The seasonal sea-ice cover of the combined Bering and Okhotsk Seas at the time of maximum ice extent is almost 2 × 106 km2 and exceeds that of any other seasonal sea-ice zone in the Northern Hemisphere. Although both seas are relatively shallow bodies of water overlying continental shelf regions, there are important geographical differences. The Sea of Okhotsk is almost totally enclosed, being bounded to the north and west by Siberia and Sakhalin Island, and to the east by Kamchatka Peninsula. In contrast, the Bering Sea is the third-largest semi-enclosed sea in the world, with a surface area of 2.3 × 106 km2, and is bounded to the west by Kamchatka Peninsula, to the east by the Alaskan coast, and to the south by the Aleutian Islands arc.

While the relationship between the regional oceanography and meteorology and the sea-ice covers of both the Bering Sea and Sea of Okhotsk have been studied individually, relatively little attention has been given to the occasional out-of-phase relationship between the fluctuations in the sea-ice extent of these two large seas. In this study, we present 3 day averaged sea-ice extent data obtained from the Nimbus-5 Electrically Scanning Microwave Radiometer (ESMR-5) for the four winters for which ESMR-5 data were available, 1973 through 1976, and document those periods for which there is an out-of-phase relationship in the fluctuations of the ice cover between the Bering Sea and the Sea of Okhotsk. Further, mean sea-level pressure data are also analyzed and compared with the time series of sea-ice extent data to provide a basis for determining possible associations between the episodes of out-of-phase fluctuations and atmospheric circulation patterns.

Previous work by Campbell and others (1981) using sea-ice concentrations also derived from ESMR-5 data noted this out-of-phase relationship between the two ice packs in 1973 and 1976. The authors commented that the out-of-phase relationship is “... surprising as these are adjacent seas, and one would assume that they had similar meteorologic environments”. We argue here that the out-of-phase relationship is consistent with large-scale atmospheric circulation patterns, since the two seas span a range of longitude of about 60°, corresponding to a half wavelength of a zonal wave-number 3, and hence are quite susceptible to changes in the amplitude and phase of large-scale atmospheric waves.

The use of many display techniques for remotely sensed glaciological data requires the reduction of the data to a regularly spaced rectangular grid of values. Most remotely sensed data are not immediately suitable for display, because the area of interest is covered by more than one set of data on mutually incompatible grids (e.g. Landsat, AVHRR), or because the data are available as profiles along widely spaced ground tracks (e.g. radio echo-sounding, satellite altimetry). In addition, data may be sparsely and randomly scattered (e.g. surface elevations from TWERLE balloons).

A variety of techniques is available to reduce data to a specified grid system. These include spatial averaging, interpolation from nearest neighbours, and surface-fitting techniques, notably polynomial fitting and bi-cubic splines. All of these are useful under differing circumstances.

The ice-filled caldera of Mount Wrangell, Alaska, provides an unusual opportunity to examine the interaction between a glacier and an active volcano. The caldera acts as a giant calorimeter, preserving a rough balance between snow precipitation at the glacier surface and bottom melting. In April 1982 we sounded the glacier using an airborne 840 MHz pulsed radar (Narod B B, Clarke G K C 1983 UHF radar system for airborne surveys of ice thickness. Canadian Journal of Earth Sciences 20(7): 1073–1086). The data were recorded on magnetic tape, then computer processed and plotted as depth, rather than time, sections.

In addition to mapping ice thickness, we detected extensive internal layers presumed to have been deposited during past eruptions of Mount Wrangell. The challenge of interpreting these internal reflectors inspired us to develop a unified interpretation model that incorporates both glaciological measurements and phenomenological equations for firn and ice. The interpretation model yields the variation with surface-normal depth z of temperature T, heat flux q, ice pressure p, density ρ, surface-normal component of flow velocity w, down-slope component of flow velocity u, depositional age ta, two-way wave travel time τ2, and two-way propagation loss P2. The following system of linear differential equations is integrated using the Runge-Kutta method:

1

2

3

4

5

6

7

8

9 together with physical property relations such as the following 10

11

12

13

14

The boundary conditions on the integration variables are T(0) = Ts,T(h) = TM, p(0) = 0, ρ(0) =ρs, w(0) = ws, u(h) = uB, t a(0) = 0, τ2(0) = 0, and P2(0) = 0, where Ts is surface temperature, TM is bottom temperature, ρs is surface density, ws is surface-normal velocity measured at the glacier surface, and uB is sliding velocity. The remaining variables are thermal conductivity K, specific heat capacity c, flow-law constant B0, creep activation energy Q, universal gas constant R, shape factor F, surface slope α, flow-law exponent n, gravity acceleration g, empirical constants m1, m2, and p* from Benson’s firn-densification law (Benson C S 1962 Stratigraphie studies in the snow and firn of the Greenland ice sheet. SIPRE Research Report 70), ice density ρI, surface value of two-dimensional flow divergence Δ0 = (∂u/∂x + ∂v/∂y)s, relative dielectric permittivity ∈, free-space electromagnetic wave velocity v0, propagation loss rate D,T0 = 273.16 °K, thermal conductivity of ice K.I, loss-law constant D0, and activation energy for propagation loss E. The units of temperature and density in the above equations are respectively K and kg m−3.

Preliminary modelling has yielded an age-depth relationship for internal reflecting layers that roughly matches the known eruption record for Mount Wrangell and extends the volcanic history by more than 200 years. High signal absorption rates restrict interpretation of the caldera geometry. Reflections from the bed are not consistently detectable beyond a depth of approximately 350 m, suggesting a loss rate of 7–8 dB per 100 m of path length. The anomalous losses are attributed to abnormally high concentrations of ionic impurities and scattering sources.

We have developed and tested a portable impulse radar for ground-based sounding of glaciers. Noteworthy characteristics of the instrument are its portability, low power consumption, digital data storage, and the ability to be operated either manually or automatically under program control. Current system specifications include a band width of 46 MHz; a sampling interval of 10.76 ns; depth precision of 0.9 m; 1024 samples per record; amplitude resolution of 8 bits; minimum recordable signal at the receive antenna equal to 0.26 mV; an operating center frequency of 8.5 MHz and an antenna-damping coefficient of 300 ohms.

The transmitter uses paired SCRs and a 12 V to 800 V converter to impress a 1200 V step on to a resistively damped dipole antenna. This pulse is triggered from the receiving system via a fibre optics cable so that each record can include the complete surface-path wavelet. The receiver unit combines a wide-band amplifier (with variable front-end attenuation) with a microprocessor-controlled data-acquisition system of our own design. The result of each sounding can be replayed as an “A-scope” display on a small, low-cost oscilloscope and stored on or retrieved from digital cassettes. In the unattended mode, records are collected at programmable intervals. The system weighs about 7.5 kg and uses dry cells or rechargeable batteries for power.

Examples were presented in a poster session of sounding profiles taken in July 1986 on Trapridge Glacier, Yukon Territory in Canada, along lines coinciding with an extensive drilling program. Although not yet fully analysed, we feel that some of the results may represent the effects of crevasses, internal features such as morainal material, varying bed features, and changes in subglacial and englacial hydrology.

We have studied acoustical reflection and scattering properties of the underside of laboratory-grown sea ice. Our purpose was to determine the morphologic characteristics of undeformed sea ice that control acoustic scattering and reflection. So our experiments included both detailed studies of the structure of the ice as well as the application of a variety of acoustic methods.

Ice sheets were grown in an outdoor pond (about 6 m by 13 m by 1.5 m) and exhibited features characteristic of undeformed, cold sea ice: an upper granular zone; a columnar zone of crystals with cross-sectional areas of about 1 cm2; vertical sheets of brine pockets; a bulk salinity of 9.1‰ distributed over 9 cm of ice; a dendritic interface at the ice/water boundary with dendrites about 0.5 mm across at the time of our measurements. Echo-amplitude fluctuations of normal-incidence sonar pings (100 kHz to 800 kHz) were measured as the sonars moved horizontally under the ice and accumulated into echo-amplitude histograms. (Data from a deteriorating ice sheet as well as data on lake ice were also collected.) We fitted the Rice probability density function (PDF) to the data and combined the resultant statistical parameter with Eckart acoustic scattering theory. The reflection coefficients calculated using this method ranged from 0.06 to 0.12, depending on environmental conditions. RMS roughness calculated using data from new sea ice was estimated to be about 0.3 mm. Because our ice thin sections show the ice to be porous and permeable at the interface with dendrites 0.5 mm thick, we suspect that the dendrites control the scattering as described by the echo-amplitude histograms. Further, we attribute the low reflection coefficients to the dendritic structure which may act as an impedance-matching zone into the columnar section of the sea ice.

Transmission measurements were performed by positioning a transducer located at the ice/air interface directly over the transducer located in the water. The total attenuation through 18 cm of ice ranged from 12 dB at 50 kHz to 70 dB at 420 kHz (signal levels were measured relative to the same path in water).

New aspects of the physics of ice movement in glaciers are presented. An analysis of the equation for the energy of a moving ice mass shows that the energy which is dissipated into heat (internal friction in a glacier) occurs not only because of average ice-velocity profiles, as it is commonly considered when simulating glaciers by continuum laminar flow. It also turns out that a significant part of the internal friction is associated with the dissipation of motion energy on internal macroheterogeneities in a glacier: at places where fractures appear and, in the case of a block structure, at points of contact between individual blocks, on ice dams, ice bridges, etc. This mechanism seems to be dominant during rapid ice flow in highly fractured glaciers.

The process was previously examined on the surging Medvezhiy Glacier in the central Pamir Mountains. On Medvezhiy Glacier the above phenomenon was particularly distinctive because of anomalously high ice-movement velocities during a calm period; that is, between consecutive surges (ice velocity amounted to 3–5 m d−1) and because of the block structure of the glacier against the background of its fracturing. Simple measurements of “instant” velocities of a large number of bench marks on the glacier surface, located across the middle flow course of the glacier, established that velocities at all points are of a fluctuating character, which outwardly resemble velocity fluctuations seen in turbulent liquid flow. It turns out that deviations of the “instant” velocity from the average velocity can amount to more than 50% of the latter. Using data obtained from periodic recordings of the “instant” velocity for fixed pairs of bench marks, which were located at different distances (so that it was possible to evaluate the difference in instant velocities), a structural function was constructed. This structural function meets the well-known, so-called Kholmogorov two-thirds law For local isotropic turbulence. This indicates that, as with turbulence, the energy of middle movement is transferred along a cascade of ice conglomerates of decreasing size down to the smallest, limited only by the dimensions of fundamental blocks. At the points of contact at dams and junctions, motion energy dissipates into heat. It is postulated that this phenomenon may have a much broader application; it may also be characteristic of the movement of any fractured mass of solid material.

These findings may be used to formulate a theory which describes the mechanism of auto-oscillations in a surging glacier. According to this theory, the pseudoturbulent character of internal friction during a quiescent period (between surges) is one of the factors which prevents the appearance of a surge as long as possible, thus ensuring the growth of a new active glacier after each ice catastrophe.

These studies also conclude research on the general concept of auto-oscillations of mountain glaciers — the so-called surging glaciers, originally formulated in the course of research on the cause of a surge on Medvezhiy Glacier in 1963. This concept was subsequently adopted and is now widely used in glaciology.

We report characteristics of Seasat altimetry signatures recorded over Antarctic sea ice. Up to four discrete zones can at times be seen in characteristic sequences in the Weddell and Ross Seas, and elsewhere. They are substantially larger than those reported in the Arctic, covering up to 2500 km at the time of maximum ice extent in 1978. Transitions between them can be abrupt, with marked changes occurring in less than a few kilometres. Some zones were found to persist through the 3 month satellite lifetime; others exhibited intermittent variations. Repeat data coverage has enabled temporal as well as spatial patterns to be investigated.

Interpretation of the geophysical cause of the patterns observed has been limited by available data. Some comparisons may be made with surface measurements of nadir back-scatter on first- and multi-year floes but these account for only a small proportion of the altimetry returns studied. Correlations with the NOAA Navy Ice Charts show significant disparities in the determination of the ice edge which may relate to the sensitivity of the altimeter to the presence of fresh ice or ice in very small quantities. Similar signatures can be found next to small coastal leads at the continental margin, an area known to be important for the growth of new ice.

The Seasat radar altimeter was designed to operate over the open ocean and encountered problems over sea ice. In particular, the on-board measurements of surface height were noisy and unreliable. As a consequence, published mean sea-surface and geoid maps based on the Seasat on-board height estimates either omit sea-ice-covered areas or include suspect data. We have identified and investigated the problems encountered by the Seasat altimeter over sea ice and have developed a technique for extracting accurate surface-height values from the sea-ice echo wave-form data. The retracking method is based upon fitting real wave forms to a library of model returns.

A number of maps of component isolines of the Maruch Glacier’s (West Caucasus) mass balance compiled on the basis of field observations during 11 years were analyzed.

The total balance of the glacier’s mass seems to be closely associated with the relationship between accumulation and ablation areas by years, while the total values of net accumulation and ablation are fully determined by the surface-area values of accumulation and ablation regions. The layer of residual snow in the nourishment area by the end of the ablation season and the layer of many-year ice melting in the discharge area changes from year to year against the points fixed in space, but their mean specific value according to the above glaciological areas remains constant from year to year.

Using the results and the remote-sensing images, in which the many-year ice and the snow surplus are well distinguished, it is possible to estimate the areas of the ablation and accumulation regions for the studied glacier and then the mass balance, net accumulation, and, naturally, the run-off from the melting many-year ice.

Estimation of the distribution within the glacier of not only “net” but also total values of accumulation and ablation is possible, if this distribution is similar from year to year, and if total values of accumulation and ablation are known at certain points. The multitude of points, for which total accumulation and ablation may be determined by remote-sensing images, is found on the equilibrium line or on the transient snow line of the glacier at any date during the ablation season. In the first instance, ablation and accumulation, the latter being here equal to the former by definition, may be calculated by the mean summer temperature of the air, extrapolated for the corresponding altitude against the nearest meteorological station. Secondly, ablation is calculated by the sum of positive temperatures of the air and by the temperature coefficient of snow melting, extrapolated for the altitude of the transient snow line.

In order to check the hypothesis of the similarity of isoline systems for accumulation and ablation, all the final values for each year and measured at different points have been normalized according to their mean value for each year.

The normalized maps turned out to be similar to each other. Using the regular net of points, we calculated the variation coefficient of normalized values. They were smaller by 0.20 for the whole area of the maps of normalized ablation and for the greater part of the map area of normalized accumulation. A map of mean normalized values was compiled for 11 years. The isoline "unit" on it coincided with the many-year firn line. With the help of this topological map, one can compile maps for distribution of ablation and accumulation on a glacier for any year or moment, for which there are data on the location of the nourishment line or snow line according to a remote-sensing image and for which air temperature at the altitude of these lines may be estimated.

In conclusion, the degree of similarity of the maps of component isolines for the glacier mass balance between different glaciers of the same morphological type was analyzed. For this, we used normalization of not only the characteristics of the glaciers’ mass balances but also of morphological characteristics (altitudinal change, width, etc.). The results point to similarity of distribution of accumulation and ablation among glaciers. This will allow us to extrapolate the principal features of the isoline maps and the described methods of calculation from the studied glaciers to unexplored ones.

The radar altimeter of the satellite Seasat has proved that ice and snow surfaces in the polar regions can return meaningful signals if the terrain is not excessively rugged or sloping. Because the use of the leading edge of the wave forms for height determination entails inherent uncertainties and, at best, provides only a single datum per wave form, the entire wave forms should be studied. Excesses or deficiencies in amplitude at various ranges within a single wave form, and the changes that occur in successive wave forms, can be analysed to yield information on the geometric and scattering properties of features observed by the altimeter.

Results from computer simulations are presented, showing how (1) a margin of sea ice (sinusoidal in the model) can be mapped, (2) the boundaries of two isolated ice floes can be outlined, (3) sea-ice concentrations can be derived within annuli about the nadir of an individual footprint, and (4) for land ice, the elevations of topographic features, together with the general slope of the ground, can be determined if an imaging instrument that operates simultaneously with the altimeter provides the outlines of these features. In examples (1) and (2), it is assumed that the ice is contiguous wherever that is possible, to permit the analytical reconstruction of the ice margin or individual ice floes in the presence of the inevitable ambiguity in the position of any feature with respect to the two sides of the satellite track. Example (4) requires that the altimeter record correctly records the strongest signals returned by ice-packs. This condition is not fulfilled by any existing radar altimeter, but it may be achieved in the next generation of these instruments. In additional examples of information from entire wave forms, the effects of crevasses and sastrugi in reducing or re-distributing the energy of the returned signals are also illustrated.

Full details of these analyses and results will be published at a later date.

Relatively little data on the distribution of Antarctic icebergs were available prior to 1980. The published literature included size data of about 5000 icebergs, and position data of 12 000 icebergs. There were indications that the size data were biased in favour of larger icebergs.

A programme of systematic iceberg observations was therefore initiated by Norsk Polarinstitutt in 198! through the SCAR Working Group on Glaciology. This programme is based on standard “blue” forms distributed to all ships going to Antarctica. The icebergs are recorded every 6 h and in Five length groups: 10–50, 50–200, 200–500, and 500–1000 m, and those over 1000 m are described individually.

The amount of data has increased greatly from the start in 1981–82. The position of 70 000 icebergs, including 50 000 that had been size classified, were on file at Norsk Polarinstitutt by December 1985, and the data set is growing rapidly. Most ships travelling to and from Antarctica now participate in collection of the data. (Fig.1 shows the locations of the icebergs sighted.) Fig. 1.

Location of iceberg observations under the programme initiated in 1981. Main ship tracks are clearly reflected. The average observation represents 14 icebergs.

The size distribution of the classified icebergs observed under this programme up to December 1985 is given in Table I: Table I

The “standard size” (length, width, and thickness) is based on our observations from three Antarctic expeditions which carried out dedicated iceberg studies. Many icebergs are of course not right-angled parallelepipedal in shape, but this is a good approximation for most of the larger icebergs.

The data are based both on visual sightings and on radar observations. Duplicate observations from a ship moving at slow or zero speed are as far as possible eliminated, both during observation, and by critical appraisal before the data are filed. The data editing also includes evaluation of data quality, especially in connection with radar observations, and comparison of positions and dimensions of the large icebergs in order to reduce to a minimum repeated observations from different vessels of icebergs >1000 m. These account for most of the iceberg mass (see Table I).

Consideration of iceberg-distribution patterns and the observed area of the Southern Ocean, and of duplicate observations, indicates more than 300 000 icebergs south of the Antarctic Convergence, with a total ice mass of about 1016 kg. Consideration of mean residence times indicates an annual iceberg production from the continent of 23–1015 kg, which is considerably higher than most other recent estimates. This also suggests that the Antarctic ice sheet is in balance.

The data indicate large regional differences in iceberg sizes, the most noticeable being between the two sides of the Antarctic Peninsula, and between the Amery Ice Shelf/ Prydz Bay area and the remainder of East Antarctica. These differences are probably mainly related to different calving sites.

About one-third of the observed icebergs are over the continental shelf of Antarctica. The total under-water area of these icebergs is two orders of magnitude less than the under-water area of the Antarctic ice shelves. The annual total iceberg melting and its effect on the water masses over the continental shelf has been calculated from ocean-water temperature variations at 200 m depth and estimated melt rates. This turns out to be an order of magnitude less than the annual effect of melting sea ice. The iceberg data considered here are probably under-represented with respect to the smallest sizes, and they do not include icebergs that have become <10 m. Inclusion of these ice bodies would increase the total melt.

Narrow field-of-view sensors on satellites monitoring solar radiation measure the reflected radiance in a particular direction. For climatic studies of the Earth’s radiation budget, the albedo is needed, which is the integral of the upward radiance over all angles divided by the downward irradiance. In order to infer the albedo from a radiance measurement at only one angle, it is necessary to know a priori the distribution of reflected radiation with angle, i.e. the bi-directional reflectance-distribution function (BRDF). The BRDF is a function of four angles: solar zenith and azimuth, and satellite zenith and azimuth. For areal or temporal averages on many natural surfaces, only three angles are needed to describe the function, because only the difference between the two azimuths is important, not their individual values. This assumption was made when developing empirical BRDFs from Nimbus-satellite data for use in the Earth Radiation Budget Experiment (ERBE). However, in large areas of the polar regions, all four angles are needed, because the sastrugi are oriented parallel to a prevailing wind direction. The BRDF shows a forward peak when the solar beam is along the direction of the sastrugi, and an enhanced backward peak when it is perpendicular. Averaging over all solar azimuths (relative to the sastrugi azimuth) causes back-scattering to be averaged together with forward-scattering. The conclusion of the ERBE analysis, that snow is the most nearly isotropic of all Earth surfaces, is therefore at least partly a spurious result of this averaging.

Measurements of the BRDF were carried out from a 23 m tower at the South Pole during January and February at 900 nm wavelength for varying azimuths between the Sun and the sastrugi fabric. The wavelength was selected near the midpoint of the solar-energy spectrum but where scattered sky radiation is negligible. Measurements were made with 10° field of view at 15° intervals in viewing zenith and azimuth angles throughout the day, at intervals of 1 h (15° of solar azimuth). For BRDF normalized such that its angular average is unity, the principal features of the results include a forward-scattering peak with a value of about five together with a side- and back-scattering lobe of 1.1 to 1.3. Variations in solar azimuth produced a skewness in BRDF which was approximately consistent with enhanced scattering at the specular angle with respect to the solar azimuth and the orientation of the principal fabric of the sastrugi pattern. The angularly averaged pattern was remarkably similar to the results of Taylor and Stowe even though their values were integrated over wavelength and were made through the atmosphere. Our studies thus suggest that, for mid- to late summer, the Taylor and Stowe results require only small corrections for sastrugi effects. This is not, however, expected to be true from sunrise through late November.

Spectral albedos showed values at visible wavelengths of 0.97 to 0.99 which agree very well with the model calculations of Wiscombe and Warren using our observed mean snow grain-sizes. Albedos for wavelengths above 1400 nm were higher than model predictions, indicating that the depth dependence of grain-size must be included in the analysis.

This research was supported by National Science Foundation grant DPP-83–16220.

The ablation of sea ice is an important feature in the global climate system. During the melt season in the Arctic, rapid changes occur in sea-ice surface conditions and areal extent of ice. These changes alter the albedo and vary the energy budgets. Understanding the spatial and temporal variations of melt is critical in the polar regions. This study investigates the spring onset of melt in the seasonal sea-ice zone of the Arctic Basin through the use of a melt signature derived by Anderson and others from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) data. The signature is recognized in the “gradient ratio” of the 18 and 37 GHz vertical brightness temperatures used to distinguish multi-year ice. A spuriously high fraction of multi-year ice appears rapidly during the initial melt of sea ice, when the snow-pack on the ice surface has started to melt. The brightness-temperature changes are a result of either enlarged snow crystals or incipient puddles forming at the snow/ice interface.

The timing of these melt events varies geographically and with time. Within the Arctic Basin, the melt signatures are observed first in the Chukchi and Kara/Barents Seas. As the melt progresses, the location of the melt signature moves westward from the Chukchi Sea and eastward from the Kara/Barents Seas to the Laptev Sea region. The timing of the melt signal also varies with year. For example, the melt signature occurred first in the Chukchi Sea in 1979, while in 1980 the signature was first observed in the Kara Sea.

There are also differences in the timing of melt for specific geographic locations between years. The melt signature varied almost 25 days in the Chukchi Sea region between 1979 and 1980. The other areas had changes in the 7–10 day range.

The occurrence of these melt signatures can be used as an indicator of climate variability in the seasonal sea-ice zones of the Arctic. The timing of the microwave melt signature has also been examined in relation to melt observed on short-wave imagery. The melt events derived from the SMMR data are also related to the large-scale climate conditions.

During spring 1984, a joint agency research effort was made to explore the use of satellite passive microwave techniques to measure snow-water equivalents in the upper Colorado River basin. This study involved the near real-time acquisition of microwave radiances from the Scanning Multichannel Microwave Radiometer (SMMR) aboard the Nimbus-7 satellite, coupled with quasi-simultaneous surface measurements of snow-pack depth and profiles of temperature, density, and crystal size within the basin. A key idea in this study was to compare, for the same space and time-scales, the SMMR synoptic physics data taken in the basin. Such a snow-measurement program was logistically difficult, but two field teams took detailed snow-pit measurements at 18 sites in Colorado, Utah, and Wyoming during the last 2 weeks of March, when the snow-pack is normally at its maximum extent and depth. These observations were coupled with snow-water-equivalent measurements from Soil Conservation Service SNOTEL sites. Microwave- gradient ratio, Gr (Gr is the difference of the vertically polarized radiances at 8 mm and 17 mm divided by the sum), maps of the basin were derived in a near real-time mode every 6 days from SMMR observations. The sequential Gr maps showed anomalously low values in the Wyoming snow-pack when compared to the other states. This near real-time information then directed the field teams to Wyoming to carry out an extensive survey, which showed that these values were due to the presence of depth hoar; the average crystal sizes were more than twice as large as in the other areas. SMMR can be used to monitor the spatial distribution and temporal evolution of crystal size in snow-packs. Also, scatter diagrams of snow-water equivalents from the combined snow-pit and SNOTEL observations versus Gr from the Wyoming part, and the Colorado and Utah part, of the basin can be used to estimate snow-water equivalents for various parts of the basin.

For more than 10 years, images obtained from the four Landsat Multispectral Scanner (MSS) bands have provided important data for mapping and glaciological studies in the inaccessible polar regions. During this period, the specifications of the MSS have remained little altered, to allow data comparability. More recently, satellites 4 and 5 of the Landsat series have been equipped additionally with Thematic Mapper (TM) sensors. The TM has 7 bands in the visible, near infra-red, mid infra-red, and thermal infra-red, together with a larger dynamic range and improved spatial resolution relative to the MSS. The aim of this paper is to compare MSS and TM computer-compatible tapes (CCTs) from a glacierized area in order to demonstrate the advantages of using TM data in glaciological applications.

The digital MSS and TM scenes compared were imaged simultaneously from Landsat 5 on 5 May 1984 over the north-west part of Spitsbergen, Svalbard (path 218, row 3). This location was selected because of the range of glaciological features present: numerous valley glaciers, the ice field of Holtedahlfonna, fast ice, and ice floes. Partially cloud-covered imagery was preferred, to allow comparison of the two sensors in terms of their ability to distinguish between clouds and snow. The time of year is also advantageous, in that Sun elevation (27°) is high enough for detector saturation to occur in MSS band 2 (Dowdeswell and McIntyre 1986). Surface-elevation data from airborne radio echo-sounding, and other ancilliary glaciological information, are also available for this part of Svalbard.

Differences in the dynamic range and the wavelengths over which TM and MSS data are collected have two main implications for glaciological studies. First, snow and snow-covered ice masses can be distinguished easily from cloud cover in TM band 5 (1.57 to 1.78 μm). Snow appears dark whereas clouds are light at this wavelength. For example, thin clouds over part of Oscar II Land in Spitsbergen became apparent. In many MSS scenes of the Antarctic, the cloud-free ice-sheet surface has been misidentified as cloud-covered during quality-control analysis. Secondly, the wider dynamic range of the TM sensors means that saturation occurs less frequently over snow than was the case with MSS imagery. Digital analysis of MSS and TM scene radiance over Spitsbergen demonstrates this fact and implies that ice-surface topographic information will only rarely be degraded in TM imagery, although TM band 1 (0.45 to 0.52 μm) is most often saturated.

The nominal spatial resolution of TM sensors is 30 m, except for the thermal infra-red band. This is a significant improvement over the 79 m by 56 m resolution of the MSS. A major advantage of this is that ice margins and ice-surface features can be more precisely identified. More accurate glacier maps can be made, and smaller variations in termini positions of outlet glaciers can be monitored. Ice-surface features, such as crevasses, are more likely to be recorded on TM imagery, and examples are shown from Spitsbergen glaciers. The identification of such features is of major importance in studies of ice-surface velocities from Landsat imagery. For sea-ice applications, the ability to identify smaller floes is also important; for example, in the analysis of floe-size distributions.

The only significant drawbacks to the use of Landsat TM data in glaciological studies are the expense, particularly in the more useful digital format, and the small amount of coverage yet available for the polar regions.

The relationship between the intensity of snow acoustic emission impulses and snow-cover stability is revealed by measuring the physical and mechanical properties of the snow cover in the starting zones of avalanches. This relationship is fundamental to the remote identification of an avalanche-hazard period.

In order to estimate the mechanical properties of a snow layer, a method of applying a rigid penetrometer equipped with a piezoelectric accelerometer is used. The viscosity coefficients of snow under destruction and the specific energy of destruction are determined. The overall effect of the different elements of destruction is assessed using both structural investigations and acoustic methods (acoustic emissions).

A comparison has been made of 46 radar-determined ice thicknesses and those resulting from seismic sounding on Bakaninbreen, Skobreen, and Paulabreen in central Spitsbergen. Significant differences were recorded between the two techniques, with 50% of the comparisons exceeding 15 m. Systematic differences between the three glaciers were also observed: on Paulabreen the seismic ice depths are consistently deeper than those determined by radio echo- sounding, whilst the opposite is true on Skobreen.

Instrumental errors from the radar (SPRI 60 MHz unit) and seismic equipment (ABEM Terraloc) are considered small or insignificant. Factors affecting the respective propagation velocities may be responsible for differences in mean thickness particularly in the case of seismic waves, although the changes are obtained from the first returns. One hypothesis to explain the differences on Paulabreen, and to a lesser degree on Bakaninbreen, is that these glaciers are underlain by a seismic low-velocity layer due to the presence of moraine or till. Unfortunately, equipment problems in the field prevented the digital logging of the seismic data and the analogue records are not of sufficient quality for detailed analyses to reveal the possible presence of a till horizon and its seismic velocities. However, observations at the snout of Paulabreen show considerable thicknesses of basal till. With a “P”-wave velocity in such a layer close to or less than that of ice acoustic returns would possibly come from the till–bedrock interface, whereas radar returns would be from the region of the ice–till boundary.

For the seismic ice depths that are shallower than the radar soundings on Skobreen an alternative explanation is required. The valley occupied by the glacier is considerably narrower than in the case of the other two glaciers. According to one detailed radio echo-sounding cross-profile, the line of the combined seismic and radar sounding was displaced to one side of the centre and deepest part of the glacier. This would result in early seismic returns from the nearest facets of the valley side rather than the subjacent bed. The radio waves, however, undergo a focussing effect in the ice, giving rise to a considerably smaller footprint. The difference in slant length between the general area of the bed viewed by the radar and that returning seismic energy is approximately +15–20 m at the location of the cross-profile. This value is of the order of the differences between the two systems and could therefore account for the observed disparity here and at the other locations.

The ability to distinguish the several major types of sea ice with active radar instruments has been well studied in recent years. The separation of sea-ice types by radar results principally from variations in radar back-scatter due to characteristic differences of these ice types in surface morphology and brine content. When sea ice is viewed with an active radar at angles greater than about 20° from nadir, undeformed ice reflects radar waves and results in a low return, while ridges, hummocks, and small-scale surface features scatter the radar waves and produce a high return. The presence of salt increases the dielectric constant of ice; penetration by radar into the ice is then negligible, and the return is essentially determined by surface morphology. The absence of salt reduces the dielectric properties of ice; radar waves can then penetrate the ice to some depth and are scattered by air bubbles and brine-drainage channels (called volume scattering), thereby enhancing the return even for roughened surfaces. All these properties vary significantly with radar frequency and polarization as well as seasonally. For example, higher radar frequencies respond to smaller-scale surface features, while lower radar frequencies penetrate further into the ice with resulting volume scattering.

The high-resolution imagery from synthetic aperture radars (SAR), mounted on aircraft, shuttle, or satellite platforms, is very effective for many sea-ice studies, including the separation of ice types. An aircraft-mounted X-band (9 GHz) SAR, for example, can discriminate smooth first-year ice, rough first-year ice, multi-year ice, and open water by the intensity (tone) of the radar returns and floe geometry. The preferred SARs to date for satellites and shuttle platforms have been L-band (1–2 GHz) systems. SAR imagery of sea ice was extensively acquired by Seasat in 1978 over the Beaufort Sea, with limited quantities obtained by the Shuttle Imaging Radar (SIR-B) over the Weddell Sea in 1984. While L-band SAR can discriminate rough and smooth ice along with roughened open water based on image intensity and floe geometry, the returns from thick first-year ice and multi-year ice are not clearly distinguishable. The fact that there is volume scattering from multi-year ice suggests that there may be textural or spatial frequency variations that could be used to separate these two major ice types in radar imagery. In order to investigate the separation of sea-ice types in the large amount of L-band SAR imagery available, image-analysis techniques including filtering and classification programs have been utilized, pointing towards an automatic classification algorithm for use in future SAR sea-ice data sets, especially from space.

An important characteristic of all SAR imagery is the presence of image speckle, a coherent form of noise caused by the random variability of scatterers across even a uniform surface. Most SAR processors reduce this effect by averaging multiple independent samples but this is done at the cost of reducing resolution. Speckle reduction can also be accomplished by filtering. Several filters have been tested including median, box, and adaptive edge filters. Each filter has different characteristics in terms of smoothing speckle and in the response to sharp gradients or edges, such as ridge or lead openings, as well as computational requirements. Optimization of each filter’s parameters has been determined by the quality of classification of each ice type.

The classification programs that have been tested are based on tone and texture image characteristics. The programs are supervised; that is, a small training area for each class is pre-selected for statistical analysis. From these statistics, the remainder of the imagery is subjected to the particular classification algorithm. The tone program separates classes based on the mean, standard deviation, and number of standard deviations of each class, and includes a Bayesian maximum-likelihood classifier for ambiguous elements. The texture program determines the statistical homogeneity of each class and the optimal segmentation of each small area into the various classes.

Remote-sensing methods in monitoring the glacierization of Mount EI‛ brus are used to produce base and dynamic maps, and to obtain quantitative information (dynamic indices) about the rate, intensity, and variations of the process. The monitoring system is divided, according to scope and territory covered, into small-scale for total glacierization and the periglacial zone, medium-scale for separate glaciers, and large-scale (detailed) for part of the glaciers or sectors of the adjoining slopes. The approximate relationship of even scales is 1 : 4.

Small-scale monitoring remote-sensing systems are important for making maps showing the complex characteristics of the glaciological system. A series of maps was produced including geographical, those of high-altitude zones, slope and exposure angles, geological, glaciomorphological, climatic (temperature, precipitation, and winds), distribution of direct solar radiation, hydrological (source of streams), seats of avalanches, and landslides. All these data serve as a cartographical basis in monitoring the glacierization of Mount EI‛ brus. They are compiled from remotely sensed and Earth-based data.

Current monitoring on a small scale includes observations of the conditions which determine the existence of the glacial system - this includes data on winter snowfall and the period of snow cover. These observations were obtained from meteorological and resource satellites, and from scanner data of medium and high resolution. Also important are observations of changes in the outline of glaciers, times of snowfall and character of the distribution of snow, and its redistribution due to avalanches and snowstorms. High-resolution space photographs, small-scale aerial photographs, and aerovisual observations provide the data for these observations. It has been determined that the area of the glaciers of Mount El‛ brus has been reduced by 1 % in the last 25 years, i.e. the rate of its deglacierization dropped sharply as compared to preceding decades.

The role of quantitative information gains importance in the medium-scale level of monitoring. Topographical maps of separate glaciers compiled from aerial photographs or data from ground stereo-photogrammetric surveys constitute the base maps at this level. The main method used in monitoring were large-scale surveys from aircraft, perspective surveys from helicopters, and phototheodolite surveys. Multi-date surveys of the glaciers provide data about the changes in their outlines and height, the character of their relief, their moraines, the amount of snow accumulation and ablation in separate years, the surface rates of ice flow and their fluctuations. The techniques by which quantitative information is obtained about changes in the glaciers are derived from processing the data of multi-date surveys. The organization and techniques of phototheodolite surveys have been improved. A theory evolved for determining the surface-ice movement by stereo-photogrammetric means and the technique for it has also improved; algorithms and programs for machine processing of the data of multi-date surveys (ground and from aircraft) have been produced

At this level of monitoring, it has been found that the retreat rate of most glaciers has slowed down and several glaciers are now in equilibrium. Several glaciers became active at the beginning of the 1970s and 1980s; this was accompanied by an increase in their height and forward movement. For example, activation of Kyukyurtlyu Glacier has been recorded (higher surface and increasing flow rate) which has caused the glacier to move forward 100 m. Surveys at an interval of 2 years recorded the beginning of the process of retreat of this glacier.

Detailed monitoring is used to detect the mechanism of the dynamic processes and to study it on local representative sectors. On a glacier it may take the form of annual surveys of its tongue, which makes it possible to observe the processes of formation of moraines and glacio-fluvial relief. Studies may also be made of the mechanism of the movement of avalanches and landslides, deducing their quantitative characteristics and appraising the results of avalanches and landslides. Multi-date surveys of sectors of the slopes provide information about processes in the periglacial zone. At this level, regularly repeated ground stereo-photogrammetric surveys are the main means of observation.

Glaciological remote-sensing monitoring provides a wealth of data for theoretical development in the field of glaciology. It makes it possible to forecast and produce warnings about hazardous processes and phenomena.

The areal distribution of glacier annual mass balance b(z) is an important characteristic of the existence of glacierization and its evolution. At present the measured value of annual mass balance at different elevations is only available for a limited number of mountain glaciers of the globe, because of the great amount of labour required for such measurements.

The analysis of long-term mass-balance measurements made at Abramova Glacier, Limmerngletscher, White Glacier, Hintereisferner, and Peyto Glacier has revealed that for each year the spatial distribution of annual mass balance is well described by quadratic equations. The main variable in these equations is altitude (z). The various parameters of these formulae are estimated by the author for mean weighted height of the ablation and accumulation areas, and for the glaciers as a whole. It is found that the parameters of annual mass balance for each glacier can be calculated from formulae which include combinations of the following variables: annual balance at one of the three weighted altitudes, maximum annual snow-line elevation, annual and seasonal amounts of precipitation, and air temperatures at nearby meteorological stations.

Therefore, in order to calculate the distribution of annual mass balance as a function of absolute altitude, it is sufficient to obtain a value for mass balance measured only at a single point on a glacier, and common meteorological observational data. A comparison of actual and calculated values of mass balance has shown good agreement between them.

Considering the successful use of aerial remote-sensing for the measurement of snow depth in mountains by means of special stakes, it is satisfactory to accept this method for the assessment of annual mass balance at the mean weighted altitude of the ablation zone. It is possible to use aerial photo-surveys or stereophotogrammetry to resolve this problem. Then annual mass balance for the whole area of a glacier is calculated by using data from one point together with data from a nearby meteorological station.