A detailed description of the seasonal cycle of Northern Hemisphere sea ice for 1974 is provided by the passive microwave data from the Nimbus 5 Electrically Scanning Microwave Radiometer (ESMR). Sea ice extent has been mapped and analyzed in eight regions of the Arctic and marginal seas. In the seasonal sea ice areas, the ice concentration is also mapped, whereas in areas of first-year and multiyear ice mixtures, the corresponding mapping is of a parameter representing a combination of ice concentration and multiyear ice fraction. The total monthly ice extent increased from a sharp minimum of 7.6 × 106 km2 in September, when the ice pack was mostly confined to the central Arctic Ocean and portions of the Greenland Sea, Kara Sea, and Canadian Archipelago, to a broad maximum of 14.4 × 106 km2 in March, when the ice cover was nearly complete in the Arctic Ocean, Hudson Bay, Kara Sea, and Canadian Archipelago and was extensive for large portions of the other peripheral seas and bays. In the areas of seasonal sea ice coverage, the average ice concentration was approximately 75% in winter, which is close to the values observed in the Southern Ocean and significantly less than the greater-than-95% concentrations observed in the central Arctic Ocean and Hudson Bay, where the ice packs are constrained by land boundaries. Midwinter decreases in ice extent for 1—2 months are noted in the regions of the Greenland Sea and the Kara and Barents Seas.

Iceland’s largest ice cap, Vatnajökull, has been the test site for a series of airborne and satellite remote-sensing studies since 1966. Various types of image data acquired by the Landsat Multispectral Scanner (MSS) and the Seasat Synthetic Aperture Radar (SAR) are assessed for their value to glaciological studies of Vatnajökull. A low Sun angle winter 1973 MSS band 7 Landsat image of Vatnajökull provides information about the distribution and size of subglacial volcanic calderas, cauldron subsidence caused by subglacial geothermal and (or) intrusive volcanic activity, and delineation of the probable position of surface ice divides. Two types of multi-spectral digital enhancements were applied to a late summer 1973 MSS image of Vatnajökull. The first type was used to prepare a planimetric base map showing the location of the principal surface features and an inventory of 38 named outlet glaciers, one internal ice cap (Öraefajökull), and two detached glaciers which comprise this complex ice cap, and to measure its area (8300 km2). The second type provides information about the position of the snow line at the approximate end of the 1973 melt season, the areas encompassed by the ice fades of the ablation area and the slush zone and wet-snow facies/percolation facies of the accumulation area. More information about the surface morphology of Vatnajökull was available from the Sow Sun angle winter and the digitally enhanced summer Landsat image of the ice cap than from the Seasat SAR image.

Daily maps of multiyear ice concentration, derived from Nimbus-7 SMMR passive microwave data, are analyzed to obtain the displacement of the multiyear ice edge and information on the convergence/divergence within the pack. The dynamic—thermodynamic sea-ice model of Hibler (1979) is run with daily time steps and with forcing by the interannually varying fields of geostrophic wind and temperature-derived thermodynamic fluxes. Model-data comparisons are made for the net drift during the months of November through January of the 1978—79, 1979—80, and 1980—81 seasons, and for the shorter-term drift during a 52 day period. Both the model and the data-based drifts for the 25 November 1978 to 28 January 1979 period differ from the classical Beaufort-gyre pattern exhibited in the other two winters. For the 52 day period of November—December 1978, both the model and the data show an eastward drift followed by a westward drift of the ice edges in the Laptev Sea, and for the 25 November 1978 to 28 January 1979 period, a net westward drift of about 250 km. Overall, the model and the data exhibit the same patterns of ice movement with marked month-to-month and large interannual variations in the drift. Good agreement is found in most regions of the central Arctic, but pronounced discrepancies occur near the edge of the total ice pack in the East Greenland Sea. During a short period of large changes in multiyear ice concentration in the central Arctic around 2 December 1980, the divergence implied by the changes in multiyear concentration is qualitatively compared with the divergence computed from the modeled velocity fields. Both the microwave data and the model results indicate similar temporal characteristics of pack-ice response during this major deformation event.

Periodically since 1950, air photographs and SLAR images have been taken of the Arctic ice shelves. The study of air photographs and SLAR images of the outer part of Milne Ice Shelf had three aims: (1) to map losses and ice re-growth at the shelf front, (2) to map the evolution of melt pools on shelf ice and multi-year land-fast sea ice, and (3) to assess the usefulness of air photographs and SLAR for these purposes. For mapping of ice calvings and subsequent sea-ice growth, both air photographs and radar images have been used sucessfully. However, air photographs are better than radar for mapping ice-surface features. The ridge-and-trough systems that characterize the surface of the ice shelf and old sea ice are clearly visible on each type of imagery but, because of their larger scale, air photographs proved to be most useful for a study of melt-pool evolution. The orientation of the melt pools is parallel to the prevailing winds which drive water along the troughs. The drainage system evolves by a process of elongation and coalesence.

The paper discusses data analysed from airborne radio echo-sounding of Svalbard glaciers at frequencies of 440 and 620 MHz. Bottom returns from depths greater than 200 m are recorded with fewer gaps if the more powerful 620 MHz radar is used, and if measurements are carried out in the spring before intensive melt on glaciers. For all relatively thin glaciers and some glaciers up to 320–625 in thick, the track with bed returns is still rather common, apparently caused by their colder temperature regime. However, because of severe scattering of radio waves, this procedure still does not solve the problems of the echo-sounding of accumulation areas of many of the larger glaciers, the ice plateau, and heavily crevassed parts of glaciers.

For considerable areas of those Spitsbergen glaciers which have a thickness greater than 200 m, internal radar reflections (IRR) were registered as a single isolated layer from depths usually ranging from ¼ to ½ of their thickness. Studies of two deep bore holes on Fridtjovbreen have- demonstrated that such IRR are related to a boundary between cold ice and water-bearing ice near the melting point. These IRR can be interpreted as indicators of a special class of two-layered or transitional glacier, and of the location within them of the ice-melt isotherm.

Airborne radio echo-sounding was carried out in order to measure the thickness of the ice sheet in the Shirase Glacier drainage basin and map the bedrock topography. It was found that the elevation of bedrock was approximately at sea-level from Shirase Glacier to 100 km up-stream of the glacier and thereafter it was 500–100 m higher. Investigation of the echo intensity reflected from the bedrock indicates that at ice thicknesses less than 1000 m absorption was about 5.2 dB/100 m, but at greater ice thicknesses echo intensity did not depend upon the ice thickness but became approximately constant. Where ice thicknesses were greater than 1000 m in the main flow area of the Shirase Glacier drainage basin, the reflection strengths of about 9 dB were greater than outside the basin. Since the increase in echo intensity was considered to be due to the existence of water, the strong echo observed in the main part of the basin supported an hypothesis that the base of the basin was wet and the ice sheet was sliding on the bedrock.

Areal snow-cover data provided by remote sensing enable the areal water equivalent at the start of the snow melt season to be evaluated. To this end, the time scale in the graphical representation of the snow coverage curves is replaced by the totalized computed daily melt depths. These refer to the seasonal snow cover at the starting date and disregard subsequent snowfalls.

A synthetic aperture radar (SAR) technique has been used to image part of the grounding-line region of Bach Ice Shelf in the Antarctic Peninsula. The radar was sledge-mounted and operated in a pulsed mode with a carrier frequency of 120 MHz. The coherently detected output was recorded photographically as in-phase and quadrature components. Because the system was essentially stationary for each measurement, there was no doppler information about the reflecting points as in the more commonly used airborne and satellite-based SARs. Instead, the phase history was used directly to identify point targets by a correlation method.

Three sounding runs were carried out over the grounding line to give views of the area from separate directions. An aperture length of 104 m was necessary to achieve 8 m resolution in the along-track direction for an ice thickness of 290 m. The mapped swath was 88 m wide. Corrections to the data were made to allow for density variations and absorption in the ice. The back-scatter coefficient showed greater variations in echo strength over grounded ice compared with floating ice and texture analysis of the radar image revealed a statistically significant difference between these two regimes.

A systematic programme of side-scan sonar and plumb- line soundings was carried out in the Weddell Sea area in 1985 to measure the under-water sides of ice shelves and icebergs. From these observations the following model is suggested for the evolution of the ice front:

• (1) Initial stage: fracturing of the ice shelves takes place along smooth, curvi-linear segments with vertical faces.

• (2) Formative stage: the freshly formed vertical face is eroded both by wave and swell action around the water line, by small calvings from the undercut, overhanging subaerial face, and by submarine melting. The melting has a minimum at 50–100 m depth, and increases with depth to a rate of around 10 m a−1 at 200 m, This is about twice the rate of erosion at the water line. The variation in melting with depth results from a combination of summer melting by near-surface water, and year-round melting by water masses that are increasingly warmer than the pressure melting-point with depth.

• (3) Mature stage: this stage is reached after a few years of exposure. The backward erosion of the face leads to a shape with a prominent under-water “nose” with a maximum projection to more than 50 m at 50–100 m depth. The ramp above this slopes upwards to meet the vertical wall about 5 m below the water line. The ice below the nose is melted back beyond the above-water face. There is no net buoyancy and ice shelves at this mature stage are generally not up-warped at the front.

Ice shelves are important regions to observe because they are likely to be sensitive indicators of climatic change. The satellite-borne radar altimetry is highly suited to ice-shelf monitoring; experience with Seasat, which flew in 1978, has demonstrated that a height-measurement precision of the order of 1 m can be obtained over ice surfaces (Brooks and Others 1983).

We identify subtle changes in altimeter wave forms associated with crevassed zones and the grounding line. Normal retracking procedures are shown to be inadequate in detecting such changes, and so methods which provide sensitive indication of the presence of these features in the sampled areas are devised. By ranging to the first return in the echo, the grounding line is identified, and by differencing this measurement with the half-peak power range, a measure of surface roughness is obtained which can be used to detect crevassed zones.

Detection of crevassed shear zones allows delimitation of distinct zones of flow in the ice shelf which can be monitored by future altimeter missions. Monitoring of the grounding-line position can provide sensitive indication of mass-balance conditions over the grounded part of the drainage basin.

The annual surface mass balance for 1983 and 1984 and the 10 year cumulative mass balances for 1975–85 were calculated for 60 south-east Alaskan and north-west British Columbia glaciers. At present, the mass balance is positive on nine, at equilibrium on nine, and negative on 42 glaciers. The ratio of glaciers with positive and equilibrium mass balance to glaciers with negative mass-balance has not changed significantly since 1946; however, the magnitude of negative balances has declined on 39 of the 42 glaciers.

The annual mass balance of south-east Alaska and north-west British Columbia glaciers cannot be measured on more than a few glaciers. This paper presents the methods and results for a mass-balance model using as input local weather records, Juneau Icefield field studies, and satellite imagery. The primary variable in mass balance from one glacier to another is the budget gradient. The budget gradient varies predictably according to three parameters: ocean proximity, surface slope, and valley width-valley height. The annual fluctuation of the budget gradient can be determined by examination of local weather records, determination of activity indexes, and delineation of the equilibrium-line gradient from the maritime to the continental part of each icefield. The latter two variables are determined using largely satellite imagery, keyed to topographic maps.

This procedure, where applicable, yielded mass-balance errors of ±0.16–0.22 m and 10 year cumulative mass-balance errors of ±0.08–0.15 m.

The physical background for the interpretation of microwave measurements on snow and ice is summarized. The angular and spectral behaviour of backscattering is shown for various snow types based on measurements carried out in the Swiss Alps. The information content of SAR images in regard to snow and glacier applications is discussed, and examples are shown for Seasat SAR and airborne SAR images. The preliminary specifications are given for an optimum SAR system for snow and glacier monitoring. The main advantage of SAR is due to its weather independence; for special applications the SAR information on the physical state of snow and ice may be of interest. Future SAR sensors can become important components in a snow and glacier monitoring system, but in order to fulfil all tasks, high-resolution optical sensors and improved passive microwave sensors will also be required.

The seasonal progression of snow melt on the Arctic pack ice is mapped from satellite shortwave imagery (0.4–1.1 micrometers) for four spring/summer seasons (1977, 1979, 1984 and 1985). This provides the first detailed information on the temporal change of the ice surface albedo in summer and of its year-to-year variability. The average surface albedo of the Arctic Basin for the years investigated falls from between 0.75 and 0.80 in early May to between 0.35 and 0.45 in late July and early August. In the central Arctic, where ice concentration remains high and ponding on the ice is limited, the July albedo ranges from 0.50 to 0.60. Overall, melt progresses poleward from the Kara and Barents Seas and from the southern Beaufort and Chukchi Seas, with the melt fronts meeting on the American side of the Pole. There are substantial year-to-year differences in the timing, duration and extent of the melt interval. The progression of melt in May and June of the earliest melt year (1977) was about 3 weeks ahead of the latest year (1979). By late July, the central Arctic was essentially snow free in 1977 and 1979, but more than 50% snow covered in 1984. Although limited in extent, our data base suggests relationships between snow melt and Arctic surface air temperatures in spring, spring cloudiness and the extent of late summer ice.

A digital radar system comprising multiple microprocessors, for use with 50 MHz radar units modified from the Scott Polar Research Institute Mark IV design, is described. The major features of the system include coherent integration of radar traces, storage of data in raw digitized form without demodulation, real-time play-back of digitized information, and high system performance resulting in good spatial sampling with integration even in airborne operations. Unfocused synthetic beam shaping also results from the integration of echoes, thus reducing clutter or incoherent scattering from the sides of the beam pattern along the profiling track.

Examples of data collected during the austral summer of 1985–86 in the Antarctic on ice stream B, in both ground and airborne programs, illustrate both the flexibility in data presentation and features present in the records.

Profiles of the ice cover in the North Water area were obtained in the winter of 1980/81 by using low-level infrared thermometry. The flight measurements were carried out from December to March. The statistical analysis of the sea ice surface temperature was carried out to yield distributions, frequencies and widths of fractures. Ice-free as well as ice-covered fractures with a maximum ice thickness of 0.4 m were analysed. Typical fracture frequencies were 0.25 per km for Lancaster Sound and 0.14 per km for Baffin Bay and the North Water area, with 90% of fractures being less than 0.6 km wide. From December to March, the fractures occupied 8.8% of the Ice cover in the North Water area, 8.7% along the Baffin Bay profile and 10% in the Lancaster Sound. In the North Water area the distance (y) between fractures for different fracture widths (x) is an exponential function of the form y=Aexp(ax) (A,a are constants), for fractures between 50 and 800 m wide. In the North Water area during winter, fractures of all widths occur 5 times more frequently than in M’Clure Strait and about 7 times more frequently than in southern Beaufort Sea. The heat loss in Lancaster Sound at the ice-air interface was found to be 40 to 100% larger due to the fractures compared to a fast ice situation in the same winter.

Run-off modelling is needed in Greenland to extend the short series of measurements. However, the delineation of hydrological basins on the Greenland ice sheet is difficult because of the lack of information about surface and subglacial drainage patterns. Low Sun-angle Landsat data have been used for mapping local surface features which has led to an improvement in basin delineations and thereby run-off simulations. Work is now in progress to map subglacial topography by electromagnetic reflection (EMR) from a helicopter. This information will be used for calculating hydraulic potentials within the basin and to assess the possibilities of future changes in drainage-basin delineation.

The impulse radar was found to be useful in surveying the internal structures of wet snow patches in mountain regions. Radar profiles revealed that the thickness of the perennial ice was 30 m, possibly the thickest in Japan. An unconformity widely extended nearly parallel to the surface at the depth of 2 to 9 m and divided the ice into two parts.

Extensive echo-sounding was carried out in east Dronning Maud Land during the 1984 field season. A 179 MHz radar with separate transmitting and receiving antennae was used and the echoes were recorded by a digital system to detect minute reflections. The results gave cross-sections of the ice sheet along traverse routes from lat. 69 °S. to 75°S, Detailed observations on the ground at Mizuho station showed that there was elliptical polarization in the internally reflected echoes when two antennae, kept in parallel with each other, were rotated horizontally. The internal echoes were most clearly distinguished when the antenna azimuth was oriented perpendicular to the flow line of the ice sheet. The internal echoes with a high reflection coefficient were detected at depths of 500–700 m and 1000–1500 m at Mizuho station. Since a distinct internal echo at a depth of 500 m coincides with a 5 cm thick volcanic ash-laden ice layer found in the 700 m ice core taken near the observation site, these echoes may correspond to the acidic ice layers formed by past volcanic events in east Dronning Maud Land.

The images of NOAA/TIROS-N APT, AVHRR and a few Landsat MSS obtained from 1980 to 1985 are analysed in this paper. It is found that the snow-cover distribution in Qilian Mountains is above 3700 m a.s.l. during winter to spring every year. There are two concentrations of snow cover. One is on Mount Leng Longling in the upper reaches of the Shiyang River and the other is located between Hala Lake and Mount Danghe Nanshan.

Based on preliminary investigations, it is known that the surface water resource in the Hexi region is 68 8 × 108 m3, of which about 24.8% is from glaciers and melting, and the snow-melt run-off is 7.63 × 108 m3, equal to 62.6% of the total amount of spring run-off.

The average value of Cv for spring run-off in the Shiyang River, Heihe River, and Shule River is 0 32 and the Cv value of snow-melt run-off in spring is 0.41, about three times as much as that of the annual run-off in the Hexi, region. A prediction model of spring snow-melt run-off at the Ying Louxia Hydrometric station in the Heihe River area can be constructed by using hydrometeorological data and snow-cover percentage for the Heihe River basin obtained from NOAA/TIROS-N APT, and AVHRR images. The prediction models (2) and (3) have been tested by the Water Resources Management Office of the Heihe River basin in the Zhangye and Flood Prevention Office of Gansu Province. The prediction accuracy is suitable for demands.

As part of a systematic analysis of Seasat radar altimetry data to obtain Antarctic ice fronts and ice-shelf elevations north of lat. 72° S., Fimbulisen (between long. 12°W. and 08°E.) and the Amery Ice Shelf (around long. 72°E.) are mapped. Interactive computer analysis is used to examine and correct the altimetry range measurements and derive the ice-front positions. Surface elevations and ice-front positions from radar altimetry are compared with ice fronts, ice rises, crevasse zones, and grounding lines identified in Landsat imagery. By comparison of the visible features in imagery and the computer-contoured elevations from radar altimetry, the radar-elevation mapping on some ice rises is confirmed, but some spurious contours are also identified. During the interval between the 1974 Landsat imagery and the 1978 radar altimetry, the central part of the Amery Ice Shelf front advanced 1.5 ± 0.6 km/a, which is in agreement with the ice-velocity measurements of 1.1 ± 0.1 km/a (Budd and others 1982), suggesting negligible calving in the central part of the ice shelf. The undulating surface and small mean slope from the grounding line to about lat. 70°S. suggest a zone of partial grounding similar to Rutford Ice Stream, On Fimbulisen, some previously unmapped ice rises are identified. The ridge of the Jutul-straumen ice tongue is shown to be about 20 m above the surrounding ice and laterally expanding as it flows northward to the ice front. Icebergs within the sea ice and a zone of shore-fast ice are also identified with the same technique used to map the ice-shelf front.