations between observed ice changes and polar climate, and estimation of the present and future contributions of the ice sheets to global sea level rise. Other scientific objectives of ICESat include: global measurements of cloud heights and the vertical structure of clouds and aerosols; precise measurements of land topography and vegetation canopy heights; and measurements of sea ice roughness, sea ice thickness, ocean surface elevations, and surface reflectivity. The Geoscience Laser Altimeter System (GLAS) on ICESat has a 1064 nm laser channel for surface altimetry and dense cloud heights and a 532 nm lidar channel for the vertical distribution of clouds and aerosols. The predicted accuracy for the surface-elevation measurements is 15 cm, averaged over 60 m diameter laser footprints spaced at 172 m along-track. The orbital altitude will be around 600 km at an inclination of 94° with a 183-day repeat pattern. The on-board GPS receiver will enable radial orbit determinations to better than 5 cm, and star-trackers will enable footprints to be located to 6 m horizontally. The spacecraft attitude will be controlled to point the laser beam to within±35 m of reference surface tracks at high latitudes. ICESat is designed to operate for 3–5 years and should be followed by successive missions to measure ice changes for at least 15 years.

Application of triple-deck theory to the prediction of glaze ice roughness formation on an airfoil leading edge

A viscous–inviscid interaction triple-deck structure is developed to describe the thermomechanical interaction of an air boundary layer with ice sheets and liquid films. Linear stability results are compared with nonlinear triple-deck computations, and a number of nonlinear simulations of air–water–ice interactions are presented. An icing instability is encountered in regimes with simultaneous wall and air cooling that is believed to admit small scale and highly irregular surface roughness. The stabilization of the smallest scale icing disturbances is obtained through the Gibbs–Thomson relation. This local thermodynamic condition relates the freezing temperature of a pure substance to the surface tension and the mean curvature of the interface and provides a short scale stabilizing mechanism for icing instability modes. Comparison with available experimental data on glaze ice roughness diameters, accreted on NACA 0012 airfoil leading edges under glaze icing conditions, is provided. It is also found in all cases computed in this study that water beads can be formed on a wetted ice surface once the water film is locally ruptured by ice roughness elements.

ERS SAR studies of sea ice signatures in the Pechora Sea and Kara Sea region

Results from a statistical analysis of 105 synthetic aperture radar (SAR) images and meteorological data are presented, covering parts of the Pechora Sea and Kara Sea in the Russian Arctic. Wind speed, air temperature, and other data were collected for the SAR sample areas, and a manual sea ice classification of the SAR samples was performed. All variables were input to different multivariate regression analyses, which were used to separate ice and water samples. In subsequent regression analyses the classes young ice and rough first-year ice were separated from water, and the two ice types were separated from one another. In the separation of all ice types from water, correlation coefficients of up to 0.90 were achieved between predicted and actual values. Correlation coefficients were as high as 0.93 between predicted and actual values for open water and young ice and for open water and rough first-year ice. When trying to separate young ice from rough first-year ice, correlation coefficients of about 0.60 were obtained. The study indicated that the mean and standard deviation of the backscattering coefficients and air temperatures were the most important information for separating water and sea ice using regression techniques.

A Bayesian classification model for sea ice roughness from scatterometer data

For sea ice in the Baltic Sea, surface scattering can be regarded as the dominant scattering mechanism at C-band. In this paper, a new statistical method is introduced for making statistical inferences about the underlying ice surface roughness on the basis of one-dimensional (1D) scatterometer data y. The central parameter in the hierarchical model applied in the context is a mixture parameter p, which indicates the degree of surface roughness in ice surface. Several questions related to the occurrence of different ice classes on a transect can be solved with the aid of the posterior distribution [p|y]. An empirical approximation for the posterior distribution is computed by using Markov Chain Monte Carlo methodology. The efficiency of the suggested approach is investigated by analyzing a C-band HH-polarization helicopter-borne HUTSCAT scatterometer data. The results provided by the statistical model show good agreement with a video-based ice type classification.

Sea-ice thickness and roughness in the Ross Sea, Antarctica

AbstractSea-ice thickness and roughness data collected on three cruises in the Ross Sea, Antarctica, showed interseasonal, regional and interannual variability. Variability was reduced to season, or age of ice floe, when sea-ice roughness values from around Antarctica were compared. There were statistically significant correlations between mean snow elevation and mean ice thickness; snow surface roughness and mean ice thickness; and snow surface roughness and ice bottom roughness, which appeared to be independent of season, geographical location and deformation history of ice floes. Our field data indicate that ice thickness can be predicted from snow elevation measurements with higher accuracy in summer. The feasibility of using snow surface roughness to infer ice thickness and ice bottom roughness is promising, and can provide us with a means to study the thickness and underside of Antarctic sea ice at good spatial and temporal resolution.

Alternative Fuels for ICF Targets

The first observation of the beta-layering phenomenon showed that it was possible to fabricate inertial confinement fusion (ICF) targets having an outer ablating shell surrounding a symmetric solid layer of DT fusion fuel. The sensitivity of fusion yield to the internal DT ice roughness is a function of many factors, one of which is the relatively low density of solid DT (0.25 g/cm3), leading to a high Atwood number for the ablator/fuel interface. This is one of the issues that has led us to consider other DT-based fuels having higher densities than pure DT but still capable of being automatically redistributed into a uniform layer by beta-layering. The two principle conditions for beta-layering redistribution, self-heating and a moderately high vapor pressure, can be found in only a few other systems. But by concentrating on hydrides of elements in the second row of the periodic chart, we can find materials which should beta-layer and which might be good candidates for fusion fuel. We exclude lithium hydride and beryllium hydride, because these materials are solids at room temperature where an automatic redistribution technique such as beta-layering would not be necessary. Therefore we begin with boron and consider the following materials:• diborane: B2(DT)3 (or B2D3T3),• methane: C(DT)2 (or CD2T2),• ammonia: N(DT)3/2 (i.e., 0.5·NDT2 + 0.5·ND2T),• water: DT0, and• hydrofluoric acid: (DT)1/2F (i.e., 0.5·kDF + 0.5·TF).