Comparing elevation and backscatter retrievals from CryoSat-2 and ICESat-2 over Arctic summer sea ice

The CryoSat-2 radar altimeter and ICESat-2 laser altimeter can provide complimentary measurements of the freeboard and thickness of Arctic sea ice. However, both sensors face significant challenges for accurately measuring the ice freeboard when the sea ice is melting in summer months. Here, we used crossover points between CryoSat-2 and ICESat-2 to compare elevation retrievals over summer sea ice between 2018&ndash;2021. We focused on the electromagnetic (EM) bias documented in CryoSat-2 measurements, associated with surface melt ponds over summer sea ice which cause the radar altimeter to underestimate elevation. The laser altimeter of ICESat-2 is not susceptible to this bias, but has other biases associated with melt ponds. So, we compared the elevation difference and reflectance statistics between the two satellites. We found that CryoSat-2 underestimated elevation compared to ICESat-2 by a median difference of 2.4 cm and by a median absolute deviation of 5.3 cm, while the differences between individual ICESat-2 beams and CryoSat-2 ranged between 1&ndash;3.5 cm. Spatial and temporal patterns of the bias were compared to surface roughness information derived from the ICESat-2 elevation data, the ICESat-2 photon rate (surface reflectivity), the CryoSat-2 backscatter and melt pond fraction derived from Seintnel-3 OLCI data. We found good agreement between theoretical predictions of the CryoSat-2 EM melt pond bias and our new observations; however, at typical roughness &lt;0.1 m the experimentally measured bias was larger (5&ndash;10 cm) compared to biases resulting from the theoretical simulations (0&ndash;5 cm). This intercomparison will be valuable for interpreting and improving the summer sea ice freeboard retrievals from both altimeters.</p

Complex evolving patterns of mass loss from Antarctica’s largest glacier

Pine Island Glacier has contributed more to sea level rise over the past four decades than any other glacier in Antarctica. Model projections indicate that this will continue in the future but at conflicting rates. Some models suggest that mass loss could dramatically increase over the next few decades, resulting in a rapidly growing contribution to sea level and fast retreat of the grounding line, where the grounded ice meets the ocean. Other models indicate more moderate losses. Resolving this contrasting behaviour is important for sea level rise projections. Here, we use high-resolution satellite observations of elevation change since 2010 to show that thinning rates are now highest along the slow-flow margins of the glacier and that the present-day amplitude and pattern of elevation change is inconsistent with fast grounding-line migration and the associated rapid increase in mass loss over the next few decades. Instead, our results support model simulations that imply only modest changes in grounding-line location over that timescale. We demonstrate how the pattern of thinning is evolving in complex ways both in space and time and how rates in the fast-flowing central trunk have decreased by about a factor five since 2007

Toward a compact underwater structured light 3-D imaging system

Thesis (S.B.)--Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 53-54).A compact underwater 3-D imaging system based on the principles of structured light was created for classroom demonstration and laboratory research purposes. The 3-D scanner design was based on research by the Hackengineer team at Rice University. The system is comprised of a low-power, open-source hardware single-board computer running a modified Linux distribution with OpenCV libraries, a DLP pico projector, camera board, and battery module with advanced power management. The system was designed to be low-cost, compact, and portable, while satisfying requirements for watertightness. Future development and applications may involve navigation systems for an autonomous underwater vehicle (AUV). An initial study of 3-D imaging methods is presented, and the strengths and drawbacks of each type are discussed. The structured light method was selected for further study for its ability to produce high-resolution 3-D images for a reasonable cost. The build of the 3-D imaging system was documented for reproducibility, and subsequent testing demonstrated its functions and ability to produce 3-D images. An instruction guide for operation of the device is provided for future classroom and laboratory use. The 3-D imaging system serves as a proof-of-concept for utilizing structured light methods to produce 3-D images underwater. Image resolution was limited by the output resolution of the pico projector and camera module. Further exploration in obtaining ultra high-resolution 3-D images may include use of a more powerful projector and a higher resolution camera board module with autofocus. Satisfactory 3-D scanning validated the performance of structured light scanning above water. However, contaminants in the water hindered accurate rendering by the system while submerged due to light scattering. Future development of a on-the-fly mapmaking system for AUV navigation should include algorithms for filtering light scattering, and hardware should based on an instantaneous structured light system utilizing the Kinect 2-D pattern method. Autofocus and increased projector brightness would also be worthwhile additions.by Geoffrey E. Dawson.S.B

A high-resolution Antarctic grounding zone product from ICESat-2 laser altimetry

The Antarctic grounding zone, which is the transition between the fully grounded ice sheet to freely floating ice shelf, plays a critical role in ice sheet stability, mass budget calculations, and ice sheet model projections. It is therefore important to continuously monitor its location and migration over time. Here we present the first ICESat-2-derived high-resolution grounding zone product of the Antarctic Ice Sheet, including three important boundaries: the inland limit of tidal flexure (Point F), inshore limit of hydrostatic equilibrium (Point H), and the break in slope (Point Ib). This dataset was derived from automated techniques developed in this study, using ICESat-2 laser altimetry repeat tracks between 30 March 2019 and 30 September 2020. The new grounding zone product has a near-complete coverage of the Antarctic Ice Sheet with a total of 21 346 Point F, 18 149 Point H, and 36 765 Point Ib locations identified, including the difficult-to-survey grounding zones, such as the fast-flowing glaciers draining into the Amundsen Sea embayment. The locations of newly derived ICESat-2 landward limit of tidal flexure agree well with the most recent differential synthetic aperture radar interferometry (DInSAR) observations in 2018, with a mean absolute separation and standard deviation of 0.02 and 0.02 km, respectively. By comparing the ICESat-2-derived grounding zone with the previous grounding zone products, we find a grounding line retreat of up to 15 km on the Crary Ice Rise of Ross Ice Shelf and a pervasive landward grounding line migration along the Amundsen Sea embayment during the past 2 decades. We also identify the presence of ice plains on the Filchner–Ronne Ice Shelf and the influence of oscillating ocean tides on grounding zone migration. The product derived from this study is available at https://doi.org/10.5523/bris.bnqqyngt89eo26qk8keckglww (Li et al., 2021) and is archived and maintained at the National Snow and Ice Data Center

Colloidal magnetic fluids as extractants for chemical processing applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2003.Includes bibliographical references.The feasibility of using high gradient magnetic separation (HGMS) to separate the Fe₃0₄ nanoparticles was studied in this work. We present a general model for nanoparticle capture based on calculating the limit of static nanoparticle buildup around the collection wires in an HGMS column. Model predictions were compared successfully with experimental results from a bench-scale HGMS column. Permanent capture of individual nanoparticles is limited by diffusion away from the wires; however, 60-125 nm aggregates of particles can be captured permanently in the bench-scale column. The model provided estimates of the minimum particle size for permanent capture of individual nanoparticles and nanoparticle aggregates.This focus of this thesis is a novel class of water-based magnetic fluids that are specifically tailored to extract soluble organic compounds from water. Magnetic fluids are colloidal dispersions of magnetic nanoparticles that do not settle in gravitational or moderate magnetic fields due to their small size and do not aggregate because of their surface coatings. These materials offer several potential advantages over traditional methods of organic separation, such as activated carbon adsorption. For example, magnetic fluids possess a large surface area for separation while avoiding porous structures that introduce a high mass transfer resistance. The magnetic fluids were prepared by precipitation and consist of a suspension of [approximately]7.5 nm diameter magnetite (Fe₃0₄) nanoparticles coated with a [approsimately]9 nm thick bifunctional polymer layer comprised of an outer hydrophilic polyethylene oxide (PEO) region for colloidal stability, and an inner hydrophobic polypropylene oxide (PPO) region for solubilization of organic compounds. Characterization of these materials revealed the particle dimensions and magnetic properties. In addition, we examined the colloidal stability of the magnetic fluids over a broad range of conditions. The structure of the polymer shell, which was examined with neutron scattering and lattice calculations, shows some evidence of segregation of the PEO and PPO chains. The magnetic fluids exhibit a high capacity for organic solutes, with partition coefficients between the polymer coating and water on the order of 10³ to 10⁵, which is consistent with values reported for solubilization of these organics in PEO-PPO-PEO block copolymer (Pluronic) micelles.by Geoffrey D. Moeser.Ph.D

Robust orienting to protofacial stimuli in autism

Newborn infants exhibit a remarkable tendency to orient to faces. This behavior is thought to be mediated by a subcortical mechanism tuned to the protoface stimulus: a face-like configuration comprising three dark areas on a lighter background. When this unique stimulus translates across their visual field, neurotypical infants will change their gaze or head direction to track the protoface [1–3] . Orienting to this low spatial frequency pattern is thought to encourage infants to attend to faces, despite their poor visual acuity [2,3] . By biasing the input into the newborn’s visual system, this primitive instinct may serve to ‘canalize’ the development of more sophisticated face representation. Leading accounts attribute deficits of face perception associated with Autism Spectrum Disorders (ASD) [4] to abnormalities within this orienting mechanism. If infants who are later diagnosed with ASD exhibit reduced protoface orienting, this may compromise the emergence of perceptual expertise for faces [5] . Here we report a novel effect that confirms that the protoface stimulus captures adults’ attention via an involuntary, exogenous process (Experiment 1). Contrary to leading developmental accounts of face perception deficits in ASD, we go on to show that this orienting response is intact in autistic individuals (Experiment 2)