ALBERT

Beschreibung: The modern and composite Pleistocene snow-lines of the Absaroka and Beartooth Mountains, Montana-Wyoming, were established from a population of 135 north-facing cirques and cirque glaciers distributed across Boulder River, Stillwater River, Rosebud, and Rock Creeks. The 3110 m elevation of the modern snow-line was established by connecting the accumulation-area ratios of 35 existing cirque glaciers. The composite Pleistocene snow-line was established by joining the mean floor elevations of the lowest abandoned cirques.Comparison of the modern and composite Pleistocene snow-lines indicates that the composite Pleistocene snow-line was depressed, at minimum, 305 m during the Pleistocene. The 305 m depression suggests that the mean Pleistocene temperatures were, at a minimum, 1.9 deg lower than the present mean annual temperatures found in the study area.Both the modern and Pleistocene snow-lines slope towards lower elevations in the Boulder and Stillwater drainages than in the Rosebud–Rock Creek drainages. Modern precipitation rates also show greater precipitation in the Boulder–Stillwater drainages than the Rosebud–Rock Creek drainages. The parallelism of the modern and composite Pleistocene snow-lines, and present precipitation rates suggest that temperature rather than precipitation is the controlling factor causing the depression of the snow-lines.The distribution of cirque elevations indicates that snow-lines fluctuated a minimum of four times during the Pleistocene.

Beschreibung: The modern and composite Pleistocene snow-lines of the Absaroka and Beartooth Mountains, Montana-Wyoming, were established from a population of 135 north-facing cirques and cirque glaciers distributed across Boulder River, Stillwater River, Rosebud, and Rock Creeks. The 3110 m elevation of the modern snow-line was established by connecting the accumulation-area ratios of 35 existing cirque glaciers. The composite Pleistocene snow-line was established by joining the mean floor elevations of the lowest abandoned cirques. Comparison of the modern and composite Pleistocene snow-lines indicates that the composite Pleistocene snow-line was depressed, at minimum, 305 m during the Pleistocene. The 305 m depression suggests that the mean Pleistocene temperatures were, at a minimum, 1.9 deg lower than the present mean annual temperatures found in the study area. Both the modern and Pleistocene snow-lines slope towards lower elevations in the Boulder and Stillwater drainages than in the Rosebud–Rock Creek drainages. Modern precipitation rates also show greater precipitation in the Boulder–Stillwater drainages than the Rosebud–Rock Creek drainages. The parallelism of the modern and composite Pleistocene snow-lines, and present precipitation rates suggest that temperature rather than precipitation is the controlling factor causing the depression of the snow-lines. The distribution of cirque elevations indicates that snow-lines fluctuated a minimum of four times during the Pleistocene.

Beschreibung: A quantitative comparison of seasonal and interannual Antarctic sea-ice coverage over the four years 1973-76 has been accomplished through the use of passive microwave imagery from the Nimbus-5 satellite. For the entire Southern Ocean both the total ice extent (area with ice concentration greater than 15%) and the actual ice area (the spatially-integrated ice concentration) have decreased over this period of 4 a, but not uniformly in all regions. From 1973 to 1976 the annual-mean value of total ice extent decreased from 13.8 × 106 km2 to 12.1 × 106 km2, yielding an average decrease of 4.0% a−1. The inter-annual difference is greatest during the spring, as the ice decays, with the decrease in the December-mean averaging 8.4% a−1, the largest of any month. The decrease in the November-mean averaged 4.5% a−1. The overall decrease was principally due to the consistent yearly decrease of ice In the Weddell Sea sector (60°W to 20°E). Other sectors show less consistency. For instance, the ice in the Ross Sea sector (130°W to 160°E) increased from 1973 to 1974 and then decreased from 1974 to 1976, and no consistent trend is apparent in the ice extent between 20°E and 160°E. The total ice extent in the Bellingshausen- Amundsen seas sector (60°W to 130°W) actually increased slightly from 1973 to 1976. The area of the open water within the ice pack behaved differently from the total ice area, Increasing each year from February to November but having no clear interannual trend. A detailed analysis of the passive microwave imagery for the Antarctic region is planned for publication in an atlas.

Beschreibung: Greenland and Antarctic ice-sheet surface elevations have been obtained from Seasat radar altimeter data after computer retracking of the return waveforms. The height of the altimeter above the surface is determined from the measured time between transmission of radar pulses and their return. The altimeter servo-tracking circuit attempted to maintain the midpoint of the ramp of the return waveform in the center of 60 time gates, each equivalent to 0.47 m in range. Waveforms representing an average of 100 pulse returns were recorded each 0.1 s, corresponding to a distance interval of 662 m on the surface. Deviations of the midpoint of the waveform ramp from the central-gate position were caused by changes in range larger than the design limits of the servo-circuit, thereby producing errors in the height indicated by the altimeter. If the deviation was greater than about 25 gates (13 m range), the waveform ramp moved outside the time gates and the servo-tracking was temporarily interrupted. These larger deviations resulted in a loss of about 30% of the data over the ice sheets. Both surface undulations and the steeper slopes near the ice-sheet edge produced range velocities sufficient to cause interruption of altimeter tracking. Waveforms that remained within the 60 gates have been corrected by a computer curve-fitting procedure applied to each waveform.Preliminary contour maps of surface elevation at 100 m contour intervals have been created for much of the East Antarctic ice sheet north of 72°S and the Greenland ice sheet south of 72°N. The standard deviation of the difference in elevation at 1 032 crossover points in the retracked Greenland elevation profiles is 1.9 m, which is largely due to radial errors in determination of the satellite position. Adjustment of the radial components of the orbits to minimize the crossover differences in select regions reduces the difference to 0.25 m, which is indicative of the optimum obtainable precision over the ice sheets. This precision is comparable to the value of 0.05 to 0.10 m obtained over the oceans where waveform averages of 1 s are used. The data are sufficiently dense to permit contouring at smaller intervals (2 to 10 m) only in the regions near the maximum latitudes of ±72°. Contouring at the smaller intervals illustrates the three dimensional characteristics of some of the observed undulations.Several methods were tested for correcting slope-induced displacements, which are typical of reflection-range measurements using a wide-angle beam. The slope-induced displacement hα2/2 is about 40 m for a satellite altitude h of 800 km and a surface slope a of 10−2. In a simulation experiment, an apparent surface profile was created by computer simulation of the altimeter measurement of an actual ice-surface profile and was then corrected for slope-induced displacement. The results show that the residual error between reconstructed and actual surfaces is about 15% of the displacement. Along the sub-satellite track the data are sufficiently dense to permit such correction for along-track slope-induced displacements caused by both undulations and regional slopes, but in the other dimension the data are generally only sufficient to permit across-track correction for regional slopes.

Beschreibung: A quantitative comparison of seasonal and interannual Antarctic sea-ice coverage over the four years 1973-76 has been accomplished through the use of passive microwave imagery from the Nimbus-5 satellite. For the entire Southern Ocean both the total ice extent (area with ice concentration greater than 15%) and the actual ice area (the spatially-integrated ice concentration) have decreased over this period of 4 a, but not uniformly in all regions. From 1973 to 1976 the annual-mean value of total ice extent decreased from 13.8 × 106 km2 to 12.1 × 106 km2, yielding an average decrease of 4.0% a−1. The inter-annual difference is greatest during the spring, as the ice decays, with the decrease in the December-mean averaging 8.4% a−1, the largest of any month. The decrease in the November-mean averaged 4.5% a−1. The overall decrease was principally due to the consistent yearly decrease of ice In the Weddell Sea sector (60°W to 20°E). Other sectors show less consistency. For instance, the ice in the Ross Sea sector (130°W to 160°E) increased from 1973 to 1974 and then decreased from 1974 to 1976, and no consistent trend is apparent in the ice extent between 20°E and 160°E. The total ice extent in the Bellingshausen- Amundsen seas sector (60°W to 130°W) actually increased slightly from 1973 to 1976. The area of the open water within the ice pack behaved differently from the total ice area, Increasing each year from February to November but having no clear interannual trend. A detailed analysis of the passive microwave imagery for the Antarctic region is planned for publication in an atlas.

Beschreibung: Greenland and Antarctic ice-sheet surface elevations have been obtained from Seasat radar altimeter data after computer retracking of the return waveforms. The height of the altimeter above the surface is determined from the measured time between transmission of radar pulses and their return. The altimeter servo-tracking circuit attempted to maintain the midpoint of the ramp of the return waveform in the center of 60 time gates, each equivalent to 0.47 m in range. Waveforms representing an average of 100 pulse returns were recorded each 0.1 s, corresponding to a distance interval of 662 m on the surface. Deviations of the midpoint of the waveform ramp from the central-gate position were caused by changes in range larger than the design limits of the servo-circuit, thereby producing errors in the height indicated by the altimeter. If the deviation was greater than about 25 gates (13 m range), the waveform ramp moved outside the time gates and the servo-tracking was temporarily interrupted. These larger deviations resulted in a loss of about 30% of the data over the ice sheets. Both surface undulations and the steeper slopes near the ice-sheet edge produced range velocities sufficient to cause interruption of altimeter tracking. Waveforms that remained within the 60 gates have been corrected by a computer curve-fitting procedure applied to each waveform. Preliminary contour maps of surface elevation at 100 m contour intervals have been created for much of the East Antarctic ice sheet north of 72°S and the Greenland ice sheet south of 72°N. The standard deviation of the difference in elevation at 1 032 crossover points in the retracked Greenland elevation profiles is 1.9 m, which is largely due to radial errors in determination of the satellite position. Adjustment of the radial components of the orbits to minimize the crossover differences in select regions reduces the difference to 0.25 m, which is indicative of the optimum obtainable precision over the ice sheets. This precision is comparable to the value of 0.05 to 0.10 m obtained over the oceans where waveform averages of 1 s are used. The data are sufficiently dense to permit contouring at smaller intervals (2 to 10 m) only in the regions near the maximum latitudes of ±72°. Contouring at the smaller intervals illustrates the three dimensional characteristics of some of the observed undulations. Several methods were tested for correcting slope-induced displacements, which are typical of reflection-range measurements using a wide-angle beam. The slope-induced displacement hα2/2 is about 40 m for a satellite altitude h of 800 km and a surface slope a of 10−2. In a simulation experiment, an apparent surface profile was created by computer simulation of the altimeter measurement of an actual ice-surface profile and was then corrected for slope-induced displacement. The results show that the residual error between reconstructed and actual surfaces is about 15% of the displacement. Along the sub-satellite track the data are sufficiently dense to permit such correction for along-track slope-induced displacements caused by both undulations and regional slopes, but in the other dimension the data are generally only sufficient to permit across-track correction for regional slopes.

Beschreibung: The repetitive synoptic ice data obtainable by satellite sensing provide a means of studying the time-dependent behavior of both sea ice and ice sheets on climatic time scales. Examples of sea-ice parameters which may be measured are extent, concentration, and multiyear fraction; and examples of ice-sheet/ice-shelf parameters are surface elevation, ice-front position, extent and duration of summer melting, and ice accumulation rates. Desired snow-cover parameters include extent and snow depth or water-equivalent depth. The unique ability of satellites to measure such ice parameters and the characteristics of the consequent data sets significantly influence the structure of ice models that can be successfully used with the data. Ice data sets recently acquired by satellite sensing are described. The past decade of sea-ice data provides a detailed description of the interannual variability of sea ice on a regional and seasonal basis. Because of the longer time scales involved in ice-sheet variations, a comparable record of ongoing ice-sheet variations has not yet been established, but important baseline data sets are being developed.

Beschreibung: The repetitive synoptic ice data obtainable by satellite sensing provide a means of studying the time-dependent behavior of both sea ice and ice sheets on climatic time scales. Examples of sea-ice parameters which may be measured are extent, concentration, and multiyear fraction; and examples of ice-sheet/ice-shelf parameters are surface elevation, ice-front position, extent and duration of summer melting, and ice accumulation rates. Desired snow-cover parameters include extent and snow depth or water-equivalent depth. The unique ability of satellites to measure such ice parameters and the characteristics of the consequent data sets significantly influence the structure of ice models that can be successfully used with the data. Ice data sets recently acquired by satellite sensing are described. The past decade of sea-ice data provides a detailed description of the interannual variability of sea ice on a regional and seasonal basis. Because of the longer time scales involved in ice-sheet variations, a comparable record of ongoing ice-sheet variations has not yet been established, but important baseline data sets are being developed.