Ice thickness measurements have been carried out at the mouths of ice streams D and E, West Antarctica using a surface-based impulse radar. These studies have been undertaken as a part of the continuing effort to understand the state of the West Antarctica ice sheet and its response to climate change. Thickness measurements will be used in the mass balance calculation currently in progress and to better understand features in the surface topography seen at low angle sun illumination in the satellite imagery. Results show that the discharge areas of ice streams D and E are thickening by approximately 1 m per year, and thus that these ice streams are probably loosing mass. Aperiodic wavelike features in the surface topography are described which pose interesting questions about migration of the grounding line and ice-stream dynamics.

The relationship between ice fabric and the internal radio-echo reflections was investigated using observation data collected at Mizuho Station, Antarctica. The data were obtained by 179 MHz radio-echo sounding and the ice fabric was measured from 700 m Mizuho ice core. The dielectric permittivity tensor at given depths in the ice sheet was calculated from the ice fabric.

The calculated dielectric permittivity tensor showed that the ice sheet at Mizuho Station is a uniaxially birefringent medium. The symmetrical axis of rotation was the same as the flow line. In such a medium, theory predicts that the electric field vectors are allowed only in the two directions parallel and perpendicular to the flow line. The prediction coincided well with the observation: a strong signal was observed only when the transmitting antenna and the receiving antenna, kept parallel to one another, were oriented parallel or perpendicular to the flow line. However, the observed signal strength in these two directions differed from one another at each depth.

It is also shown that the power reflection coefficient due to the variation of ice fabric with depth is of approximately the same order as that due to the density change and is large enough to produce the predominant internal radio-echo reflections.

Dielectric anisotropy in ice Ih was investigated at 9.7 GHz with the waveguide method. The measurement of dielectric permittivity was made using single crystals collected from Mendenhall Glacier, Alaska. The result of the measurement shows that ϵ′‖c, the real part of dielectric permittivity parallel to the c axis, is larger than ϵ′⊥c the real part of dielectric permittivity perpendicular to the c axis. This tendency is similar to that at low frequencies in the region of the Debye relaxation dispersion. It can be proposed that ϵ′‖c>ϵ′⊥c in the HF, VHF and microwave frequency range. ϵ′‖c and ϵ′‖c depend slightly upon temperature but the dielectric anisotropy, ∆ϵ′=ϵ′‖c-ϵ′⊥c, is constant and becomes 0.037 (±0.007). Based on the present results, a simple caculation indicates that the maximum power reflection coefficient caused by the dielectric anisotropy is about −50 ∼ −80 dB, which is significantly larger than the power reflection coefficient observed in the ice sheet by radio-echo sounding, about −70 ∼ −80 dB. This leads to a conclusion that dielectric anisotropy is one of the dominant causes of internal reflections.

The authors have developed an experimental device in a cold laboratory with the purpose of measuring optical parameters of natural snow depending on grain-size, impurity content and density. Snow samples were prepared from homogeneous layers in order to measure the radiative properties of clearly identified snow types. The first part of this paper describes the working assumptions and the experimental device. In the second part, experiment results are described and discussed. We have compared albedo measurements of different natural snow types with theoretical values derived from physical optics, based on Mie scattering. The albedo evolution of three different snow types submitted to temperature-gradient metamorphism is analyzed.

A model for estimating the radiation balance of alpine snowfields is presented. Shortwave and longwave downward flux densities are computed for sloping surfaces using a modified version of the two-stream radiative transfer scheme of Zdunkowski and others (1982). Surface albedo and thermal exitance values are estimated using Landsat-5 Thematic Mapper (TM) imagery and digital terrain data. The LOWTRAN7 radiative transfer code is utilized in order to remove atmospheric effects in satellite imagery as well as calculating solar irradiance within TM spectral bands, for the determination of the near-nadir reflectance of snow. Under a Lambertian assumption, near-nadir reflectance measurements obtained from a few TM bands are used to calculate the total hemispherical reflectance (albedo) of snow. The net all-wave radiation of snowfields is then simulated for the complete day on the same date as that of the Landsat overflight. The model is tested using Landsat TM data acquired in late June 1984, and results compared with field measurements acquired on Niwot Ridge, Colorado, U.S.A. Preliminary results are very encouraging but problems remain in the estimation of surface albedo from near-nadir satellite reflectance measurements of TM. These problems are discussed and recommendations for future model improvements are given.

Modeling microwave back-scattering and emission from snow-packs requires the knowledge of snowpack characteristics and their dynamics to select an appropriate model. Both theory and field data show that microwave back-scattering coefficients and brightness temperatures are sensitive to parameters describing snow-microstructure. Stereological methods and other techniques can be applied to images of sections cut from undisturbed snow, and are used to obtain accurate and unbiased estimates of snow-microstructure parameters for discrete scatterer modeling. Assuming that the ice particle-size distribution can be characterized as a log-normal distribution function, we show that the parameters describing the distribution can be obtained from section images. The results show that, in addition to snow density and ice-particle size, the particle-size variation has great effect on dry-snow extinction properties. The optically equivalent ice-particle size for Rayleigh scattering in a snowpack with grain-size variations can be determined from the stereological measurements from snow sections.

The radiative-transfer model developed by Zwally (1977) is modified and coupled to a one-dimensional time-dependent temperature model, to calculate the seasonal variation in brightness temperature. By comparing this with observed records, the radiative properties of firn can be determined. By retaining scattering as a source term in the radiative transfer function, agreement between model-derived scattering and absorption coefficients and those calculated from the Mie/Rayleigh scattering theory can be obtained. The horizontal brightness temperature is not linked to the vertical one through a constant power reflection coefficient.

Snow-cover monitoring using passive microwave remote sensing methods has been shown to be seriously limited under melt conditions when the snowpack becomes wet. A wet snow indicator has been developed using DMSP SSM/I 37 GHz dual-polarization data for the open prairie region of western Canada. The indicator is used to identify areas of wet snow and discriminate them from areas of snow-free land. Validation and testing efforts have illustrated that the addition of the indicator to the current SSM/I snow water equivalent algorithm provides a more accurate representation of spatial snow coverage throughout the winter season for the open prairie region. The improved spatial and temporal information resulting from the use of the indicator enhances both climatological and hydrological analyses of snow-cover conditions using passive microwave data. Although the wet snow indicator has only been validated for the open prairie region of western Canada, it may also be applicable to other regions of similar terrain and vegetative characteristics. However, in areas of dense vegetation, such as the boreal forest, the performance of the indicator is poor due to the generally low 37 GHz polarization differences of the vegetation cover.

Differences in radio-echoes received from the base of the ice sheet covering two different mountain areas in Dronning Maud Land, East Antarctica, are herein interpreted as differences in the physical properties in the ice and differences in basal conditions. An abnormally strong signal can be caused either by unusually little absorption of radio waves within the ice or by water at the base of the glacier.

One of the areas studied is totally covered by a 400–1400 m thick ice sheet. The altitude of the surface is about 2800 m a.s.l. The second area is a nunatak area situated relatively closer to the coast. The ice thickness in the valleys is about 500 m and the altitude of the surface is about 400 m.

The annual mean surface temperature for the areas is about −30° and −17°C respectively. In both cases ice depths are moderate and flow rates low, and it is thus probable that the ice is frozen to the bed.

However, in the colder locality I believe the strong echo is caused by low or negligible rate of mass flux. In the lower area strong bottom echoes are believed to be a cause of negative mass balance at the surface of the blue ice areas. The negative surface mass balance reverses the vertical component of ice flow and it also changes the temperature distribution within the ice. Basal melting conditions may thus occur at locations where the ablation rate is high. This process may also have important implications on the development of land forms under cold ice sheets.

The rapid retreat and disintegration of the Larsen Ice Shelf sector extending north of Seal Nunataks (65° S), documented from the mid 1970s onwards by remote sensing, is presented and related to the Antarctic Peninsula climatic warming recorded over several past decades. A 1975 KOSMOS satellite photograph and a series of LANDSAT MSS and TM images taken in 1978, 1979, 1986, 1988 and 1989 were used to monitor the retreat of the ice shelf between Seal Nunataks and Prince Gustav Channel. The ice shelf has decreased by more than 30% during the period 1975–89 within the Christensen Island to Cape Longing region. Measurements of the ice front position carried out in the field during late 1991 indicate that the recession between Lindenberg Island and Sobral Peninsula is still continuing, in some places at a rate of up to 2.5 km a−1.

Satellite passive microwave observations can provide unique mesoscale (25 km) information on snowpack properties; however, the mountainous terrain of the upper Colorado River basin compounds the difficulty of the problem. Nevertheless, observations of this region from the Scanning Multichannel Microwave Radiometer (SMMR) have provided unique, synoptic, mesoscale snowpack information from 1979 to 1987 on the snowpack extent. For this nine-year period, the SMMR 18 and 37 GHz brightness temperature observations, combined to form a parameter called NGR, show the average maximum snowpack extent covers 70% of the basin and occurs on water year day 130 (mid-February). The minimum snowpack extent took place in 1981 and covered 35% of the basin. The maximum snowpack extent took place in 1979 and covered 99% of the basin. Summation of the NGR values from each SMMR mesoscale pixel within the basin provides an index of the regional snowpack properties on both an intra- and inter-annual basis and exhibits behavior similar to the snowpack extent. When compared to the nine-year average, 1981 is the minimum year and 1979 is the maximum year. Furthermore, the sum over the basin of the annual maximum NGR from each pixel correlates with the annual discharge, r = 0.6. This correlation increases to 0.8 when digital elevation data are used to characterize each SMMR pixel and only the April through July discharge is used in the regression. Hence, this study combines the small scale elevation data with the mesoscale SMMR observations to investigate the basin-wide or regional snowpack characteristics and its hydrology.

Spring snow melt over the Beaufort and East Siberian seas is examined using visible-band DMSP imagery, SMMR brightness temperatures, and surface air temperatures for the years 1979, 1980 and 1984–86. Regional melt onset, as identified from the SMMR data, typically begins in early June, but can vary by up to two weeks between years. The subsequent pace of melt, estimated from the difference in days between the SMMR signal and visual identification of melt features in the DMSP data, exhibits a similar range. Interannual differences in snow melt are examined with respect to variations in cloud cover and geostrophic meridional winds. While over the Beaufort Sea, early melt onset and rapid melt progression may be favored by a combination of limited cloud cover and, as inferred from the wind data, strong northward air advection, no firm conclusions can be drawn from the available data. Presumably, potential relationships between melt and atmospheric forcings tend to be masked by additional factors, such as variations in initial snow depth and uncertainties in the data sets.

In order to interpret changes in the Greenland ice sheet as indications of climatic variation, it is necessary to place observations of local changes in a regional context. This requires a comprehensive monitoring effort which addresses both the inland ice and changes in the ice margin. This paper describes the design of a program for regional investigation of the Greenland ice sheet using synthetic aperture radar (SAR), and discusses the utility of SAR data for detection of change in the ice sheet margin.

Comprehensive coverage of the Greenland ice sheet by ERS-1 SAR will allow us to map the boundaries of snow facies on the ice sheet and investigate recent changes in the ice margin. We will use geo-referenced images to map the current boundaries of snow facies, providing a baseline which can be used to detect future change.

We demonstrate the utility of SAR for detecting recent changes in the ice margin. SAR images clearly show the ice edge, moraines, and ice marginal lakes. These features can be compared with published maps and earlier images in order to document changes in the margin of the ice sheet. We show evidence for recession of a section of the western margin of the ice sheet. The recession, which occurred between 1938 and 1978, ranges from a few hundred meters at high elevations to several kilometers at calving faces in both ice marginal lakes and fjords.

ERS-1 SAR will provide the first opportunity to pursue a comprehensive investigation of the state of the Greenland ice sheet. The ability to address conditions on the ice and to look at margin changes in a systematic way will allow us to develop a stronger framework for interpreting changes in the ice sheet in terms of climatic variation.

The angular dependence and polarization behaviour of back-scattering and emission of polar firn at 5.2 GHz and 10.3 GHz were measured during an oversnow traverse in Dronning Maud Land, Antarctica. The signatures emphasize the importance of snow stratification in the interpretation of microwave remote sensing measurements. Highest backscattering coefficients and little angular variations were observed for refrozen firn near the coast. In permanently dry snow, areas with high accumulation rates and homogeneous snow morphology showed low backscattering coefficients and high emissivities. Pronounced layering and related density variations in low accumulation zones resulted in increased polarization differences of brightness temperatures and increased like-polarized backscattering coefficients. This behaviour is confirmed by the analysis of C-band scatterometer measurements of the Active Microwave Instrument aboard the European Space Agency's ERS-1.

Four different algorithms for retrieving ice concentration from passive microwave imagery are applied to SSM/I data collected over the Weddell Sea in September 1989. Comparison of the results along a typical transect from the ice edge to the coast shows point-wise differences of up to 45% in ice concentration. The observed differences can largely be explained by the different combinations of the 19 and 37 GHz, horizontal and vertical polarization, data channels utilized by each algorithm. Through their frequency/polarization signatures, and with the aid of coincident surface observations, the differences are further related to ice type, specifically the presence of grease ice or thin ice, and surface conditions.

A method to detect sub-pixel-scale polynyas with passive microwave data has been developed. Models for the brightness temperature transition from sea ice to shelf ice, with and without a coastal polynya, are generated by simulating the satellite overflight across the coast. The models are compared with image data along transects across the coast; correlation coefficients are used to determine the best-fit model and thereby detect the influence of open water. Polynya areas derived from a single SSM/I overpass compare well with coincident AVHRR data showing a mean difference of 156 km2. A longer time series based on daily averaged SSM/I data over a study area adjacent to research station Halley indicates that the opening and closing of polynyas are highly correlated with station wind data.

We measured the velocities of six glacier tongues and a few tongues within ice shelves distributed around the Antarctic coastline by determining the displacement of crevasse patterns seen on sequential Landsat images. The velocities range from less than 0.2 km a−1 for East Antarctic ice-shelf tongues to more than 2.5 km a−1 for the Thwaites Glacier Tongue. All glacier tongues show increases in velocity toward their distal margins. In general, the tongues of glaciers draining the West Antarctic ice sheet have moved significantly faster than those in East Antarctica. This observation may be significant in light of the hypothesized possible disintegration of the West Antarctic ice sheet.

Full hemispheric monitoring of snow cover has been possible since the advent of meteorological satellites in the 1960s. Since that time visible satellite imagery has been used to chart the extent of northern hemisphere snow on a weekly basis. An analysis of monthly snow areas from a consistent U.S. National Oceanic and Atmospheric Administration (NOAA) set, dating back to 1972, finds that hemispheric snow cover has been well below means for the 1972–91 interval since the middle of 1987. Since the late 1970s multichannel data from satellite-borne passive microwave sensors have been used to estimate hemispheric snowpack extent and depth or water equivalent. Microwave extents generally run considerably lower than NOAA visible values due to microwave difficulties in detecting patchy or wet snow, snow lying on unfrozen ground, and snow in forested regions. The relative simplicity of observing hemispheric snow cover from satellites, the potential of integrating these and other sources of data using geographical information system techniques, the critical role that snow cover has in the global heat budget, and the expected role of snow feedbacks in anthropogenic climate change support the continued diligent monitoring of snow cover.

Co-located sets of AVHRR and SSM/I passive microwave imagery are used to estimate ice surface temperatures and to infer cloud cover in the Arctic. Physical temperatures are determined from the AVHRR data by modeling atmospheric and surface conditions. The resulting field-of-view temperatures are converted to ice surface skin temperatures by adjusting for ice concentration calculated using the SSM/I data. By selecting AVHRR-derived temperatures for clear sky conditions, “effective” emissivities of first-year and multi-year ice are calculated. Given these emissivities, microwave brightness temperatures, and proportions of first-year and multi-year ice as estimated using the NASA Team Algorithm, physical temperatures of the sea ice/snow surface are calculated that are, in theory, relatively independent of cloud conditions. The resulting ice temperatures are used to delineate a portion of the cloud cover in the AVHRR data. The advantages of this approach are that only a fairly small amount of AVHRR data are needed to calibrate the SSM/I imagery that can then be used to calculate a time-series of temperatures on a large scale.

Two geodetic airborne laser altimeter (ALA) systems coupled to Global Positioning System receivers acquired submeter-resolution topographic profiles of the lower parts of Breidamerkurjökull and Skeidarárjökull, Iceland, in May 1989 and September 1991 (Skeidarárjökull) and of Jakobshavns Isbræ, Greenland, in April 1992. Maximum measured crevasse depths on Breidamerkurjökull, Skeidarárjökull and Jakobshavns Isbræ were 20.7, 36.1 and 50.2 m, respectively. Crevasse spacings were 43 m (45 crevasses km−1) for Breidamerkur jökull, 46–51 m (25 crevasses km−1) for Skeidarárjökull and 20–40 m (lower part) or 50–80 m (upper part) of Jakobshavns Isbræ (27 crevasses km−1). Surface slopes were ~2.4° for the lower 11 km of Breidamerkurjökull, ~0.8° for the lower 10 km of Skeidarárjökull and 1.55° for the lower 28 km of Jakobshavns Isbræ (with a range of 0.55° for the final 17 km to ~6.3° for a steep central part several km in length). Average longitudinal strain-rate values, estimated by assuming a bulk ice temperature of 0°C and a density of 880 kg m−3, ranged from 0.12 a−1 for Breidamerkurjökull, to 0.63 a−1 for Skeidarárjökull; values for Jakobshavns Isbræ fell between 1.3 and 1.7 a−1. Remote sensing of glacier microtopography by ALA offers a potential new tool for determining crevasse morphology, spatial density and spacing, meter-scale local slopes, long-wavelength gradients and derived strain rates.