on the use of the dual frequency envisat altimeter to determine snowpack properties of the antarctic ice sheet abstract the primary purpose of ice sheet altimetry is to monitor the changes in ice sheet topography which may impact on global sea level however the altimetric signal is sensitive to different properties of the snowpack and therefore can also be used to determine these properties the radar altimeter onboard the european space agency s envisat satellite provides a dual frequency dataset at ku 13 6 ghz and s band 3 2 ghz in this paper these signals are studied over the antarctic ice sheet during the 4 first years of the mission 2002 2006 in order to retrieve snowpack properties the altimeter signal can be described by 4 classical waveform parameters the 4 year time series of all these parameters are decomposed into a linear and a seasonal time component the linear component is almost constant the distribution of the mean parameters over the antarctic ice sheet shows that the altimeter signal is sensitive to small scale mm surface roughness for the first time the amplitudes and phases of the seasonal variations are characterized the s band amplitudes are greater than the ku band and the phase varies over the entire ice sheet previous studies suggested that the seasonal variations of the altitude from the altimeter are created by a decrease of the snowpack height through compaction the dual frequency observations shown here suggest that this hypothesis is too simple instead the altitude variations observed in the altimetric signal are not created by the snowpack height change but are more likely caused by the seasonal change of the snow properties which cause a different response between the s and ku bands therefore both the linear and the seasonal variations of the altimetric signal can be used to retrieve snowpack properties here we compare the dual frequency envisat signal with a model of the altimetric echo over the antarctic ice sheet the model combines a surface model with a sub surface model for both the s and ku bands the brown model brown g s 1977 the average impulse response of a rough surface and its applications ieee transactions on antennas and propagation 25 1 is used to describe the interaction of the radar wave with the snow surface the backscatter coefficient of the surface is derived using the iem method fung a k 1994 microwave scattering and emission models and their applications boston ma artech house the sub surface signal takes into account both the layering effects and the scattering caused by the homogeneous media which is composed of small snow grains the model is tested in two areas of the antarctic plateau which present very different waveform parameters the sensitivity of the radar signal to the different snowpack properties is investigated the analysis of the waveform behaviours shows that the sub surface signal can be completely masked by the small scale surface roughness signal finally the temperature and surface density effects are investigated in order to explain the seasonal variations of the altimetric signal both the temperature and the compaction rate of the snow change seasonally temperature is shown to impact on the ku band signal furthermore the compaction rate of the snow surface can explain all of the seasonal variation characteristics observed at both the s and ku bands the seasonal change of compaction rate in the snow creates a change in the waveform shape that can bias the altitude in particular the snow compaction can induce a bias in the retrieved altimetric altitude of more than 80 cm for the ku band and 1 5 m for the s band this work underlines that the altitude time series needs to be corrected for the shape of the altimetric echo over ice sheets

snow density along the route traversed by the japanese swedish antarctic expedition 2007 08 during the japanese swedish antarctic traverse expedition of 2007 08 we measured the surface snow density at 46 locations along the 2800 km long route from syowa station to wasa station in east antarctica the mean snow density for the upper 1 or 0 5 m layer varied from 333 to 439 kg m 3 over a region spanning an elevation range of 365 3800 m a s l the density variations were associated with the elevation of the sampling sites the density decreased as the elevation increased moving from the coastal region inland however the density was relatively insensitive to the change in elevation along the ridge on the antarctic plateau between dome f and kohnen stations because surface wind is weak in this region irrespective of elevation the wind speed was suggested to play a key role in the near surface densification the results of multiple regression performed on the density using meteorological variables were significantly improved by the inclusion of wind speed as a predictor the regression analysis yielded a linear dependence between the density and the wind speed with a coefficient of 13 5 kg m 3 m s 1 1 this relationship is nearly three times stronger than a value previously computed from a dataset available in antarctica our data indicate that the wind speed is more important to estimates of the surface snow density in antarctica than has been previously assumed

density log of a 181 m long ice core from berkner island antarctica a 181 m long ice core was drilled at 79 36 51 s 45 43 28 w near the summit of berkner island antarctica 886 m a s l berkner island is located between the filchner and ronne ice shelves and the ice near the summit shows little lateral flow the density of the ice core was measured every 3 mm along its length using attenuation of a gamma ray beam which gave an absolute accuracy of 2 as expected there is a general density increase with depth the maximum densities of 900 kg m 3 being reached just above 100 m depth comparison with the electrical conductivity method ecm shows density variations with the same wavelength as the annual signals which can be seen in the ecm log higher acidity during summer in the shallowest part of the core the density of winter layers is higher than that of summer layers a relationship which is reversed at greater depth we assume that the densification rates for the two types of firn are different similar density phenomena were observed on ice cores from greenland showing that such phenomena are not a local effect

rapid change of snow surface properties at vostok east antarctica revealed by altimetry and radiometry we present results of snow surface properties using the envisat dual frequency altimeter at s 3 2 ghz and ku 13 6 ghz bands and the amsr e microwave radiometer at frequencies ranging between 6 and 36 ghz in the vostok region east antarctica the altimetric time series observed between 2002 and 2008 show variations at 3 different time scales daily seasonal and inter annual that correlate directly with variations in the snow surface properties in this study we focus on the analysis of the rapid daily event occurring on february 14th 2005 that created a jump of the backscatter coefficient of up to 5 3 db at the s band and 2 5 db at the ku band the ratio of v h polarization brightness temperature slowly decreased in december and january 2005 and suddenly increased on february 14th 2005 the origin of this rapid event is investigated using aws data from vostok station altimetric and radiometric data simultaneously both snow surface density and roughness are found to vary during this event this event is shown to be synchronous with strong wind occuring during a period of anomalous wind direction and the presence of surface hoar these particular conditions certainly modified the snow surface roughness and thus impacted the altimetric signal we finally investigate the impact of this event on the calculation of the regional ice sheet mass balance using different corrections of height with echo shape variations it is shown to be negligible only if the full echo shape correction legresy et al 2006 is used

dual frequency altimeter signal from envisat on the amery ice shelf in antarctica radar altimeter measurements are sensitive to dielectric and penetration properties of the sensed medium snow such that the spacecraft s altitude can be biased since 2002 relatively low frequency radar measurements over the amery ice shelf east antarctica have been acquired using the envisat dual frequency altimeter at s 3 2 ghz and ku 13 6 ghz bands which penetrate a few meters into the firn the altimeter signal is however modified in summer by the presence of snowfilled crevasses indeed the specularity of the snow surfaces in summer makes the altimetric signal sensitive mostly to nadir echoes that increases the ratio between the crevasse signal and the surrounding ice shelf signal at nadir crevasses are distinguished by differences in backscattering behavior compared with the surrounding ice shelf signal crevasses are characterized by a strong backscatter coefficient at ku band and anomalies in the s band altitude estimation these two characteristics make snowfilled crevasses detectable by the dual frequency altimeter of envisat we first retrieve the geometric properties of the crevasses using a hyperbolic shape function created by strong crevasse backscatter in the ku waveform measurements from this retrieved crevasse signal and further waveform analysis we assess the properties of the snow surface and its sub surface the crevasse due to its small size compared to the altimeter footprint is found to be an excellent target to study snow properties of the ice shelf the anomalies in the s band altitude measurements over crevasses can then be explained by the presence of a double echo in the s band waveforms this echo is attributed to a reflection at the base of the snowbridge where we see evidence of sub surface echos in the individual altimeter waveforms based on this observation a methodology is developed to estimate the thickness of the snowbridge we calculate the penetration depths in the summer snow surface of the amery at ku band that is found to be around 6 m

microstructure dependent densification of polar firn derived from x ray microtomography the densification of dry polar snow and firn results in a continuous increase of density with depth accompanied by significant density fluctuations within seasonal layers density measurements of high spatial resolution reveal a persistent minimum of density fluctuations in the vicinity of the snow firn transition 0 55 0 65 g cm 3 in firn core records in this study we give an explanation for the fluctuation minimum by applying a new method of x ray microtomography to obtain three dimensional 3 d structural data of a greenland firn core at 13 different depths between 10 and 78 m a set of 16 samples of 40 cm total length for each depth interval was measured a reconstructed firn segment of 40 cm covers 1 2 years of snow accumulation using digital image analysis techniques different structural parameters are estimated including 3 d pore and particle sizes and specific surface areas it is shown that the densification rates of snow and firn layers consisting of coarse particles are much higher than those consisting of fine particles within the same depth interval this causes a density crossing of fine and coarse grained layers with a minimum of density variations at the crossover point this crossing over implies that formerly dense layers in the seasonal density signal are not of the same origin as dense layers in the deeper part of the firn column and that the seasonal density signal will totally change shape with depth it is speculated that in coarse and fine grained firn the dominant mechanism of densification acts over different regimes of density

instruments and methods in situ measurements of firn compaction profiles using borehole optical stratigraphy we have developed a technique in which we use a borehole video camera and post processing software to make a record of the optical brightness as a function of depth in polar firn we call this method borehole optical stratigraphy to measure firn compaction we note the positions of optical features on the borehole wall detected by an initial baseline log we track the displacements of these features in subsequent logs the result provides a measurement of the relative vertical motion and thus compaction of the firn over the survey period we have successfully used this system at summit greenland to measure the depth distribution of firn column shortening experienced in a borehole over three 1 year periods the uppermost 30 m of the firn at summit is compacting as predicted by a simple steady state model implying that the firn density profile at summit is at or close to steady state over the past 70 years

instruments and methods a physically based method for correcting temperature profile measurements made using thermocouples high frequency diurnal temperature variations occur simultaneously at multiple depths separated by meters of snow in at least several and probably many arctic and antarctic thermocouple datasets these temperature variations cannot be caused by heat conduction from the surface because their amplitudes are too large and there is no phase lag with depth and they cannot be caused by heat advection because the air flux required is greater than is available rather the simultaneous temperature variations stvs appear to originate within the box that houses the data logger as thermocouple like offset voltages wire heating or thermistor error the stvs can be corrected by requiring that the temperatures vary smoothly with time at the greatest depth at which temperature is measured the correction voltage determined in this fashion when applied to the thermocouples at other depths corrects the entire dataset the method successfully removes stvs with 24 hour period that are up to 3 88c in amplitude and is superior to the averaging techniques commonly used to correct thermocouple data because it introduces no spurious non physical temperature variations the correction method described can be applied to all thermocouple data where temperature measurements have been made at depths 0 5 m into the snowpack the corrections should allow more physical process and parameter information to be extracted more confidently from existing firn temperature data identification of the stvs and their probable cause also suggests how better data might be collected in the future

snow crystal growth with varying surface temperatures and radiation penetration the temperature field is derived for a sinusoidally varying surface temperature with varying solar radiation penetration the growth rates of snow crystals are calculated to explain the rapidly growing layers of faceted crystals i e depth hoar that form just below the surface at high altitudes and in polar snow the solutions also show that a layer of wet snow can exist just below the surface even on days when the surface temperature remains sub freezing

modeling of firn compaction for estimating ice sheet mass change from observed ice sheet elevation change changes in ice sheet surface elevation are caused by a combination of ice dynamic imbalance ablation temporal variations in accumulation rate firn compaction and underlying bedrock motion thus deriving the rate of ice sheet mass change from measured surface elevation change requires information on the rate of firn compaction and bedrock motion which do not involve changes in mass and requires an appropriate firn density to associate with elevation changes induced by recent accumulation rate variability we use a 25year record of surface temperature and a parameterization for accumulation change as a function of temperature to drive a firn compaction model we apply this formulation to icesat measurements of surface elevation change at three locations on the greenland ice sheet in order to separate the accumulation driven changes from the ice dynamic ablation driven changes and thus to derive the corresponding mass change our calculated densities for the accumulation driven changes range from 410 to 610kgm 3 which along with 900kgm 3 for the dynamic ablation driven changes gives average densities ranging from 680 to 790kgm 3 we show that using an average or effective density to convert elevation change to mass change is not valid where the accumulation and the dynamic elevation changes are of opposite sign

the connectivity of crystallite agglomerates in low density firn at kohnen station dronning maud land antarctica in this study the three dimensional 3 d microstructure of polar firn is investigated by means of x ray microfocus computer tomography mct basic topological properties including the euler and coordination numbers are derived from the reconstructed 3 d volume images it is shown that sample volumes of about 4 cm 3 are representative for polar firn in terms of their connectivity the connectivity function defined as the change of euler number with structure size is calculated via image processing routines it is used to split the ice phase at small bridges into single crystallite agglomerates the bond size distributions and the mean size of the agglomerates are estimated all mct measurements were carried out on the uppermost 9 m of a shallow firn core b35 drilled during the 2005 06 field campaign at kohnen station dronning maud land dml antarctica the results are compared with estimates from classical two dimensional 2 d surface section observations the 3 d approach confirms the linear relationship between coordination number and density which hitherto has only been derived from 2 d observations layers of buried snow dunes show a stronger connectivity than layers of moderate crystal size and density the formation of agglomerates made of crystallites is a common feature of polar firn in dml it is proposed that the growth of agglomerates leads to reduced critical densities for the transition between the densification regime of grain boundary sliding and plastic deformation

ice sheet elevation changes caused by variations of the firn compaction rate induced by satellite observed temperature variations 1982 2003 changes in the surface elevation of the greenland and antarctic ice sheets and ice shelves caused by variations in the rate of firn compaction are calculated with a time dependent firn densification model driven by two decades 1982 2003 of satellite observed monthly surface temperatures the model includes the effects of melting and refreezing both the direct changes in density and the subsequent effects on the densification rate as previously shown the temperature dependent rate of densification is largest in summer but changes in winter temperatures also have a significant effect over the last decade climate warming has enhanced the rate of compaction and lowered the average surface elevation of greenland by 1 8 cma 1 and most of west antarctica by 1 9 cma 1 in east antarctica a small cooling raised the average surface elevation by 0 14 cma 1

snow accumulation variability and random walk how to interpret changes of surface elevation in antarctica because snow accumulation above an ice sheet is a cumulative process a random fluctuation of snow accumulation can lead to significant variability in ice sheet mass or elevation without being linked to a long term climatic change moreover ice sheet mass balance cannot be measured directly we can determine the ice sheet surface elevation through altimetric data but due to the densification process the snow accumulation variability affects the surface elevation variations more than the mass variations we investigated the effect of the recent snow accumulation variability on both ice sheet mass and volume at the time scale of the satellite era there is more than 10 chance of measuring an artificial trend greater than 15 of the mean accumulation rate from a 10 year elevation series in our simplified model the impact of snow accumulation variability only depends on the ratio between the snow densification time and the variability time period it decreases with the densification rate and increases with the variability frequency so that the induced error is maximum for high frequency noise in the central part of antarctica finally our results showed that knowledge of snow accumulation variability for the past 5 10 years is needed to eliminate the delayed induced error d 2004 elsevier b v all rights reserved

interannual variations of shallow firn temperature at greenland summit firn temperature profiles are calculated in a thermal model using continuous surface temperatures derived from automatic weather station aws data and passive microwave data in the greenland summit region during the period 1987 1999 the results show that significant interannual variations of mean summer june to august and annual temperatures occur in the top 15 m in addition to the normal seasonal cycle of firn temperature at 5 m depth the seasonal cycle is damped to 13 of the surface seasonal amplitude but even at 15 m about 1 or 0 6 c of the seasonal cycle persists both summer and mean annual temperatures decrease from 1987 to 1992 followed by a general increasing trend interannual variability is 5 c at the surface but only is only dampened to 3 2 c at 10 m depth and 0 7 c at 15 m depth dampening of the interannual variability with depth is slower than dampening of the seasonal cycle because of the longer time constant of the interannual variation the warmer spring and summer temperatures experienced in the top 5 m due to both the seasonal cycle and interannual variations affect the rate of firn densification which is non linearly dependent on temperature during the 12 year period 1987 1999 the mean annual surface temperature is 29 2 c and the mean annual 15 m temperature is 30 1 c which is more than 1 c warmer than a 15 m borehole temperature representing the period of about 1959 and warmer than the best fit temperature history by alley and koci back to 1500 a d

improving surface boundary conditions with focus on coupling snow densification and meltwater retention in large scale ice sheet models of greenland snowpack changes during the melt season are often not incorporated in modelling studies of the surface mass balance of the greenland ice sheet densification of snow accelerates when meltwater is present due to percolation and subsequent refreezing and needs to be incorporated in ice sheet models for ablation calculations in this study simple parameterizations to calculate surface melt snow densification and meltwater retention are included as surface boundary conditions in a large scale ice sheet model of greenland coupling the snow densification and meltwater retention processes achieves a separation of volume and mass changes of the surface layer in order to determine the surface melt contribution to runoff experiments for present day conditions show that snow depth at the onset of melt mean annual near surface air temperature and the mean density of the annual snow layer are key factors controlling the quantity and spatial distribution of meltwater runoff above the equilibrium line on the greenland ice sheet

ground based measurements of spatial and temporal variability of snow accumulation in east antarctica the east antarctic ice sheet is the largest highest coldest driest and windiest ice sheet on earth understanding of the surface mass balance smb of antarctica is necessary to determine the present state of the ice sheet to make predictions of its potential contribution to sea level rise and to determine its past history for paleoclimatic reconstructions however smb values are poorly known because of logistic constraints in extreme polar environments and they represent one of the biggest challenges of antarctic science snow accumulation is the most important parameter for the smb of ice sheets smb varies on a number of scales from small scale features sastrugi to ice sheet scale smb patterns determined mainly by temperature elevation distance from the coast and wind driven processes in situ measurements of smb are performed at single points by stakes ultrasonic sounders snow pits and firn and ice cores and laterally by continuous measurements using ground penetrating radar smb for large regions can only be achieved practically by using remote sensing and or numerical climate modeling however these techniques rely on ground truthing to improve the resolution and accuracy the separation of spatial and temporal variations of smb in transient regimes is necessary for accurate interpretation of ice core records in this review we provide an overview of the various measurement techniques related difficulties and limitations of data interpretation describe spatial characteristics of east antarctic smb and issues related to the spatial and temporal representativity of measurements and provide recommendations on how to perform in situ measurements

modeled seasonal variations of firn density induced by steady state surface air temperature cycle seasonal variations of firn density in ice sheet firn layers have been attributed to variations in deposition processes or other processes within the upper firn a recent high resolution mm scale density profile measured along a 181 m core from antarctica showed small scale density variations with a clear seasonal cycle that apparently was not related to seasonal variations in deposition or known near surface processes gerland and others 1999 a recent model of surface elevation changes zwally and li submitted produced a seasonal variation in firn densification and explained the seasonal surface elevation changes observed by satellite radar altimeters in this study we apply our 1 d time dependent numerical model of firn densification that includes a temperature dependent formulation of firn densification based on laboratory measurements of grain growth the model is driven by a steady state seasonal surface temperature and a constant accumulation rate appropriate for the measured antarctic ice core the modeled seasonal variations in firn density show that the layers of snow deposited during spring to mid summer with the highest temperature history compress to the highest density and the layers deposited during later summer to autumn with the lowest temperature history compress to the lowest density the initial amplitude of the seasonal difference of about 0 13 reduces to about 0 09 in five years and asymptotically to 0 92 at depth which is consistent with the core measurements

mass changes of the greenland and antarctic ice sheets and shelves and contributions to sea level rise 1992 2002 changes in ice mass are estimated from elevation changes derived from 10 5 years greenland and 9 years antarctica of satellite radar altimetry data from the european remote sensing satellites ers 1 and 2 for the first time the dh dt values are adjusted for changes in surface elevation resulting from temperature driven variations in the rate of firn compaction the greenland ice sheet is thinning at the margins 42 2g t a 1 below the equilibrium line altitude ela and growing inland 53 2g t a 1 above the ela with a small overall mass gain 11 3g t a 1 0 03 mm a 1 sle sea level equivalent the ice sheet in west antarctica wa is losing mass 47 4g t a 1 and the ice sheet in east antarctica ea shows a small mass gain 16 11 gt a 1 for a combined net change of 31 12 gt a 1 0 08 mm a 1 sle the contribution of the three ice sheets to sea level is 0 05 0 03 mm a 1 t he antarctic ice shelves show corresponding mass changes of 95 11 gt a 1 in wa and 142 10 gt a 1 in ea thinning at the margins of the greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate the marked thinnings in the pine island and thwaites glacier basins of wa and the totten glacier basin in ea are probably ice dynamic responses to long term climate change and perhaps past removal of their adjacent ice shelves the ice growth in the southern antarctic peninsula and parts of ea may be due to increasing precipitation during the last century

kinematic model for the opening of the sea of japan and the bending of the japanese islands we have analyzed paleomagnetic data from japan reported over the past 25 years in combination with marine geophysical and on land geologic data to reconstruct the opening of the sea of japan and the bending of honshu from these data we propose that the sea of japan opened by a two stage back arc spreading event 1 in the early to middle tertiary japan began to rift off the asian coast as a single rigid block rotated clockwise approximately 25 and formed the japan basin 2 in the miocene the yamato and tsushima basins opened and shortly thereafter 19 ma the japan arc collided with another island arc in central hokkaido at this stage honshu began to behave as two rigid blocks southwest honshu and northeast honshu southwest honshu continued to rotate clockwise approximately 45 as the yamato and tsushima basins continued to open northeastern honshu rotated counterclockwise approximately 40 as hokkaido was held fixed by the arc collision while the yamato basin continued to open these two events the opening of the tsushima and yamato basins and the arc collision in hokkaido resulted in the early to middle miocene bending of the japan arc observed in the paleomagnetic data after 12 ma back arc spreading ceased

paleomagnetic evidence for the age and extent of middle tertiary counterclockwise rotation dixie valley region west central nevada paleomagnetic data obtained from oligocene to lower miocene igneous rocks and middle miocene basaltic rocks at fifteen localities from a region surrounding dixie valley in west central nevada indicate that parts of the area experienced counterclockwise vertical axis rotation and these data provide constraints on the extent and timing of rotation counterclockwise vertical axis rotation probably exceeding 30 is indicated for oligocene to lower miocene rocks in the central part of the study area calculated rotations increase with increasing age of the oligocene to lower miocene rocks indicating that vertical axis rotation was concurrent with ash flow deposition about 23 33 ma paleomagnetic data indicate that middle miocene basaltic rocks about 10 17 ma postdate significant counterclockwise vertical axis rotation although this interpretation is complicated because the rocks were erupted episodically in pulses that apparently were short lived with respect to geomagnetic secular variation counterclockwise rotation was related to a deformation event that predated development of the present basin and range physiography of the area the total amount of crustal rotation in tertiary time in the area is poorly known rotation estimates calculated from time averaged mean directions that incorporate data from a broad range of the oligocene to lower miocene units probably underestimate total tertiary rotation because the mean directions include data from units that postdate much of the rotation for example a rotation estimate for a composite sequence from the central part of the study area is 23 15 whereas estimates from older and younger halves of the sequence are 37 21 and 11 16 respectively paleomagnetic data indicate that oligocene to lower miocene rocks at some localities in the northern and southern parts of the study area e g the golconda canyon locality probably did not experience significant tertiary counterclockwise rotation although the paleomagnetic data alone are insufficient to fully define the geographic area subjected to tertiary counterclockwise rotation the data suggest that a discrete subregion was affected rather than the entire basin and range province