Priestley Glacier Research Articles

Active and Passive Microwave Remote Sensing of Priestley Glacier, Antarctica

Active and Passive Microwave Remote Sensing of Priestley Glacier, Antarctica

Observing glacier dynamics with low-cost, multi-GNSS positioning in Victoria Land, Antarctica

Abstract This study examines the performance of low-cost, low-power GNSS positioning systems for glacier monitoring in high-latitude environments. We compare the positioning performance of co-located low-cost u-blox ZED-F9P GNSS units (a few hundred USDs) and survey-grade Trimble R10 units (> $10,000 USD) under stationary (on land) and dynamic (on glacier) conditions near Terra Nova Bay, Antarctica. Low-cost and survey-grade systems yield almost identical error magnitudes under short (3 m), medium (34 km) and long (390 km) baseline kinematic-positioning scenarios. We further examined the efficacy of low-cost GNSS for glaciological applications by installing four u-blox and two Trimble receivers on Priestley Glacier to observe tide-modulated ice flexure. All receivers successfully detected subtle tidal oscillations with amplitudes < 3 cm, consistent with the predicted phasing from a tide model. These experiments offer a strong rationale for the widespread use of low-cost receivers to expand and densify GNSS monitoring networks, both in Antarctica and in glaciated regions worldwide.

Elastic wave propagation in anisotropic polycrystals: inferring physical properties of glacier ice

An optimization problem is proposed for inferring physical properties of polycrystals given ultrasonic (elastic) wave velocity measurements, made across multiple sample orientations. The feasibility of the method is demonstrated by inferring both the effective grain elastic parameters and the grain c -axis orientation distribution function (ODF) of ice-core samples from Priestley glacier, Antarctica. The method relies on expanding the ODF in terms of a spherical harmonic series, which allows for a non-parametric estimation of the sample ODF. Moreover, any linear combination of the Voigt (strain) and Reuss (stress) homogenization scheme is allowed, although for glacier ice, the exact choice is found to matter little for bulk elastic behaviour, and thus for inferred physical properties, too. Finally, the accuracy of the inferred grain elastic parameters is discussed, including the well-posedness and shortcomings of the inverse problem, relevant for future adoptions in glaciology, geology and elsewhere.

Ultrasonic and seismic constraints on crystallographic preferred orientations of the Priestley Glacier shear margin, Antarctica

Abstract. Crystallographic preferred orientations (CPOs) are particularly important in controlling the mechanical properties of glacial shear margins. Logistical and safety considerations often make direct sampling of shear margins difficult, and geophysical measurements are commonly used to constrain the CPOs. We present here the first direct comparison of seismic and ultrasonic data with measured CPOs in a polar shear margin. The measured CPO from ice samples from a 58 m deep borehole in the left lateral shear margin of the Priestley Glacier, Antarctica, is dominated by horizontal c axes aligned sub-perpendicularly to flow. A vertical-seismic-profile experiment with hammer shots up to 50 m away from the borehole, in four different azimuthal directions, shows velocity anisotropy of both P waves and S waves. Matching P-wave data to the anisotropy corresponding to CPO models defined by horizontally aligned c axes gives two possible solutions for the c-axis azimuth, one of which matches the c-axis measurements. If both P-wave and S-wave data are used, there is one best fit for the azimuth and intensity of c-axis alignment that matches the measurements well. Azimuthal P-wave and S-wave ultrasonic data recorded in the laboratory on the ice core show clear anisotropy of P-wave and S-wave velocities in the horizontal plane that match that predicted from the CPO of the samples. With quality data, azimuthal increments of 30∘ or less will constrain well the orientation and intensity of c-axis alignment. Our experiments provide a good framework for planning seismic surveys aimed at constraining the anisotropy of shear margins.

Tidal Modulation of a Lateral Shear Margin: Priestley Glacier, Antarctica

We use high resolution, ground-based observations of ice displacement to investigate ice deformation across the floating left-lateral shear margin of Priestley Glacier, Terra Nova Bay, Antarctica. Bare ice conditions allow us to fix survey marks directly to the glacier surface. A combination of continuous positioning of a local reference mark, and repeat positioning of a network of 33 stakes installed across a 2 km width of the shear margin are used to quantify shear strain rates and the ice response to tidal forcing over an 18-day period. Along-flow velocity observed at a continuous Global Navigation Satellite Systems (GNSS) station within the network varies by up to ∼30% of the mean speed (±28 m a−1) over diurnal tidal cycles, with faster flow during the falling tide and slower flow during the rising tide. Long-term deformation in the margin approximates simple shear with a small component of flow-parallel shortening. At shorter timescales, precise optical techniques allow high-resolution observations of across-flow bending in response to the ocean tide, including across-flow strains on the order of 10–5. An elastodynamic model informed by the field observations is used to simulate the across-flow motion and deformation. Flexure is concentrated in the shear margin, such that a non-homogeneous elastic modulus is implied to best account for the combined observations. The combined pattern of ice displacement and ice strain also depends on the extent of coupling between the ice and valley sidewall. These conclusions suggest that investigations of elastic properties made using vertical ice motion, but neglecting horizontal displacement and surface strain, will lead to incorrect conclusions about the elastic properties of ice and potentially over-simplified assumptions about the sidewall boundary condition.

Microstructure and Crystallographic Preferred Orientations of an Azimuthally Oriented Ice Core from a Lateral Shear Margin: Priestley Glacier, Antarctica

A 58 m long azimuthally oriented ice core has been collected from the floating lateral sinistral shear margin of the lower Priestley Glacier, Terra Nova Bay, Antarctica. The crystallographic preferred orientations (CPO) and microstructures are described in order to correlate the geometry of anisotropy with constrained large-scale kinematics. Cryogenic Electron Backscatter Diffraction analysis shows a very strong fabric (c-axis primary eigenvalue ∼0.9) with c-axes aligned horizontally sub-perpendicular to flow, rotating nearly 40° clockwise (looking down) to the pole to shear throughout the core. The c-axis maximum is sub-perpendicular to vertical layers, with the pole to layering always clockwise of the c-axes. Priestley ice microstructures are defined by largely sub-polygonal grains and constant mean grain sizes with depth. Grain long axis shape preferred orientations (SPO) are almost always 1–20° clockwise of the c-axis maximum. A minor proportion of “oddly” oriented grains that are distinct from the main c-axis maximum, are present in some samples. These have horizontal c-axes rotated clockwise from the primary c-axis maximum and may define a weaker secondary maximum up to 30° clockwise of the primary maximum. Intragranular misorientations are measured along the core, and although the statistics are weak, this could suggest recrystallization by subgrain rotation to occur. These microstructures suggest subgrain rotation (SGR) and recrystallization by grain boundary migration recrystallization (GBM) are active in the Priestley Glacier shear margin. Vorticity analysis based on intragranular distortion indicates a vertical axis of rotation in the shear margin. The variability in c-axis maximum orientation with depth indicates the structural heterogeneity of the Priestley Glacier shear margin occurs at the meter to tens of meters scale. We suggest that CPO rotations could relate to rigid rotation of blocks of ice within the glacial shear margin. Rotation either post-dates CPO and SPO development or is occurring faster than CPO evolution can respond to a change in kinematics.

Grounding‐Zone Flow Variability of Priestley Glacier, Antarctica, in a Diurnal Tidal Regime

AbstractTidal modulation of ice streams and their adjacent ice shelves is a real‐world experiment to understand ice‐dynamic processes. We observe the dynamics of Priestley Glacier, Antarctica, using Terrestrial Radar Interferometry (TRI) and GNSS. Ocean tides are predominantly diurnal but horizontal GNSS displacements also oscillate semi‐diurnally. The oscillations are strongest in the ice shelf and tidal signatures decay near‐linearly in the TRI data over 10 km upstream of the grounding line. Tidal flexing is observed 6 km upstream of the grounding line including cm‐scale uplift. Tidal grounding line migration is small and 40% of the ice thickness. The frequency doubling of horizontal displacements relative to the ocean tides is consistent with variable ice‐shelf buttressing demonstrated with a visco‐elastic Maxwell model. Taken together, this supports previously hypothesized flexural ice softening in the grounding‐zone through tides and offers new observational constraints for the role of ice rheology in ice‐shelf buttressing.

Numerical modelling of geothermal heat flux and ice velocity influencing the thermal conditions of the Priestley Glacier trough (northern Victoria Land, Antarctica)

Deep geothermal contributions can affect the thermal equilibrium of glacial systems, which can induce changes in their thermal state. These changes can be influenced by glacier dynamics and by the different ice flow velocities experienced over time, which dictate the static and dynamic interactions with glacial trough bedrock. To assess such interactions and to weight the effects of ice mass thermalisations on bedrock, 2D multistage numerical modelling was performed for Priestley Glacier (an outlet glacier) in the East Antarctic glacial system (northern Victoria Land), which has experienced extreme heat flux variations related to rifting and late Cenozoic volcanism. The thermal evolution of Priestley Glacier over the last ~500 ka was modelled considering the mutual contributions of local and deep thermal sources through a parametric variation in the geothermal heat flux. The ice mass velocity was indirectly taken into account by assuming an ice persistence time over the bedrock under timelled considering the mutual contributions of local and deep thermal sources through a parametric variation in the geothermal heat flux. The ice mass velocity was indirectly taken into account by assuming an ice persistence time over the bedrock under time-dependent modelling conditions. Ice thickness and geothermal heat fluxes, varying from 50 to 120 mW/m2, were conditions are strongly linked in determining the thermal state of the glacier base and consequently its impact on basal erosion. According to the adopted ice stationing levels, heat flux conditions not exceeding 70 mW/m2 are needed for preserving a dry-based glacier only assuming velocity conditions equal to or lower than those observed to date along Priestley Glacier. Assuming increased velocity conditions, the glacier sensitivity to heat fluxes decreases, and high to very high heat fluxes are not sufficient to cause glacier base transitions to wet states.

Identification of Phyllosilicates in the Antarctic Environment Using ASTER Satellite Data: Case Study from the Mesa Range, Campbell and Priestley Glaciers, Northern Victoria Land

In Antarctica, spectral mapping of altered minerals is very challenging due to the remoteness and inaccessibility of poorly exposed outcrops. This investigation evaluates the capability of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite remote sensing imagery for mapping and discrimination of phyllosilicate mineral groups in the Antarctic environment of northern Victoria Land. The Mixture-Tuned Matched-Filtering (MTMF) and Constrained Energy Minimization (CEM) algorithms were used to detect the sub-pixel abundance of Al-rich, Fe3+-rich, Fe2+-rich and Mg-rich phyllosilicates using the visible and near-infrared (VNIR), short-wave infrared (SWIR) and thermal-infrared (TIR) bands of ASTER. Results indicate that Al-rich phyllosilicates are strongly detected in the exposed outcrops of the Granite Harbour granitoids, Wilson Metamorphic Complex and the Beacon Supergroup. The presence of the smectite mineral group derived from the Jurassic basaltic rocks (Ferrar Dolerite and Kirkpatrick Basalts) by weathering and decomposition processes implicates Fe3+-rich and Fe2+-rich phyllosilicates. Biotite (Fe2+-rich phyllosilicate) is detected associated with the Granite Harbour granitoids, Wilson Metamorphic Complex and Melbourne Volcanics. Mg-rich phyllosilicates are mostly mapped in the scree, glacial drift, moraine and crevasse fields derived from weathering and decomposition of the Kirkpatrick Basalt and Ferrar Dolerite. Chlorite (Mg-rich phyllosilicate) was generally mapped in the exposures of Granite Harbour granodiorite and granite and partially identified in the Ferrar Dolerite, the Kirkpatrick Basalt, the Priestley Formation and Priestley Schist and the scree, glacial drift and moraine. Statistical results indicate that Al-rich phyllosilicates class pixels are strongly discriminated, while the pixels attributed to Fe3+-rich class, Fe2+-rich and Mg-rich phyllosilicates classes contain some spectral mixing due to their subtle spectral differences in the VNIR+SWIR bands of ASTER. Results derived from TIR bands of ASTER show that a high level of confusion is associated with mafic phyllosilicates pixels (Fe3+-rich, Fe2+-rich and Mg-rich classes), whereas felsic phyllosilicates (Al-rich class) pixels are well mapped. Ground truth with detailed geological data, petrographic study and X-ray diffraction (XRD) analysis verified the remote sensing results. Consequently, ASTER image-map of phyllosilicate minerals is generated for the Mesa Range, Campbell and Priestley Glaciers, northern Victoria Land of Antarctica.

Post-LGM dynamic deglaciation along the Victoria Land coast, Antarctica

The post-Last Glacial Maximum (LGM) deglaciation of Antarctica holds important clues for understanding past environmental changes and predicting future changes in the Antarctic Ice Sheet. Cosmogenic nuclide exposure dating of glacial erratics documents the spatial and temporal glacier changes during the most recent deglaciation. We collected 55 erratic cobbles from the eight glaciated benches on Inexpressible Island, which is at the terminal area of Priestley Glacier, Terra Nova Bay, Victoria Land, to elucidate its post-LGM deglaciation pattern. Analyses of the 10Be ages and 26Al/10Be ratios suggest that Priestley Glacier underwent ∼254 m of lowering during the mid-Holocene, between 8.9 and 5.9 ka. This lowering rate (∼0.09 m a−1) is faster than those observed along other Victoria Land outlet glaciers to the north (Tucker and Aviator), but slower than the one to the south (Mackay). The post-LGM lowering of these outlet glaciers was triggered by marine ice sheet instability, with their asynchronous onsets of deglaciation derived from their diachronous response times to the southwestward migration of the grounding-line retreat until their synchronous termination of deglaciation at ∼6 ka. A post-LGM deglaciation model with the southwestward migration of the western Ross Ice Shelf grounding line provides the best match to the terrestrial exposure dating results of the thinning patterns of the outlet glaciers along the Victoria Land coast.

Burial and exhumation of the Eisenhower Range, Transantarctic Mountains, based on thermochronological, sedimentary rock maturity and petrographic constraints

The Eisenhower Range is a N–S trending mountain range in the Transantarctic Mountains (TAM) adjacent to the NW Ross Sea Embayment. New AFT and apatite (U–Th–Sm)/He (AHe) data from vertical basement profiles supplemented by paleotemperature and pressure estimates derived from Beacon sandstones provide new quantitative results on regional burial evolution and first regional constraints on basin inversion and exhumation processes. AFT ages between 32±2 and 259±18Ma and AHe ages of 37±3–173±16Ma correlate positively with sample elevations. Thermal history modeling of these data and complementary thermal indications detect heating of the paleosurface on the Eisenhower Range to temperatures ≥80°C subsequent to Ferrar magmatism, and constrain Late Eocene rapid cooling. Regression of modeled paleotemperatures against sample elevations refers to a high Jurassic (~45°C/km) and a moderate Cretaceous–Eocene (28±8°C/km) geothermal gradient. The texture of Beacon sandstones supports strong mechanical compaction that requires a higher overburden than preserved in the stratigraphic record. Modeled paleotemperatures and pressures suggest basement burial that increases from Late Jurassic (0.7–1.1km) to Eocene (1.8–2.1km). The overburden comprises 0.7–1.1km cumulative Beacon/Ferrar rocks and 0.7–1.4km of post-Ferrar sediments. Rapid cooling of the whole sample suite between ~35 and 30Ma implies fast erosion of the post-Ferrar sediments and (re-) exposure of underlying magmatic rocks. Subsequent differential sample cooling to present-day surface temperature infers ongoing exhumation by glacial incision enhanced by isostatic response to basin inversion. Decreasing amounts of exhumation from the coast (>3km) toward the interior (1.5–2.2km) point to backstepping incision along the fault controlled Priestley Glacier. Substantial exhumation of the Eisenhower Range since the Late Eocene is hence triggered by both tectonic and climatic factors, superimposed by considerable lithological influence during the initial exhumation stage.

Magnetic study of an Antarctic weathering profile on basalt: Implications for recent weathering on Mars

In order to better interpret the Martian surface weathering-related mineralogy, we focused on a relevant analogue of the Martian subsurface in terms of lithology and paleoclimate: the Jurassic-aged Ferrar dolerite (Priestley Glacier, Transantarctic Mountains), weathered in cold and dry climate. Together with chemical and mineralogical studies, rock magnetic properties were investigated and completed with Mössbauer measurements. Weathering of the decimetric block is evidenced by chemical profiles showing an increase in Fe content (from 10.5 in the core to 13 wt.% in the surface) and a decrease in Si (from 57 to 53 wt.%, respectively). According to mineralogical, thermomagnetic and hysteresis properties, the main opaque mineral is inherited titanomaghemite, with a concentration about 1%. Enhancement in low field magnetic susceptibility (from 4 · 10 − 6 to 10 · 10 − 6 m 3/kg, respectively) and saturation magnetization (from 0.44 to 0.96 Am 2/kg, respectively) indicates the neoformation of metastable maghemite. Neoformed red ferric (oxy)hydroxides are abundant in the surface. High field susceptibility normalized to iron concentration and Mössbauer spectra indicate a replacement of Fe 2+ from primary silicates (pyroxene) by poorly crystalline antiferromagnetic Fe 3+ (oxy)hydroxides. Thus, highly magnetic titanomaghemite and maghemite coexist with nanosized ferric (oxy)hydroxides and primary silicates, in accordance with Martian in situ observations. Therefore, this study supports the formation of the Martian regolith as resulting from a slow weathering process in near present day conditions.

A new fully three-dimensional numerical model for ice dynamics

AbstractThe problem of describing ice dynamics has been faced by many researchers; in this paper a fully three-dimensional model for ice dynamics is presented and tested. Using an approach followed by other researchers, ice is considered a non-linear incompressible viscous fluid so that a fluid-dynamic approach can be used. The model is based on the full three-dimensional Stokes equations for the description of pressure and velocity fields, on the Saint-Venant equation for the description of the free-surface time evolution and on a constitutive law derived from Glen’s law for the description of ice viscosity. The model computes the complete pressure field by considering both the hydrostatic and hydrodynamic pressure components; it is time-evolutive and uses high-order numerical approximation for equations and boundary conditions. Moreover it can deal with both constant and variable viscosity. Three theoretical tests and two applications to Priestley Glacier, Antarctica, are presented in order to evaluate the performance of the model and to investigate important phenomena of ice dynamics such as the influence of viscosity on pressure and velocity fields, basal sliding and flow over perturbed bedrocks. All these applications demonstrate the importance of treating the complete pressure and stress fields.

Sea‐ice conditions in the Ross Sea during spring 1996 as observed on SAR and AVHRR imagery

The structure and properties of the sea‐ice field in the western Ross Sea during the spring of 1996 are evaluated using data from the ERS‐2 Synthetic Aperture Radar (SAR) and the Advanced Very High Resolution Radiometer (AVHRR) sensors. A multispectral classification method enabled separation of six ice types including fast ice, new ice, smooth first‐year ice, rough first‐year ice, thin new ice/wind roughened open water and glacial ice. In Terra Nova Bay it was observed that the size of the recurring polynya depended strongly on the strength of the consistent offshore winds arising from the strong katabatic wind flow down the Priestley and Reeves Glaciers. It appears that the presence and size of this feature is more dependent on atmospheric than on oceanographic forcing. The Terra Nova Bay Polynya is frequently not ice free, but covered, at least in part, by thin, new sea ice. During katabatic events, rows of new ice, formed in response to Langmuir circulations, are observed oriented perpendicular to the coastline. These structures are swept downstream where, under less turbulent conditions, they consolidate to form pancake ice and thicker icefloes. Elsewhere over the continental shelf sea‐ice conditions changed little in over a month, indicating that much of the ice in this area is landfast.

Mechanisms of Antarctic katabatic currents near Terra Nova Bay

The evolution of Antarctic katabatic wind in the vicinity of Terra Nova Bay has been studied using a three-dimensional, hydrostatic, limited area model. Realistic simulations of a summer katabatic event, using different horizontal resolutions, have been performed by employing a sequential self-nesting procedure. The high-resolution orography data allow a description of the structure of the main valleys and of the two different flows descending from the broad Reeves Glacier and (with less accuracy) channelled into the much narrower Priestley Glacier valley, respectively. The mechanism of katabatic wind development has been investigated by means of a Lagrangian diagnostic study performed along trajectories flowing down from the above-mentioned glaciers. Heat balance and kinetic energy balance have been evaluated accurately in order to asses the characteristics of the two katabatic currents that may superpose once they reach the Nansen Ice Sheet.

Diurnal Variations of the Temperature and Their Influenceon Wind Regime in a Confluence Zone of Antarctica

The behaviour of the wind field at the confluence zone of Terra Nova Bay during a transition period from winter to summer is considered. To explain some observed features the influence of diurnal variations are considered to integrate and complete the analysis based on annual and seasonal variations. Diurnal variations in the buoyancy difference between two main air masses coming from the Revees and Priestley glaciers are assumed to contribute to diurnal alteration of the wind direction at the Nansen ice sheet. The data recorded at three automatic weather stations (AWS) available that period are analysed. Using the wavelet analysis procedure, it is shown that the contribution of one-day variations of some basic meteorological parameters and their gradients (in particular of temperature and pressure) is important and comparable with synoptic ones. On the base of the diurnal variations some aspects of the local circulation have been explained. The basis for the further studies related to the evolution of katabatic winds in this area are also presented.

Palaeomagnetic and geochronological results from the Cambro-Ordovician Granite Harbour Intrusives inland of Terra Nova Bay (Victoria Land, Antarctica)

Palaeomagnetic investigations and Rb–Sr dating were carried out on samples from two plutons from the Granite Harbour Intrusives of the Transantarctic Mountains inland of Terra Nova Bay. The Rb–Sr whole rock–biotite ages from Teall Nunatak (475±4, 483±4 Ma), a quartz-diorite pluton cropping out to the south of Priestley Glacier, are older than that from the Mount Keinath monzogranite (450±4 Ma), which is located to the north of the glacier. These results are consistent with the literature data, which suggest that during the last phases of the Ross Orogeny the cooling rate of the basement was significantly lower to the north than to the south of Priestley Glacier. The Teall Nunatak quartz-diorite is characterized by a stable magnetization, whose blocking-temperature spectrum ranges from 530 to 570 °C. At one site, the stable magnetization is screened by a large secondary component of opposite polarity, removed by thermal demagnetization below 300 °C. The characteristic directions after thermal demagnetization yielded a southern pole located at lat. 11°S, long. 21°E. The magnetization of Mount Keinath monzogranite consists of several components with overlapping stability spectra. A characteristic direction was isolated at one site only, obtained by demagnetizing the specimens in the temperature range from 380 to 460 °C. Comparison with the other East Antarctica poles shows that those from Victoria Land are very well grouped and give a reliable early Ordovician palaeopole (lat. 5°S, long. 23°E, with K=196 and A95=3.7°), whereas the poles from Wilkes, Enderby and Dronning Maud Land are dispersed. We tentatively advance the hypothesis that the dispersion reflects different magnetization ages due to the slow cooling of these regions during the last stages of the Ross Orogeny.

Comparison between glacier ice velocities inferred from GPS and sequential satellite images

Measurements derived from remote-sensing research and Held surveys have provided new ice-velocity data for David Glacier-Drygalski Ice Tongue and Priestley and Reeves Glaciers, Antarctica. Average surface velocities were determined by tracking crevasses and other patterns moving with the ice in two sequential satellite images. Velocity measurements were made for different time intervals (1973-90, 1990-92, etc.) using images from various satellite sensors (Landsat 1 MSS, Landsat TM, SPOT XS). in a study of the dynamics of David Glacier-Drygalski IceTongue and Priestley and Reeves Glaciers, global positioning system (GPS) measurements were made between 1989 and 1994. A number of points were measured on each glacier: five points on David Glacier, three on Drygalski Ice Tongue,two on Reeves Glacier—Nansen Ice Sheet and two on Priestley Glacier. (lomparison of the results from GPS data and feature-tracking in areas close to image tie-points shows that errors in measured average velocity from the feature-tracking may be as little as +15-20 m a−1. in areas far from tie-points, such as the outer part of Drygalski Ice Tongue, comparison of the two types of measurements shows differences of about ± 70 m a−1.

A statistical analysis of wind in Terra Nova Bay, Antarctica, for the austral summers 1988 and 1989

A statistical analysis of wind frequencies has been done analyzing Sodar and Automatic Weather Stations (AWS) data recorded during two successive summer campaigns in Terra Nova Bay area, Antarctica. A preliminary analysis performed using AWS data for the same periods showed that no particular differences occurred between the two summers. This encouraged a comparison between sodar data at the two sites and the AWS as they would have been recorded at the same time. The local circulation is largely dominated by the katabatic winds originating by radiative cooling of near-surface air along the slopes of Reeves (270°) and Priestley (330°) glaciers. The surface wind frequencies showed a strong dependence on the station location confirming the preminent role of the orography on the air flow. Sodar data evidenced that a considerable variation of wind intensity and direction may occur in the upper levels and highlighted some aspects of the three-dimensional air motions. The contemporary analysis of the sodar profiles and AWS surface measurements allowed us to bring out some peculiarities of the local summer circulation related to the variation of the relative buoyancy of the two flows blowing from the Priestley and Reeves glaciers. During the summer, contrary to the winter, the airstream issuing from the Priestley does not cross the Northern Foothills and flows into the Nansen Ice Sheet at the confluence of Reeves and Priestley glaciers. Consequently, at the coastal site stations, the observed air flow is attributable to the Reeves glacier, which is more buoyant and capable, because of the greater air mass involved and of the sufficient speed to cross the Northern Foothills. Moreover, it has been shown how the Northern Foothills prevent barrier winds, running parallel to Transantarctic Mountains from reaching the coastal site where the Italian base is located. On the contrary a peak for barrier wind was observed when sodar was on Nansen Ice Sheet.

Complexity of the wind field as observed by a sodar system and by automatic weather stations on the Nansen Ice Sheet, Antarctica, during summer 1988-1989: two case studies

In the Antarctic boundary layer, some phenomena appear to be particularly enhanced with respect to those at midlatitudes. Thermal stratifications are present almost all year and can substain drainage flows of different types. Several episodes of flows with different potential temperature and consequently with different buoyancy were observed on the Nansen Ice Sheet by a sodar. Two case studies are presented in which a comparison with the data of the automatic weather stations located on the Reeves and the Priestley glaciers as well on the Nansen Ice Sheet have been used. As a main result, we saw that during the summer time, the flow from the Reeves glacier may superimpose the one from the Priestley, in contrast with the current opinion based on the automatic weather station measurements averaged over the entire year. Also, an episode of barrier-katabatic flow interaction is shown in which the first is negatively buoyant in comparison with the second. Since the wind profile frequently showed a great variability with height, the importance of using remote sensing sensors in studies that realistically want to model the dynamics of the Antarctic boundary layer has been emphasized. DOI: 10.1034/j.1600-0889.1992.00017.x