Akins, Alex B., Alan B Tanner, Andreas Colliander, Nicole-Jeanne Schlegel, Kenza Boudad, and Igor Yanovsky, January 2025: A sparse synthetic aperture radiometer constellation concept for remote sensing of Antarctic ice sheet temperature. IEEE Transactions on Geoscience and Remote Sensing, 63, DOI:10.1109/TGRS.2025.3534466. Abstract
We present a concept for UHF/L band (0.5-2 GHz) remote sensing of Antarctic ice sheet internal temperature using a highly sparse synthetic aperture radiometer constellation. This concept leverages the relative stability of ice sheet thermal emission over long temporal periods to gradually assemble a collection of array baselines which are jointly transformed to develop large image facets. We formulate a calculation of minimum array complexity based on the desired sensitivity, spatial resolution, and time available for observations. We determine from this calculation that such a system can achieve 1-10 kilometer spatial resolution (significantly finer than the program of record) over monthly to yearly timescales with as few as 10-20 elements; even fewer elements are required for observing only the ice sheet center. The inverse problem of reconstructing image facets from mixed-pointing and mixed-configuration observations is posed using a Fourier domain data constraint with a total variational regularization in the image domain. This approach enables image formation from heterogeneous observations while mitigating artifacts. We present a notional constellation design for three satellites which could accomplish the necessary baseline sampling by rotating the phase and semi-major axis of spacecraft relative positions in planar circular orbits. We demonstrate image formation with observing system simulations leveraging predictions of Antarctica’s multi-wavelength brightness temperature computed from ice sheet thermomechanical and radiative transfer models.
Felikson, Denis, David R Rounce, John Fasullo, Angelica R Rodriguez, Surendra Adhikari, Brett Buzzanga, Sönke Dangendorf, Robert E Kopp, Richard B Lammers, J T Reager, Doug Brinkerhoff, Beáta Csathó, Manuela Girotto, Benjamin D Hamlington, Erik R Ivins, Praveen Kumar, Eric Larour, R Steven Nerem, Sophie Nowicki, Nicole-Jeanne Schlegel, Jan-Erik Tesdal, and Matthew Weathers, et al., September 2025: Progress and future directions in constraining uncertainties in sea-level projections using observations. Nature Climate Change, 15, DOI:10.1038/s41558-025-02437-4. Abstract
Coastal planning, mitigation and adaptation efforts rely on credible sea-level projections generated by physical models. However, the large uncertainties in these projections pose a challenge for policymakers. Here we provide an overview of the main sources of uncertainty in model projections of sea-level change on multi-decadal to centennial timescales and we offer perspectives on the use of observations to narrow uncertainties. We propose several directions for future research, including improvements in emerging emulation techniques, more systematic quantification of uncertainty structure within both observations and models, lengthening observational records of processes, and expanding collaborations across physical and social sciences. Advancements in these areas are urgently needed, as the next phase of the Intergovernmental Panel on Climate Change assessment cycle gets underway.
Hossan, Alamgir, Andreas Colliander, Baptiste Vandecrux, Nicole-Jeanne Schlegel, Joel Harper, Shawn Marshall, and Julie Z Miller, October 2025: Retrieval and validation of total seasonal liquid water amounts in the percolation zone of the Greenland ice sheet using L-band radiometry. The Cryosphere, 19(10), DOI:10.5194/tc-19-4237-20254237-4258. Abstract
Quantifying the total liquid water amounts (LWAs) in the Greenland ice sheet (GrIS) is critical for understanding GrIS firn processes, mass balance, and global sea level rise. Although satellite microwave observations are very sensitive to ice sheet melt and thus can provide a way of monitoring the ice sheet melt globally, estimating total LWA, especially the subsurface LWA, remains a challenge. Here, we present a microwave retrieval of LWA over Greenland using enhanced-resolution L-band brightness temperature (TB) data products from the National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) satellite for the 2015–2023 period. L-band signals receive emission contributions deep in the ice sheet and are sensitive to the liquid water content (LWC) in the firn column. Therefore, they can estimate the surface-to-subsurface LWA, unlike higher-frequency signals (e.g., 18 and 37 GHz bands), which are limited to the top few centimeters of the surface snow during the melt. We used vertically polarized TB (V-pol TB) with empirically derived thresholds to detect liquid water and identify distinct ice sheet zones. A forward model based on radiative transfer (RT) in the ice sheet was used to simulate TB. The simulated TB was then used in an inversion algorithm to estimate LWA. Finally, the retrievals were compared with the LWA obtained from two sources. The first source was a locally calibrated ice sheet energy and mass balance (EMB) model, and the second source was the Glacier Energy and Mass Balance (GEMB) model within NASA's Ice-sheet and Sea-level System Model (ISSM). Both models were forced by in situ measurements from six automatic weather stations (AWSs) of the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and the Greenland Climate Network (GC-Net) located in the percolation zone of the GrIS. The retrievals show generally good agreement with both the references, demonstrating the potential for advancing our understanding of ice sheet physical processes to better project Greenland's contribution to the global sea level rise in response to the warming climate.
Hossan, Alamgir, Andreas Colliander, Nicole-Jeanne Schlegel, Joel Harper, Lauren Andrews, Jana Kolassa, Julie Z Miller, and Richard I Cullather, November 2025: Evaluation of wet snow dielectric mixing models for L-band radiometric measurement of liquid water content in Greenland's percolation zone. The Cryosphere, 19(11), DOI:10.5194/tc-19-6077-20256077-6102. Abstract
The effective permittivity of wet snow and firn links the snow microphysics to its radiometric signature, making it essential for accurately estimating the liquid water amount (LWA) in the snowpack. Here, we compare ten commonly used microwave dielectric mixing models for estimating LWA in wet snow and firn using L-band radiometry. We specifically focus on the percolation zone of the Greenland Ice Sheet (GrIS), where the average volume fraction of liquid water is between 0 % and 6 %. We used L-band brightness temperature (TB) observations from the NASA Soil Moisture Active Passive (SMAP) mission in an inversion-based framework to estimate LWA, applying different dielectric mixing formulations in the forward simulation. We compared the effective permittivities of the mixing models over a range of conditions and evaluated their impact on the LWA retrieval. We also compared the LWA retrievals to the corresponding LWA from two state-of-the-art Surface Energy and Mass Balance (SEMB) models. Both SEMB models were forced with in situ measurements from automatic weather stations (AWS) of the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and Greenland Climate Network (GC-Net) located in the percolation zone of the GrIS and initialized with relevant in situ profiles of density, stratigraphy, and sub-surface temperature measurements. The results show that the mixing models produce substantially different real and imaginary parts of the dielectric constant, significantly impacting the LWA retrieved from the TB. The correspondence with the SEMB-derived LWA varied by model and site, with correlation coefficients ranging from 0.67 to 0.98 and RMSD values between 5.4 and 23.9 mm. Overall, the power law-based empirical models demonstrated better performance for 2023 melt season. The analysis supports informed selection of dielectric mixing models for improved LWA retrieval accuracy.
Houriez, Luc, Eric Larour, Lambert Caron, and Nicole-Jeanne Schlegel, et al., October 2025: Reinforced ridges in Thwaites Glacier yield insights into resolution requirements for coupled ice sheet and solid Earth models. The Cryosphere, 19(10), DOI:10.5194/tc-19-4355-20254355-4372. Abstract
Grounding line retreat in the Amundsen Sea Embayment (ASE) is expected to drive the largest Antarctic contribution to sea-level rise over the coming centuries. In this region, low mantle viscosity accelerates the solid Earth's viscoelastic response to ice mass loss, leading to a stabilizing feedback via bedrock uplift and local sea-level fall: effects governed by gravitation, rotation, and deformation (GRD) processes. These stabilizing effects can be enhanced by the presence of ridges and confinements, which have been identified in ASE but can only be represented by using high model resolutions. Here, we investigate how coupled ice sheet–GRD simulations respond to (i) ice sheet model resolution, (ii) GRD spatial resolution, and (iii) the coupling interval between the two systems. We consider two model setups with distinct mesh structures, surface mass balance (SMB) forcings, and basal melt parametrizations. Our findings underscore the importance of feedback mechanisms at kilometer scales and decadal to sub-decadal timescales. Resolving bedrock topography at 2 km instead of 1 km raises the projected sea level by 7.1 % in 2100 and lowers it by 18.8 % in 2350. In our most conservative setup, we find that bedrock uplift delays grounding line retreat by up to 30 years on ridges located 34 and 75 km upstream of Thwaites Glacier's current grounding line. This mechanism plays a key role in reducing Thwaites' sea-level contribution by up to 53.1 % in 2350. These findings underscore the critical need to reduce uncertainties in bedrock topography.
