Undeformed Sea Ice Research Articles

Numerical simulation for cutoff draft of sea ice ridge keels based on a novel optimal modeling with nonlinear-statistical constraints

<abstract> <p>Optimal identification and numerical models are powerful tools that have been widely used in geoscience research for many years. In this study, we proposed a novel optimal method to simulate a key parameter (cutoff draft) of the ridge keels due to dynamic deformation of sea ice at bottom. The sea ice ridges were measured in the Northwestern Weddell Sea of Antarctic, by a helicopter-borne electromagnetic-induction (EM) system. An optimal model with nonlinear-statistical constraints was developed, by taking deviations between the theoretical and measured keel draft (spacing) distributions as the performance criterion, and cutoff draft as the identified parameter. The properties of the optimal model and the existence of the optimal parameter were demonstrated. We identified that the optimal cutoff draft was 3.78 m via an optimal numerical algorithm, this value was then employed to separate the ridge keels from the ice bottom. Finally, the relationship between the mean keel draft and frequency (number of keels per km) was analyzed, and the result showed that this relationship was modeled well by a logarithmic function with a correlation coefficient of 0.7. The present optimal modeling method will provide a new theoretical reference for separating accurately the ridge keels from undeformed sea ice bottom, and analyzing the relationship between the morphologies of sea ice surface and bottom and the inversions of sea ice bottom draft and ice thickness by the surface height.</p> </abstract>

2014 summer Arctic sea ice thickness and concentration from shipborne observations

ABSTRACTA series of shipborne sea ice observations were performed during the Chinese National Arctic Research Expedition in the Pacific Arctic sector between 2 August 2014 and 1 September 2014. Undeformed sea ice thickness (SIT) as well as area fractions of open water, melt pond, and sea ice (Aw, Ap, and Ai) were monitored using downward-oriented and oblique-oriented cameras. The results show that SIT varied between 20 and 220 cm throughout the whole cruise, with the average and standard deviation equaling 104.9 and 29.1 cm, respectively. Mean Aw and Ai were 0.52 and 0.44 in the marginal ice zone, respectively, while mean Aw decreased to 0.23 and mean Ai increased to 0.73 in the pack ice zone. Limited variation between 0 and 0.32 in Ap was seen throughout the whole cruise. Shipborne sea ice concentration was then rectified and mapped across a large transect to validate estimates derived from the satellite sensors Special Sensor Microwave Imager/Sounder (SSMIS) (25 km) and AMSR2 (25 km). Overestimations were 9.5% and 9.9% for SSMIS and AMSR2 compared with measurements, respectively. The mean areal broadband surface albedo based on shipborne survey increased from 0.07 to 0.66 along the transect between 72°N and 81°N.

Retrieval of the thickness of undeformed sea ice from simulated C-band compact polarimetric SAR images

Abstract. In this paper we introduce a parameter for the retrieval of the thickness of undeformed first-year sea ice that is specifically adapted to compact polarimetric (CP) synthetic aperture radar (SAR) images. The parameter is denoted as the "CP ratio". In model simulations we investigated the sensitivity of the CP ratio to the dielectric constant, ice thickness, ice surface roughness, and radar incidence angle. From the results of the simulations we deduced optimal sea ice conditions and radar incidence angles for the ice thickness retrieval. C-band SAR data acquired over the Labrador Sea in circular transmit and linear receive (CTLR) mode were generated from RADARSAT-2 quad-polarization images. In comparison with results from helicopter-borne measurements, we tested different empirical equations for the retrieval of ice thickness. An exponential fit between the CP ratio and ice thickness provides the most reliable results. Based on a validation using other compact polarimetric SAR images from the same region, we found a root mean square (rms) error of 8 cm and a maximum correlation coefficient of 0.94 for the retrieval procedure when applying it to level ice between 0.1 and 0.8 m thick.

Porosity of growing sea ice and potential for oil entrainment

Abstract The pore space in the bottom-most layers of growing sea ice is directly connected to the ocean beneath, allowing for fluid exchange while providing a sheltered environment for sea-ice microbial communities. Because of its role as a habitat and its high porosity and permeability, potential entrainment of oil into this pore space is of broader concern. We estimate the ice volume that can potentially be infiltrated by oil and other buoyant pollutants in surface ocean water evaluating several years of sea ice measurements on undeformed landfast first-year sea ice at Barrow, Alaska. This ice is representative of undeformed sea ice in areas targeted for offshore oil development. The calculated ice volume is related to crude oil entrainment volumes with empirical relationships derived from field and laboratory measurements. We synthesize 12 years of sea-ice core salinity data and 6 years of quasi-continuous sea ice temperature profile measurements to derive the seasonal evolution of ice thickness and temperature gradients in sea ice. Porosity profiles are calculated from temperature and salinity profiles. Based on previous observations, an oil penetration depth is defined by a porosity threshold of 0.1 to 0.15. Ice thickness is found to increase from 0.6 m in January to its maximum of 1.5 m in May, and average temperature gradients at the ice–water interface range from − 15 °C/m in January to − 2 °C/m in May. Depending on ice temperature conditions, derived depths of fluid penetration range from 0.02 to 0.10 m in January to 0.12 to 0.25 m in May for a porosity threshold of 0.10. These penetration depths are approximately halved for a porosity threshold of 0.15. For average temperature conditions, expected entrainment of crude oil is less than 2 L/m2 in January and may be as high as 5 to 10 L/m2 in May. Accessible ice volume and entrainment potential are expected to increase during warm spells and with the opening of brine channel networks in late spring. Considering inhomogeneous spread and pooling of oil under ice, entrainment in warm sea ice is expected to add approximately 20% to previous estimates of the under-ice pooling capacity.

3D EM modelling of Antarctic sea ice pressure ridges

The use of airborne electromagnetic (AEM) surveys to measure Antarctic sea ice thickness is being investigated as a regional mapping tool to aid global climate studies. Undeformed sea ice floes are modelled as 1D structures. However, sea ice pressure ridge sails (in air) and keels (undersea) form convenient 2D and 3D targets. In order to use AEM techniques to study the distribution of ice thickness, it is first necessary to test if realistic sea ice structures can be modelled electromagnetically. Pressure ridge structures from drillhole sampling can in some cases be constructed from simple block models. Finite element and integral equation 3D EM modeling programs can then be applied to these structures to test for agreement, and to predict the AEM response, including topography (sails). EM modelling of frequency domain systems shows that good agreement is obtained at certain frequencies and that a vertical coaxial (VCX) transmitter?receiver coil configuration laterally resolves the ice keel better than a horizontal coplanar (HCP) configuration. Cross-sectional dimensions of the sails and keels are relatively small and mesh design is critical in order to avoid discretisation errors. Software and hardware constraints may limit successful EM modeling of multiple keels that combine sub-metre structures and coarser structures. For a given mesh, discretisation errors may lead to noisy EM response profiles depending on the frequency, coil geometry, and component (in-phase or quadrature). Adaptive mesh structures may be required to minimise discretisation errors.

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

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

Application of the dense medium radiative transfer theory for calculating microwave emissivities of different sea ice types

The dense medium radiative transfer theory (DMRT) was used to calculate microwave emissivities of different undeformed sea ice types in the Arctic and Antarctic. The computed results were compared with measurements. More often taken for describing snow, we show that the DMRT can be applied to both sea ice and snow cover. While doing so the choice of appropriate parameters needed in the DMRT is discussed. As a result a multi-layered, uniform model for a variety of sea ice types including snow cover is obtained.

Attenuation of sound through laboratory grown saline ice

Attenuation of acoustic waves transmitted through saline ice was studied in situ as ice formed in a large, outdoor pool. Amplitude of sonar pings (50–800 kHz) transmitted normal to the ice sheet surface were measured after passing through ice about 18 cm thick and having an average salinity of about 4 parts per thousand. Columnar crystals in the body of the ice sheet and a fine, dendritic structure near the base of the ice sheet (dendrites were about 0.5 mm thick) characterize the ice as being nearly identical to young, undeformed sea ice found naturally covering the polar oceans. Additional measurements were performed on newly grown saline ice about 3 cm thick. Measured attenuations through the 18-cm-thick ice ranged from about 20 dB at 50 kHz to 80 dB at 420 kHz. For the 3-cm-thick ice, total attenuation at 188 kHz was about 12 dB (compared to about 40 dB for the 18-cm-thick ice). That this represents an absorption of the acoustic wave by the ice is evidenced by two other observations: first, a low reflection coefficient measured on newly grown ice (about 0.04 at 188 kHz); second, measurements of scattered energy back into the transmitting region showed little redistribution of energy into nonspecular directions. Based on the observed decrease of attenuation per unit thickness of ice, significant absorption is attributed to the lower, porous and permeable portions of the ice. Presumably the important absorption mechanisms are associated with the pure ice dendrites and alternating planes of brine pockets.

Acoustical Reflection and Scattering from the Underside of Laboratory-Grown Sea Ice: Measurements and Predictions (Abstract)

We have studied acoustical reflection and scattering properties of the underside of laboratory-grown sea ice. Our purpose was to determine the morphologic characteristics of undeformed sea ice that control acoustic scattering and reflection. So our experiments included both detailed studies of the structure of the ice as well as the application of a variety of acoustic methods. Ice sheets were grown in an outdoor pond (about 6 m by 13 m by 1.5 m) and exhibited features characteristic of undeformed, cold sea ice: an upper granular zone; a columnar zone of crystals with cross-sectional areas of about 1 cm2; vertical sheets of brine pockets; a bulk salinity of 9.1‰ distributed over 9 cm of ice; a dendritic interface at the ice/water boundary with dendrites about 0.5 mm across at the time of our measurements. Echo-amplitude fluctuations of normal-incidence sonar pings (100 kHz to 800 kHz) were measured as the sonars moved horizontally under the ice and accumulated into echo-amplitude histograms. (Data from a deteriorating ice sheet as well as data on lake ice were also collected.) We fitted the Rice probability density function (PDF) to the data and combined the resultant statistical parameter with Eckart acoustic scattering theory. The reflection coefficients calculated using this method ranged from 0.06 to 0.12, depending on environmental conditions. RMS roughness calculated using data from new sea ice was estimated to be about 0.3 mm. Because our ice thin sections show the ice to be porous and permeable at the interface with dendrites 0.5 mm thick, we suspect that the dendrites control the scattering as described by the echo-amplitude histograms. Further, we attribute the low reflection coefficients to the dendritic structure which may act as an impedance-matching zone into the columnar section of the sea ice. Transmission measurements were performed by positioning a transducer located at the ice/air interface directly over the transducer located in the water. The total attenuation through 18 cm of ice ranged from 12 dB at 50 kHz to 70 dB at 420 kHz (signal levels were measured relative to the same path in water).

Acoustical Reflection and Scattering from the Underside of Laboratory-Grown Sea Ice: Measurements and Predictions (Abstract)

We have studied acoustical reflection and scattering properties of the underside of laboratory-grown sea ice. Our purpose was to determine the morphologic characteristics of undeformed sea ice that control acoustic scattering and reflection. So our experiments included both detailed studies of the structure of the ice as well as the application of a variety of acoustic methods.Ice sheets were grown in an outdoor pond (about 6 m by 13 m by 1.5 m) and exhibited features characteristic of undeformed, cold sea ice: an upper granular zone; a columnar zone of crystals with cross-sectional areas of about 1 cm2; vertical sheets of brine pockets; a bulk salinity of 9.1‰ distributed over 9 cm of ice; a dendritic interface at the ice/water boundary with dendrites about 0.5 mm across at the time of our measurements. Echo-amplitude fluctuations of normal-incidence sonar pings (100 kHz to 800 kHz) were measured as the sonars moved horizontally under the ice and accumulated into echo-amplitude histograms. (Data from a deteriorating ice sheet as well as data on lake ice were also collected.) We fitted the Rice probability density function (PDF) to the data and combined the resultant statistical parameter with Eckart acoustic scattering theory. The reflection coefficients calculated using this method ranged from 0.06 to 0.12, depending on environmental conditions. RMS roughness calculated using data from new sea ice was estimated to be about 0.3 mm. Because our ice thin sections show the ice to be porous and permeable at the interface with dendrites 0.5 mm thick, we suspect that the dendrites control the scattering as described by the echo-amplitude histograms. Further, we attribute the low reflection coefficients to the dendritic structure which may act as an impedance-matching zone into the columnar section of the sea ice.Transmission measurements were performed by positioning a transducer located at the ice/air interface directly over the transducer located in the water. The total attenuation through 18 cm of ice ranged from 12 dB at 50 kHz to 70 dB at 420 kHz (signal levels were measured relative to the same path in water).

Acoustical reflection and scattering from the underside of laboratory grown sea ice: Measurements and predictions

Acoustical reflection and scattering properties of the underside of undeformed sea ice which was grown in an outdoor pond were studied. Echo amplitude fluctuations of normal incidence sonar pings (100–800 kHz) were measured as the sonars moved horizontally under the ice and accumulated into echo amplitude histograms. The Rice probability density function (PDF) was fit to the data and the resultant statistical parameter was combined with the Eckart acoustical scattering theory to estimate an rms roughness of the water/ice interface to be 0.3 mm. Because the ice thin sections showed the ice to be porous and permeable at the interface with dendrites 0.5 mm thick, it appeared that the dendrites controlled the scattering. The average reflection coefficient was of the order 0.05. The low reflection coefficient (low compared to the 0.35 value which is predicted from the bulk properties of sea ice) was attributed to the dendritic structure which was porous and permeable at the water/ice interface. From the data and modeling done, scattering, and, hence, echo fluctuations, for normal incidence sonars of various frequencies and beamwidths were also predicted.

Four years of low-altitude sea ice broad-band backscatter measurements

The ability to use radar to discriminate Arctic Sea ice types has been investigated using surface-based and helicopter-borne scatterometer systems. The surface-based FM/CW radar operated at 1.5 GHz and at multiple frequencies in the 8-18-GHz region. Measurements were made at angles of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10\deg</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70\deg</tex> from nadir. The helicopter-based radar operated at the 8-18-GHz frequencies with incidence angles of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0\deg</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60\deg</tex> . Extensive surface-truth measurements were made at or near the time of backscattar measurement to describe the physical and electrical properties of the polar scene. Measurements in the 8-18-GHz region verify the ability to discriminate multiyear, thick first-year, thin first-year, and pressure-ridged sea ice and lake ice. The lowest frequency, 9 GHz, was found to provide the greatest contrast between these ice categories, with significant levels of separation existing between angles from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15\deg</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70\deg</tex> . The radar cross sections for like antenna polarizations, VV and HH, were very similar in absolute level and angular response. Cross-polarization, VH and HV, provided the greatest contrast between ice types, The 1.5-GHz measurements showed that thick first-year, thin first-year, and multiyear sea ice cannot be distinguished at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10\deg</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60\deg</tex> incidence angles with like polarization, VV, by backscatter alone; but that undeformed sea ice can be discriminated from pressure-ridged ice and lake ice. The effect of snow cover on the backscatter from thick first-year ice was also investigated. It contributes on the order of 0 to 4 dB, depending on frequency and incidence angle; the contribution of the snow layer increased with increasing frequency. Snow cover on smooth lake ice was found to be a major backscatter mechanism. Summer measurements demonstrate the inability to extend the knowledge of the backscatter from sea ice under spring conditions to all seasons.

Surface-Based Scatterometer Results of Arctic Sea Ice

Radar backscatter measurements were made of shorefast sea ice near Point Barrow, AK, in May 1977, with a surface-based FM-CW scatterometer that swept from 1-2 GHz and from 8.5-17.5 GHz. The 1-2 GHz measurements showed that thick first-year and multiyear ice cannot be distinguished at 10-70° incidence angles, but that undeformed sea ice can be discriminated from pressure ridges and lake ice. Results also indicate that frequencies between 8-18 GHz have the ability to discriminate between thick first-year, multiyear, and lake ice. Cross polarization was found to be a better discriminator than like polarization. In addition, at these latter frequencies the differential scattering cross section σ° was found to have an approximately linearly increasing frequency response.