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.

The accuracy of the travel-time–velocity and travel-time–depth profile derived from ground-penetrating radar (GPR) common-midpoint (CMP) surveys at different frequencies is investigated for the first time ever by direct comparison with the profile calculated from high-resolution dielectric-profiling (DEP) ice-core data. In addition, we compare two travel-time profiles calculated from ice-core density data by means of different dielectrical mixture models with the DEP-based profile. CMP surveys were carried out at frequencies of 25,50,100 and 200 MHz near the new European deep-drilling site DML05 in Dronning Maud Land, Antarctica, during the 1998/99 field season. An improved scanning capacitor for high-resolution DEP and a γ-densiometer for density measurements were used to determine the complex dielectric constant and the density at 5 mm increments along the ice core B32, retrieved in 1997/98 at DML05. The comparisons with DEP- and density-based velocity series show that the CMP velocity series are slightly higher but asymptotically approach the core-based velocities with depth. Root-mean-square differences of the DEP velocity series range between 8% for the 25 MHz CMP and 2% in the case of the 200 MHz survey. Density-based velocities differ from the DEP velocities by 51 %. The travel-time–depth series calculated from the interval velocities show a better agreement between all series than the velocity series. Differences are 5.7–1.4% for the 25 and 200 MHz CMP measurements, and <0.6% for the density data. Based on these comparisons, we evaluate the accuracy with which the depth of electromagnetic reflectors observed in common-offset profiles can be determined, and discuss reasons for the observed differences between CMP- and core-based profiles. Moreover, we compare the errors determined from the field measurements with those estimated from GPR system characteristics to provide a measure that can be used to estimate the accuracy of GPR analyses for the planning of GPR campaigns. Our results show that CMP surveys are a useful technique to determine the depth of radar reflectors in combination with common-offset measurements, especially on a region-wide basis.

Abstract. In September 2019, the research icebreaker Polarstern started the largest multidisciplinary Arctic expedition to date, the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) drift experiment. Being moored to an ice floe for a whole year, thus including the winter season, the declared goal of the expedition is to better understand and quantify relevant processes within the atmosphere–ice–ocean system that impact the sea ice mass and energy budget, ultimately leading to much improved climate models. Satellite observations, atmospheric reanalysis data, and readings from a nearby meteorological station indicate that the interplay of high ice export in late winter and exceptionally high air temperatures resulted in the longest ice-free summer period since reliable instrumental records began. We show, using a Lagrangian tracking tool and a thermodynamic sea ice model, that the MOSAiC floe carrying the Central Observatory (CO) formed in a polynya event north of the New Siberian Islands at the beginning of December 2018. The results further indicate that sea ice in the vicinity of the CO (<40 km distance) was younger and 36 % thinner than the surrounding ice with potential consequences for ice dynamics and momentum and heat transfer between ocean and atmosphere. Sea ice surveys carried out on various reference floes in autumn 2019 verify this gradient in ice thickness, and sediments discovered in ice cores (so-called dirty sea ice) around the CO confirm contact with shallow waters in an early phase of growth, consistent with the tracking analysis. Since less and less ice from the Siberian shelves survives its first summer (Krumpen et al., 2019), the MOSAiC experiment provides the unique opportunity to study the role of sea ice as a transport medium for gases, macronutrients, iron, organic matter, sediments and pollutants from shelf areas to the central Arctic Ocean and beyond. Compared to data for the past 26 years, the sea ice encountered at the end of September 2019 can already be classified as exceptionally thin, and further predicted changes towards a seasonally ice-free ocean will likely cut off the long-range transport of ice-rafted materials by the Transpolar Drift in the future. A reduced long-range transport of sea ice would have strong implications for the redistribution of biogeochemical matter in the central Arctic Ocean, with consequences for the balance of climate-relevant trace gases, primary production and biodiversity in the Arctic Ocean.

ABSTRACT. Since the austral summer of 1994^95 the Alfred Wegener Institute has carried out airborne radio-echo sounding (RES) measurements in Antarctica with its newly designed RES system. Since 1995^96 an ongoing pre-site survey for an ice-coring drill site in Dronning Maud Land has been carried out as part of the European Project for Ice Coring in Antarctica. The survey covers an area of 948 000 km 2 , with >49 500 km of airborne RES obtained from >200 hours of flight operation flown during the period 1994^97. In this paper, first results of the airborne RES survey are graphically summarized as newly derived maps of the ice thickness and subglacial topography, as well as a three-dimensional view of surface and subglacial bed and outcrop topography, revealing a total ice volume of 1.48610 6 km 3 .

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