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We present the design and field test results for a 600 to 900 MHz polarimetric ice penetrating radar that can be operated on the ground or from an airborne platform. This system is part of a development to build a dual band (VHF/UHF) polarimetric ice sounding radar suite. The VHF radar operates over 140-215 MHz and is essentially a modified version of the multi-channel 3D imaging system reported in [1]. The UHF radar, the focus of this work, is an adaptation of the CReSIS Accumulation Radar, which operates from 600 to 900 MHz [2]. The radar system uses a custom-designed, dual-polarized 4x4 antenna array with increased peak and average transmit power levels, which together provide additional sensitivity with respect to prior system renditions. The UHF radar incorporates a new receiver [3] that uses controlled analog compression via RF limiters to increase the instantaneous dynamic range. We designed the instrument setup to be towed by snowmobiles and operated at nominal speeds of 4 to 8 m/s. The relatively slow motion helps improve SNR through an increase in coherent averaging due to the longer dwell time. Although the focus of the field test is on ground-based work, the electronics are designed to also support airborne operation. 

Uncertainty about sea-level rise is dominated by uncertainty about iceberg calving, mass loss from glaciers or ice sheets by fracturing. Review of the rapidly growing calving literature leads to a few overarching hypotheses. Almost all calving occurs near or just downglacier of a location where ice flows into an environment more favorable for calving, so the calving rate is controlled primarily by flow to the ice margin rather than by fracturing. Calving can be classified into five regimes, which tend to be persistent, predictable, and insensitive to small perturbations in flow velocity, ice characteristics, or environmental forcing; these regimes can be studied instrumentally. Sufficiently large perturbations may cause sometimes-rapid transitions between regimes or between calving and noncalving behavior, during which fracturing may control the rate of calving. Regime transitions underlie the largest uncertainties in sea-level rise projections, but with few, important exceptions, have not been observed instrumentally. This is especially true of the most important regime transitions for sea-level rise. Process-based models informed by studies of ongoing calving, and assimilation of deep-time paleoclimatic data, may help reduce uncertainties about regime transitions. Failure to include calving accurately in predictive models could lead to large underestimates of warming-induced sea-level rise. ▪ Iceberg calving, the breakage of ice from glaciers and ice sheets, affects sea level and many other environmental issues. ▪ Modern rates of iceberg calving usually are controlled by the rate of ice flow past restraining points, not by the brittle calving processes. ▪ Calving can be classified into five regimes, which are persistent, predictable, and insensitive to small perturbations. ▪ Transitions between calving regimes are especially important and with warming might cause faster sea-level rise than generally projected. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 51 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 

Abstract Hercules Dome is a prospective ice‐core site due to its setting in the bottleneck between East and West Antarctica. If ice from the last interglacial period has been preserved there, it could provide critical insight into the history of the West Antarctic Ice Sheet. The likelihood of a continuous, well‐resolved, easily interpretable climate record preserved in ice extracted from Hercules Dome depends in part on the persistence of ice‐flow dynamics at the divide. Significant changes in ice drawdown on either side of the divide, toward the Ross or Ronne ice shelves, could change the relative thickness of layers and the deposition environment represented in the core. Here, we use radar sounding to survey the ice flow at Hercules Dome. Repeated radar acquisitions show that vertical velocities are consistent with expectations for an ice divide with a frozen bed. Polarimetric radar acquisitions capture the ice‐crystal orientation fabric (COF) which develops as ice strains, so it depends on both the pattern of ice flow and the time over which flow has been consistent. We model the timescales for COF evolution, finding that the summit of Hercules Dome has been dynamically stable in its current configuration, at least over the last five thousand years, a time period during which the Antarctic ice sheet was undergoing significant retreat at its margins. The evident stability may result from a prominent bedrock ridge under the divide, which had not been previously surveyed and has therefore not been represented in the bed geometry of coarsely resolved ice‐sheet models. 

Ice sheets reshape Earth’s surface. Maps of the landscape formed by past ice sheets are our best tool for reconstructing historic ice sheet behavior. But models of glacier erosion and deposition that explain mapped features are relatively untested, and without observations of landforms developing in situ, postglacial landscapes can provide only qualitative insight into past ice sheet conditions. Here we present the first swath radar data collected in Antarctica, demonstrating the ability of swath radar technology to map the subglacial environment of Thwaites Glacier (West Antarctica) at comparable resolutions to digital elevation models of deglaciated terrain. Incompatibility between measured bedform orientation and predicted subglacial water pathways indicates that ice, not water, is the primary actor in initiating bedform development at Thwaites Glacier. These data show no clear relationship between morphology and glacier speed, a weak relationship between morphology and basal shear stress, and highlight a likely role for preexisting geology in glacial bedform shape. 

Abstract Bedforms of Thwaites Glacier, West Antarctica both record and affect ice flow, as shown by geophysical data and simple models. Thwaites Glacier flows across the tectonic fabric of the West Antarctic rift system with its bedrock highs and sedimentary basins. Swath radar and seismic surveys of the glacier bed have revealed soft‐sediment flutes 100 m or more high extending 15 km or more across basins downglacier from bedrock highs. Flutes end at prominent hard‐bedded moats on stoss sides of the next topographic highs. We use simple models to show that ice flow against topography increases pressure between ice and till upglacier along the bed over a distance that scales with the topography. In this basal zone of high pressure, ice‐contact water would be excluded, thus increasing basal drag by increasing ice‐till coupling and till flux, removing till to allow bedrock erosion that creates moats. Till carried across highlands would then be deposited in lee‐side positions forming bedforms that prograde downglacier over time, and that remain soft on top through feedbacks that match till‐deformational fluxes from well upglacier of the topography. The bedforms of the part of Thwaites surveyed here are prominent because ice flow has persisted over a long time on this geological setting, not because ice flow is anomalous. Bedform development likely has caused evolution of ice flow over time as till and lubricating water were redistributed, moats were eroded and bedforms grew.