snow megadune fields on the east antarctic plateau extreme atmosphere ice interaction large dune fields occupy more than 500 000 km 2 of the east antarctic plateau the megadunes or long wavelength surface ripples have amplitudes of only a few meters wavelengths of a few kilometers and parallel crests which can extend one hundred kilometers they occur in areas characterized by low accumulation extensively recrystallized snow and strong scattering of the microwave part of the spectrum dune crests are oriented perpendicular to the regional katabatic wind direction snow megadunes are unlikely to be formed by simple wind transport of snow particles

a snow transport model for complex terrain as part of the winter environment in middle and high latitude regions the interactions between wind vegetation topography and snowfall produce snow covers of non uniform depth and snow water equivalent distribution a physically based numerical snow transport model snowtran 3d is developed and used to simulate this three dimensional snow depth evolution over topographically variable terrain the mass transport model includes processes related to vegetation snow holding capacity topographic modification of wind speeds snow cover shear strength wind induced surface shear stress snow transport resulting from saltation and suspension snow accumulation and erosion and sublimation of the blowing and drifting snow the model simulates the cold season evolution of snow depth distribution when forced with inputs of vegetation type and topography and atmospheric foreings of air temperature humidity wind speed and direction and precipitation model outputs include the spatial and temporal evolution of snow depth resulting from variations in precipitation saltation and suspension transport and sublimation using 4 years of snow depth distribution observations from the foothills north of the brooks range in arctic alaska the model is found to simulate closely the observed snow depth distribution patterns and the interannual variability

surface drag over the snow surface of the antarctic plateau 1 factors controlling surface drag over the katabatic wind region the drag coefficient of the snow surface over the antarctic plateau is evaluated through direct measurement of reynolds stress on the mizuho plateau east antarctica in the austral summer the estimated roughness height z0 varies from 10 l to 10 4 cm even under near neutral conditions large shear stress appears in light wind followed by increased turbulent intensity in the katabatic wind region of the plateau z0 shows symmetric changes with wind direction the average value of z0 in the smoothest direction is 0 0004 cm which is the minimum value previously reported and it increases to 0 015 cm for 40 rotation of wind direction toward the roughest direction the directional dependence of z0 is similar at three stations located nearly 100 km apart unlike the results of jackson and carroll at the south pole the smoothest direction deviates 20 from the mean sastrugi axes and agrees with the direction of the prevailing high wind the 4 m neutral drag coefficient is estimated to be 0 8 10 3 for the smoothest direction and 1 5 10 3 for the roughest direction the effect of snow drift is unimportant a generalized discussion of the results is given in a companion paper

deep air convection in the firn at a zero accumulation site central antarctica abstract ice cores provide unique archives of past atmospheres and climate but interpretation of trapped gas records and their climatic significance has been hampered by a poor knowledge of the prevalence of air convection in the firn layer on top of polar ice sheets in particular the phasing of greenhouse gases and climate from ice cores has been obscured by a discrepancy between empirical and model based estimates of the age difference between trapped gases and enclosing ice which may be due to air convection here we show that deep air convection 23 m occurs at a windy near zero accumulation rate site in central antarctica known informally as the megadunes site 80 77914 s 124 48796 e deep convection is evident in depth profiles of air withdrawn from the firn layer in the observed pattern of the nitrogen isotope ratio 15 n 14 n the argon isotope ratio 40 ar 36 ar and in the mixing ratios of the anthropogenic halocarbons methyl chloroform ch 3 ccl 3 and hfc 134a ch 2 fcf 3 transport parameters diffusivities were inferred and air was dated using measured carbon dioxide co 2 and methane ch 4 mixing ratios by comparing with the law dome atmospheric record which shows that these are the oldest firn air samples ever recovered co 2 mean age 1863 ad the low accumulation rate and the consequent intense metamorphism of the firn due to prolonged exposure to seasonal temperature cycling likely contribute to deep air convection via large grain size and vertical cracks that act as conduits for vigorous air motion the megadunes site provides a possible modern analog for the glacial conditions in the vostok dome fuji and dome c ice core records and a possible explanation for lower than expected 15 n 14 n ratios in trapped air bubbles at these times a general conclusion is that very low accumulation rate causes deep air convection via its effect on firn structural characteristics

modis based mosaic of antarctica moa data sets continent wide surface morphology and snow grain size abstract we present digital image mosaics of surface morphology and optical snow grain size for the antarctic continent and surrounding islands assembled from 260 moderate resolution imaging spectroradiometer modis images the products are derived from modis band 1 red light 647 nm and band 2 infrared 857 nm orbit swath data acquired during the 2003 2004 austral summer multiple images of all areas are combined in a data cumulation scheme to improve spatial resolution and increase radiometric content of the mosaics the component images were de striped geo registered and re sampled to the projection grid using the modis swath to grid toolbox ms2gt software a 125 m ground equivalent polar stereographic projection was used identical to previous radar image mosaics and similar to several other continent wide data sets geo location accuracy of the final mosaics is better than 125 m swath acquisition times were limited to the range 05 00 13 30 ut to provide broad but uniform mosaic illumination regions of cloud cover blowing snow and intense forward scattering were masked prior to compositing the moa snow grain size data set image provides the first continent wide semi quantitative map of mean summer snow firn and blue ice grain size optical grain size radius was determined using a normalized difference index derived from modis band 1 and band 2 rad values atmospheric correction adjustments for directional reflectance variation and snow grain size estimation were accomplished by a look up table generated by a combination of published models of snow spectral reflectance and atmosphere radiative transfer validation of the snow grain size image is provided by in situ spectra of snow covered sea ice made in october 2003 we use the new data sets to determine major ice shelf areas and shelf edge coastline and ice grounding line vector files and determine areas of selected blue ice regions these data sets are available for download at http nsidc org data moa

the role of wind and splash erosion in inland drift sand areas in the netherlands abstract european drift sand areas reached their maximum size in the 19th century from the late 19th until the mid 20th century most drift sand areas in europe were successfully reforested today drift sand formation can still be studied in the netherlands however due to change in land use their limited size eutrophication and colonisation by invasive exotics the area occupied by active drift sand is rapidly declining and with it its unique fauna and vegetation between 2001 and 2005 experiments and observations were carried out at the inland drift sand area kootwijkerzand central netherlands to know more about the extent the intensity and the role of wind and splash erosion in a drift sand ecosystem measurements were carried out on the horizontal sediment flux the vertical accumulation flux the raising and lowering of the surface the organic matter content of the topsoil the saltation activity and the weather conditions an attempt was also made to evaluate the effect of various measures to re activate erosion without sufficient erosion the scattered drift sand vegetation will rapidly change into closed grass dominated vegetation the role of erosion in the drift sand ecosystem is therefore crucial erosion slows down the succession rate because it causes the regular deposition of nutrient poor sediment and facilitates the regular renewal of the vegetation by creating new locations with extremely poor conditions for colonisation blow outs on bare deflation plains wind erosion is the dominant process here the transport generated by splash drift can be of the same magnitude as during a moderate wind erosion event but splash drift events are less frequent and also shorter than wind erosion events they normally represent less than 10 of the total sediment transport in the scattered drift sand vegetation areas on the other hand splash erosion and splash drift dominate here the organic matter content of the transported and later deposited sediment is equal to the organic matter content of the parent soil showing that splash drift has been the main transport mechanism biological crusts developed in these areas may later reduce the influence of splash high transport rates are needed to reactivate wind erosion in vegetated areas under the present conditions the erosion activity in most drift sand areas is small heavy wind erosion is only observed during extreme storms

observations theory and modeling of the differential accumulation of antarctic megadunes antarctic megadunes are characterized by significant spatial differences in accumulation rate with higher accumulation on the windward side and near zero accumulation on the lee side this leads to spatial differences in physical properties of snow and surface roughness as well as to the upwind migration of the megadunes while previous studies agree that megadunes are a result of complex interactions between wind topography and snow it is not clear how they form or why they accumulate on the windward side here we use icesat observations dimensional analysis and atmospheric flow modeling to investigate what conditions are responsible for the accumulation patterns and upwind migration of the megadunes first we use icesat data to quantify the pattern of differential surface elevation change across the megadunes we then use dimensional analysis based on supercritical flow theory and atmospheric flow modeling to show that the megadunes topography and a stable atmosphere will always lead to upwind dune migration we show that a combination of persistent katabatic winds strong stability and spatial variability in surface roughness is responsible for the accumulation on the upwind slope and hence the upwind migration of the megadunes we further show that spatial differences in surface roughness are the primary control on accumulation magnitudes and hence dune migration velocity the dune migration velocity in turn influences the degree of snow metamorphism and the physical properties of snow that are relevant for paleoclimate records our findings pertain to the ongoing evolution of the megadunes but their genesis remains an open question

revisiting the linear theory of sand dune formation a linear stability theory for dune and antidune formation is presented that implements a rotational two dimensional flow model as in previous linear theories the phase lag between sediment transport and bed elevation remains the main mechanism driving instability however it is shown that this phase lag varies significantly in a neighbourhood of the bed moreover since the layer in which sediments are moving has a finite though small thickness it is assumed that the perturbations of the fluid stress driving bedload transport should be evaluated at the top of the layer itself it is shown that such an apparently minor modification of the classical approach alters remarkably the balance between stabilizing and destabilizing effects that drives the instability process this also allows a clarification of the debated role of bedslope on the formation of dunes and antidunes following the above ideas antidunes are shown to form in the absence of suspended sediment load and without any effect associated with sediment inertia the present analysis ultimately allows a successful unification of the theories of dune and antidune formation

atmospheric flow over a succession of nearly two dimensional ridges and valleys observations of neutral static stability flow in a nearly two dimensional valley are presented upstream and downstream of the valley are a succession of similar ridges and valleys the valley sides have a slope of about 32 to the horizontal and when the wind is nearly perpendicular to the valley there is a marked flow separation the observations of mean flow and turbulence are compared with numerical simulations of flow over periodic two dimensional ridges and valleys

antidunes on steep slopes 1 when rates of subcritical flow are increased on gentle slopes the bed successively produces ripples dunes and flat beds antidunes defined here as all bed undulations for which the surface gravity waves are in phase with the bed profile appear only in high flow rates and may be found in some extreme natural flow events inversely on steep slopes flume experiment s 1 approximately flows are supercritical and antidunes are observed to appear just after the beginning of sediment motion and to disappear for high flow rates using data from new experiments we seek to improve the prediction of antidune geometry on steep slopes an equation for antidune wavelength was deduced from dimensional analysis and fitted to new experimental data the equation was successfully evaluated using a data set extended to 167 values with data from the literature obtained on both steep and gentle slopes this equation gives results very similar to the usual analytical equation from kennedy when tested on the results from gentle slope experiments but proved to be better adapted for antidune wavelength on steep slopes

late holocene land surface change in a coupled social ecological system southern iceland a cross scale tephrochronology approach this work is supported by a uk natural environment research council nerc phd studentship ne f00799x 1

faceted crystal formation in the northeast greenland low accumulation region snow pit analysis in the low accumulation region in northeast greenland indicated a mean annual accumulation of 12 1 6 cm it also revealed numerous faceted layers that were not related to the summer horizon of previous years in general the faceted crystals were found under a wind crust these crusts appear to originate during the winter snow and air temperature data from automatic weather stations suggest that the katabatic storms are responsible for the formation of the wind crusts and the faceted layers frequent abrupt temperature changes up to 30 c were observed within two days due to turbulent mixing of cold air inversions the near surface air temperature variations are correlated with wind speed the latter leading the former by 6 9 hours for increasing wind and 1 4 hours for decreasing wind because of low accumulation katabatic storms affect the same snow surface for several months causing repeated air temperature fluctuations this leads to strong vapor pressure gradients and faceted crystal growth in the top snow layer on average we found two faceted layers in addition to the summer layer per year for a 4 year record in addition to its climatological significance in the snow record the formation of these layers is especially important to understand for the interpretation of microwave satellite observations of the firn

overview of the spaceborne imaging radar c x band synthetic aperture radar sir c x sar missions abstract the spaceborne imaging radar c x band synthetic aperture radar sir c x sar the most advanced imaging radar system to have flown in earth orbit was carried in the cargo bay of the space shuttle endeavour in april and october 1994 sir c x sar simultaneously recorded data at three wavelengths l c and x bands 23 5 cm 5 8 cm and 3 1 cm respectively in addition the full polarimetric scattering matrix was obtained at l and c band over a variety of terrain and vegetation types scientists are using multifrequency polarimetric sir c x sar data in studies of geology hydrology ecology oceanography and radar remote sensing techniques the october sir c x sar flight also included acquisition of experimental repeat pass interferometry data which have been used to generate digital elevation models and to detect surface motions in volcanic tectonic and glacial terrains results from sir c x sar clearly show the increased value of using multiparameter and interferometric capabilities to characterize earth s surface and vegetation cover and to generate geophysical products compared with optical sensors or single channel radars alone

discussion of field evidence and hydraulic modeling of a large holocene jokulhlaup at jokulsa a fjollum channel iceland by douglas howard sheryl luzzadder beach and timothy beach 2012 this paper discusses howard et al 2012 who reconstruct the peak discharge of a glacial outburst flood or jokulhlaup for part of the jokulsa a fjollum in north central iceland they propose that this flood was the largest on earth weconsider that the magnitude of the jokulhlaup proposed by howard et al 2012 warrants much more robust field evidence and demands more carefully parameterised hydraulic modelling for these reasons we firstly i present their study in the context of previous research ii highlight issues with attributing landforms and sediments to jokulhlaups and iii consider uncertainty regarding the timing and magnitude of jokulhlaups along the jokulsa a fjollum we argue herein that whilst a range of landforms and sediments that are attributable to jokulhlaups can be observed along the jokulsa a fjollum these are not necessarily diagnostic of jokulhlaups secondly we critically discuss iv the major underlying assumptions of their study and v their calculations and subsequent interpretations these assessments lead us to consider that the proposal by howard et al 2012 of the largest flood on earth is highly unrealistic especially when due consideration is given to a possible source area and a trigger mechanism

an inventory of active subglacial lakes in antarctica detected by icesat 2003 2008 through the detection of surface deformation in response to water movement recent satellite studies have demonstrated the existence of subglacial lakes in antarctica that fill and drain on timescales of months to years these studies however were confined to specific regions of the ice sheet here we present the first comprehensive study of these active lakes for the antarctic ice sheet north of 868 s based on 4 5 years 2003 08 of nasa s ice cloud and land elevation satellite icesat laser altimeter data our analysis has detected 124 lakes that were active during this period and we estimate volume changes for each lake the icesat detected lakes are prevalent in coastal antarctica and are present under most of the largest ice stream catchments lakes sometimes appear to transfer water from one to another but also often exchange water with distributed sources undetectable by icesat suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow thus these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds but also may withdraw water at other times to slow flow

snow megadunes in antarctica sedimentary structure and genesis 1 megadune fields occupy large areas in the interior of the east antarctic ice sheet and are the result of unusual snow accumulation and redistribution processes they therefore are important to surface mass balance and ice core interpretation field observations gps gpr and surface measurements have provided a detailed description of megadune sedimentation and morphology over a 70 km2 area located 200 km east of dome c a combination of remote sensing analysis using landsat and satellite radar altimetry and field measurements indicate that slope in the prevailing wind direction spwd and climatic conditions play a crucial role in megadune genesis the megadune areas tend to be characterized by slightly steeper regional slope and the presence of highly persistent katabatic winds the megadunes represent 2 to 4 m amplitude waves of 2 to 5 km wavelength formed by variable net accumulation ranging between 25 leeward faces to 120 windward faces of the accumulation in adjacent nonmegadune areas leeward faces are characterized by glazed sastrugi free surfaces and extensive depth hoar formation windward faces are covered by large rough sastrugi up to 1 5 m in height

inhomogeneous precipitation distribution and snow transport in steep terrain 1 the inhomogeneous snow distribution found in alpine terrain is the result of wind and precipitation interacting with the snow surface over topography we introduce and explain preferential deposition of precipitation as the deposition process without erosion of previously deposited snow and thus in absence of saltation a numerical model is developed describing the relevant processes of saltation suspension and preferential deposition the model uses high resolution wind fields calculated with a meteorological model arps the model is used to simulate a 120 h snow storm period over a steep alpine ridge for which snow distribution measurements are available the comparison to measurements shows that the model captures the larger scale snow distribution patterns and predicts the total additional lee slope loading well however the spatial resolution of 25 m is still insufficient to capture the smaller scale deposition features observed the model suggests that the snow distribution on the ridge scale is primarily caused by preferential deposition and that this result is not sensitive to model parameters such as turbulent diffusivity drift threshold or concentration in the saltation layer

stably stratified shear flow over low hills summary an analysis is presented for the mean flow over hills with low slopes for various kinds of stable stratification in the approach flow the froude number f based on the height is always greater than unity the analysis is based on dividing up the flow into different regions with different dynamics we concentrate on the flow in the inviscid outer region of the flow it is shown that in the lower part of the outer region the middle layer where the shear is large the upwind velocity profile and the shape of the hill rather than the upwind density profile largely determine the vertical perturbation velocity in the upper layer the structure of the flow depends largely on the form of the upwind density profile for example this largely determines the location of maximum surface wind new solutions for stable conditions are obtained when there is a constant density gradient when n ub 0 z2 and when there is an elevated inversion layer in the first and third of these cases we compare the results with the hydrostatic approximation and also obtain new general formulae for the near surface winds an approximate formula is developed for surface winds at arbitrary values of f 1 for uniform stable stratification

determination of aeolian transport rates of volcanic soils in iceland abstract sandy deserts cover 20 000 km2 in iceland consisting primarily of volcanic materials with basaltic volcanic glass being the main constituent wind erosion is severe in the country causing dust pollution with widespread aeolian redistribution affecting most icelandic ecosystems and sand movement over vegetated areas in the form of advancing sand fronts we quantified wind erosion using bsne field samplers and automated sensors over several years at two sites with contrasting environments the study sites are holsfjoll with andic soil materials in the arid northeast highlands 1200 mm both areas show similar annual aeolian transport of 120 670 kg m 1 yr 1 aeolian flux in storms at the ne site was 3 43 kg m 1 h 1 on average with up to 200 kg m 1 h 1 during gusts multiple regression shows potential flux of 200 kg m 1 h 1 during intense storms of 20 m s 1 at 2 m height the research shows major aeolian activity in the humid south iceland height distribution curves indicate considerable transport high above the surface at both sites 60 cm stable height distribution curves for each location allow for measurements using single dust trap over long periods the research explains the intense activity of advancing sand fronts in iceland and the significance of continuously recharged sand sources for maintaining severe wind erosion in humid areas of iceland

solar heating rates and temperature profiles in antarctic snow and ice observations of temperature maxima at about 10 cm depth in cold antarctic snow during summer have previously been explained by proposing that solar heating is distributed with depth whereas thermal infrared cooling is localized at the surface the solid state greenhouse an increase in temperature from the surface to 10 cm depth t 4 k found by rusin 1961 on the antarctic plateau was successfully reproduced by schlatter 1972 in a combined radiative transfer and heat transfer model

the drangajokull ice cap northwest iceland persisted into the early mid holocene abstract most glaciers and ice caps in iceland experienced rapid deglaciation in the early holocene reaching a minimum extent during the holocene thermal maximum here we present evidence of the holocene glacial history from lake sediment cores retrieved from seven threshold lakes around the drangajokull ice cap in the vestfir ir peninsula nw iceland the sediment cores show on off signals of glacial meltwater activity as minerogenic material deposited from glacial meltwater alternates with organic rich material gyttja deposited without glacial meltwater we base the chronology of the sediment cores on 14c ages and geochemical identification of key tephra layers with known ages a 25 cm thick layer of the saksunarvatn tephra in lake skorarvatn indicates that the northern part of the ice cap had reached a similar size as today or was smaller already by 10 2 cal kyr bp however 14c ages of lake sediment cores from the highlands southeast of drangajokull suggest that this part of the ice cap was larger than today until 7 8 7 2 cal kyr bp even today the drangajokull ice cap has a different behavior than the main ice caps in iceland characterized by a very low glaciation limit because palaeoclimatic proxies show an early mid holocene temperature optimum in this part of iceland we suggest that the persistence of drangajokull into the early holocene and possibly also the entire holocene was due to high winter precipitation

icesat antarctic elevation data preliminary precision and accuracy assessment 1 since first light on february 20th 2003 nasa s ice cloud and land elevation satellite icesat has derived surface elevations from 86 n to 86 s latitude these unique altimetry data have been acquired in a series of observation periods in repeated track patterns using all three geoscience laser altimeter system glas lasers here we focus on antarctic ice sheet elevation data that were obtained in 2003 2004 we present preliminary precision and accuracy assessments of selected elevation data and discuss factors impacting elevation change detection we show that for low slope and clear sky conditions the precision of gla12 laser 2a release 21 data is 2 1 cm and the relative accuracy of icesat elevations is 14 cm based on crossover differences

on the glaciological meteorological and climatological significance of antarctic blue ice areas this paper presents the current state of our knowledge about antarctic blue ice areas scattered widely over the antarctic continent yet covering only approximately 1 of its surface area blue ice areas have some peculiar characteristics that make them of particular interest for planetary geologists glaciologists meteorologists and climatologists for instance blue ice areas are subject to net ablation mainly by sublimation since any accumulation is easily swept off their smooth surfaces by winds moreover they are significantly warmer than their surroundings and the wind over them is generally more gusty an important feature of blue ice is its relatively low albedo 0 56 compared with that of snow 0 80 blue ice is probably considerably smoother than snow in the aerodynamic sense with roughness lengths as low as 3 10 6 m which indicates that blue ice is probably the only permanent natural surface over which the air flow is mainly in the aerodynamic smooth regime further since it is likely that very old ice is exposed at the surface of some blue ice areas this ice would be of great interest for paleoclimatic studies it has also been suggested that blue ice areas are sensitive to climate change this would make them useful as antarctic climate indicators because they can easily be monitored by satellite finally this paper addresses several gaps in our knowledge about blue ice areas and gives suggestions for future blue ice area research

analysis of antidune migration direction 1 antidunes are bed forms characteristic of upper regime unidirectional flows contrary to bed forms developed in lower regime flows antidunes not only can propagate in the downstream direction but can propagate upstream or remain stationary in this work we analyze the stable hydraulic conditions that determine each of the three antidune movement possibilities and a new theory is developed for distinguishing the occurrence of each of these conditions the theory is developed from an energy balance over a symmetrical antidune for this the bernoulli equation is applied between the antidune crest and antidune trough and the pressure head is corrected to account for the centrifugal forces generated by curvilinear flow manipulation of the fundamental equations produces an antidune mobility dimensionless number fa dependent on the froude number the mean water depth and the antidune wavelength the critical value of the antidune mobility number fa 1 corresponds to the stationary condition while values higher and lower than the critical correspond to downstream and upstream propagation respectively theory predicts that for a given froude number downstream migrating antidunes are formed for higher water depth wavelength ratios than for upstream migrating antidunes likewise by introducing a restriction for the maximum stationary wave height above antidunes theory predicts that downstream migrating antidunes could attain steeper height wavelength ratios than upstream migrating antidunes comparison with published experimental data in literature largely showed agreement between theory and experimental observations

shoreline erosion and aeolian deposition along a recently formed hydro electric reservoir blondulon iceland abstract erosion and deposition processes have been active along the shores of the blondulon hydro electric reservoir in the icelandic highlands since its formation nearly two decades ago in this study bluff development and distribution of aeolian material along the shores of the reservoir is related to erosive forces and source materials the 57 km 2 reservoir was formed in the glacial river blanda in 1991 and enlarged in 1996 bluff erosion and aeolian deposition has been monitored since 1997 field observations meteorological data gis analyses and calculations of wave power were used to measure and describe erosive processes along the new shoreline wave induced bluff erosion was most rapid during the first years after impoundment and was most active at sites with glacial till substrate high cumulative wave power and long fetches along the dominant wind direction in recent years relatively high erosion rates have continued at bluffs of fluvioglacial material under low cumulative wave power the fluvioglacial material has low resistance to wave activity creating unstable bluffs whereas bluffs made of glacial till are more stable aeolian sandy sediment mainly volcanic tephra eroded from soils has been deposited at sites open towards dry strong winds during low water levels in inlets of low wave energy slope and aspect against wind direction restricted the sediment distribution to 212 000 m 2 21 ha of the heathland along the reservoir by 2008 a volume of 11 000 m 3 had accumulated the continued redistribution of aeolian sediments and processes of bluff erosion were controlled by the water level fluctuations in the reservoir

extreme firn metamorphism impact of decades of vapor transport on near surface firn at a low accumulation glazed site on the east antarctic plateau snow and firn properties control the transport of vapor gases and water between the atmosphere and the underlying strata an understanding of this transport and the properties that control it is important for predicting air snow transfer of chemical species and for interpreting ice cores remote sensing images of east antarctica show large areas of alternating light and dark bands these low amplitude long wavelength features have glazed downwind faces and rough upwind faces and are called megadunes the first linked measurements of the permeability and the associated microstructure for a glazed area within a well defined megadune area are reported in this paper permeability and density were measured along with grain scale properties derived from digital image processing of preserved thick sections at this cold low accumulation glazed site a clear layering pattern exists in the top meter the firn density ranges from 0 24 to 0 50 g cm 3 permeability measurements range from 50 x 10 10 to 200 x 10 10 2 several times greater than corresponding profiles from warmer higher accumulation sites like siple dome antarctica it is shown that buoyancy driven natural convection may be important in post depositional processes in very cold low accumulation sites like this

continental scale simulation of the antarctic katabatic wind regime abstract katabatic winds are a common feature of the lower antarctic atmosphere although these drainage flows are quite shallow there is increasing evidence that the low level circulations are an important component in establishing large scale tropospheric motions in the high southern latitudes three dimensional numerical simulations of the antarctic katabatic wind regime and attendant tropospheric circulations have been conducted over the entire continent to depict the topographically forced drainage patterns in the near surface layer of the atmosphere results of the simulation enable a mapping of katabatic wind potential and identification of coastal regions which may experience anomalously intense katabatic winds a large upper level cyclonic circulation forms rapidly in response to the evolving katabatic wind structure in the lower atmosphere suggesting that the drainage circulations are an important component in prescribing the resulting resulting circumpolar vortex these results imply that som

impacts of an accumulation hiatus on the physical properties of firn at a low accumulation polar site 1 recent field investigations of a megadune region of east antarctica provide evidence that differences in grain size thermal conductivity and permeability across a megadune profile are due to spatial accumulation variability in the absence of significant microclimate variations the megadunes are low amplitude 2 8 m long wavelength 2 5 km bands with perceptible but low accumulation less than 40 mm water equivalent weq yr 1 and accumulation hiatus within several kilometers proximity as determined by remote sensing surface feature classification and ground penetrating radar profiling our hypothesis that accumulation rate impacts the extent of temperature gradient driven metamorphic growth in low accumulation rate sites is supported by measurements of various firn physical properties relatively small differences in accumulation rate less than 40 mm weq yr 1 result in large differences in physical properties including grain size thermal conductivity and permeability which are apparent in satellite based microwave data from both passive and active sensors the differences in physical snow structure between low accumulation areas and accumulation hiatus areas in the near surface are sufficiently distinct that evidence of past accumulation hiatus should be observable in the physical and chemical properties of an ice core record

the icelandic rofabard soil erosion features soil erosion and desertification are severe problems in iceland erosion processes are numerous and more than one can occur at each site resulting in many erosional forms erosion forms and an erosion severity scale are the basis for a recent national survey of erosion in iceland one of the most distinctive erosion forms in iceland is an erosion escarpment termed rofabard in icelandic rofabards are formed in thick but non cohesive andosols that overlie more cohesive materials such as glacial till or lava the relatively loose andosols beneath the root mat are undermined creating escarpments or rofabards the rofabards retreat as a unit with a fully vegetated and rich ecosystem on top but leaving barren desert in their place rofabards are common within a 20 000 km2 area the agricultural research institute and soil conservation service erosion database suggests that erosion associated with rofabards has denuded 15 000 30 000 km2 of land that was previously fully vegetated and had fertile andosols but is now mostly desert erosion rates associated with rofabards are reported as the loss of vegetated land with andosol mantle measured as hectares per square kilometre per year this measure of erosion has more meaning for icelandic landscapes than the traditional tonnes per hectare per year estimated losses of andosol cover in rofabard areas for the whole country are currently about 230 ha a 1 this rate is about 10 times lower than the rate needed to cause estimated losses of andosol mantle in rofabard areas since settlement 1125 years ago during peak years of soil erosion losses were probably several thousand hectares per year but the erosion rates slowed down as extensive andosol areas have become barren deserts copyright 2000 john wiley sons ltd

physical weathering and modification of a rhyolitic hyaloclastite in iceland fragmental volcanic glass or hyaloclastite is a common glaciovolcanic eruption product that is formed in large abundance during basaltic andesitic and rhyolitic subglacial eruptions the physical weathering of rhyolitic hyaloclastites differs notably from basaltic hyaloclastites due to differences in cementation and edifice consolidation as rhyolitic glasses are also much rarer comparatively little is known about their physical weathering and fracturing characteristics in the presented study we provide a process oriented analysis of the physical modification of subglacially erupted rhyolitic hyaloclastites from the blahnukur edifice in torfajokull iceland frost weathering experiments were performed to determine how vesicular glass particles fragment to finer particle sizes the surficial porosity of the glass drives such frost weathering through the process of pore pressurisation and was quantified using high pressure mercury intrusion uniaxial compression experiments were carried out to understand how the glass structure responds to the application of external stress the observed fracturing in both experimental treatments was found to adhere to fractal statistics which allowed the compression experiments to be used in conjunction with the frost weathering experiments for inferring the fracturing characteristics of rhyolitic volcanic glasses transport processes by wind and gravity were simulated by long duration abrasion experiments in rock tumblers through granular avalanching but these low energy particle interactions were not found to significantly abrade particles a notable result from our fragmentation experiments was the production of 10 m particles this size range is considered respirable and illustrates how physical weathering can continuously create potentially harmful ash textures a process which is often overlooked in health hazard assessments after volcanic eruptions fragmentation by post eruptive weathering can lead to overestimations of the fine ash fraction produced by syneruptive fragmentation and granulometric studies therefore need to be appreciative of the effects of such secondary fracturing processes

characterizing the height profile of the flux of wind eroded sediment wind erosion causes severe environmental problems such as aeolian desertification and dust storms in arid and semiarid regions reliable prediction of the height profile of the wind eroded sediment flux is crucial for estimation of transport rates verification of computer models understanding of particle modified wind flows and control of drifting sand this study defined the basic height profile for the flux of wind eroded sediment and the coefficients that characterize its equation nine grain size populations of natural sand at different wind velocities were tested in a wind tunnel to measure the flux of sediment at different heights the resulting flux profiles resemble a golf club with a small back turn where the flux increases with increasing height within 20 mm above the surface if the small back turns are neglected the flux profiles can be expressed by an exponential decay function q text r z a text e bz r where qr z is the dimensionless relative flux of sediment at height z which follows the exponential decay law proposed by previous researchers for aeolian saltation three coefficients a creep proportion a relative decay rate and an average saltation height are proposed to characterize the height profile coefficients a and b in the above equation represent the creep proportion and relative decay rate as a function of height respectively coefficient a varies widely depending on grain size and wind velocity but averages 0 09 it is suggested that the grain size and wind velocity must be specified when discussing creep proportion coefficients a and b are nearly linearly correlated and decrease as grain size and wind velocity increase the average saltation height the average height sediment particles can reach was a function of grain size and wind velocity and was well correlated with coefficients a and b

air flow over a two dimensional hill studies of velocity speed up roughness effects and turbulence wind tunnel measurements have been made of the streamwise mean and turbulent velocities over a rough bell shaped two dimensional hill with height h and maximum slope 0 26 placed in a neutrally stable boundary layer of thickness 10 h and roughness length zo 0 02 h close agreement is found between the mean velocity and predictions obtained from taylor s 1977 computational model and jackson and hunt s 1975 analytical linearized model for locations at or upwind of the hill top but not in the wake examination of the models shows that the shear stresses are only important in an inner region close to the hill surface so that as suggested by jackson and hunt 1975 the perturbation to the mean flow outside this region is essentially inviscid the theory shows that over very rough surfaces such as wooded or urban terrain the height of this inner region can be of the same order as the height of the roughness elements so that in our experiments no measurements could be made in this region in a second experiment flow over a smooth hill on a rough surface was studied the additional increase of wind speed over the hill top can be estimated by assuming a linear superposition of the velocity changes produced by the changes in elevation and in surface roughness in this case rough to smooth in the lee of a hill however the change in roughness significantly alters the flow with flow separation being suppressed and here a linear superposition is not appropriate finally we consider why observed changes in turbulence structure close to the surface differ from those well above the surface calculations of these changes based on the simple theoretical arguments of equilibrium shear layers and rapidly distorted turbulent flows agree well with turbulence measurements in wind tunnels and in the field

the surface windfield over the antarctic ice sheets the intense radiative cooling of air over the ice slopes of antarctica generates a surface wind regime that is strongly controlled by topography and plays a key role in determining the behaviour of the atmosphere and ocean in high southern latitudes1 7 resultant surface winds are intimately linked to the orientation of the ice terrain fig 1 and display the highest degree of persistence found on earth the close coupling between wind and topography allows estimation of the former if the latter is known with some precision here we report on time averaged near surface airflow over the antarctic continent during winter diagnosed from a recent accurate synthesis of terrain slopes and from estimates of the lower atmospheric temperature structure the simulated drainage pat tern exhibits strong spatial variability with the airflow concentrated into several zones near the coastal margin these confluence regions are responsible for strong persistent katabatic winds over downstream coastal stretches and are indicative of zones of greatest katabatic potential

impact pressures and flow regimes in dense snow avalanches observed at the vallee de la sionne test site 1 a fundamental problem in avalanche engineering is to determine the impact pressures exerted on structures this task is complicated because snow avalanches flow in a variety of regimes primarily depending on snow temperature and moisture content in this paper we address this problem by analyzing measured impact pressures flow velocities and flow depths of five vallee de la sionne avalanches the measurements are made on a 20 m high tubular pylon instrumented with high frequency pressure transducers and optoelectronic velocity sensors in the observed avalanches we find both subcritical and supercritical flow regimes typical froude numbers were smaller than 6 the subcritical regime fr 1 is characterized by a flow plug riding above a highly sheared basal layer the measured pressures are large and velocity independent in contradiction to calculation procedures pressure fluctuations increase with flow depth indicating a kinematic stick slip phenomena which is largest at the basal layer supercritical flow regimes 1 fr 6 are characterized by a sheared flow all over the avalanche depth in this regime the impact pressure is velocity dependent we derive relationships governing impact pressure as a function of the froude number and therefore flow regime encompassing all the observed avalanches

a consideration of the dune antidune transition in fine gravel hydraulic data defining the dune antidune transition in fine gravel are compared with potential flow theory and information is drawn from published experiments and field based studies attention is given to both transitional bedforms and the development of downstream migrating antidunes in the latter case most data pertain to sand beds and not to gravel empirical data provide some weak support for the theoretical notion that the transition occurs at progressively lower froude numbers at greater relative depths although a critical froude number of 0 84 may reasonably be applied for the beginning of the dune to antidune transformation lag effects and a possible depth limitation ensure that transitional bedforms may persist across a broad range of froude numbers from 0 5 to 1 8 this latter observation has great relevance for palaeohydraulic estimates derived from outcrop data whereas the application of theoretical bedform existence fields based upon potential flow theory to fine gravel was previously purely speculative the addition of experimental and field data to these plots provides a degree of confidence in applying stability theory to practical geological problems for the first time laboratory data pertaining to downstream migrating gravel antidunes are compared with theory these bedforms have been reported from certain experimental near critical flows above sand or gravel beds but have been observed infrequently in natural streams however there are no detailed studies from natural rivers and only a few contentious identifications from outcrops nevertheless the limited hydraulic data conform to theoretical expectations

extent of low accumulation wind glaze areas on the east antarctic plateau implications for continental ice mass balance persistent katabatic winds form widely distributed localized areas of near zero net surface accumulation on the east antarctic ice sheet eais plateau these areas have been called glaze surfaces due to their polished appearance they are typically 2 200km2 in area and are found on leeward slopes of ice sheet undulations and megadunes adjacent leeward high accumulation regions isolated dunes are generally smaller and do not compensate for the local low in surface mass balance smb we use a combination of satellite remote sensing and field gathered datasets to map the extent of wind glaze in the eais above 1500m elevation mapping criteria are derived from distinctive surface and subsurface characteristics of glaze areas resulting from many years of intense annual temperature cycling without significant burial our results show that 11 2 1 7 or 950 143 103 km2 of the eais above 1500m is wind glaze studies of smb interpolate values across glaze regions leading to overestimates of net mass input using our derived wind glaze extent we estimate this excess in three recent models of antarctic smb at 46 82 gt the lowest input model appears to best match the mean in regions of extensive wind glaze