Sail Height Research Articles

Thickness and surface‐properties of different sea‐ice regimes within the Arctic Trans Polar Drift: Data from summers 2001, 2004 and 2007

Large‐scale sea‐ice thickness and surface property data were obtained in three summers and in three different sea‐ice regimes in the Arctic Trans‐Polar Drift (TPD) by means of helicopter electromagnetic sounding. Distribution functions P of sea‐ice thickness and of the height, spacing, and density of sails were analyzed to characterize ice regimes of different ages and deformations. Results suggest that modal ice thickness is affected by the age of a sea‐ice regime and that the degree of deformation is represented by the shape of P. Mean thickness changes with both age and deformation. Standard error calculations showed that representative mean and modal thickness could be obtained with transect lengths of 15 km and 50 km, respectively, in less deformed ice regimes such as those around the North Pole. In heavier deformed ice regimes closer to Greenland, 100 km transects were necessary for mean thickness determination and a representative modal thickness could not be obtained at all. Mean sail height did not differ between ice regimes, whereas sail density increased with the degree of deformation. Furthermore, the fraction of level ice, open melt ponds, and open water along the transects were determined. Although overall ice thickness in the central TPD was 50% thinner in 2007 than in 2001, first‐year ice (FYI) was not significantly thinner in 2007 than FYI in 2001, with a decrease of only 0.3 m. Thinner FYI in 2007 only occurred close to the sea‐ice edge, where open water covered more than 10% of the surface. Melt pond coverage retrieved from laser measurements was 15% in both the 2004 MYI regime and the 2007 FYI regime.

Sea-ice thickness and mass at Ice Station Belgica, Bellingshausen Sea, Antarctica

Ice Station Belgica was commenced in late winter 2007 in the Bellingshausen Sea as part of Sea Ice Mass Balance in Antarctica (SIMBA), an IPY 2007 cruise on the research vessel N.B. Palmer. A primary objective was to build on the work of previous Antarctic drift station experiments to geophysically characterize sea ice in terms of thickness, surface and ice bottom morphology, and ultimately area-unitized mass. A 24 day drift station was established at approximately 70°S and 93°W in mixed first-year and multi-year ice with three geophysical study sites selected on a 5 km 2 floe. A comprehensive time series assessment of elevation-surveyed transects ranging from 100 m to 300 m in length included snow surface elevation, snow depth, electromagnetic (EM) profiling, and direct drilling for ice draft and ice freeboard. Additional work included a snow surface morphology characterization of a 100 m×300 m area between the primary time series EM transects. Correlation of EM ice thicknesses with collocated drilled ice thickness yielded equations for the correction of EM underestimation of thick deformed ice, particularly at pressure ridges. Mean ice thickness from corrected EM was compared to isostatic ice thickness calculated from surface elevation, snow depth, ice freeboard and respective snow, slush, ice, and sea water densities. Results were consistent, with mean ice thicknesses for multi-year ice of 2.35 m, 2.34 m, and 2.41 m, with similar variance, for corrected EM, drilling, and buoyancy methods respectively. Additionally, a mean ice thickness of 2.31 m was calculated from ASPeCt observations of the ice field associated with the floe, using the method incorporating mean sail heights and fractional coverage of surface deformities or ridging. Temporal series assessment of ice freeboard indicated a slightly negative mean ice freeboard (<0.04 m), with clear evidence of new snow-ice formation from the freezing of slush. The three distinct snow and ice regions assessed on the Belgica floe had mean corrected EM ice thickness of 0.52±0.04 m (±1 std. deviation), 0.92±0.17 m, and 2.35±1.37 m, and mean snow depths of 0.08±0.03 m, 0.36±0.09 m, and 0.68±0.31 m respectively. Each ice type represented a sizable fraction of the floe's total area (∼20%, 40%, and 40% respectively from visual estimates) reflecting a complex dynamic and thermodynamic history of formation, as well as the difficulty in characterizing even a single floe by a single class or mean value for thickness and snow depth. Implications of these results are discussed with regards to the resolution of satellite-based altimetry and snow depth products and efforts to generate and validate satellite sea ice and snow thickness products.

Hummocks near the North Pole 35 drifting station

Study of hummocks is a very important applied and scientific problem. The main objective of this work is representing new information on structure of hummocks that were studied in the spring time at the North Pole 35 drifting station. In this paper, methodology and results of studying several hummocks with different morphology are considered. The maximum hummock keel draft varied from 3.5 to 14 m, while the maximum sail height was between 2.5 and 3.9 m. The hummock porosity and porosity of the unconsolidated part of its keel varied in the range of 6–18 and 4–26%, respectively. The average thickness of the hummock consolidated layer stayed in the range from 1.8 to 2.4 m. It was found that relatively small sail heights do not affect the consolidated layer thickness, however higher sails influence on the consolidated layer size. The higher the hummock sail, the less its consolidated layer thickness is.

Comparison of different ridge formation models of Arctic Sea ice with observations from laser profiling

AbstractSea ice deforms under convergent and Shear motion, causing rafting and ridging. This results in thicker ice than could be formed by thermodynamic growth only. Three different approaches to Simulating the formation of pressure ridges in a dynamic–thermodynamic continuum model are considered. They are compared with and evaluated by airborne laser profiles of the Sea-ice Surface roughness. The respective characteristic of each of the three ridging Schemes is (1) a prognostic equation for deformation energy from which ridge parameters are derived; (2) a redistribution function, Shifting ice between two categories, level and ridged, combined with a Monte Carlo Simulation for ridge parameters; and (3) prognostic equations for ridge density and height, resulting in the formation of ridged-ice volume. The model results Show that the ridge density is typically related to the State of ice motion, whereas the mean Sail height is related to the parent ice thickness. In general, all of the three models produce realistic distributions of ridges. Finally, the Second ridging Scheme is regarded as the most appropriate for climate modelling, while the third Scheme has advantages in Short-term Sea-ice forecasting.

First-year ridge interaction with the Molikpaq in the Beaufort Sea

This paper presents details of the failure behaviour and loads associated with the interaction of first-year ridges with the offshore structure Molikpaq. An analysis was made of ridge interactions during 2 years of deployment of the Molikpaq in the Canadian Beaufort Sea. The first-year ridge interactions were complex and characterized by a complicated sequence of ice failures that are largely dependent on the particulars of the ridge and the interaction event. Typical ridge failure modes included spine failures, failing behind the ridge, shearing, stopping and mixed modal. Typical load levels ranged from 30 to 100 MN for first-year ridges with sail heights up to 2.5 m. For these ridges, the ridge line load [ P LL in MN/m] was related to the ridge sail height [ H s in m] by P LL=0.36 H s+0.25.

Measurements of temperature distribution, consolidation and morphology of a first-year sea ice ridge

A ridge was located in the Van Mijen fjord, Spitsbergen in the end of February 1998. The ridge was 15 m long, 10 m wide, and had a maximum sail height of 1 m. The initial keel depth was 4.4 m. The temporal and spatial temperature distribution of a cross-section was monitored. The temperatures were compared with meteorological conditions and changes in the adjacent level ice. The consolidation of the ridge started at formation and continued until mid May 1998. Six thermistor strings were installed in a cross-section of the ridge. The temperature measurements showed a sharp change in temperature gradient at a certain depth, and below this depth the temperatures were very close to the freezing point of the surrounding sea water. The consolidated layer reached an average thickness of minimum 1.3 times the level ice thickness. The consolidated layer had a `W' shape. The average porosity of the consolidated- and the partly consolidated layer was 3.4%, of which the majority probably was in the partly consolidated layer.

Four stages of pressure ridging

The pressure ridging process is simulated using a two‐dimensional particle model. Blocks are broken from an intact sheet of relatively thin lead ice pushed against a thick, multiyear floe at a constant speed. The blocks of ice rubble accumulate to form the ridge sail and keel. During the simulations the energy consumed in ridge growth, including dissipation, is explicitly calculated. On the basis of the results of simulations performed with the model, the ridging process can be divided into four distinct stages. The first stage begins with an intact sheet of lead ice impacting a floe and ends when the sail reaches its maximum height. In the second stage the ridge keel deepens and widens. The stage ends when the maximum keel draft is reached. In the third stage the direction of growth is leadward creating a rubble field of more or less uniform thickness. The third stage ends when the supply of thin ice is exhausted. In the fourth stage the rubble field is compressed between converging floes. The results of simulations establish the dependence of ridging energetics in the first and second stages on the thickness of the ice sheet and the amount of ice pushed into the ridge. The average profiles of the simulated ridges delineate the growth process in the first, second, and third stages. The energetics and profiles of the fourth stage were described by Hopkins et al. [1991]. Lead ice extents of up to 1300 m are pushed into ridges to determine maximum sail heights, keel drafts, and ridging forces.

Sea ice ridging in the eastern Weddell Sea

In August 1986, sea ice ridge heights and spatial frequency in the eastern Weddell Sea were measured using a ship‐based acoustical sounder. Using a minimum ridge sail height of 0.75 m, a total of 933 ridges were measured along a track length of 415 km. The ridge frequency varied from 0.4 to 10.5 ridges km−1. The mean height of the ridges was found to be about 1.1 m regardless of the ridge frequency. These results are compared to other ridging statistics from the Ross Sea and found to be similar. Comparison with Arctic data, however, indicates that the height and frequency of the ridges are considerably less in the Weddell Sea than in the Arctic. Whereas in the Arctic the mean ridge height tends to increase with the ridge frequency, we found that this was not the case in the Weddell Sea, where the mean ridge height remained constant irrespective of the ridge frequency. Estimates of the contribution of deformed ice to the total ice thickness are generally low except for a single 53‐km section where the ridge frequency increased by an order of magnitude. This resulted in an increase in the equivalent mean ice thickness due to ridging from 0.04 m in the less deformed areas to 0.45 m in the highly deformed section. These values were found to be consistent with values obtained from drilled profile lines during the same cruise.

Measurements of ridge sails in the Beaufort Sea

Ice pressure ridges in the southern Beaufort Sea near Tuktoyaktuk were surveyed in April 1986. Sail cross-section profiles and ice-block dimensions of ridges of extreme heights were measured at several locations along the outer edge of the landfast ice. Statistical distributions of sail heights as well as correlations between sail dimensions and between ice block dimensions are obtained. Geographical distribution of sail dimensions and longitudinal changes along individual ridges are examined. Key words: ridges, landfast ice, sea ice, ridge sails, ridge statistics, Beaufort Sea.

Structure-ridge interaction

The interaction of a massive ice-resistant bottom-founded offshore structure with a multiyear pressure ridge embedded in and adfrozen to a multiyear ice floe is considered in this paper from a deterministic point of view. For the sake of calculating global ice loading on structures, a rectangular model of multiyear pressure ridges is proposed and discussed. It is shown that during the structure-ridge interaction, when the structure has an inclined surface, a combined twisting-bending mode of ridge failure can occur. As a result, a biaxial state of stress is to be considered at any point within an arbitrarily chosen cross-section of the ridge. The emphasis in the paper is given to the twisting component of the twisting-bending mode. All results are presented in a closed analytical form in terms of sail height, ridge length, and strength and mechanical properties of ridges on a large scale.

Morphological investigations of first-year sea ice pressure ridge sails

Sea ice pressure ridge sail heights and the dimensions of ice blocks that comprised the sails were measured for 30 ridges in April, 1980. The ridges were located from 30 to 200 km offshore in the Prudhoe Bay, Alaska region. Sail height was found to be a function of the thickness of the ice in the ridge. A reasonable relationship shows that height is dependent on the square root of block thickness. The data also verify that the ratio of sail height to ice block thickness is much larger for ridges composed of thin ice than for those composed of thick ice. Ridge width and cross-sectional area are also found to be related to block thickness. For the largest blocks in the ridge, block surface area is related to thickness squared, as would be expected from plate deflection theory. Examination of lateral variation of height and width along the ridge shows an expected large variation, particularly with height. No geographic variations of individual pressure ridge morphology were discernible from these data, but a laser profile taken during the study period shows that the mean heights and numbers of ridges decrease as the distance from the coast increases.

Sea-ice topography of the Arctic Ocean in the region 70° W to 25° E

In October 1976 a cooperative experiment was made to survey the sea-ice topography in the European sector of the Arctic Ocean. H. M. submarine Sovereign acquired 4000 km of ice draft data by using an upward-looking sonar, while a Canadian Forces aircraft flew along the submarine’s track and acquired 2200 km of ice elevation data by using a laser profilometer. The two types of profile were processed in corresponding 100 km section lengths, and the following statistical analyses and comparisons were made: (i) Probability density functions o f ice draft and elevation. Each distribution shows a peak for young ice and for undeformed multi-year ice. At large ice thicknesses the distributions take the analytical form of a negative exponential. The mean drafts enable two distinct geographical ice regimes to be identified. There is an ‘offshore zone’ of very heavy pressure ridging extending up to 400 km from the coasts of Ellesmere Island and of north Greenland, with mean ice draft in the range 5.0 to 7.5 m , while out in the central Arctic Ocean the mean ice draft is lower (3.9—5.1 m) and the characteristics of the ice cover remain homogeneous over a length scale of 1000 km. The transition between the two regimes is abrupt, taking place in less than 25 km. Data from the same part of the central Arctic taken in March 1971 showed a mean ice draft 0.3 m lower, while data from the central Beaufort Sea showed a mean draft more than 0.8 m lower. (ii) Level ice distributions. Ice with a local gradient of less than 1 in 40 was defined as level ice, and used as an indicator of the quantity and thickness distribution of undeformed (i.e. thermodynamically grown) ice in the Arctic Ocean. The distribution has a mode at 3.0—3.1 m draft, and level-ice percentages are in the range 30—40 (bottom side) and 70—80 (top side) in the offshore zone, and 45—55 (bottom) and 85-95 (top) in the central Arctic. Thus about half of the Arctic ice cover consists of deformed ice. (iii) Pressure ridge spacings. The spacings of ridge keels fit a negative exponential distribution, characteristic of randomness, except at close spacings where there is a deficit of keels (explained as a geometrical effect) and at very large spacings where there is an excess (due to the contribution of polynyas). The distribution of sail spacings exhibits these two effects, but also differs from a random distribution at moderate sail separations. (iv) Ridge elevations and drafts. Keel drafts fit a law of form P( h ) d oc exp ( — Ah 2 ) d h , except for an excess of keels at drafts beyond 20 m. There is a positive correlation between mean keel draft and keel frequency. There are 3.5—4.5 keels per kilometre with draft exceeding 9 m in the offshore zone and 2—3 in the central Arctic. Sail elevations fit a law of form P( h ) d h x exp ( — A h ) d h , with a positive correlation between mean sail elevation and sail frequency. The sail elevation and ice elevation distributions can be related by assuming that all thick ice is contained in pressure ridges of triangular cross section. (v) Keel—sail comparison. For the 21 corresponding 100 km sections there are positive correlations between mean sail height and mean keel draft, and between keel frequency and sail frequency. From these it is possible to convert a sail distribution (computed from a laser profile) into a keel distribution, enabling sea ice bulk characteristics to be derived from airborne surveys alone. (vi) Leads and polynyas . A lead was defined as a continuous sequence of depth points greater than 5 m long and not exceeding 1 m draft. The number density n ( d ) of leads per kilometre of width d m fits the power law n ( d ) — 15 d -2 . Exceptionally wide leads were concentrated in the offshore zone and in the marginal ice zone close to the open water of the Greenland Sea.

Multi-year pressure ridges in the Canadian Beaufort Sea

Pressure ridges and hummocks are the largest of the sea ice relief features seen in the ice cover which characterizes the Canadian Beaufort Sea. Due to their massiveness and structural integrity, multi-year pressure ridges are generally considered to be the most formidable formations in this area and represent not only a significant impediment to shipping but also a major problem in the design of offshore structures. Here, the findings of a field study designed to generate fundamental data on multi-year pressure ridges in the near shore zone of the Canadian Beaufort Sea are presented. The study investigated the geometry of eleven floating multi-year ridges or ridge fragments and the sail height and keel depth of four additional multi-year ridge fragments. The cross-sections of multi-year ridges with total thicknesses varying between 9.6 and 41.8 m were examined and the results suggest that they can be adequately represented by one ridge model with a constant sail to keel ratio and geometry. It is also shown that the ice comprising multi-year ridges is solid with the interblock voids existing at the time of their formation being completely filled with ice. The data obtained from this study is being used in the engineering design of exploration and production systems for the Beaufort Sea. In the shallow waters of this area, exploratory drilling from artificial islands has been carried out since 1973 and, since 1976, the exploration effort has extended into the deeper waters of the Beaufort Sea using drillships.

Structure of a Multi-Year Pressure Ridge

Three transverse profiles across a large pressure ridge located in the Beaufort Sea are presented. The ridge sail extended 4m. above sea level and the ridge keel 13 m. below. The cross-sections of the ridge keel can be described as roughly semi-circular. This suggests that form drag coefficients for flow transverse to the long axes of multi-year ridges may be as high as 0.8. Examination of several salinity, temperature and brine-volume profiles shows that much of the ice in the ridge has a very low salinity and is quite strong. All the inter-block voids that initially existed in the ridge at the time of its formation have been completely filled with ice. These observations, coupled with icebreaking experience indicate that multi-year ridges are, indeed, significant obstacles to even the largest icebreaking ship and should be avoided if possible. A very large first year ridge with a sail height of 12.8 m. is also described. This is the largest free-floating ridge yet measured.

Statistical aspects of sea-ice ridge distributions

A theoretical distribution function for pressure-ridge, sail heights and keel depths is derived from fundamental assumptions about the randomness of the ridges. It is shown that the distribution function for ridge spacings (distance between ridges) can also be predicted from the assumption of spatially random occurrence. The suggested distribution functions are, in form, negative exponentials of the ridge height (or depth) squared and the ridge spacing, respectively. Extremely good fits were achieved to extensive data collected from sonar profiles of the lower surface of the pack ice and to laser profiles, as well as visual roughness data from the upper ice surface. Using these models, it is possible to completely characterize the ridging, in a one-dimensional sense, by two parameters: 〈N〉, the mean number of ridges per unit length, and 〈h〉, the mean ridge height (or depth). In addition, there is a linear correlation between 〈N〉 and 〈h〉. This suggests that maps showing the distribution of 〈N〉 or 〈h〉 over an ocean covered with pack ice can be used to statistically characterize both the spacing and the height distribution of the ridges.