Snow Dunes Research Articles

Radiometric validation of the Atmospheric Infrared Sounder over the Antarctic Plateau

Validation of infrared satellite instruments, after they have been launched into orbit, has traditionally relied on views of the relatively warm sea surface. The Antarctic Plateau provides a complementary validation target from space because it is also homogeneous over large areas, yet it is relatively cold. During summer, cloud‐free conditions occur often and the atmospheric humidity is very low with values of total column water vapor around 1 mm. Radiance validation experiments were conducted at Dome C, Antarctica (75°S, 123°E; 3280 m), in January 2003 and again in December 2003/January 2004 for the Atmospheric Infrared Sounder (AIRS). The Polar Atmospheric Emitted Radiance Interferometer (PAERI) measured upwelling and downwelling infrared radiance at the surface during overpasses of NASA's Earth Observing System Aqua satellite. Radiosondes were launched coincidentally with many of these overpasses. These data are used to calculate the outgoing spectral radiance at the top of the atmosphere (TOA), which is then compared with spectral radiance measurements from AIRS in selected frequency bands. At the 95% confidence level, the brightness temperature differences are −0.12 ± 0.24 K (M‐07), −0.15 ± 0.23 K (M‐08), and −0.13 ± 0.24 K (M‐09). These results validate the AIRS radiances at low temperatures (230 to 250 K) relative to the TOA calculations between 788 and 973 cm−1. The cold bias may be partially explained by subvisible or undetected clouds. Ground‐based validation experiments over the Antarctic Plateau can yield valuable information about the low‐radiance performance of infrared satellite instruments. However, one must use ground‐based instruments with a large enough field of view to average over the significant radiance variations that exist across individual snow dunes. Validation of AIRS at low radiance is important because many of the geophysical parameters that are of scientific interest are actually retrieved from low radiance values, including upper tropospheric humidity and properties of high clouds.

Millimeter wave radar with clutter measurements

A portable millimeter wave test radar system, also suitable for battery operation, gives interesting possibilities for clutter recordings at hard-to-reach sites. The designed system covers all common radar frequencies from the Ka- to V-bands and enables spatial detection of targets or clutter elements within an adjustable time gate, whereby spatial clutter profiles of rain can be analyzed. The construction allows full operation with non-scanning antennas as well. This is advantageous when measuring temporal RCS variations of selected targets or surface clutter from snow dunes.

Devon Island's Oriented Landforms as an Analog to Illinois-Type Paha

Illinois-type paha are unusual regional landforms located in northwestern Illinois, USA, consisting of inactive, oriented, oblong ridges composed of either loess or bedrock capped by loess. A recent study reidentified the paha of Illinois as relict oriented, snowmelt interfluves, formed by erosion from snowdrifts or snow dunes occupying oriented valleys alternating with inter-valley ridges during the cold phase(s) of the Pleistocene Epoch. Devon Island, located in the high Arctic, exhibits similar regional, oriented inter-valley ridges and adjacent oriented valleys with the added feature of overlying, oriented snowbanks or snow dunes. The similarities between both regional landforms include morphology, dimensions, parallelism, orientation to the (paleo)wind; and the fact that the oriented dissection was formed irrespective of the steepest direction of the general slope. Oriented nival landforms with the same peculiar characteristics also exist on Resolute, Cornwallis Island, adjacent to Devon Island. Nivation compounded by indirect influence of the wind is proposed as the causal agent for all of the previously mentioned landforms.

Tracking subpixel-scale sastrugi with Advanced Land Imager

High radiometric resolution imagery from the Advanced Land Imager (ALI) can detect the spatial reflectance variations associated with sastrugi (snow dunes) on ice sheets. This is shown, in theory, by considering the appropriate SNR ratio of ALI as compared with eight-bit optical sensors and, in practice, with multitemporal ALI imagery. Comparison of ALI imagery spanning 7.5 months also shows that the reflectance pattern associated with sastrugi can persist over this interval offering the potential of their use for measuring ice sheet velocity in uncrevassed regions.

Snow dunes and glazed surfaces in Antarctica: new field and remote-sensing data

AbstractAs part of the International Trans-Antarctic Scientific Expedition project, the Italian Antarctic Programme undertook two traverses from the Terra Nova station to Talos Dome and to Dome C. Along the traverses, the party carried out several tasks (drilling, glaciological and geophysical exploration). The difference in spectral response between glazed surfaces and snow makes it simple to identify these areas on visible/near-infrared satellite images. Integration of field observation and remotely sensed data allows the description of different mega-morphologic features: wide glazed surfaces, sastrugi glazed surface fields, transverse dunes and megadunes. Topography global positioning system, ground penetrating radar and detailed snow-surface surveys have been carried out, providing new information about the formation and evolution of mega-morphologic features. The extensive presence, (up to 30%) of glazed surface caused by a long hiatus in accumulation, with an accumulation rate of nil or slightly negative, has a significant impact on the surface mass balance of a wide area of the interior part of East Antarctica. The aeolian processes creating these features have important implications for the selection of optimum sites for ice coring, because orographic variations of even a few metres per kilometre have a significant impact on the snow-accumulation process. Remote-sensing surveys of aeolian macro-morphology provide a proven, high-quality method for detailed mapping of the interior of the ice sheet’s prevalent wind direction and could provide a relative indication of wind intensity.

Low microwave brightness temperatures in central Antarctica: observed features and implications

AbstractThe microwave brightness temperature of the ice sheet in Antarctica has a few narrow, elongated features of very low microwave brightness temperatures. The contrast inside and outside of these areas is at least 10 K at 37 GHz. These features are persistent throughout the year and from year to year and occur at 6.6,10.7,18 and 37 GHz. The exceptionally large frequency gradients observed in this narrow area suggest that very large snow grains exist in the snow cover. Past traverse data revealed that the entire surrounding region undergoes large temperature gradients that drive high sublimation causing very large snow grains to form and the formation of both surface and depth hoar. A detailed study of former snow pits showed that, at the place of the unique features, the snow grains from the surface down to the first 2 m are the largest observed over the region. A digital elevation model shows the surface is nearly flat, and corresponds to a subglacial depression in the underlying bed. Previously reported kilometer-scale snow dunes are observed up- and downslope of the level area, but not within the level area, where the brightness temperature is lowest. In this katabatic zone, the drift-snow redistribution and the natural wind-shadow induced by the slope depression create a protected area where the snow grains can grow into very large grains.

Vast snow dunes frozen in time

New and old satellite data are helping glaciologists understand some of the curious features on the Antarctic continent. Comparison of declassified data from the 1960s with modern satellite images has revealed great expanses of snow dunes that have apparently not moved in the past 30 years. Other high-resolution images have exposed the source of giant streams of ice that flow into the sea.

Book reviews: Mader, D. 1995: Aeolian and adhesion morphodynamics and phytoecology in recent coastal and inland sand and snow flats and dunes from mainly North Sea and Baltic Sea to Mars and Venus (2 volumes). Frankfurt am Main: Peter Lang. liv + 2348 pp. £150.00 cloth.ISBN: 3 631 48258 2

Book reviews: Mader, D. 1995: Aeolian and adhesion morphodynamics and phytoecology in recent coastal and inland sand and snow flats and dunes from mainly North Sea and Baltic Sea to Mars and Venus (2 volumes). Frankfurt am Main: Peter Lang. liv + 2348 pp. £150.00 cloth.ISBN: 3 631 48258 2

The lichen flora and habitats of the syowa region, continental antarctica

The lichen flora and habitats in the Syowa region of continental Antarctica were surveyed. The distribution patterns of lichens appear to reflect habitat salinity, derived from wind-blown sea spray. Some halotolerant species are established in saline habitats where snow dunes are well developed. Snow and ice ameliorate the salinity of the habitat and provide a major source of moisture for lichens. Wind-borne drift snow carried by katabatic winds or occasional snow showers also provide the source of moisture to lichens growing on dry rock habitats in the Syowa region.

The lichen flora and habitats of the Syowa region, continental Antarctica

The lichen flora and habitats in the Syowa region of continental Antarctica were surveyed. The distribution patterns of lichens appear to reflect habitat salinity, derived from wind-blown sea spray. Some halotolerant species are established in saline habitats where snow dunes are well developed. Snow and ice ameliorate the salinity of the habitat and provide a major source of moisture for lichens. Wind-borne drift snow carried by katabatic winds or occasional snow showers also provide the source of moisture to lichens growing on dry rock habitats in the Syowa region.

Reflection of solar radiation by the Antarctic snow surface at ultraviolet, visible, and near‐infrared wavelengths

The variation of snow albedo with wavelength across the solar spectrum from 0.3 μm in the ultraviolet (UV) to 2.5 μm in the near infrared (IR) was measured at Amundsen‐Scott South Pole Station during the Antarctic summers of 1985–1986 and 1990–1991. Similar results were obtained at Vostok Station in summer 1990–1991. The albedo has a uniformly high value of 0.96–0.98 across the UV and visible spectrum, nearly independent of snow grain size and solar zenith angle, and this value probably applies throughout the interior of Antarctica. The albedo in the near IR is lower, dropping below 0.15 in the strong absorption bands at 1.5 and 2.0 μm; and it is quite sensitive to grain size and somewhat sensitive to zenith angle. Near‐IR albedos were slightly lower at Vostok than at South Pole, but day‐to‐day variations in the measured grain size due to precipitation, drifting, and metamorphism were found to cause temporal variations in near‐IR albedo larger than those due to any systematic geographical change from South Pole to Vostok. The spectrally averaged albedos ranged from 0.80 to 0.85 for both overcast and clear skies, in agreement with measurements by others at South Pole and elsewhere in Antarctica. Using a two‐layer radiative transfer model, the albedo can be explained over the full wavelength range. Tests were made to correct for systematic errors in determining spectral albedo. Under clear skies at about 3000‐m elevation the diffuse fraction of downward irradiance varied from 0.4 in the near UV to less than 0.01 in the near IR; knowledge of this fraction is required to correct the measured irradiance for the instrument's deviation from a perfect cosine‐response. Furthermore, the deviation from cosine response is itself a function of wavelength. Under clear skies a significant error in apparent albedo can result if the instrument's cosine collector is not parallel to the surface; e.g., if the instrument is leveled parallel to the horizon, but the local snow surface is not horizontal. The soot content of the snow upwind of South Pole Station was only 0.3 ng/g. It was somewhat greater at Vostok Station but was still too small to affect the albedo at any wavelength. Bidirectional reflectance at 0.9‐μm wavelength, measured from a 23‐m tower at the end of summer after the sastrugi (snow dunes) had diminished, showed a pattern remarkably similar to the spectrally averaged pattern obtained from the Nimbus 7 satellite.

Air flow and dry deposition of non-sea salt sulfate in polar firn: Paleoclimatic implications

Non-sea salt sulfate aerosol (NSS) is an important factor for the Earth's albedo because it backscatters solar radiation and is the major cloud condensation nucleus over oceans. At Vostok, Antarctica, NSS concentration shows an increase in glacial period ice of 20–46% that cannot be attributed to changes in accumulation rate. The additional NSS may be due to enhanced dry deposition of NSS by topographic windpumping during the windy glacial periods. We model the volume flux of air into snow due to barometric pressure changes and air flow over surface microrelief. The Gormley-Kennedy equation approximately describes how aerosols advected into the snow pack are removed from the air by diffusion to the snow matrix. Barometric pressure and wind speed data from several polar sites have been used to quantify the vertical volume flux of air and mass flux of NSS. Model results indicate that air flow over small sastrugi, wind carved snow dunes commonly found on ice caps, is the dominant dry deposition mechanism for NSS. Paleo wind speed and surface roughness can significantly influence the aerosol record in ice cores.

Observations of the Antarctic ice sheet with the Seasat scatterometer: relation to katabatic-wind intensity and direction

AbstractThe Seasat A satellite scatterometer radar, initially designed to measure ocean-wind intensity and direction, also provided observations on the Antarctic ice sheet. The signal of the back-scatter coefficient decreases strongly from 10 to −20 dB when the incidence angle of the observations increases from 0° to 65°. An additional 5 dB signal is found, which is correlated with the direction and intensity of katabatic winds, independent of the incidence angle and polarization of the signal. By using simplified models of the volume-scattering within the snowpack (which is mostly sensitive to snow grain-size) and surface-scattering from the air-snow interface (which depends on roughness), it is evident that the signal of the scatterometer could result from the effects of snow dunes at low incidence angle, and of micro-roughness and volume back-scatter at incidence angles greater than 25°. The instrument therefore provides a means of measuring the direction and intensity of katabatic winds.

Observations of the Antarctic ice sheet with the Seasat scatterometer: relation to katabatic-wind intensity and direction

AbstractThe Seasat A satellite scatterometer radar, initially designed to measure ocean-wind intensity and direction, also provided observations on the Antarctic ice sheet. The signal of the back-scatter coefficient decreases strongly from 10 to −20 dB when the incidence angle of the observations increases from 0° to 65°. An additional 5 dB signal is found, which is correlated with the direction and intensity of katabatic winds, independent of the incidence angle and polarization of the signal. By using simplified models of the volume-scattering within the snowpack (which is mostly sensitive to snow grain-size) and surface-scattering from the air-snow interface (which depends on roughness), it is evident that the signal of the scatterometer could result from the effects of snow dunes at low incidence angle, and of micro-roughness and volume back-scatter at incidence angles greater than 25°. The instrument therefore provides a means of measuring the direction and intensity of katabatic winds.

Application of image cross-correlation to the measurement of glacier velocity using satellite image data

Image-to-image cross-correlation software is applied to pairs of digital satellite images to map the velocity field of moving ice. This technique uses small-scale glacial surface features, such as crevasse scars and snow dunes, as markers on the surface of the moving ice. Displacements of the surface features are mapped by selecting small image areas centered on distinct features, or by dividing a large area of densely featured glacial surface into a grid of areas, and searching a subsequent image for matching areas using a cross-correlation algorithm. Interpolation of the peak correlation values allows the displacements to be measured to subpixel accuracy, resulting in very precise velocity measurements. Cross-correlation is also applied to provide image coregistration in areas devoid of bedrock exposures. In such areas, subtle large-scale topographic undulations in the ice surface, related to underlying bedrock structure, may be correlated by using large image areas and low-pass filtered images. Both types of applications are demonstrated, using Ice Stream D and Ice Stream E in West Antarctica as test areas. A high-resolution map of the velocity field of the central portion of Ice Stream E, generated by the displacement-measuring technique, is presented. The use of cross-correlation software is a significant improvement over previous manually-based photogrammetric methods for velocity measurement, and is far more cost-effective than in situ methods in remote polar areas.

The Heat and Mass Balance of Snow Dunes on the Central Antarctic Plateau

The heat-balance components of the faces of snow blocks were measured at “Plateau” station on the Antarctic plateau using micro-meteorological instrumentation. Heat-balance considerations and sublimation observations indicate considerable mass loss from the sloping snow surfaces during summer but little loss from horizontal surfaces. This process tends to level the irregularities of the snow surface. The conclusions are applicable to the snow dunes forming the general accumulation pattern on the Antarctic plateau.

The Heat and Mass Balance of Snow Dunes on the Central Antarctic Plateau

The heat-balance components of the faces of snow blocks were measured at “Plateau” station on the Antarctic plateau using micro-meteorological instrumentation. Heat-balance considerations and sublimation observations indicate considerable mass loss from the sloping snow surfaces during summer but little loss from horizontal surfaces. This process tends to level the irregularities of the snow surface. The conclusions are applicable to the snow dunes forming the general accumulation pattern on the Antarctic plateau.

Further observations on sastrugi, snow dunes and the pattern of surface winds in Antarctica

Surface winds are responsible for the two main types of feature observed on the Antarctic plateau: (a) sastrugi, produced by katabatic (gravity) winds, and (b) snow dunes, which are raised by blizzard winds. The two are often associated in the sense that the severest patches of sastrugi seem to be eroded out of dunes deposited by a previous blizzard. However, sastrugi are by no means limited to this; they may occur more or less continuously, merging one into the next over a wide area of plateau.

The Rolls on the Ellesmere Ice Shelf

Surface rolls are characteristic features of both the Ellesmere ice shelf and the floating ice islands derived from it. These surface features are described and their origin and evolution discussed. Various theories of origin are considered which involve such forces as pressure from the polar pack, movement of glaciers, temperature variations, tidal movements and wind. Of these, wind action appears the most likely to have caused the rolls, and it is suggested that their development was analogous to the formation of seif dunes in desert. "They should be regarded as fossil snow dunes that have been perpetuated by the annual drainage of melt-water."