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The year 2000 has witnessed the calving of several remarkably large icebergs in the Ross and Weddell seas of Antarctica, including the calving of B-l 5, possibly the largest iceberg yet to be observed. Here we present satellite imagery that records the calving, and precursor events leading up to the detachment of B-l 5 from the Ross Ice Shelf in late March 2000. These images suggest that the evolution of a network of large rifts on the ice shelf over the decade prior to calving has controlled both the time of calving and the giant size of B-15. What controls this evolution in turn remains a mystery.

This is the second of two papers summarizing the glaciological observations made on the southern and central parts of the Greenland Ice Sheet during two field campaigns conducted during the 1980s by scientists at The Ohio State University's Byrd Polar Research Center. Three large clusters of survey stations, as well as a strain grid leading to the Dye-3 borehole, were deployed and surveyed in 1980, and resurveyed the next summer (1981). In 1987 and 1989, similar surveys were conducted in the Summit region as part of the GISP2 site selection. The primary objectives of these field campaigns were to measure absolute surface velocities and to obtain 20-30 year average accumulation rates from shallow firn cores. At a number of the survey sites, shallow cores were retrieved to determine accumulation rates and firn temperatures were measured in some of the boreholes. Each core was sampled for density and gross beta-radioactivity. The latter reveals specific stratigraphic horizons associated with nuclear fallout from atmospheric testing of thermonuclear devices in the 1950s and early 1960s. Measurements of oxygen isotopic ratios allowed identification of the seasonal cycles preserved in the firn that provide annual markers distinguishing individual accumulation years. While most of the final results, such as surface velocities and accumulation rates, have been published previously in various reports and papers, the raw data and basic details of the observations are not easily accessible. The purpose of this review is to present these data, including data hitherto unpublished in a single place, and to evaluate and review their quality for those who have used or plan to use these observations. The report augments these observations by comparison with more recent measurements. Together with the data on surface velocity and ice-sheet geometry presented in the first part, these glaciological data sets represent one of the first large-scale surveys conducted on the Greenland Ice Sheet and, as such, provide an invaluable benchmark for change detection.

This report is the first of two papers summarizing the glaciologal observations made on the southern and central parts of the Greenland Ice Sheet during two field campaigns conducted during the 1980s by scientists at The Ohio State University's Byrd Polar Research Center. Three large clusters of survey stations, as well as a strain grid leading to the Dye-3 borehole, were deployed and surveyed in 1980, and resurveyed the next summer (1981). In 1987 and 1989, similar surveys were conducted in the Summit region as part of the GISP2 site selection. The primary objectives of these field campaigns were to measure absolute surface velocities and to obtain 20-30 year average accumulation rates from shallow firn cores. Velocities were determined using the U.S. Navy Navigational Satellite System, otherwise known as the TRANSIT system. To obtain accurate positions for survey stations on the ice sheet, the short-arc method was used, in which satellite tracking stations at two stationary coastal stations were operated simultaneously with tracking stations on the ice sheet. Positions of the ice-sheet tracking stations are calculated relative to the coastal stations and the fixed coordinates of these reference stations are determined from point positioning using the precise orbit parameters. Repeating this procedure at a later date allows ice sheet motion to be determined.

Numerical modeling based on radiative transfer theory and observations of grain growth with depth have indicated that passive microwave brightness temperatures over the polar ice sheets should be a function of accumulation rate. Using accumulation-rate time series from nine shallow cores in the dry-snow facies in central Greenland, we find that multi-year averages of mean annual SMMR brightness temperatures and accumulation rates are highly correlated. The 18 GHz vertically polarized channel is found to be the best predictor of accumulation rate. If the 18V brightness temperatures and the measured annual accumulation rates are averaged over 1979-1986, a linear regression to these multi-year average values reproduces the corresponding measured accumulation rates to within about ±5%. This regression also was used to compute annual accumulation rates at the nine core sites from the corresponding average annual SMMR brightness temperature. Measured and predicted annual accumulation rates agree to within ±10% if the mean annual passive microwave data is lagged by one year. Finally, we find that the predicted total annual accumulation over the 150 × 150 km study area agrees well with the corresponding value computed from the measured accumulation rate; both for an eight-year average and for annual values.

The year 2000 has witnessed the calving of several remarkably large icebergs in the Ross and Weddell seas of Antarctica, including the calving of B-15, possibly the largest iceberg yet to be observed. Here we present satellite imagery that records the calving, and precursor events leading up to the detachment of B-l5 from the Ross Ice Shelf in late March 2000. These images suggest that the evolution of a network of large rifts on the ice shelf over the decade prior to calving has controlled both the time of calving and the giant size of B-15. What controls this evolution in turn remains a mystery.

During September and October, 1989, we made backscatter measurements over sea ice in the Weddell Sea using a ship-based 13·5-GHz radar at incidence angles from 0° to 30° with VV polarization. The objectives of these measurements were to document microwave signatures of Antarctic ice morphologies and to determine the geophysical processes responsible for microwave scattering. The frequency of 13·5 GHz is similar to that used on operational satellite radar altimeters. We found that two characteristics of the snow cover dominated the measured backscatter coefficients (σ0). A thick snow cover increased σ0 at oblique incidence due to volume scatter. Slush at the snow/ice interface increased σ0 by about 8 dB at normal incidence because the smooth surface of the slush layer represents a stronger dielectric boundary. Our results indicate that it may be possible to monitor the formation and subsequent freeze-up of this slush layer using satellite radars. Field observations have shown that this slush can be a significant source of new ice formation, contributing as much as 16 per cent to the total sea ice volume found in the Weddell Sea.

Passive microwave satellite data provide extremely important information about the climate and surface conditions in the often cloudy high latitude regions of the Earth. Available since 1978, multichannel passive microwave data have great potential for long term climate monitoring. In order to ensure consistent data sets for such long term monitoring, the relations between the microwave brightness temperatures from similar sensors on successive satellite platforms must be understood. In this study the 19,22. and 37GHz channels of the Defense Meteorological Satellite Program (DMSP) F8 and F11 Special Sensor Microwave Imager (SSMI) instruments are compared. While the analysis shows that the two data sets are highly correlated with correlation coefficients greater than 0·98. the consistency between the two data sets can be improved by applying small corrections in the order of I deg K. Two sets of regression coefficients are provided for adjusting the F11 data to the F8 baseline.

Snow and ice research during the past quadrennial covers a wide range of topics varying from the climatic effects of large ice sheets and sea ice covers to applied problems such as the icing of power lines and communication facilities. This review focuses in more detail on three topics of the many subjects investigated to provide a more coherent look at the advances achieved and prospects for the future. These are: the influences of layers in seasonal snow covers; research in ice mechanics on freshwater and sea ice; and remote sensing of polar ice sheets. These topics provide useful examples of the general needs in snow and ice research applicable to most areas, e.g. better representation in models of detailed processes, careful controlled laboratory experiments to quantify processes, and field studies to provide the appropriate context for interpretation of processes from remote sensing.

We performed experiments to collect radar backscatter data at Ku (13.4GHz) and C bands (5.3GHz) over simulated sea ice at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) during the 1990 and 1992 winter seasons. These experiments were conducted over bare saline ice grown in an indoor tank and an outdoor pond facility. The radar data were calibrated using a complex vector calibration scheme to reduce systematic effects. In conjunction with the radar measurements we measured ice physical propertiesThese measurements demonstrate that the dominant backscatter mechanism for bare saline ice is surface scattering. Both the copolarized and cross-polarized measurements compare favourably with the predictions of surface scattering models at two frequenciesDuring the 1992 indoor tank experiment we applied four successive layers of snow (about 2.5 cm each) to the saline ice sheet after the ice thickness had reached about 12 cm. The backscatter at normal incidence dropped by l5dB and the backscatter at 45° increased by 11 dB with the introduction of the first snow layer. The application of three more layers, each of approximately 2.5 cm depth, did not alter the radar signature significantly. By modelling and direct observation we found that the initial change in the signature was caused by a roughening of the surface at the snow-ice interface and the change in dielectric contrast at the snow-ice interface.

AbstractGeophysical and glaciological data compiled since the IGY have led to a new picture of ice shelf dynamics, so that the ice shelf is now viewed as a dynamic system that responds strongly to changes in ice input and grounding line fluctuations, and that may be crucial to the existence of the inland ice sheet.Measurements over the last twenty years have revealed the following characteristics about the shelf. Sonic logging in a drill hole in the ice demonstrates a striking anomaly in wave velocity that may be associated with the passage of the ice over the grounding line. Seismic shear wave velocities show anisotropic characteristics that may reflect either crystal anisotropy or stress anisotropy, or both. The apparent dielectric constant in the ice is significantly less than expected from laboratory studies and may exhibit real regional variations. The mass output from the ice shelf is only about half as great as the glacier input through the Transantarctic Mountains plus the surface accumulation on the shelfand its West Antarctic drainage basin, yet measurements on the ice shelf indicate that the mass balance is near zero. This could be consistent with recovery from a past surge of the West Antarctic inland ice. The submarine topography is dominated by broad ridges and valleys extending in an unbroken pattern from the open Ross Sea past the Ross Ice Shelf to the grid eastern part of the West Antarctic inland ice. Convolutions in the ice thickness suggest turbulent flow. Detailed examination of thickness variations, of present-day ice movement rates, of the record of past flow lines revealed by radar soundings, and of isostatic gravity anomalies has led to a picture of Holocene retreat and fluctuations in grounding line positions during the last 1500 years.

We used a coherent radar depth sounder operating at 150MHz to collect ice thickness data on outlet glaciers in northwestern Greenland. The radar data were collected in conjunction with laser surface elevation measurements and were tagged with GPS information for accurate geolocation. The radar signals were corrupted by multiple echoes between the aircraft and the ice surface, as well as between the ice surface and the ice-bedrock interface. We applied the homomorphic deconvolution technique to remove multiple echoes successfully and have identified the grounding line of a long ice shelf in northwestern Greenland.

Sonar echo amplitude data have been collected at carrier frequencies of 188 and 120 kHz from the underside of different sea ice types. Histograms of normal incidence echo amplitudes were formed from over 90 samples of each ice type. Experiments were conducted on saline ice grown in an outdoor pond under relatively controlled conditions at the USA Cold Regions Research and Engineering Laboratory (CRREL) and on the sea ice cover in the Fram Strait. Analysis shows marked variations (about a factor of 5) in the magnitude of the coherent reflection coefficients as congelation ice at the bottom of an ice sheet evolves from a growing dendritic interface to an ablating, thermally altered interface. Larger differences (about a factor of 10) are observed between growing congelation ice and slush ice, used to simulate frazil. These results indicate that important variations in acoustic regime exist in areas where different ice types are intermingled.