Sea Ice Thickness Estimates Research Articles

Sea ice as an evaluation target for HEM modelling and inversion

We investigate helicopter electromagnetic (HEM) inversion schemes applied to synthetic and measured HEM sea ice profiling data. Direct HEM data-to-ice-thickness inversion is compared to three different formal, least squares layered earth inversion algorithms. By making several approximations, it is possible to directly invert a single channel measurement (i.e., the in-phase or quadrature component of a single frequency measurement) to obtain an estimate of sea ice thickness. Measurements from multiple input channels, however, can be used in a layered earth inversion to simultaneously recover several model parameters such as sea ice thickness, sea ice conductivity and sub-ice bathymetry. Synthetic data sets for a particular two-frequency HEM system showed that simple least squares inversion algorithms produce reliable estimates of sea ice thickness in cases where the ice is thicker than 3 m. These methods could also recover acceptable estimates of sea ice thickness when a thin, conductive, partially melted sea ice layer was present, and could determine shallow, sub-ice bathymetry in brackish water. As expected, 1D transformations and inversions of synthetic data for a three-dimensional pressure ridge keel structure contained artifacts, notably broadening of the apparent width of the keel. Prior to inverting a field data set acquired over rather thin (~ 0.5 m) Antarctic sea ice, we found it necessary to recalibrate the phase angle of the measurements using a phasor diagram-based method. Direct transformation of a single channel from the recalibrated data set produced more accurate estimates of sea-ice thickness than formal inversion of multi-channel data. We suggest that the least squares inversion methods are inferior in this situation because of the particular characteristics of the two-frequency HEM system used in this evaluation; the extreme differences in sensitivity of high and low frequency data components, the overall low sensitivity to sea ice conductivity (especially for thin ice), and the partially low signal-to noise ratios of the measurements. The data sets used in this study will be made available to the public to allow alternate inversion approaches to be applied and evaluated. It is suggested that inclusion of parameter bounds and other forms of regularization could help to improve the inversion results.

ICESat measurements of sea ice freeboard and estimates of sea ice thickness in the Weddell Sea

Sea ice freeboard heights in the Weddell Sea of Antarctica are derived from the Ice, Cloud, and Land Elevation Satellite (ICESat) laser altimeter measurements, which have a unique range precision to flat surfaces of 2 cm within 70 m footprints spaced at 172 m along track. Although elevations of flat surfaces can be obtained to an accuracy of ∼10 cm (1σ) per footprint, direct determination of freeboard heights is precluded by errors in knowledge of the geoid and temporal variability of the ocean surface. Therefore freeboards are determined relative to an ocean reference level detected over areas of open water and very thin ice within the sea ice pack using an along‐track filtering method. The open water/thin ice selections show good agreement in the combined analysis of ICESat segments and Envisat Synthetic Aperture Radar (SAR) imagery. The average residual between the ICESat‐measured ocean level and the EGM96 geoid is 1.4 m. Estimates of snow depth on the sea ice from AMSR‐E passive microwave data along with nominal densities of snow, water, and sea ice are used to estimate sea ice thickness. Four periods of ICESat data in May–June (fall) and October–November (late winter) of 2004 and 2005 between longitudes 298°E and 360°E are analyzed. In the fall the mean freeboards are 0.28 m in 2004 and 0.29 m in 2005, and the mean thicknesses are 1.33 m in 2004 and 1.52 m in 2005. In late winter the freeboards grew to 0.37 m in 2004 and 0.35 in 2005, and the thicknesses grew to 2.23 m in 2004 and 2.31 m in 2005. The interannual differences in freeboard are small, and the larger interannual change in estimated thickness mainly represents differences in the snow depth estimates. Seasonal changes in the spatial patterns of freeboard and thickness over the 4 months correlate with the expected circulation of sea ice in the Weddell Sea, as indicated by sea ice velocity fields.

Seasonal ice area and volume production of the Arctic Ocean: November 1996 through April 1997

The RADARSAT Geophysical Processor System (RGPS) produces estimates of sea ice motion, deformation, and thickness of the Arctic Ocean sea ice cover from time‐sequential synthetic aperture radar (SAR) imagery acquired by RADARSAT. Based on these products, we summarize the evolution of a region of the Arctic Ocean ice cover in terms of its area change, ice volume production, and multiyear ice coverage over a 6‐month period between 7 November 1996 through 30 April 1997. This region encompasses a large part of the Western Arctic Ocean. The initial ice area covers ∼2.52 × 106 km2. At the end of April, this region covers ∼2.59 × 106 km2, a net divergence of ∼3%. Over the period, thin ice (0–20 cm) typically covers <2% of this area. The RGPS estimates only the thickness distribution of sea ice volume produced by openings and closings of the ice cover since the initial observation. Thus, only the ice volume of the seasonal ice cover can be determined. Between November 1996 and April 1997, ∼1000 km3 (∼0.4 m) of sea ice is produced. The amount stored in undeformed ice and ridged ice is dependent on the character of the redistribution function. The RGPS products also provide estimates of multiyear ice coverage. From the time‐series of multiyear ice coverage, we estimate that nearly 83% of the area is covered by multiyear ice. For more detailed examination of the regional dependence of ice‐cover deformation, volume production, and MY ice coverage, the larger region is divided into five subregions. Where possible, we provide an analysis of the errors in the above estimates. In cases where quantification of the uncertainty is not feasible, we list the potential error sources. There are remaining sources of uncertainty in these calculations that remain unquantified, but at this time, there are no adequate in situ or remote‐sensing data for comprehensive evaluation of the above estimates.

Antarctic sea ice thickness measurements using a ship-borne electromagnetic induction device

We perform a theoretical analysis of Antarctic sea ice thickness estimates made using a ship-borne electromagnetic instrument. Inversion of theoretical one-dimensional sea ice models assuming a conductive half-space model has shown that the ship-borne system is able to determine thickness to within 10% for level ice up to ~2 m thick. However, the presence of a thin conducting brine layer within the ice, common during the Antarctic summer, results in a large error in thicknesses estimated assuming perfectly-resistive ice. An analysis of the effect of system attitude on the electromagnetic response has shown that pitch and roll of up to 10° can result in significant errors in the interpreted ice thickness in areas of thin sea ice. A preliminary three-dimensional model study of the EM-31 system has shown that it can yield good estimates of maximum sea ice keel thickness. However, sea ice thickness is significantly over- or underestimated near sharp vertical boundaries in the three-dimensional model.

Ice thickness derived from high‐resolution radar imagery

A new analysis makes it possible to produce basin‐scale estimates of sea ice age and thickness from observations of ice motion in radar images of the Arctic Ocean. Anticipated from sequential synthetic aperture radar (SAR) imagery over a series of winters are Arcticwide fields of ice motion and estimates of ice age and thickness. This analysis technique is implemented in the RADARSAT Geophysical Processor System (RGPS),a data processing system dedicated to analyzing large volumes of SAR imagery of sea ice [Kwok, 1998].Currently, we have only crude estimates on how much ice there is, how it varies in space and time, where ice is produced and where it melts, and how rapidly it is transported from place to place. SAR provides an amazingly detailed look at sea ice, but the level of detail is so great that it is not at first apparent how to utilize the data for improving sea ice data sets and models.

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.

Sounding sea ice thickness using a portable electromagnetic induction instrument

Field trials using a man‐portable, commercially available, electromagnetic induction (EMI) sounding instrument, with a plug‐in data processing module for the remote measurement of sea ice thickness, are discussed. The processing module was made to allow for the direct determination of sea ice thickness and to show the result in a numerical display. The processing module system was capable of estimating ice thickness within 10 percent of the the true ice value for ice from about 0.7 to 3.5 m thick, the thickest of undeformed ice in our study area. However, since seawater under the Arctic pack ice has relatively uniform conductivity (2.55 ± 0.05 S/m), a simplified method can be used for estimating sea ice thickness using just an EMI instrument. This technique uses only the EMI conductivity measurement, is easy to put into use, and does not rely on theoretically derived look‐up tables or phasor diagrams, which may not be accurate for the conditions of the area.