The third symposium on Remote Sensing of Snow and Ice, organized by the International Glaciological Society, took place in Boulder, Colorado, 17–22 May 1992. As part of this meeting a total of 21 papers was presented on snow and ice applications of Advanced Very High Resolution Radiometer (AVHRR) satellite data in polar regions. Also during this meeting a NASA sponsored Workshop was held to review the status of polar surface measurements from AVHRR. In the following we have summarized the ideas and recommendations from the workshop, and the conclusions of relevant papers given during the regular symposium sessions. The seven topics discussed include cloud masking, ice surface temperature, narrow-band albedo, ice concentration, lead statistics, sea-ice motion and ice-sheet studies with specifics on applications, algorithms and accuracy, following recommendations for future improvements. In general, we can affirm the strong potential of AVHRR for studying sea ice and snow covered surfaces, and we highly recommend this satellite data set for long-term monitoring of polar process studies. However, progress is needed to reduce the uncertainty of the retrieved parameters for all of the above mentioned topics to make this data set useful for direct climate applications such as heat balance studies and others. Further, the acquisition and processing of polar AVHRR data must become better coordinated between receiving stations, data centers and funding agencies to guarantee a long-term commitment to the collection and distribution of high quality data.

Landsat 5 Thematic Mapper observations are used to derive the horizontal distribution of reflectance over Hintereisferner in Austria. To analyze temporal evolution of surface reflectance, three different dates are considered. The retrieval method adopts the linear relation between the planetary and surface reflectance, which follows directly from the principles of invariance. Constants in this linear relation are determined using a simple two-stream radiation model for each of the three days under consideration. From a comparison with observations the retrieved albedo is shown to compare favorably with ground observations.

A helicopter-mounted low frequency ice-radar has been developed for the depth sounding of temperate glaciers. The radar consists of standard transmitter and digital receiver equipment. The long antennae are supported on a special aluminium and fibreglass construction which hangs 20 m below the helicopter. The radar has been used on Engabreen, an outlet glacier of the Svartisen Ice Cap in northern Norway, where ice thicknesses of up to 350 m were obtained. The results have been used to construct a map of bed topography of the lower part of Engabreen. This map is largely consistent with ice thickness data obtained by drilling.

Large scale melting phenomena like meltwater drainage channels and meltwater accumulation basins or frozen “lakes” have been surveyed on the land ice mass in Dronning Maud Land, Antarctica. These melting features were also detected in a Landsat Thematic Mapper (TM) image recorded on 12 February 1990. Image processing techniques such as principal component (PC) analysis, band ratioing and histogram-equalizing are carried out to emphasize the melting phenomena. Interestingly, a histogram-equalized single TM Band 5 image appears to be a good discriminator, leaving the melting phenomena as bright areas in the TM scene. The largest frozen “lake” is close to 1km wide while some of the drainage channels stretch more than 5 km. There is also a comparison of satellite-derived and in situ reflectance and temperature. The atmospheric corrected satellite data underestimate surface snow albedo by about 16% while TM measurements of surface temperature are 2° to 6°C lower than that measured on the ground.

Bidirectional radiance measurements of various snow-covered surfaces have been acquired using the Advanced Solid-State Array Spectro-radiometer (ASAS) which was flown on the NASA C-130 aricraft over Glacier National Park, Montana on 14 March 1991. A frozen lake, a snow-covered meadow, a mixed coniferous/deciduous forest and a dense coniferous forest were studied. Knowledge of the bidirectional reflectance characteristics of various snow-covered surfaces is key to calculation of albedo from satellite sensors in the future. While the reflectance characteristics of pure snow targets have been measured from the ground, aircraft and satellites, very few bidirectional reflectance data exist for surface covers of mixed snow and vegetation. Analysis of the 14 March 1991 ASAS data show that the anisotropic reflectance of snow was strong over the snow-covered frozen lake but slightly dampened over the meadow because of the presence of low vegetation. Over the dense coniferous forest, strong backscatter was observed while higher radiances in both the forward and aftward sensor view zenith directions was measured over the mixed forest. Snow that was known to be beneath the coniferous forest canopy was not detected in our data.

Six multispectral SPOT images and one Landsat Thematic Mapper image of the central part of the Sør Rondane Mountains, Dronning Maud Land, Antarctica, were obtained over a five year period (1986–90) and studied for their potential in mapping the glacierized surface. From the thermal IR reflectances of Landsat TM a map of ice and snow temperatures was obtained, reflecting large-scale topography. The “middle” IR and “near” IR bands (in descending order of importance) enhance markedly the variations on the snow surface itself. This can be most clearly seen in the accumulation area north and south of the range where the snow cover is abundant.

From multispectral SPOT imagery a glacio-morphological map was created. Some aspects of this map are discussed with respect to the significance of the ablation in this isolated mountain area, where the main ice stream is diverted around the range. Finally, an analysis of multitemporal images led to the development of a classification tool to quantify the extent of ablation and accumulation areas. Also, by this method drift snow could be distinguished from the permanent snow cover.

An ice-surface temperature retrieval algorithm for the Greenland ice sheet was developed using NOAA 11 thermal radiances from channels 4 and 5. Temperature, pressure and humidity profiles, cloud observations and skin temperatures from the Swiss Federal Institute of Technology (ETH) camp, located at the equilibrium line altitude at 49°17′W, 69°34′N, were used in the LOWTRAN 7 model. Through a statistical analysis of daily clear sky profiles, the coefficients that correct for the atmospheric effects were determined for the ETH-Camp field season (May to August). Surface temperatures retrieved by this method were then compared against the in situ observations with a maximum difference of 0.6 K. The NOAA 11 narrow-band planetary albedo values for channels 1 and 2 were calculated using pre-launch calibration coefficients. Scattering and absorption by the atmosphere were modelled with LOWTRAN 7. Then, narrow-band albedo values for the AVHRR visible and near infrared channels were compared with in situ high resolution spectral reflectance measurements. In the visible band (580–680 nm), AVHRR-derived narrow-band albedo and the in situ measurements corrected with radiative transfer model LOWTRAN 7 showed a difference of less than 2%. For the near infrared channel (725–1100 nm) the difference between the measured and modelled narrow-band albedo was 14%. These discrepancies could be either the result of inaccurate aerosol scattering modelling (lack of the in situ observation), or the result of sensor drift due to degradation.

Undulating topography on the East Antarctic ice sheet was clearly revealed by NOAA AVHRR. The following three patterns of undulating topography were detected by using high-pass filtered images from the visible to thermal infrared channels. In coastal regions (below 2000 m a.s.l.), undulation can be clearly detected by the fluctuation of reflectance in visible channel. It has wavy structure with spacing less than 10 km and alignment at a right angle to the ice-flow lines. In the katabatic zone (from 2000 m a.s.l. to 3000 m a.s.l) well defined fluctuations of albedo stpectrum can be seen with spacing from 10 km to 20 km, aligned at right angles to the ice-flow lines or prevailing katabatic wind direction. Ground-survey data show that the undulating topography is associated with large variations of net accumulation rate. On the inland plateau (above 3000 m a.s.l.), undulation can be clearly seen in the fluctuation of thermal infrared channel in winter. Ground-survey data show that the signal corresponds to the undulating topography. The alignment of the undulation is at a right angle to the ice-flow lines and the spacing is longer than 20 km. The characteristics of these undulations represent the ice-flow dynamics and accumulation anomaly.

Analysis of satellite images (Landsat 1 MSS, 4 TM and SPOT 1 XS), of U.S. Navy aerial photographs (TMA) and of U.S.G.S. maps has made it possible to assess a number of parameters relating to the surface dynamics (between 1956 and 1990) of eight ice tongues and of two ice shelves in the Terra Nova Bay area. The study shows that between 1960–63 and 1972–73 there was a decided decrease in the areas of some of the tongues and shelves: −53 km2 for the Aviator Glacier Tongue and −124 km2 for the Nansen Ice Sheet. On the other hand, the areas generally increased in the period 1972–73 to 1988. An exception to this situation is the small Hells Gate ice shelf whose area diminished by 1.15 km2 between 1956–57 and 1988. The calculated surface velocity of the ice tongues shows that they increase from the grounding line to their outer limit. Values of longitudinal strain rate generally increase from the grounding line to just after the start of the ice tongue, after which they diminsh proceeding towards its outer limit. Integration of the areal values with radio-echo sounding data has enabled the ice discharges of the southern flow of the David Glacier (12km3a−1) and of the Aviator Glacier Tongue (0.62 km3 a−1) to be calculated. Furthermore, on the basis of the data available, basal melting values of between 25 cm a−1 and 100 cm a−1 are deduced for these two ice tongues, and bottom freezing values of 20 cm a−1 for the Drygalski Ice Tongue. Different spectral responses of the glacial areas have made it possible to discriminate ablation areas from those of accumulation and to differentiate various typologies of ice (glacier ice, melt lake ice, and sea ice formed at the ice shelf-ocean interface).

Climatological and hydrological investigations in mountainous regions frequently require delineation of the areas covered by snow and glacial ice. Active microwave sensors can discriminate snow and glacier from other targets, are effective under all weather conditions, and have a spatial resolution compatible with the topographic variation in alpine regions. Using data acquired with the NASA AIRSAR, which operates at three frequencies and four combinations of polarization, we examine the usage of synthetic aperture radar (SAR) to map snow- and glacier-covered areas. In order to assess the available SARs that operate from satellites, we analyze single-frequency, single-polarization data, and we compare our results with images from the Landsat Thematic Mapper obtained under clear conditions only a few days after the SAR flight. Single polarization, C-band (5.3 GHz) SAR data can discriminate between areas covered by wet snow from those that are ice-free, but do not easily separate glacier ice from snow and rock. L-band SAR (1.25 GHz) data cannot discriminate between snow and glacier ice. The accuracy (74%) of SAR is high enough to justify its use as the data source in areas that are too cloud-covered to obtain reliable data from the thematic mapper.

The potential of satellite radar altimetry for high-resolution mapping of Antarctic ice streams is evaluated, using retracked and slope-corrected data from the Lambert Glacier and Amery Ice Shelf area, East Antarctica, acquired by Geosat during the Exact Repeat Mission (ERM), 1986–89. The map area includes lower Lambert Glacier north of 72.18°S, the southern Amery Ice Shelf, and the grounded inland ice sheet on both sides. The Geosat ERM altimetry is found to provide substantially more complete coverage than the 1978 Seasat altimetry, due to improved tracking. Variogram methods are used to estimate the noise levels in the data as a function of position throughout the map area. The spatial structure in the data is quantified by constructing experimental variograms using altimetry from the area of the grounding zone of Lambert Glacier, which is the area chiefly of interest in this topographically complex region. Kriging is employed to invert the along-track height measurements onto a fine-scale 3 km grid. The unsmoothed along-track Geosat ERM altimetry yields spatially continuous maps showing the main topographic features of lower Lambert Glacier, upper Amery Ice Shelf and the adjacent inland ice sheet. The probable position of the grounding line of Lambert Glacier is identified from a break in slope at the grounded ice/floating ice transition. The approximate standard error of the kriged map is inferred from the data noise levels.

A Landsat Thematic Mapper scene over Terre Adélie, Antarctica, is used to estimate the snow grain-size. New calibration coefficients are computed for the TM scene, using sea-water reflectance. Topographic effects are corrected with the visible band TM2. Atmospheric effects are taken into account. The snow theoretical reflectance is calculated with the Wiscombe and Warren (1980) model. Estimated snow grain-sizes are similar to those given in the literature.

The Special Sensor Microwave/Imager (SMM/I) instrument provides daily measures of microwave brightness temperatures Tb over the polar regions. Data are available from 1987 to the present and have a demonstrated utility for sea-ice studies. In this paper we investigate applications to polar ice sheets with a view towards correlating patterns of Tb with ice-sheet elevation. We go on to discuss the Tb signature of processes linked to temperature anomalies and accumulation zone boundaries.

Our approach consists of combining SSM/I Tb values provided on CD-ROM by the National Snow and Ice Data Center (NSIDC) with a digital elevation model (DEM) of Antarctica derived originally from the Scott Polar Research Institute Antarctic Map Folio. We focused on 37 GHz data which is mapped onto a 316 × 332 polar stereographic grid at 25 km resolution. The DEM consists of a 281 × 281 array of elevation values with a 20 km resolution. The DEM was resampled to be compatible with the SSM/I data by transforming the elevation data from the original projection place to geodetic coordinates. The elevation data were than transformed onto the SSM/I projection plane. Elevation data were interpolated to yield ice-sheet surface heights at 25 km intervals coinciding with SSM/I point Tb assignments.

By co-registering the two data sets, it is possible to “drape” the brightness temperature data over the elevation data. The resulting image highlights the relationship (and variations) between ice-sheet topography and brightness temperature.

The glacier-covered test site Rofental in the Austrian Alps was surveyed by the AIRSAR of the NASA/Jet Propulsion Laboratory in August 1989 and June 1991. A few days after the AIRSAR survey in 1989 Landsat TM and SPOT images were acquired, which provide the opportunity to compare visible and radar data. Field activities during the SAR surveys included measurements of dielectrical, structural, and surface roughness properties and the deployment of corner reflectors. After polarimetric and radiometric calibration of the SAR data, polarimetric signatures of the main target types, snow, glacier ice and moraines, were derived. Pronounced effects of surface roughness were observed at C- and L-band for artificially roughened snow surfaces. Parallel AIRSAR swaths of the 1991 survey provide the basis for studies of incidence angle effects. Due to the increasing penetration depth, the unpolarized backscattering components of the wet snow areas increase from the C- to the P-band. The multifrequency backscattering properties of the various surfaces offer the best means for target discrimination.

Russian satellite imagery of the polar regions has recently become available to western scientists through Sojuzkarta. The KFA-1000 photographic camera is of particular interest to glaciologists due to its high resolution (a nominal 5 m). Digitized photographic products from this instrument are compared with digital data from Landsat and SPOT series satellites for a partly glacierized area of northwest Spitsbergen. Comparison of KFA-1000 data with detailed maps of man-made structures at the settlement of Ny Ålesund demonstrates that the resolution of the photographic imagery is about 3 m, but scanner resolution limited our digital data to about 6 m. Significantly less detail can be resolved on Landsat TM imagery and Landsat MSS data fail to resolve any of the structures. KFA-1000 data are compared with Landsat TM and MSS images and SPOT HRV multispectral imagery for several tidewater glaciers in Spitsbergen. KFA-1000 imagery is of a significantly higher geometric resolution than the other sensors, allowing the clear identification of individual crevasses and other ice surface features. KFA-1000 scenes from 1985 and 1988 are used to measure ice marginal fluctuations for several northwest Spitsbergen glaciers, and the onset of a surge can also be identified. This imagery has a 60% overlap between scenes and the heighting accuracy of the stereoscopic data is calculated at 45 m. Radiometric analysis of KFA-1000 data is restricted to relative brightness values, since no absolute calibration is available. The photographic products appear speckled, and the range and standard deviation of normalized pixel brightness values over snow is greater than for equivalent Landsat TM data. The very high spatial resolution of the KFA-1000 camera is its principal attraction for glaciologists.

The study of surface features and processes on glaciers, ice caps and ice sheets often requires multitemporal data in order to take account of the dynamic behaviour of snow and ice. Satellite remote sensing provides an important means of acquiring such data sets from polar regions, but the effectiveness of visible (VIS) and near-infrared (NIR) wavebands is severely limited by the frequent presence of cloud cover. As well as preventing direct imaging of the surface, cloud cover can be difficult to discriminate from snow and ice because of its similarly high reflectivity. This paper describes a detailed quantitative study of the limitations imposed by cloud cover. The number of potentially useful images which could have been acquired from a study area in Svalbard for the April-September period for 1980–89 is determined. Meteorological data from two stations analyzed over this study period show that fewer than 10% of the available satellite overpasses occurred during cloud-free periods. A cloud cover level of three octas or less, used to define high quality imagery, occurred 29% and 32% of the time at the two stations. Markedly seasonal variations are found which can be explained in terms of the regional climate such that during the months August and September the probability of obtaining a cloud-free Landsat image every year is effectively zero. Analysis of several years of Landsat Quick Look images for other regions of Svalbard confirms these findings. However, a similar series for the Scoresby Sund region of East Greenland shows a much higher percentage of low cloud scenes and no marked seasonality.

Remote sensing has provided a means of obtaining estimates of snow-covered area, yet traditional methods have had difficulty mapping snow in shaded and vegetated areas. Spectral mixture analysis is a linear mixture modeling technique that shows promise for mapping land surface covers, particularly when imaging spectrometer data are used. Applying this technique to AVIRIS data collected over the Sierra Nevada, California, we have estimated the fraction of snow cover in each pixel, even in areas that are shaded or forested. This modeling technique enables us to map snow cover at the sub-pixel level and provides a means of estimating the errors associated with the calculation.

The microwave emissivity of two snow covers was measured in Alaska in March, 1990. Observations were made on taiga snow near Fairbanks that was 0.83 m thick with a 0.55 m thick basal layer of depth hoar. Other measurements were made on the tundra snow cover at Imnaviat Creek north of the Brooks Range which was 0.27 to 0.64 m thick and consisted of two or more wind slabs overlying a depth hoar layer 0.14 to 0.26 m thick. Density, crystal structure, and grain size were similar in tundra and taiga depth hoar layers.

Emissivity was measured at 18.7 and 37 GHz using radiometers mounted on a 1.5 m tall bipod. Measurements were made on undisturbed snow, and then several snow layers were removed and additional measurements were made. This sequence was repeated until all snow had been removed. Effective emissivity values for the full snow depth ranged from 0.6 (37 GHz, H-pol) to 0.95 (18.7 GHz, V-pol) and were similar for both taiga and tundra snow covers. For both snow covers, there was a marked reduction in the effective emissivity (eeff) from that of the underlying ground with a maximum reduction of about 30%. All of the reduction was found to occur within the depth hoar layer. Maximum reduction in eeff could be caused by a depth hoar layer 0.3 m thick. Overlying wind slab or new snow were nearly “invisible”, increasing the effective emissivity only by a small amount due to self-emittance. Thus, it was difficult to distinguish the two different snow covers on the basis of their emissivity, since both contained 0.3 m of depth hoar or more.

Spaceborne data are becoming sufficiently extensive spatially and sufficiently lengthy over time to provide important gauges of global change. There is a potentially long record of microwave brightness temperature from NASA's Scanning Multichannel Microwave Radiometer (SMMR), followed by the Navy's Special Sensor Microwave Imager (SSM/I). Thus it is natural to combine data from successive satellite programs into a single, long record. To do this, we compare brightness temperature data collected during the brief overlap period (7 July-20 August 1987) of SMMR and SSM/I. Only data collected over the Antarctic ice sheet are used to limit spatial and temporal complications associated with the open ocean and sea ice. Linear regressions are computed from scatter plots of complementary pairs of channels from each sensor revealing highly correlated data sets, supporting the argument that there are important relative calibration differences between the two instruments. The calibration scheme was applied to a set of average monthly brightness temperatures for a sector of East Antarctica.

To assess future global change, monitoring of the climate system through observation and analysis of seasonal and interannual fluctuations of climate variables is necessary. Cryospheric elements such as snow cover are often seen as sensitive indicators and integrators of basic climate conditions and hence an indicator of regional and global change. Snow-cover elements which may serve as signals of variability and change are discussed with respect to the effective use of conventional and remotely sensed information. Conventional data are shown to be effective for assessing questions of temporal variability, but are limited for spatial variability. Passive microwave satellite data make an important contribution by providing spatial and temporal information on snow water equivalent (SWE) and the regional distribution of snowpack extent and state. Use of NIMBUS-7 SMMR (Scanning Multichannel Microwave Radiometer) data to develop a time series of SWE is assessed as a complement to conventional data. Limitations of SMMR coverage compared to DMSP SSM/I (Special Sensor Microwave/Imager) coverage for production of SWE maps for climate change analysis are discussed. Although there are limitations during early season snow cover, information derived from passive microwave data is shown to be able to map and compute the areal coverage of SWE allowing interannual comparison of the amount of water available, the date of peak accumulation and the associated spatial distribution. However, the satellite data record is still too short to establish any definitive trend in snow-cover variability.