C33C-0355 1340h
Co-Variations of Ice Sheet Elevation and Slope with Accumulation, Radar Backscatter, and Temperature in West Antarctica
Maps of surface slope and elevation of the West Antarctic Ice Sheet (WAIS) may be refined by applying calibrated image pixel brightnesses to existing DEMs. Using these image-enhanced elevation models, and existing models of regional katabatic air flow, we investigate the relationship of accumulation, radar backscatter, and temperature to surface slope in the catchment area of the Ross Embayment ice streams. MODIS (Moderate Resolution Imaging Spectroradiometer) images were combined with the RAMP (Radarsat Antarctic Mapping Project) DEM and airborne laser altimeter profiles (from the ALICE/CASERTZ/SOAR aerogeophysics programs) to create a new 250-meter resolution slope and elevation DEM of the study region. The DEM fully resolves the surface undulation field of 2 - 10 km spatial scale, and permits an investigation of air-surface interaction effects in greater detail. Coupling this slope map with the regional mean wind pattern (Bromwich, pers. comm.), we compare local slope variations in the wind direction with accumulation variations derived from a 180-km ground-penetrating radar profile (Spikes, Hamilton, and Arcone, pers. comm.) and point accumulation measurements (Giovinetto, and Vaughan, pers. comm.). We also compare wind-direction slope and accumulation with radar backscatter intensity derived from the RAMP AMM-1 data set. Correlation between RAMP backscatter and accumulation along the traverse is very high (r = 0.8). Regions of higher accumulation are associated with lower backscatter. Accumulation varied as much as 3-fold over just 15 km in regions of significant local relief. Both backscatter and accumulation show high correlation (r= 0.7) with surface slope in the mean wind direction: higher accumulation occurs on windward slopes. This relationship suggests that slope, backscatter, and mean wind direction may be used to infer accumulation in high spatial detail throughout the continent. Mean surface temperature under clear-sky conditions also shows a significant correlation with surface structure, in this case elevation. Using 60 clear-sky AVHRR images, we compile a map of mean surface temperature under clear-sky conditions. Locally high areas on the ice sheet show consistently warmer surface temperatures, by 1 to 4 C, under clear-sky conditions. Katabatic winds arise from surface radiative cooling under clear conditions; so the relationship between surface elevation and clear-sky temperature is an indicator of inversion-layer air-flow. We infer that katabatic airflow dominates accumulation via snow re-distribution and may also impact mean annual snow temperature via its interaction with surface morphology at the 2 - 10 km spatial scale. We hypothesize that flow of the inversion layer slows over the upwind face of hills, causing a loss of entrained sediment (snow). Greater concentration of subsurface layering (wind-crust, hoar, or glaze layers per meter depth in the firn) on low-accumulation lee faces leads to higher radar backscatter. Steep, inverted lapse rates in the inversion layer result in persistent temperature-elevation relationships of several degrees C over elevation changes of just a few tens of meters.
C33C-0356 1340h
Close Resemblance Between Local Summer Insolation, O$_{2}$/N$_{2}$ and Total Air Content from the Dome Fuji Ice Core, Antarctica
O$_{2}$/N$_{2}$ ratio from ice cores has shown depleted values compared to the atmosphere due to selective exclusion of O$_{2}$ during bubble formation at the base of firn. The long record from the Vostok ice core revealed that O$_{2}$/N$_{2}$ ratio records the local summer insolation. Insolation may affect physical properties of the firn near the surface, which later determines how much O$_{2}$/N$_{2}$ is fractionated the during bubble close-off process. We present here a supportive record of O$_{2}$/N$_{2}$ ratio for the last 340 kyr along the Dome Fuji ice core, Antarctica, which shows variations similar to the summer insolation at $77\deg$S. Moreover, the variation of total air content (TAC) in the Dome Fuji core resembles that of O$_{2}$/N$_{2}$. High TAC and high O$_{2}$/N$_{2}$ ratio appear at times of low summer insolation. Since the TAC variation is too large to be explained by the elevation change at the Dome Fuji site in the past, a possible cause is variation of the so-called ``lock-in zone'' thickness on the orders of several meters. The lock-in zone is a region 0-10 m thick at the bottom of firn where horizontal impermeable layers prevent vertical gas mixing. At times of low insolation, the firn would retain inhomogeneities such as wind crusts and high-density layers. These small-scale inhomogeneities do not affect bulk density very much but may help trap the gases at a lower bulk density (and higher porosity and thus TAC) through formation of a thicker lock-in zone than in times of high insolation. High insolation would homogenize the firn structure through recrystallization. O$_{2}$/N$_{2}$ ratio would be less depleted if there is a lock-in zone within the total close-off zone, because O$_{2}$ molecules once excluded from bubbles would eventually be re-trapped in the ice in the lock-in zone.
C33C-0357 1340h
Precise Elevation Profiles Across Antarctic Megadunes
Covering more than 500,000 square kilometers of Antarctica, megadune features are related to persistent, down slope winds and have a subtle but characteristic morphology that is best defined by precise elevation data. In this paper, we will discuss ICESat precision laser altimetry data that details some of these features near an NSF-OPP funded research site occupied during the 2002-2003 and 2003-2004 field seasons. Work there to-date shows megadunes having 1) amplitudes from about 2 to 8 meters, 2) wavelengths of approximately 2 to 6 kilometers, 3) crest lengths of up to 100 kilometers, 4) a steeper and rougher upwind face, and 5) a smoother and longer backslope. See other talks in this session (e.g. Scambos et al.,) for additional details of their characteristics and formation. In this harsh environment, satellite altimetry is an excellent way to gather the elevation data needed to define these features. The ICESat mission acquires elevation data with a measurement spacing of about 172 m along its orbital track and can repeat a track to within approximately 50 meters. When skies are clear, ICESat measures laser spot elevations with 2 to 3 centimeter precision over these slowly-changing, undulating ice sheet features. Clouds, although rare in this area, can degrade the elevation data precision by more than a meter. Data obtained during ICESat's first four operational periods clearly documents the megadune and other ice sheet features in the region and enables the impact of clouds and the overall precision of this data to be quantified. ICESat data calibration and validation is being aided by high-precision GPS data acquired on sled traverses in the field area. The combination of ICESat data with a recently completed Antarctic MODIS mosaic will allow additional aspects of these features to be discussed.
http://icesat.gsfc.nasa.gov/
C33C-0358 1340h
Relating SAR Backscatter Variations to Ice Sheet Surface Conditions Along the US ITASE Traverse
Radar backscatter over Antarctica varies according to the physical properties of the near surface, such as surface roughness, grain size, density, stratigraphy and moisture content. These physical properties are intrinsically linked to larger scale properties including ice divide location, topography, accumulation rate, ice flow and surface melt patterns. Comparing brightness variations in radar imagery with field observations of surface properties enables a fuller understanding of backscatter patterns and makes SAR mosaics powerful tools for exploring ice sheet processes and behavior. In this study, the RADARSAT-1 Antarctic Mapping Project (RAMP) mosaic and recent ERS ASAR imagery will be used. Detailed surface topography maps and shallow radar profiles along the >5,000 km of US ITASE traverse routes in West Antarctica are used to study causes of radar backscatter variations. A series of isolated ice cores, connected to one another by isochronous horizons tracked with ground penetrating radar, yield accumulation rates along the traverse routes. Here, we correlate satellite radar brightness variation with surface topography, accumulation rate and density along the ground traverses, then proceed to infer the surface characteristics of other regions of the ice sheet.
C33C-0359 1340h
Firn density profile at Megadunes, East Antarctica, calls for an improved densification model for low accumulation sites.
We report a density profile of the firn at Megadunes, East Antarctica (80$^{o}$78'S; 124$^{o}$50'E). The Megadunes site is characterized by a low mass accumulation rate (2.9 g/cm$^{2}$/yr) and cold temperature (mean annual = -49.5$^{o}$C). Validating existing firn densification models using a density profile of such a site is important because such models, either purely empirical or mechanistic, have been calibrated with few sites that are analogous to a glacial condition (ultralow accumulation and temperature). We make use of CO$_{2}$ concentrations in the lock-in (or non-diffusive) zone to obtain the accumulation rate, assuming that the gas in the lock-in zone ages at the same rate as the surrounding ice [Battle et al., 1996], and assuming that the gas enclosure rate must equal the long-term mean accumulation rate. This estimate is preliminary and may change when results from beta analysis of cores become available. Our density profile shows that the widely used pure empirical model by Herron and Langway [1980] overestimates the close-off depth by 16 %, and the semi-mechanistic model by Pimienta and Barnola [Barnola et al., 1991] also overestimates the close-off depth by 26 %. Our study at the Megadunes site indicates that the $\delta$$^{15}$N of N$_{2}$ paradox for glacial ice from East Antarctica would have partly resulted from a poorly calibrated densification model for a `glacial-like' condition and a thick convective zone as we observed at the Megadunes site [Severinghaus et al., in prep].
C33C-0360 1340h
Relating GPR to In Situ and Stratigraphically Determined Accumulation Rates at South Pole Station
Spatial and temporal variation in accumulation rates is poorly constrained yet is a primary indicator of climate trends on ice sheets. Several methods have been used to obtain accumulation rate data with varying strengths and weaknesses. The principal mechanisms of quantifying accumulation rate data have been through stratigrpahic analysis of ice cores, or through in situ accumulation stake measurements. In situ measurements have a severely limited temporal scope and often inadequate spatial resolution to be relevant to meso-scale climate reconstructions. Stratigrpahic analysis allows for more developed temporal resolution and time series length, but is not a direct measurement and also represents a small spatial area. We propose that ground penetrating radar can be an effective tool to bridge the gap between the limited spatial signature of ice cores and dearth of in situ measurements. South Pole Station has the highest density of in situ accumulation rate measurements both temporally and spatially. One of the most notable examples of this is the accumulation stake network maintained at South Pole Station by The Ohio State University. We conducted a land based traverse along two lines of this accumulation grid while operating a 400MHz ground penetrating radar unit. By identifying significant isochrones within the radar data we have determined the relative accumulation rate for the years between the isochrones. This technique facilitates a high density of data collection with relative ease and also allows for the accumulation rate record to be extended beyond the limited amount of time for which there is in situ data. By performing this experiment at South Pole we can relate the geophysical data to direct measurements of accumulation rate as well as accumulation rates determined from stratigraphic analysis of snow pits and cores in the vicinity.
C33C-0361 1340h
Kilometer-scale texture of the West Antarctic Ice Sheet
Although it is remarkably flat in comparison to other geologic surfaces, the ice-surface of West Antarctica has a surface texture at linear scales of a few kilometers and larger. This texture has been accurately measured with NASA's GLAS instrument; I present a spectral analysis that characterizes how the kilometer-scale texture of the surface varies across the ice sheet. I distinguish surface spectra that reflect fast-flowing ice from spectra that reflect creeping ice that is frozen to its bed, and demonstrate how the spectral characteristics can be explained by mathematical modeling of ice dynamics in combination with an understanding of the statistics of land surfaces. This analysis presents a powerful tool for making educated guesses about the flow regime in parts of Antarctica where it is not as well understood as it is in West Antarctica.