C31C-01 INVITED 08:00h
Accumulation Variability and Wind-borne Snow Transport Across Lyddan Ice Rise, Antarctica
Redistribution of snow by the wind is the major contributor to spatial variability in snow accumulation over Antarctica on scales from metres to a few kilometres. Even small variations in wind speed associated with very gentle topography can give rise to large accumulation variations as a result of the highly nonlinear relationship between wind speed and snow transport. In this talk we examine the relationships between wind, topography and snow accumulation using observations made across an Antarctic ice rise. Lyddan Ice Rise is an approximately two-dimensional ridge, about 15 km wide, that rises 130m above the surrounding ice shelves. Surveys carried out using conventional stake measurements and ground penetrating radar reveal surprisingly large accumulation variations across this relatively gentle feature. On the scale of the ice rise itself, there is a gradual decline in accumulation moving from the ice shelf on the climatologically-upwind side to the climatologically downwind slope, where accumulation is reduced to around 65% of its upwind value. Superimposed on this broad-scale gradient are large (20-30%), localized variations in accumulation on a scale of around 1 km that appear to be associated with local variations in surface slope of around 0.01. The observed variations in accumulation agree well with calculations of snow redistribution made using wind measurements from automatic weather stations and winds derived from an airflow model, lending support to the hypothesis that snow redistribution is the major control on accumulation at this location. We discuss how our observations may relate to studies of accumulation variability in other parts of Antarctica, particularly the "megadune" fields of East Antarctica.
C31C-02 INVITED 08:20h
Wind-Driven Sublimation Impact on Surface Mass Balance and Ice Core Interpretation in East Antarctica
Temporal and spatial variability of snow accumulation are input parameters in mass balance studies. They are key issues of paleoclimatic reconstructions from ice cores. As part of the International TransAntarctic Scientific Expedition and of the Franco-Italian Concordia Station collaboration, field survey along traverses and spot research at selected sites have been performed over the East and NE Dome C drainage area (East Antarctica). Different methods were adopted, compared and integrated (stake farm, core analysis, snow radar, surface morphology, remote sensing) to develop an understanding of the climatic and surface conditions. The most prominent unexpected results have been discovered through ground survey coupled with satellite observations. Our idea of Antarctica as a flat continent with a homogenous snow accumulation variability has radically altered. Snow precipitation is homogeneous at a large scale (hundreds of km2), but the wind-driven sublimation phenomena controlled by slope along the prevalent wind direction have considerable impact on the spatial distribution of snow at short (tens of m) and medium (km) spatial scales. The maximum value of snow accumulation at one site is very highly correlated with firn temperature and represents the snow precipitation minus ablation not induced by wind. The high variability of surface mass balance is mainly due to ablation processes driven by katabatic winds (wind-driven sublimation); a few strong wind events can greatly decrease the mass through snowdrift sublimation, especially during summer. The spatial variability of snow accumulation at the km scale is one order of magnitude higher than temporal variability at the decadal/secular scale. Ablation processes of snow on short and long spatial scales have a significant impact on snow grain size and post-depositional losses of chemical species by re-emission and therefore on interpretation of ice core palaeoclimatic series. Where snow accumulation rate is relatively high, snow is buried quickly and initial chemistry composition is preserved. On the contrary, in ablation areas, snow is exposed to lengthy vapour exchange with the atmosphere therefore increasing the possibility of re-emission and UV-decomposition processes. The reconstruction of past climates based on firn/ice cores drilled in areas with high snow accumulation spatial variability is more complicated.
C31C-03 INVITED 08:40h
Satellite, Observational, Meteorological and Thermal Records From Two Sites in the Antarctic Megadunes - Stability of Atmospheric Forcing, Thermal Cracking, and the Seasonal Evolution of the Thermal Profile
An NSF-OPP funded research site in the megadunes occupied during the 2002-2003 and 2003-2004 field seasons provided an opportunity to monitor wind speed and direction, atmospheric pressure, air temperature, and the evolution of the thermal profile in the firn. In the first season this was done on the lee face of a megadune; in the second season it was done there and at an additional site on the windward face. Wind speed and temperature fluctuations were well correlated at the two sites with little lag. The thermal profiles provide a picture of the cold wave penetration at both sites. Firn in these areas was significantly recrystallized (see abstract by Courville et al., this session), had a surface character that included both large sastrugi (windward slopes) and very smooth surfaces (lee slopes), and showed numerous thermal contraction cracks that were likely sites of vertical air movement. In the first season the smooth lee slope was covered by a thin glaze; the spatial extent of this glaze and the surface roughness variations are detectable in satellite imagery from this period. Large area MODIS-based image maps show the dominance of katabatic-wind-generated features in the dune field. Satellite-based microwave emission time series show the source of emission to be extremely shallow and/or characteristic of rapid cooling to near isothermal conditions; these patterns have been used to map the extent of recrystallized firn. This will be revisited in light of the new time series of firn thermal profiles.
C31C-04 09:00h
Antarctic Megadunes: Characteristics and Formation
We review field geophysical, meteorological, and remote sensing data covering Antarctica's 'megadune' regions with the purpose of constraining formation models for the features. Megadunes are striped accumulation variations, oriented perpendicular to mean katabatic windflow, with hieghts ranging from 2 to 8 meters, and crest spacing from 2 to 6 km. Crest ridges have lateral extents of up to 100 km. Upwind faces are steeper than downwind faces, and are characterized by very large, eroded sastrugi. Surface 'glazes' of ice, with coarse recrystalized grains in the subsurface, are present in the lee faces. Dunes are widespread across the East Antarctic plateau, although laterally extensive dune fields occur in just a few regions. Strong variations in surface roughness and snow grain size between crest/upwind faces and trough/downwind faces are evident in albedo and radar or visible-light backscatter. Field measurements at a site 400 km southeast of Vostok station (80.78 deg S, 124.5 deg E) provide insight into dune origin, longevity and migration. Detailed surface topography from GPS confirms height and width of dunes inferred earlier using ICESat. Internal layering of megadunes (imaged using ground-penetrating radar) shows sigmoidal layers of higher accumulation along the dune crests and windward faces. Dunes accrete new layers in the upwind direction. Radar layer structures, consisting of a 6- to 15-meter-thick sequence of accumulation layers separated by erosive or very low accretion glaze layers, are visible to at least 70 meters below the surface. Given an estimated mean accumulation of 20 - 30 kg/m2 over the dune region, each dune sequence represents approximately 250 years of time. High accretion occurs over roughly 1/3 of an active dune field surface. This implies that surfaces on the lee-side dune face spend between 150 and 200 years exposed to near-surface air and temperature variations before burial by the next advancing dune face. GPS ice motion measurements indicate an ice flow of 4 m/yr at bearing 130, almost perpendicular to the mean wind direction of 226. Dune winds are dominated by flow in this direction (+/- 15 degrees) at 8 - 12 m/s for 10 months of the year. Several possible models for dune formation are considered. Katabatic wind flow of a near-surface air layer clearly controls snow redistriburtion. Previous studies have shown that dunes form in a narrow range of regional surface slope (0.0010 - 0.0015), implying that a specific range of winds speeds may determine formation. One possibility is that wave-like boundary layer effects in the katabatic flow create the dunes. Compaction effects due to the long period of low accumulation in the glaze regions may amplify dune topography. An 'anti-dune' (a term from fluvial geomorphology) model of dune formation is forwarded.
C31C-05 09:15h
Anomalous accumulation rates resulting from ice flow over Lake Vostok
The accumulation rate of snow is crucial to the development of accurate age-depth models for ice-cores. The dating of the Vostok ice-core generally assumes that accumulation rates vary linearly between the core site and the ice divide 250 km to the west [Jouzel et al., 1996; Lorius et al., 1985; Petit et al., 1999], an assumption which impacts the timing of prominent climatic transitions. We present evidence for a local accumulation rate anomaly at the ice surface above the western shoreline of Lake Vostok. A significant thickening between isochronous layers results from this geographically fixed high accumulation zone which can be stratigraphically traced to a depth of 820-1100 m in the Vostok ice-core, a portion known for its high accumulation rates and paleoclimate records that deviate from other Antarctic ice-core records. This non-climatic accumulation anomaly in the Vostok ice-core impacts the flow dependent age models and subsequent interpretations of sequencing of global climate shifts during the last glacial. These previously unreported geographically fixed accumulation rate anomalies are introduced into ice-cores drilled away from ice domes (i.e., Byrd and Vostok) and should be considered in age depth models.
C31C-06 09:30h
Firn Physical Characteristics and Impact on Interstitial Convection and Diffusion in the Megadunes of East Antarctica
Snow and firn properties and layering control interstitial transport of gases and vapor at all levels in the firn, along with air-snow transport in the near-surface firn. Knowledge of the properties and an understanding of interstitial transport mechanisms are important for interpretation of ice core and firn air records. Climate factors such as temperature and accumulation rate influence the physical properties of the firn, affecting feedbacks between the properties of the firn and interstitial transport mechanisms. In this paper we report on measurements of the physical properties of the firn in a megadunes region, and we model several types of convection likely to occur there. Megadunes are low relief, long wavelength wave-like features covering an extensive amount of Eastern Antarctica. The region experiences very low temperatures, -45 to -60 C mean annual temperature, and very low accumulation rates, 7 to 35 mm yr-1, which leads to extreme metamorphism of the firn crystals as they remain exposed to the surface for long periods of time. We have measured the transport properties of permeability and diffusivity, along with density, stratigraphy and microstructure characteristics of the top 30m of an ice core drilled in a megadunes region of East Antarctica. Permeability, density and grain scale measurements for the top several meters were also made in the field at a site adjacent to the ice core drilling. The permeability of the megadunes site is several times higher than that measured at Siple Dome. Diffusivity, measured with 10 cm resolution down the length of the core, follows an overall trend which is similar to the permeability profile. Crystal photography of several different types of microstructure encountered is shown. Multidimensional finite element modeling results of interstitial air movement due to ventilation, and also due to buoyancy-induced natural convection are presented. Ventilation, enhanced by cracks typical at low accumulation sites, is likely to influence near-surface properties, sublimation, and interstitial gas transport. Because of the extreme firn metamorphism and increased permeability, natural convection is also likely to play a large role in deep convection with interstitial transport at this and other low-accumulation sites.
C31C-07 09:45h
Do Deep Convective Zones Exist in Low-Accumulation Firn?
Trapped air in deep ice cores from the Antarctic plateau (Vostok, Dome Fuji) exhibits anomalously low gravitational fractionation during glacial periods, as inferred from \delta$^{15}$N and \delta$^{40}$Ar. These isotopic signals reflect the thickness of the stagnant air column in the firn, where molecular diffusion dominates over convection as a vertical transport mechanism. These signals have been used in prior studies as paleoindicators of the firn thickness at the time of bubble close-off. However, a near-surface layer of intense air convection may dominate molecular diffusion sufficiently that isotopic fractionation is prevented. This "convective zone" could explain the discrepancy seen in Vostok/Dome Fuji between expected firn thickness (from densification models) and inferred diffusive column thickness, if the convective zone were 30 m thick during glacial periods. Importantly, accumulation rates were very low during these times (1-2 cm/yr). The existence of deep convective zones has been debated, but not yet observed in firn air studies. Certain low-accumulation regions of the Antarctic plateau today exhibit very large grain sizes at and near the surface, as inferred from satellite passive microwave studies (Fahnestock et al., GRL). Permeability of the firn in these regions should be orders of magnitude higher than in normal firn, suggesting the hypothesis that deep convective zones exist in these regions. In January 2004 we sampled firn air for measurement of \delta$^{15}$N and \delta$^{40}$Ar profiles at one such site, where snow megadunes exist. Initial results suggest a convective zone of approximately 20 m thickness.