Lasse Rabenstein

Cross-validation of polynya monitoring methods from multisensor satellite and airborne data: a case study for the Laptev Sea

Wind-driven coastal polynyas in the polar oceans are recognized as regions of extensive new ice formation in the cold season. Hence, they may play an increasing role in the uncertain future of the sea-ice budget in the polar oceans. The Laptev Sea polynyas in the Siberian Arctic are well recognized as being significant ice producers and might gain special attention with regards to ice volume changes in the Arctic. Long-term monitoring and characterization of these polynyas require stable methods to detect the area of open water and the growth, thickness, and evolution of thin ice. We examine different parameters and methods to observe polynya area and thin ice thickness during a prominent polynya event in the Laptev Sea in April 2008. These are derived from visible, infrared, and microwave satellite data. Airborne electromagnetic ice thickness measurements with high spatial resolution and aerial photography taken across the polynya are used to assess the feasibility of the methods for long-term and large-scale polynya monitoring within this area. Our results indicate that in the narrow flaw polynyas of the Laptev Sea the coarse resolution of commonly used microwave channel combinations provokes sources of error through mixed signals at the fast- and pack-ice edges. Polynya monitoring results can be significantly improved using enhanced resolution data products. This implies that previously suggested methods for the retrieval of polynya area, thin ice thickness, and ice production are not transferable in space and time. Data as well as method parameterizations have to be chosen carefully to avoid large errors due to regional peculiarities.

Sea ice production and water mass modification in the eastern Laptev Sea

A simple polynya flux model driven by standard atmospheric forcing is used to investigate the ice formation that took place during an exceptionally strong and consistent western New Siberian (WNS) polynya event in 2004 in the Laptev Sea. Whether formation rates are high enough to erode the stratification of the water column beneath is examined by adding the brine released during the 2004 polynya event to the average winter density stratification of the water body, preconditioned by summers with a cyclonic atmospheric forcing (comparatively weakly stratified water column). Beforehand, the model performance is tested through a simulation of a well-documented event in April 2008. Neglecting the replenishment of water masses by advection into the polynya area, we find the probability for the occurrence of density-driven convection down to the bottom to be low. Our findings can be explained by the distinct vertical density gradient that characterizes the area of the WNS polynya and the apparent lack of extreme events in the eastern Laptev Sea. The simple approach is expected to be sufficiently rigorous, since the simulated event is exceptionally strong and consistent, the ice production and salt rejection rates are likely to be overestimated, and the amount of salt rejected is distrusted over a comparatively weakly stratified water column. We conclude that the observed erosion of the halocline and formation of vertically mixed water layers during a WNS polynya event is therefore predominantly related to wind- and tidally driven turbulent mixing processes.

Water content estimates of a first-year sea-ice pressure ridge keel from surface-nuclear magnetic resonance tomography

Abstract The porosity of a sea-ice pressure ridge keel is an important but poorly known variable, relevant for determining the mass budget and evolution of the Arctic sea-ice cover. Determination of keel porosity from drillholes is time-intensive and only yields limited information because of their limited lateral extent. Since the porosity within a keel equals its liquid water content, surface-nuclear magnetic resonance (surface-NMR) methods can be used to estimate porosity within such features. Surface-NMR tomography measurements were made in April 2011 using seven surface coil positions across a first-year pressure ridge on landfast sea ice near Barrow, Alaska, USA. The inversion results indicate water contents of 30 ± 7% and 40 ±10% in the ridge’s shallow and deep parts, respectively. These values are much higher than those obtained from drillholes, which are ∼10% and ∼27%, respectively. In contrast to drilling, surface-NMR tomography yields average porosity values for the entire subsurface volume. However, the inversion process is sensitive to the electrical conductivity distribution; uncertain conductivity estimates limit the reliability of the inverted water contents. Nevertheless, the results suggest that ridge porosities obtained from invasive measurements such as drilling may lead to substantially overestimated sea-ice volume.

Comparison of helicopter-borne thin sea ice thickness profiles with polarimetric signatures of dual-pol Terrasar-X data

In this paper first results of a sensitivity study using dual polarimetric TerraSAR-X data for ice thickness estimation are presented. The sea ice thickness reference data set was measured, coincident to the SAR data take, by means of a helicopter-borne EM sounding device on April 28, 2008 in the Russian Arctic. For some of the signatures, namely the complex correlation coefficient, a relation to ice thickness could be found that is theoretically predicted for L-band SAR. The first results show, that the new generation of polarimetric space borne SAR sensors like TerraSAR-X may open a new opportunity for thin sea ice thickness monitoring from space.

Improved 1D inversions for sea ice thickness and conductivity from electromagnetic induction data: Inclusionof nonlinearities caused by passive bucking

The porosity of sea ice is a fundamental physical parameter that governs the mechanical strength of sea ice and the mobility of gases and nutrients for biological processes and biogeochemical cycles in the sea ice layer. However, little is known about the spatial distribution of the sea ice porosity and its variability between different sea ice types; an efficient and nondestructive method to measure this property is currently missing. Sea ice porosity is linked to the bulk electrical conductivity of sea ice, a parameter routinely used to discriminate between sea ice and seawater by electromagnetic (EM) induction sensors. Here, we have evaluated the prospect of porosity retrieval of sea ice by means of bulk conductivity estimates using 1D multifrequency EM inversion schemes. We have focused on two inversion algorithms, a smoothness-constrained inversion and a Marquardt-Levenberg inversion, which we modified for the nonlinear signal bias caused by a passive bucking coil operated in such a highly conductive environment. Using synthetic modeling studies, 1D inversion algorithms and multiple frequencies, we found that we can resolve the sea ice conductivity within � 0.01 S∕m. Using standard assumptions for the conductivity-porosity relation of sea ice, we were able to estimate porosity with an uncertainty of � 1.2%, which enables efficient and nondestructive surveys of the internal state of the sea ice cover.

Geophysical Characterization of the Furggwanghorn Rock Glacier, Switzerland

Degradation of alpine permafrost due to changing mean annual air temperatures can act as trigger for landslides and other ground instabilities. For a better understanding of the underlying thermo-hydromechanical processes an interdisciplinary research project has been set up. An extensive geophysical and monitoring campaign was carried out on rock glacier in the Turtmann valley, Canton Valais, Switzerland over the last two years to investigate its internal structure. We employed seismic refraction tomography, electrical resistivity tomography and ground-penetrating radar. Additionally seven boreholes were drilled to a depth of 25m and equipped with temperature sensors and inclinometers. Results from the seismic tomography show a lateral very heterogeneous zone below an active layer of 3-4m thickness. The bedrock depth could not be detected over large parts of the profiles. On the electrical tomograms we can clearly distinguish between ice-free zones at the front and the flanks of the rock glacier and an ice-rich zone in the central part. Several internal shear horizons could be identified on the radar profiles. Most of them could be tracked over several profiles. Deformation measurements in a nearby borehole show that the horizon at about 15m depth is currently active.

Noise Sources for a Fixed Wing Airborne EM System, Quantified by Means of 3D Finite Element Modelling

With a 3D finite element model study the authors quantified noise occuring during aeroplane based electromagnetic measurements for the purpose of sea ice thickness determination. Namely, these are pitch and roll of the aircraft, electromagnetic coupling between aeroplane and ocean, and wing flexure. All effects are significantly amplified by the presence of the conducting sea water and for flight heights of 30 m over the ocean these effects can change the signal by about 10 percent or, in the case of wing flexure, by 100 percent. Roll, pitch and wing flexure signals have an inductive and a geometric contribution. For highly quantitative measurements such as sea-ice thickness, where a vertical resolution of 0.1 m is desired, all these effects must be taken into account. Most of the strong wing flexure signal appears on the in-phase component only, henceforth the quadrature component should be taken for sea ice thickness retrieval even when it encounters a weaker ocean response signal than the in-phase component.