A glimpse beneath Antarctic sea ice: observation of platelet-layer thickness and ice-volume fraction with multi-frequency EM

In Antarctica, ice crystals (platelets) form and grow in supercooled waters below ice shelves. These platelets rise, accumulate beneath nearby sea ice, and subsequently form a several meter thick, porous sub-ice platelet layer. This special ice type is a unique habitat, influences sea-ice mass and energy balance, and its volume can be interpreted as an indicator of the health of an ice shelf. Although progress has been made in determining and understanding its spatio-temporal variability based on point measurements, an investigation of this phenomenon on a larger scale remains a challenge due to logistical constraints and a lack of suitable methodology. In the present study, we applied a lateral constrained Marquardt-Levenberg inversion to a unique multi-frequency electromagnetic (EM) induction sounding dataset obtained on the ice-shelf influenced fast-ice regime of Atka Bay, eastern Weddell Sea. We adapted the inversion algorithm to incorporate a sensor specific signal bias, and confirmed the reliability of the algorithm by performing a sensitivity study using synthetic data. We inverted the field data for sea-ice and platelet-layer thickness and electrical conductivity, and calculated ice-volume fractions within the platelet layer using Archie’s Law. The thickness results agreed well with drillhole validation datasets within the uncertainty range, and the ice-volume fraction yielded results comparable to other studies. Both parameters together enable an estimation of the total ice volume within the platelet layer, which was found to be comparable to the volume of landfast sea ice in this region, and corresponded to more than a quarter of the annual basal melt volume of the nearby Ekström Ice Shelf. Our findings show that multi-frequency EM induction sounding is a suitable approach to efficiently map sea-ice and platelet-layer properties, with important implications for research into ocean/ice-shelf/sea-ice interactions. However, a successful application of this technique requires a break with traditional EM sensor calibration strategies due to the need of absolute calibration with respect to a physical forward model

The surface energy balance of early summer land-fast sea ice in Atka Bay, Antarctica

In-situ measurements of the land-fast sea ice energy balance are scarce. We present a data set that comprises eddy-covariance measurements of sensible and latent heat as well as measurements of the sea-ice temperature gradient, long-wave and short-wave radiation measurements over land-fast sea ice in Atka Bay, Antarctica. With this setup we are able to monitor all components of the sea-ice energy budget. Additionally, we also measured the turbulent flux of CO2 over sea ice. This 37 day-long data set is evaluated for the transition period from austral winter to summer (November to December 2012) with regard to atmospheric stability and the general weather conditions. Results for the eddy-covariance measurements show an average sensible heat flux of 6.45+-10.72 W/m2 and a latent heat flux of 12.71+-9.48 W/m2 (with one standard deviation respectively) for low pressure/high wind-speed conditions. The average net radiation is 44.37+-41.54 W/m2 and for the CO2 flux an average of -3.35+-3.37μmol/m2 was measured. During high pressure/low wind-speed conditions an average of -3.03+-10.48 W/m2 and 10.76+-10.52 W/m2 was recorded for the sensible and latent heat flux, while the average net radiation and the CO2 flux are 35.63+-56.70 W/m2 and -1.95+-1.72μmol/m2 respectively. The fast ice is therefore found as a sink of CO2 for both situations

The impact of early summer snow properties on Antarctic landfast sea ice X band backscatter

Up to now, snow cover on Antarctic sea ice and its impact on radar backscatter, particularly after the onset of freeze/thaw processes, are not well understood. Here we present a combined analysis of in situ observations of snow properties from the landfast sea ice in Atka Bay, Antarctica, and high-resolution TerraSAR-X backscatter data, for the transition from austral spring (November 2012) to summer (January 2013). The physical changes in the seasonal snow cover during that time are reflected in the evolution of TerraSAR-X backscatter. We are able to explain 76–93% of the spatio-temporal variability of the TerraSAR-X backscatter signal with up to four snowpack parameters with a root-mean-squared error of 0.87–1.62 dB, using a simple multiple linear model. Over the complete study, and especially after the onset of early-melt processes and freeze/thaw cycles, the majority of variability in the backscatter is influenced by changes in snow/ice interface temperature, snow depth and top-layer grain size. This suggests it may be possible to retrieve snow physical properties over Antarctic sea ice from X-band SAR backscatter

Ice platelets below Weddell Sea landfast sea ice

Basal melt of ice shelves may lead to an accumulation of disc-shaped ice platelets underneath nearby sea ice, to form a sub-ice platelet layer. Here we present the seasonal cycle of sea ice attached to the Ekström Ice Shelf, Antarctica, and the underlying platelet layer in 2012. Ice platelets emerged from the cavity and interacted with the fast-ice cover of Atka Bay as early as June. Episodic accumulations throughout winter and spring led to an average platelet-layer thickness of 4m by December 2012, with local maxima of up to 10 m. The additional buoyancy partly prevented surface flooding and snow-ice formation, despite a thick snow cover. Subsequent thinning of the platelet layer from December onwards was associated with an inflow of warm surface water. The combination of model studies with observed fast-ice thickness revealed an average ice-volume fraction in the platelet layer of 0.25+-0.1. We found that nearly half of the combined solid sea-ice and ice-platelet volume in this area is generated by heat transfer to the ocean rather than to the atmosphere. The total ice-platelet volume underlying Atka Bay fast ice was equivalent to more than one-fifth of the annual basal melt volume under the Ekström Ice Shelf

Monitoring land-fast sea ice in the Weddell Sea – A contribution to the Antarctic Fast Ice Network

Continuous observations of sea-ice and its snow cover are crucial to understand key processes and predict changes in the polar regions. In the pack-ice zone of the Southern Ocean, gathering these data is most challenging due to logistical constraints. In contrast, immobile sea ice fastened to the coast and ice shelves around Antarctica is relatively easy to probe from nearby coastal stations. During IPY 2007/08, several international partners grouped together in the Antarctic Fast Ice Network (AFIN) to provide the scientific community with continuous observations of fast-ice areas around the Antarctic coastline. Since 2010/11, we contribute to AFIN with a suite of measurements on the seasonal fast ice of Atka Bay, in the eastern Weddell Sea. Through its geographical location near the Ekström Ice Shelf, the fast ice is influenced by ocean-ice shelf interaction and is generally covered with a thick and highly variable snow cover. Here we present the concept and selected results of our ongoing monitoring program, where we combine traditional sea-ice measurements (drillings, coring, snow pits) with automated stations/buoys and remote sensing by satellites (MODIS, SAR)

Ice Shelf - Sea Ice Interaction: A Case Study

Here we present results from our investigation on the influence of the Ekström Ice Shelf on the land-fast sea ice of Atka Bay, eastern Weddell Sea. • Ice platelets emerge from the cavity and interact with the fast-ice of Atka Bay as early as June. Episodic accumulations throughout the winter lead to an average platelet-layer thickness of 4 m in December, with local extrema of 10 m. • Additional buoyancy prevents surface flooding and snow-ice formation despite thick -now cover. • The seasonal cycle shows a maximum thickness in December, and a subsequent thinning, which is associated with an inflow of warm water masses. • The combination of model studies with observed fast-ice thickness reveals an average ice-volume fraction of the platelet layer of 0.26. • Half of the combined solid sea-ice and ice-platelet volume in this area is generated by heat loss to the ocean rather than to the atmosphere, equivalent to more than one fifth of the annual basal melt volume under the Ekström Ice Shelf

Developments in frequency domain AEM; tackling drift and noise with a multicomponent, ferrite-core, receiver tipplet

The polar oceans' sea ice cover is a challenging geophysical target to map. Current state of practice helicopter-electromagnetic (HEM) ice thickness mapping is limited to 1D interpretation due to common procedures and systems that are mainly sensitive to layered structures. We present a new generation Multi-sensor, Airborne Sea Ice Explorer (MAiSIE) to overcome these limitations. As the actual sea ice structure is 3D and in parts heterogeneous, errors up to 50% are observed due to the common 1D approximation. With MAiSIE we present a new EM concept based on one multi frequency transmitter loop and a three component receiver coil triplet without bucking The small weight frees additional payload to include a line scanner (lidar) and high accuracy INS/dGPS. The 3D surface topography from the scanner with the EM data at from 500 Hz to 8 kHz, in x, y, and z direction, will increase the accuracy of HEM derived pressure ridge geometry significantly. Experience from two field campaigns shows the proof-of-concept with acceptable sensor drift and receiver sensitivity. The preliminary 20 ppm noise level @ 4.1 kHz is sufficient to map level ice thickness with 10 cm precision for sensor altitudes below 13 m