Haas, Christian
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Item type:Publication, Advancing microwave radar retrievals of snow depth on sea ice: toward full characterisation of the snow and sea-ice layers(2022-03-16); ; ; Snow is a key factor in the sea-ice and Earth's climate systems that modifies the physical, climatic, and biogeochemical processes taking place. One of its most important impacts is in regulating sea-ice growth and melt. Despite its importance, little is known about the spatial and temporal distribution of snow depth on sea ice on the regional to global scales. Snow is tightly coupled to the highly dynamic sea-ice and atmospheric conditions and it is, therefore, very heterogeneous and constantly evolving both in space and in time. As a spatially and temporally representative, global, year-round product of snow depth observations on sea ice does not exist to this date, applications often have to rely on climatological values that do not necessarily hold true in the rapidly warming global climate. The unknown properties directly translate into the uncertainty of the result. This dissertation takes on the ambitious goal of working toward full characterisation of the snow and sea-ice layers. To achieve that, the focus is on advancing microwave radar retrievals of snow depth on sea ice. Enhanced snow depth observations will enable improving other measurements of sea-ice related parameters, most importantly sea-ice thickness, and in joint analysis of coincident sea-ice measurements estimating sea-ice bulk density becomes possible. In the first step, field experiments with ground-based C and K band pulse radars are carried out to investigate microwave penetration into the snow cover. The results show the K band microwaves expectedly reflect from the snow surface while the C band microwaves penetrate closer to the snow–sea-ice interface potentially enabling dual-frequency snow depth retrieval in less than half of the studied cases and only on first-year ice. In the second step, radar measurements of snow depth on sea ice are upscaled by using an airborne radar in the western Arctic Ocean in 2017–2019. A high-sensitivity, ultra-wideband, frequency-modulated continuous-wave (FMCW) radar is integrated to the instrument configuration of the Alfred Wegener Institute's (AWI) IceBird sea-ice campaigns. Snow depth retrievals with a custom algorithm based on signal peakiness from the radar measurements at a low altitude of 200 ft show good consistency against high altitude measurements at 1500 ft, which are comparable to previous acquisitions. At the nominal low altitude of the IceBird surveys, the small, two-metre radar footprint increases the spatial resolution and reduces the effect of off-nadir targets. Validation against ground measurements reveal a sub-centimetre mean bias, which is below the sensor resolution. As the main result of this step, the AWI IceBird surveys are now capable of discriminating between the snow and sea-ice layers. In the third step, the full AWI IceBird sensor configuration, including airborne laser, radar, and electromagnetic induction sounding instruments, is exploited by collocating the coincident thickness and freeboard measurements and tracking the locations of air–snow, snow–sea-ice, and sea-ice-water interfaces for more than 3000 km along survey paths over different sea-ice types. Assuming values for snow and sea-water densities and that the sea-ice cover is in isostatic equilibrium, it is possible to derive sea-ice bulk density. The results show that the ice-type averaged densities for first-year and multi-year ice are higher than and do not differ as much as widely used values from previous studies. This highlights the demand of algorithms to adapt to changing sea-ice density in satellite altimetry retrievals of sea-ice thickness. Finally, a negative-exponential parametrisation of sea-ice bulk density is derived using sea-ice freeboard as the predictor variable for future applications. In conclusion, this dissertation takes important advancing steps in characterising the snow and sea-ice layers. Previously, the airborne AWI IceBird surveys carried out in late-winter were only able to measure the combined thickness of the snow and sea-ice layers but now, after successful integration of the FMCW radar and in combination with the airborne laser scanner measurements, it is possible to track the locations of all three interfaces bounding the snow–sea-ice system. Such airborne multi-instrument measurements of snow depth, sea-ice thickness, and freeboard are important data sets in their own right to complement the scarce observations of sea-ice related parameters in remote areas of the polar regions, but a joint analysis allows deriving further key parameters like sea-ice density. The results of this dissertation can be applied to improve retrievals of geophysical sea-ice parameters from the soon 30-year long satellite altimetry data record, which in turn will contribute to enhance monitoring the climate-sensitive sea-ice cover and modelling future projections of the changing global climate.doctoral thesis676 313 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Seasonal evolution and spatial variability of sea ice properties from multi-frequency electromagnetic induction sounding(2026-01-29); ; ; This thesis advances electromagnetic (EM) induction sounding methods for measuring sea ice thickness and internal layers, focusing on the sub-ice platelet layer (SIPL) and slush. The SIPL is a layer of loosely aggregated ice crystals beneath Antarctic sea ice, characterized by exceptionally high biological activity, substantial regional contributions to sea ice mass, and its role as an indicator of ice shelf--ocean interactions. In the first study, we analyzed approximately 1000 km of multi-frequency EM survey data collected within a single season on fast ice in Atka Bay, eastern Weddell Sea, with a GEM-2 towed by a snowmobile. This unprecedented dataset allowed us to resolve both SIPL thickness and ice plus snow thicknesses. Using an open-source inversion framework, detailed spatial maps of SIPL thickness and conductivity were produced. SIPL solid fractions were estimated from SIPL conductivities and ranged from 0.11 to 0.28. Calibration in a zero-conductivity environment minimized sensor drift and offsets. Validation against drill hole measurements showed that both EM-derived SIPL thickness and ice plus snow thickness were accurate within a few decimeters. Results revealed regional patterns of platelet ice accumulation and provided the first bay-wide SIPL map including ice shelf fringes, with modal thicknesses around 5 m, local minima around 2 m and previously uninvestigated local maxima reaching up to 9 m in the south-eastern bay. In the second study, we use the GEM-2 to investigate slush from sea water flooding snow-covered sea ice. In Antarctica, this promotes snow-ice formation, contributing up to 25 % of ice thickness in young first-year ice in the Weddell Sea, while in the Arctic, it is rarer but may become more common under climate change, additionally posing travel safety concerns for Arctic communities. We showed that multi-frequency EM sounding can resolve slush and ice plus snow thickness jointly, with the optimal frequency combination achieving mean absolute errors below 5 cm for slush up to 60 cm in inversions of modelled data. Field surveys in Qikiqtarjuaq, Nunavut, Canada, confirmed robustness under variable conditions for slush up to 20 cm. A reliable height-step calibration routine using a wooden ladder was established and calibration parameters remained stable over two weeks for a GEM-2 sensor of the latest generation. In Antarctica, the combination of platelet ice and surface slush measurements could enable more accurate assessments of sea ice mass balance changes. In the Arctic, this work represents an initial step towards developing an operational system that can support local communities in mapping hazardous areas. Because access to potentially hazardous ice, particularly thin or slush-covered areas, is restricted when using an EM sensor mounted on a sled pulled by a snowmobile, we tested a drone-based system to extend its operational reach. A GEM-2 combined with custom altitude and attitude monitoring was flown as suspended load or sled-towed by a drone over landfast sea ice near Qikiqtarjuaq, Nunavut, Canada. Drone hover tests caused high EM noise at a 4 m distance, decreasing below thresholds required for 5 cm thickness resolution at 7 m. An in-flight calibration was tested, and the resulting calibrated EM signals differed only slightly from those obtained with ladder-based calibration, confirming effective calibration without the use of a ladder. When the GEM-2 was used with the drone, the thickness profiles agreed well with drill hole measurements, with mean thickness differences below 10 cm and closely matching thickness distributions. Flight stability was high, with sensor roll and pitch standard deviations below 3° and maxima around ±10°, ensuring reliable sensor orientation. These results confirm that the drone-borne EM sensor can provide accurate ice thickness measurements while minimizing operator risk and enabling surveys over thin or hazardous ice. Together, these studies advance EM methods for mapping sub-ice platelet and slush layers and introduce innovative approaches for airborne thickness surveys. By providing high-resolution, field-validated tools, this work enhances our ability to monitor and understand the mass balance and structural complexity of polar sea ice and improve safety on sea ice.doctoral thesis50 60 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Spatial heterogeneity and seasonal evolution of surface properties and radiative fluxes of Arctic sea ice(2024-05-31); ; ; ; In recent years, the Arctic sea ice has experienced a significant decline, characterised by the smaller extent, longer melt season, and a shift from thick multi-year ice to thinner first-year ice. As a result, more solar radiative energy is deposited into the Arctic sea ice and the ocean underneath, further enhancing sea ice melt and ocean heat. When the Arctic is transitioning from melt onset to freeze onset, the sea ice surface spatial variability becomes stronger, altering the spatial distribution of radiative energy deposition. Understanding the seasonal evolution and spatial variability of solar radiative fluxes is a key step to broadening our knowledge of the changing Arctic sea ice. In this thesis, I investigate the year-round changes in solar radiative fluxes within the Arctic sea ice, both temporally and spatially. I examine the changes in optical properties during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition (MOSAiC) in 2020. This thesis utilises a wide range of sensors and platforms, ranging from long-term continuous point measurement, to weekly under-ice mapping of light field, and to ice-floe size parameterization. This thesis highlights the spatial variability of the solar radiative fluxes of Arctic sea ice: under the same atmospheric condition and located on the same ice floe, different locations show highly variable evolution. The largest variability is in the middle of the melt season, due to the changing melt pond coverage and status. The sea ice types and surface conditions are crucial for the sea ice energy budget, thus further controlling the melting process. This thesis provides a comprehensive 3-dimensional view of the sea ice radiative fluxes and improves the parameterization of sea ice optical properties. Also, by investigating the effects of spatial surface variability, which is a function of time and area, this thesis guides future observations of the new Arctic sea ice regime. This study bridges in-situ observation to floe-size parameterisation, advances our understanding of the upscaling of solar radiative energy fluxes both onto and through the Arctic sea ice, and deepens our understanding of the impact of sea ice heterogeneity on the large-scale energy budget of the melting Arctic sea ice.doctoral thesis270 783 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Impacts of snow and surface conditions on radiation fluxes through Arctic sea ice during different seasons(2022-01-31); ; ; Sea ice and its snow cover play a key role within the climate and ecosystem. Due to global environmental changes which are amplified in the Arctic Ocean, its sea-ice cover will primarily consist of thin and young sea ice with a reduction in extent. In particular, the area where snow accumulates reduces and the fraction of melt-pond covered sea ice and of openings in the sea-ice cover such as leads increase. Those changes of the surface conditions strongly influence the partitioning of solar radiation. The main objective of this dissertation was to establish relationships between the surface conditions that are observed and expected to dominate in the future Arctic and under-ice radiation. A deeper and broader knowledge of such relationships is especially necessary in spring and autumn during which the under-ice radiation can have significant impacts on the annual energy budget. To achieve that, field measurements collected using a variety of instruments during three campaigns for three different sea-ice types, locations, and seasons were analysed and interpreted. A main result was to derive a new parametrization for snow depth retrieval from spectral under ice-radiation measurements. This was successfully achieved with an accuracy of approximately 5 cm for two ice types, in two locations, during two seasons. In contrast to the established theory that melt ponds act as bright windows to the underlying ocean, it was possible to document and analyse cases where a thicker snow cover accumulated on melt ponds compared to on adjacent bare ice. This resulted, surprisingly, in lower levels of under-ice radiation underneath the melt ponds than underneath bare ice. New analyses of relationships between thermodynamics and optics of a refreezing lead and thin ice suggest that radiative transfer in thin ice is often not accurately accounted for using bulk formulations, as they are applicable for thicker ice. The initial states of the lead’s opening and refreezing need to be treated separately and cannot generally be considered windows into the ocean. This dissertation extended our knowledge of the relationships between snow and surface conditions and under-ice radiation. The results point towards impacts on sea-ice energy balance, ocean heat content, thermodynamic ice growth, and ice-and ocean-associated ecosystem activity.doctoral thesis484 169 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Sea ice deformation and sea ice thickness change(2022-11-01); ; ; The Arctic Ocean is undergoing a major transition from a year-round sea ice cover to ice-free summers with global consequences. Sea ice thickness is at the center of the ongoing changes because the thickness regulates key processes of the Arctic climate system and in the last six decades, the mean thickness has more than halved. With the most scientific attention on the increased melting and delayed freezing of Arctic sea ice, dynamic thickness change caused by sea ice deformation has remained less studied. Dynamic thickness change alters the sea ice thickness through colliding floes that raft or form pressure ridges or floes breaking apart resulting in leads. Because sea ice grows faster in open water and under thin ice, new ice formation is enhanced in those leads compared to the surrounding ice during the growth season. Because thinner ice is easier to break and move, the ongoing thinning of Arctic sea ice may result in more ridges and leads, which, in turn, could increase ice thickness in winter. However, our limited quantitative understanding of dynamic thickness change has hampered any robust prediction if and to which extent such increased dynamic thickening in winter could mitigate summer thinning in the warming Arctic. To address this gap, we need more robust estimates of the current magnitude as well as a better understanding and representation of the different processes in state-of-the-art sea ice models. Thus, the overarching goal of this thesis is to resolve and quantify dynamic thickness change and to link it to the corresponding sea ice deformation. I focus on the freezing period addressing the following research questions: (1) How large is the dynamic contribution to the mean sea ice thickness in different dynamic regimes? (2) How is deformation shaping the ice thickness distribution? (3) How can high-resolution microwave synthetic aperture radar (SAR) satellite data be used to estimate dynamic thickness change? I answer them in two regional case studies: a unique month-long deformation event during the closing of a polynya north of Greenland and in the Transpolar Drift along the drift track of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. The combination of available high-resolution electromagnetic (EM) induction sounding ice thickness data and high-resolution deformation data offer unique research opportunities to study the highly localized and intermittent dynamic thickness changes. My results show that dynamic thickness change plays an important role in both convergent and divergent drift regimes. Studying the polynya closing event reveals that convergence can locally double the thickness of young, thin (<1 m) ice and restore the mean thickness of 2 m of the surrounding multi-year ice within one month. In more divergent regimes like the Transpolar Drift, new ice formation in leads contributes 30% to the sea ice mass balance. There are indicators that this fraction may increase in a more seasonal Arctic sea ice cover. Besides the mean changes, I show how deformation shapes the ice thickness distribution (ITD) with a particular focus on the transfer of observational results into modeling concepts. I identify the ice that participates in ridging, show that the current ridging parameterization in state-of-the-art models is not able to reproduce the observed changes in the shape of the ITD, and suggest an updated parameterization that relates the shape of the ITD proportionally to the observed deformation. Lastly, I demonstrate that SAR-derived deformation can successfully be used to describe sea ice dynamics and to estimate the dynamic contribution to the ice thickness on regional scales. In conclusion, this dissertation substantially advances our understanding of dynamic thickness change with robust and quantitative estimates. The high-resolution EM ice thickness data with simultaneously collected high-resolution deformation data provide an excellent opportunity to deepen our process understanding and to evaluate and improve the modeling of the dynamic processes shaping the ITD. With the increasing availability of SAR data in the Arctic and the presented deformation datasets and methods, new opportunities are opening up to derive dynamic thickness change on Arctic-wide scales and to study the temporal trends in dynamic thickness change over the last decade.doctoral thesis883 877