William J. Campbell

Time-dependence of sea-ice concentration and multiyear ice fraction in the Arctic Basin

The time variation of the sea-ice concentration and multiyear ice fraction within the pack ice in the Arctic Basin is examined, using microwave images of sea ice recently acquired by the Nimbus-5 spacecraft and the NASA CV-990 airborne laboratory. The images used for these studies were constructed from data acquired from the Electrically Scanned Microwave Radiometer (ESMR) which records radiation from earth and its atmosphere at a wavelength of 1.55 cm. Data are analyzed for four seasons during 1973–1975 to illustrate some basic differences in the properties of the sea ice during those times. Spacecraft data are compared with corresponding NASA CV-990 airborne laboratory data obtained over wide areas in the Arctic Basin during the Main Arctic Ice Dynamics Joint Experiment (1975) to illustrate the applicability of passive-microwave remote sensing for monitoring the time dependence of sea-ice concentration (divergence). These observations indicate significant variations in the sea-ice concentration in the spring, late fall and early winter. In addition, deep in the interior of the Arctic polar sea-ice pack, heretofore unobserved large areas, several hundred kilometers in extent, of sea-ice concentrations as low as 50% are indicated.

Microwave remote sensing of sea ice in the AIDJEX Main Experiment

During the AIDJEX Main Experiment, April 1975 through May 1976, a comprehensive microwave sensing program was performed on the sea ice of the Beaufort Sea. Surface and aircraft measurements were obtained during all seasons using a wide variety of active and passive microwave sensors. The surface program obtained passive microwave measurements of various ice types using four antennas mounted on a tracked vehicle. In three test regions, each with an area of approximately 1.5 × 104 m2, detailed ice crystallographic, dielectric properties, and brightness temperatures of first-year, multiyear, and first-year/multiyear mixtures were measured. A NASA aircraft obtained passive microwave measurements of the entire area of the AIDJEX manned station array (triangle) during each of 18 flights. This verified the earlier reported ability to distinguish first-year and multiyear ice types and concentration and gave new information on ways to observe ice mixtures and thin ice types. The active microwave measurements from aircraft included those from an X- and L-band radar and from a scatterometer. The former is used to study a wide variety of ice features and to estimate deformations, while both are equally usable to observe ice types. With the present data, only the scatterometer can be used to distinguish positively multiyear from first-year and various types of thin ice. This is best done using coupled active and passive microwave sensing.

Arctic sea-ice variations from time-lapse passive microwave imagery

This paper presents: (1) a short historical review of the passive microwave research on sea ice which established the observational and theoretical base permitting the interpretation of the first passive microwave images of Earth obtained by the Nimbus-5 ESMR; (2) the construction of a time-lapse motion picture film of a 16-month set of serial ESMR images to aid in the formidable data analysis task; and (3) a few of the most significant findings resulting from an early analysis of these data, using selected ESMR images to illustrate these findings.

Remote sensing of the Fram Strait marginal ice zone

Sequential remote sensing images of the Fram Strait marginal ice zone played a key role in elucidating the complex interactions of the atmosphere, ocean, and sea ice. Analysis of a subset of these images covering a 1-week period provided quantitative data on the mesoscale ice morphology, including ice edge positions, ice concentrations, floe size distribution, and ice kinematics. The analysis showed that, under light to moderate wind conditions, the morphology of the marginal ice zone reflects the underlying ocean circulation. High-resolution radar observations showed the location and size of ocean eddies near the ice edge. Ice kinematics from sequential radar images revealed an ocean eddy beneath the interior pack ice that was verified by in situ oceanographic measurements.

Observations of the polar regions from satellites using active and passive microwave techniques

Publisher Summary This chapter discusses the application of active and passive microwave techniques for observations of the Polar Regions from satellites. Sequential satellite synthetic aperture radar (SAR) observations can be used to measure sea-ice kinematics to accuracy hitherto attainable only by using manned drift stations equipped with Navsat gear. In this chapter, a method is described by which the dual-polarized, multispectral radiances from the SMMR permit the determination of sea-ice concentration, multiyear ice fraction-defined as the fraction of the ice concentration which has survived at least one summer, and an ice temperature which is a weighted mean-thermodynamic temperature of the radiating portion of the ice. Some results obtained with this dual-polarized, multispectral approach are presented. An analysis is presented of the altimeter wave-height and wind-speed measurements from July to October 1978, which is during the Antarctic winter. Sea-ice observations by nimbus-7 are discussed. Observations of ocean waves in the Antarctic, and Seasat altimeter observations of ice sheets are also described.

Simultaneous Passive and Active Microwave Observations of Near-Shore Beaufort Sea Ice

The use of active and passive microwave imagery in combination is the optimum way to observe the morphology and dynamics of near shore ice. Active and passive microwave data from aircraft that are described in this paper are also compared to the ESMR (Electrically Scanning Microwave Radiometer) imagery of the Nimbus-5 satellite. The information thus obtained shows how the data to be received from the SAR (Synthetic Aperture Radar) and SMMR (Scanning Multichannel Microwave Radiometer) and on Seasat A and Nimbus G have the potential of providing a vastly increased understanding of the near shore ice of the Beaufort Sea.

Ocean eddy structure by satellite radar altimetry required for iceberg towing

Abstract Models for the towing of large tabular icebergs give towing speeds of 0.5 knots to 1.0 knots relative to the ambient near surface current. Recent oceanographic research indicates that the world oceans are not principally composed of large steady-state current systems, like the Gulf Stream, but that most of the ocean momentum is probably involved in intense rings, formed by meanders of the large streams, and in mid-ocean eddies. These rings and eddies have typical dimensions on the order of 200 km with dynamic height anomalies across them of tens-of-centimeters to a meter. They migrate at speeds on the order of a few cm/sec. Current velocities as great as 3 knots have been observed in rings, and currents of 1 knot are common. Thus, the successful towing of icebergs is dependent on the ability to locate, measure, and track ocean rings and eddies. To accomplish this systematically on synoptic scales appears to be possible only by using satelliteborne radar altimeters. Ocean current and eddy structures as observed by the radar altimeters on the GEOS-3 and Seasat-1 satellites are presented and compared. Several satellite programs presently being planned call for flying radar altimeters in polar or near-polar orbits in the mid-1980 time frame. Thus, by the time tows of large icebergs will probably be attempted, it is possible synoptic observations of ocean rings and eddies which can be used to ascertain their location, size, intensity, and translation velocity will be a reality.

Annually-Averaged Polar Sea-Ice Extents During 1978–1987: Northern Extent Decrease and Southern Extent Oscillation

A two-dimensional finite element model has been applied to Rutford Ice Stream, Antarctica, and part of Ronne Ice Shelf into which the ice stream flows. The model is an extension of one describing ice-shelf flow, and relies on vertical shear in the ice stream being small in some mathematically defined sense. This is equivalent to requmng the vertical shear to be confined to a basal layer or a deformable substrate. Although there is no direct observational evidence for such a layer beneath Rutford Ice Stream, extensive surface surveys and estimates of the strength of the overlying ice show that some dynamically equivalent mechanism must occur. If basal shear stress is parameterised in terms of the thickness and viscosity of a linearly viscous substrate, as may be the case beneath Ice Stream B in Antarctica, then going upstream from the grounding line, the thickness of this layer must decrease, and the viscosity increase (to retain a realistic thickness at the upstream limit), in order to reproduce the observed surface velocities. This physically reasonable picture is currently adopted as a working hypothesis. Vertical shear in the body of the ice stream appears to be negligible for approximately 70 km above the grounding line . Sensitivi ty tests show that , in this lower section, ice shelf back stress is an important restraining influence . A 10% reduction in back stress would produce an immediate 15% increase in groundin g line flux. Further upst ream, however, higher surface slopes and slightly lower surface velocities sugges t that the neglect of vertical shear may be less appropriate. The effect of a reduction in ice-she lf back stream is not felt in this region immediatel y, as th e gravitational driving force is almost balanced by local basal shear trac tion . A complex surface morphology has been revealed by satellite imagery below the grounding line of Rutford Ice Stream. On the basis that this may be evidence of time dependent behaviour, the finite element model is being used to investigate the origin of the pattern. iceshelf back stress, basal melting , mass flux from tributary glaciers and substrate properties can all be varied in phys ically realistic ways to try to reproduce, qualitatively, the observed surface morphology.