combination of laser and radar altimeter height measurements to estimate snow depth during the 2004 antarctic amsr e sea ice field campaign 1 among the most important parameters needed to evaluate the present and future state of antarctic sea ice cover is the ice thickness the retrieval of ice thickness using remote sensing techniques has been hampered by the absence of a capability to remotely measure snow thickness covering the sea ice data sets collected with johns hopkins applied physics laboratory s delay doppler phase monopulse d2p radar and nasa s airborne topographic mapper atm scanning lidar during nasa s antarctic amsr e sea ice field campaign over the bellingshausen sea are used to demonstrate the potential of a remote sensing technique for retrieval of snow cover thickness from an airborne platform the technique takes advantage of the fact that the radar is most sensitive to the snow ice interface while the lidar responds to the highly optically reflective snow surface the difference between the radar and laser determined surfaces yields an estimate of snow thickness that appears to be reasonably consistent with expected values the technique requires careful registration of the instrument footprints because there was no absolute range calibration of the lidar due to predeployment scheduling difficulties the vertical offset between the instruments was resolved by determining the difference between measurements over leads elsewhere over sea ice the radar defined surface is generally below the laser defined surface consistent with the radar defining the snow ice interface over a relatively small portion of the data we observed opposite relationship between the sensor defined surfaces for which we discuss a plausible physical explanation

slope induced errors in radar altimetry over continental ice sheets ranges obtained by radar altimetry from a satellite to a sloping surface on the earth are not measurements of the surface elevation at the subsatellite point instead the reflecting point is displaced upslope from the subsatellite point causing a slope induced error between the true range to the subsatellite point and the indicated range for the seasat 1 altimeter this range error is nearly 80 m for a 0 8 degree slope ice sheet surface slopes are frequently large enough to cause errors of 10 m or more in addition undulations in the surface about a mean slope cause pronounced variations in these errors altimeter measurements of ranges to modeled irregular surfaces are simulated and two correction schemes are used to reconstruct the modeled surfaces from the simulated data the results illustrate the fundamental limitations inherent in single beam radar altimetry for mapping irregular surfaces in the simpler two dimensional case for which cross track slopes are neglected a relocation scheme which constructs a surface consistent with the altimeter ranges removes 85 of the rms error the mean error along profiles of about 75 km or longer is usually reduced more than the rms error an alternate slope correction scheme which uses the local slope to calculate the expected error is less effective in the two dimensional case however over a simulated three dimensional surface where groundtracks are widely spaced and cross track slopes are significant the slope correction method must be used in at least the cross track direction the effectiveness of the three dimensional correction depends on the relative size of the errors caused by the local slope on surface undulations in comparison to the regional slope

ku band radar penetration into snow cover on arctic sea ice using airborne data satellite radar altimetry provides data to monitor winter arctic sea ice thickness variability on interannual basin wide scales when using this technique an assumption is made that the peak of the radar return originates from the snow ice interface this has been shown to be true in the laboratory for cold dry snow as is the case on arctic sea ice during winter however this assumption has not been tested in the field we use data from an airborne normal incidence ku band radar altimeter and in situ field measurements collected during the cryosat validation experiment cryovex bay of bothnia 2006 and 2008 field campaigns to determine the dominant scattering surface for arctic snow covered sea ice in 2006 when the snow temperatures were close to freezing the dominant scattering surface in 25 of the radar returns appeared closer to the snow ice interface than the air snow interface however in 2008 when temperatures were lower the dominant scattering surface appeared closer to the snow ice interface than the air snow interface in 80 of the returns

a comparison of greenland ice sheet volume changes derived from altimetry measurements we compare rates of surface elevation change on the greenland ice sheet derived from european remote sensing satellite 2 ers 2 radar altimeter data with those obtained from laser altimeter data collected over nearly the same time periods radar altimeter data show more rapid thickening 9 1 cm a 1 above 1500 m elevation in the north and 3 1 cm a 1 above 2000 m in the south than the laser estimates possibly caused by a lifting of the radar reflection horizon associated with changes in the snowpack such as those caused by progressively increased surface melting as summer temperatures rise over all the ice sheet above 2000m this results in an ers derived volume balance 75 15 km 3 a 1 more positive than that from laser data this bias between laser and radar estimates of elevation change varies spatially and temporally so cannot at present be corrected without independent surveys such as those presented here at lower elevations comparison of detailed repeat laser surveys over jakobshavn isbrae with ers results over the same time interval shows substantial ers underestimation of ice thinning rates this results partly from missing data because of bad radar waveforms over the very rough surface topography and partly from the tendency for large radar footprints to sample preferentially local high points in the topography thus missing regions of most rapid thinning along glacier depressions

combined airborne laser and radar altimeter measurements over the fram strait in may 2002 knowledge of sea ice thickness is critical for the prediction of future climate and for assessing the significance of changes in thickness sea ice thickness can be calculated from radar or laser satellite altimetry measurements of freeboard however a lack of knowledge of snow depth introduces significant uncertainties into these calculations this paper compares the first coincident airborne laser and radar altimetry data over sea ice collected during the laser radar altimetry lara field campaign lara was a flight of opportunity that provided valuable data to explore techniques to validate satellite measurements of ice freeboard and the possibility of combining laser and radar measurements over snow covered sea ice to calculate the snow depth two new methods were created to analyse these data sets a new radar retracker and a radar power simulator which models radar returns from the laser data we present the first quantitative analysis of data from the lara laser and radar altimeters and demonstrate the potential of combining laser and radar altimetry to estimate snow depth lara elevation estimates compare well with elevations from the radar altimeter onboard ers 2 at the sub meter level and the study provides lessons for future validation of satellite altimetry data over sea ice laser elevations are consistently higher than the radar elevations over snow covered sea ice as lara was a flight of opportunity no coincident in situ measurements were available nevertheless the difference between the reflecting surface of the laser and radar is consistent with snow depth from climatology and the analysis techniques developed in this paper will be useful for future radar and laser altimetry comparisons

a ground based radar backscatter investigation in the percolation zone of the greenland ice sheet abstract satellite radar altimeters and scatterometers deployed over ice sheets experience backscatter from the surface and from within the snowpack termed surface and volume backscatter respectively in order to assess the errors in satellite altimeter measurements it is vital to know where the return is originating from in the snowpack this return can vary spatially and temporally seasonal variations in the volume backscatter can be a major complicating factor in the radar return from the percolation zone ground based step frequency radar was deployed in the percolation zone of the greenland ice sheet at 1945 m elevation 69 51n 47 15w previous measurements in this area made by scientists from the byrd polar research centre and the university of kansas undertaken prior to summer melt events have shown the strongest backscatter from ice features at around 1 m depth buried beneath the previous end of summer surface in autumn 2004 radar measurements in the ku band with bandwidths of 1 and 8 ghz were made alongside detailed stratigraphic observations within a 1 km 2 site the radar results revealed no continuous reflecting horizons in the upper 3 5 m of the firn shallow cores and snowpits also indicated that there were no spatially continuous stratigraphic horizons across the study site an average electromagnetic wave velocity of 2 11 0 05 10 8 m s 1 was determined for the upper metre of the firn surface and volume backscatter at vertical incidence were calculated using a standard model the contribution of the surface backscatter to the total backscatter was on average 6 db higher than that of the volume backscatter however at the higher 8 ghz bandwidth the strongest return frequently originated not from the surface but from within the upper 30 cm of the snowpack most probably from thin ice layers at 1 ghz bandwidth these ice layers were not always resolved their return merged with the surface return causing it to broaden with the peak and leading edge moving down modelling using density and thickness measurements from shallow cores and snowpits showed that the backscatter from these shallow thin ice layers could be stronger than the surface return owing to constructive interference from the top and base of the layers

18o concentrations in sea ice of the weddell sea antarctica we present data on ice texture salinity and delta o 18 obtained from identical sections of ice cores during the winter weddell sea project 1986 on rv polarstern from july through august 1986 in the longitude range between 5 degrees w and 7 degrees e we find no uniquely definable relationship between delta o 18 values and ice texture in a particular section however most of the snow ice as well as some sections of frazil ice ar found to have negative delta o 18 concentrations this is due to varying degrees of admixtures of meteoric ice snow and sea water during formation of snow ice in contrast to common assumptions our results seem to indicate that a snow cover contributes positively to sea ice growth rather than slowing down the overall growth rate based on a simple model we have estimated the contributions of meteoric ice mean of 3 3 and the combined meteoric ice sea water fraction a minimum of 7 6 to the total ice thickness for the majority of the sampled floes although this is only a moderate contribution to the overall mass balance in the absence of congelation growth it nevertheless enhances ice growth in general this hypothesis is dependently supported by our snow and ice thickness data wadhams and others 1987 which demonstrate that the depression of the snow ice interface below the water line i e a negative freeboard and the formation of snow ice is a common occurrence in the weddell sea therefore we hypothesize that the major part of the observed apparent increase in ice thickness between our inbound and outbound tracks of wwsp 86 may not be derived from regular thermodynamically driven congelation growth but rather from the snow ice component in floes of the weddell sea

fine resolution radar altimeter measurements on land and sea ice satellite radar altimeter ra measurements are important for continued monitoring of rapidly changing polar regions in 2010 the european space agency launched cryosat 2 carrying siral a ku band ra with objectives of determining the thickness and extent of sea ice and the topography of the ice sheets one difficulty with ku band radar surveys over snow and ice is unknown penetration of ra signal into snow cover improving our understanding of the interactions of ra signals with snow and ice is needed to produce accurate elevation products to this end we developed a low power ultrawideband 12 18 ghz ra for airborne surveys to provide fine resolution measurements capable of detecting both scattering from the surface and layers within sea ice and ice sheets these measurements provide a means of identifying the dominant scattering location of lower resolution ra measurements comparable to satellite based instruments we generated two products a full bandwidth waveform fbw to identify scattering targets at fine resolution and a reduced bandwidth waveform rbw to represent conventional ra measurements retrackers are used to generate height estimates over various surface conditions for comparisons over ice sheets the leading edge tracker provided consistent ice surface elevation measurements between the fbw and rbw results however there were significant differences between the results from the centroid tracker over sea ice the location of the dominant return between the results from snow covered sea ice is highly variable this paper provides an overview of ra surveys in polar regions a description of the cresis system and a discussion of the results

radar measurements of melt zones on the greenland ice sheet surface based microwave radar measurements were performed at a location on the western flank of the greenland ice sheet here firn metamorphosis is dominated by seasonal melt which leads to marked contrasts in the vertical structure of winter and summer firn this snow regime is also one of the brightest radar targets on earth with an average backscatter coefficient of 0 db at 5 3 ghz and an incidence angle of 25 by combining detailed observations of firn physical properties with ranging radar measurements we find that the glaciological mechanism associated with this strong electromagnetic response is summer ice lens formation within the previous winter s snow pack this observation has important implications for monitoring and understanding changes in ice sheet volume using space borne microwave sensors

sea ice albedo climate feedback mechanism abstract the sea ice albedo feedback mechanism over the arctic ocean multiyear sea ice is investigated by conducting a series of experiments using several one dimensional models of the coupled sea ice atmosphere system in its simplest form ice albedo feedback is thought to be associated with a decrease in the areal cover of snow and ice and a corresponding increase in the surface temperature further decreasing the areal cover of snow and ice it is shown that the sea ice albedo feedback can operate even in multiyear pack ice without the disappearance of this ice associated with internal processes occurring within the multiyear ice pack e g duration of the snow cover ice thickness ice distribution lead fraction and melt pond characteristics the strength of the ice albedo feedback mechanism is compared for several different thermodynamic sea ice models a new model that includes ice thickness distribution the ebert and curry model the maykut and untersteiner model and the semtner level 3 an

an ultra wideband microwave radar for measuring snow thickness on sea ice and mapping near surface internal layers in polar firn sea ice is generally covered with snow which can vary in thickness from a few centimeters to 1 m snow cover acts as a thermal insulator modulating the heat exchange between the ocean and the atmosphere and it impacts sea ice growth rates and overall thickness a key indicator of climate change in polar regions snow depth is required to estimate sea ice thickness using freeboard measurements made with satellite altimeters the snow cover also acts as a mechanical load that depresses ice freeboard snow and ice above sea level freeboard depression can result in flooding of the snow ice interface and the formation of a thick slush layer particularly in the antarctic sea ice cover the center for remote sensing of ice sheets cresis has developed an ultra wideband microwave radar capable of operation on long endurance aircraft to characterize the thickness of snow over sea ice the low power 100 mw signal is swept from 2 to 8 ghz allowing the air snow and snow ice interfaces to be mapped with 5 cm range resolution in snow this is an improvement over the original system that worked from 2 to 6 5 ghz from 2009 to 2012 cresis successfully operated the radar on the nasa p 3b and dc 8 aircraft to collect data on snow covered sea ice in the arctic and antarctic for nasa operation icebridge the radar was found capable of snow depth retrievals ranging from 10 cm to 1 m we also demonstrated that this radar can be used to map near surface internal layers in polar firn with fine range resolution here we describe the instrument design characteristics and performance of the radar

antarctic sea ice elevation from satellite radar altimetry 1 in situ measurements of sea ice thickness from ship and upward looking sonar are used to assess the potential for satellite radar altimetry to provide information on antarctic sea ice thickness a climatology of satellite ice elevation estimates is compared to an antarctic sea ice thickness climatology made from the antarctic sea ice processes and climate aspect data set in addition monthly regional satellite ice elevation estimates are compared to uls ice draft data the results show reasonable spatial agreement between the satellite and in situ data and show regional signals of change in ice elevation in line with that which would be expected the results show some promise for providing information on antarctic ice thickness from radar altimetry missions such as cryosat however further studies into snow and ice density and the radar penetration into the antarctic snow cover are required

asiras airborne radar resolves internal annual layers in the dry snow zone of greenland 1 we present data from the european space agency s airborne sar interferometric radar altimeter system asiras flown during the cryovex 2004 field calibration validation campaign and new high resolution depth profiles of snow density measured in the field by neutron scattering we combine these data to calculate the depth of internal reflecting horizons in the asiras data the high resolution density data allow us to identify annual layers in the snow density profile and correlate their peaks with the reflecting horizons we use the thickness of the annual layers combined with the density profile to determine the spatial and temporal pattern of snow accumulation along the radar track for a period of 6 years from 1995 2002 our mean annual accumulation rate is 0 47 0 09 ma 1 water equivalent in agreement with the value determined from a core taken in this location in 1992 similarly our inter annual variability shows the same trends as recent model estimates over the entire ice sheet because asiras was designed to mimic as closely as possible the characteristics of the sar interferometric radar altimeter siral the principal payload of cryosat the detection of internal layering with asiras illuminates the possibility of detecting internal layers with a space borne radar and thus the possible application of this technique to the dry snow zones of antarctica greenland and smaller ice bodies

a new high resolution digital elevation model of greenland fully validated with airborne laser altimeter data a new digital elevation model of the greenland ice sheet and surrounding rock outcrops has been produced at 1 km postings from a comprehensive suite of satellite remote sensing and cartographic data sets height data over the ice sheet were mainly from ers 1 and geosat radar altimetry these data were corrected for a slope dependent bias that had been identified in a previous study the radar altimetry was supplemented with stereophotogrammetric data sets synthetic aperture radar interferometry and digitized cartographic maps over regions of bare rock and where gaps in the satellite altimeter coverage existed the data were interpolated onto a regular grid with a spacing of 1 km the accuracy of the resultant digital elevation model over the ice sheet was assessed using independent and spatially extensive measurements from an airborne laser altimeter that had an accuracy of between 10 and 12 cm in a comparison with the laser altimetry the digital elevation model was found to have a slope dependent accuracy ranging from 1 04 1 98 m to 0 06 14 33 m over the ice sheet for a slope range of 0 0 1 0 the mean accuracy over the whole ice sheet was 0 33 6 97 m over the bare rock areas the accuracy ranged from 20 to 200 m dependent on the data source available the new digital elevation model was used as an input data set for a positive degree day model of ablation the new elevation model was found to reduce ablation by only 2 compared with using an older 2 5 km resolution model which suggests that resolution induced errors in estimating ablation are no longer important

rapid response of modern day ice sheets to external forcing abstract the great ice sheets covering antarctica and greenland were traditionally believed to take thousands of years to respond to external forcing recent observations suggest however that major changes in the dynamics of parts of the ice sheets are taking place over timescales of years these changes were not predicted by numerical models and the underlying cause s remains uncertain it has been suggested that regional oceanic and or atmospheric warming are responsible but separating the influence and importance of these two forcings has not been possible in most cases the role of atmospheric versus oceanic control remains uncertain here we review the observations of rapid change and discuss the possible mechanisms in the light of advances in numerical modelling and our understanding of the processes that may be responsible

highly branched isoprenoids reveal onset of deglaciation followed by dynamic sea ice conditions in the western amundsen sea antarctica abstract the amundsen sea drainage sector of the west antarctic ice sheet wais is widely regarded as a candidate for triggering potential wais collapse the grounded ice sheet drains into the amundsen sea embayment and is thereby buttressed by its fringing ice shelves which have thinned at an alarming rate satellite based observations additionally reveal a considerable long term decrease in sea ice cover in the amundsen sea over the last two decades although the long term significance of this trend is unclear due to the short instrumental record since the 1970s in this context investigations of past sea ice conditions are crucial for improving our understanding of the influence that sea ice variability has on the adjacent marine environment as well as any role it plays in modulating ice shelf and ice sheet dynamics in this study we apply novel organic geochemical biomarker techniques to a marine sediment core from the western amundsen sea shelf in order to provide a valuable long term perspective on sea ice conditions and the retreat of the getz ice shelf during the last deglaciation we analysed a specific biomarker lipid called ipso25 alongside a phytoplankton biomarker and sedimentological parameters and additionally applied diatom transfer functions for reconstructing palaeo sea ice coverage this multi proxy data set reveals a dynamic behaviour of the getz ice shelf and sea ice cover during the deglaciation following the last ice age with potential linkages to inter hemispheric seesaw climate patterns we further apply and evaluate the recently proposed pipso25 approach for semi quantitative sea ice reconstructions and discuss potential limitations

using the temporal variability of satellite radar altimetric observations to map surface properties of the antarctic ice sheet the problem of measuring surface height and snowpack characteristics from satellite radar altimeter echoes is investigated in this paper we perform an analysis of the ers1 altimeter dataset acquired during a 3 day repeat orbit the analysis reveals that there are temporal variations in shapes of the radar altimeter echo and that these variations are linked to meteorological phenomena the time and space scales over which these variations apply are a few to tens of days and a few hundred kilometres respectively this phenomenon if not accounted for can create error in the height measurement a numerical echo model is used to recover snowpack characteristics by taking advantage of the temporal variations of the radar echoes a map of penetration depth of the radar waves in the ku band over the antarctic continent is obtained and suggests that grain size produces the dominant effect on radar extinction in the snowpack at this frequency finally a procedure is proposed to correct the height measurement within the context of ice sheet mass balance survey

the accuracy of digital elevation models of the antarctic continent abstract the accuracy of two widely used digital elevation models of antarctica was assessed using data from the geoscience laser altimeter system onboard icesat the digital elevation models were derived from satellite radar altimeter and terrestrial data sets the first termed jlb97 was produced predominantly from ers 1 data while the second termed rampv2 included other sources of data in areas of high relief and poor coverage by ers 1 the accuracy of the models was examined as a function of surface slope and original data source large errors in excess of 100 m were ubiquitous in both models in areas where terrestrially derived elevation data had been used but were more extensive in rampv2 elsewhere the systematic error bias was found to be a monotonic function of slope for jlb97 with a more complex less predictable bias for rampv2 the magnitude of the global slope dependent bias ranged from less than a metre to slightly over 10 m but with much larger regional deviations the random error ranged from about 1 m to over 100 m depending on the dem and slope the random error was consistently over a factor two larger for rampv2 compared to jlb97

a comparison of recent elevation change estimates of the devon ice cap as measured by the icesat and envisat satellite altimeters we have used surface elevation measurements acquired by the ice cloud and land elevation satellite geoscience laser altimeter system glas and envisat radar altimeter 2 ra 2 satellite altimeters to assess the elevation change of the 13 700 km2 devon ice cap dic in arctic canada between 2002 and 2008 we present algorithms for the retrieval of elevation change rates over ice caps using data acquired from these satellites a comparison of glas elevation data to those acquired by the ra 2 shows reasonable agreement between the two instruments the root mean square elevation change difference was 56 cm and the correlation coefficient between the two data sets was 0 68 using only ra 2 elevation measurements which are spatially and temporally more continuous we determined the elevation change rate of the areas of the dic where the surface geometry allows the ra 2 retracker to maintain lock this includes most of the dic excluding large parts of the eastern half of the ice cap the elevation change rate was found to be insignificant given a statistical estimate of the measurement error 0 09 0 29 m a we also present an assessment of the regional variations of the dic elevation change including a significant 0 71 0 49 m a elevation change rate of the 1980 km2 western arm furthermore we present evidence of a localized 2 m drop in the surface elevation of the south croker bay glacier during summer 2007 this drop is apparent within both satellite data sets and we interpret this signal to reflect a sudden speedup of the glacier

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