Internal ice stress

 

A governing factor in the exchange of energy between the atmosphere, sea ice cover, and ocean is the thickness distribution of the ice cover. A complex series of dynamic and thermodynamic processes defines the thickness distribution. Key to the dynamic processes is the mechanical behavior of the ice cover, which described the relationship between stress and deformation. During the SHEBA field programs we took measurements of the internal ice stress to improve the understanding of the mechanical behavior of the ice pack through direct measurements. This work was done in collaboration with Jim Overland, of NOAA’s Pacific Marine Environmental Laboratory, who deployed position buoys to monitor ice deformation.

The stress and deformation array covered a100-km-across region centered on Ice Station SHEBA. It was installed at the start of the field program, in October 1997. Stresses were measured using gauges designed specifically for this application. The gauge is a stiff, thick-walled steel annulus, that houses a vibrating wire assembly. Changes in the frequency of vibration of the wire define the stress being applied to the annulus. The stress sensor is frozen into the ice cover by drilling a hole through the ice. The stress sensors were located within the interior of the stress and deformation array at 10 different sites. Seven of these sites were placed approximately 0.5 km apart along the boundary of the SHEBA floe. The remaining three were installed on separate floes that were located about 15 km from the main floe. Measurements from each sensor describe the time series of ice stress at a single point in the ice cover.

Remote site 4, stress site in foreground, radiometer in back.

Stress sensor array on SHEBA home floe.

 

 

Remote site stress and deformation array (scale is km). Stress sensors were located at sites 9, 10, and 11.

 
Stress sensor locations:

Baltimore: Three sensors. Lead: Stress sensor installed in 36-cm-thick lead ice at a depth of 21 cm. This refrozen lead was adjacent to the SHEBA floe. Time series begins 10/31/97, ends 7/9/98. Also located at this site was a thermistor string, thickness gauges, and a PAM site. Multiyear floe: 136-cm-thick multiyear ice Top: Stress sensor was installed, at a depth of 28 cm. Time series begins 10/31/97, ends 7/9/98. Also located near this site were thickness gauges. Bottom: Stress sensor was at a depth of 109 cm. Time series begins 10/31/97, ends 9/15/98. Also located near this site were thickness gauges.

Wilmington: One sensor installed in 104-cm-thick multiyear ice at a depth of 35.3 cm. This site was near the edge of the SHEBA home floe. Times series begins 10/31/97, ends 5/1/98.

Dover: One sensor installed in 156-cm-thick multiyear ice at a depth of 50.5 cm. This site was near the edge of the SHEBA floe. Time series begins 10/14/87, ends 5/17/98.

Milford: One sensor installed in multiyear ice near edge of the SHEBA floe. Time series begins 10/14/97, ends 5/17/98. Top: Stress sensor was installed in 93-cm-thick multiyear ice, at a depth of 24.0 cm. Bottom: Stress sensor was installed in 108-cm-thick multiyear ice, at a depth of 100 cm.

Rehoboth: One sensor installed in 148-cm-thick multiyear ice, at a depth of 46 cm. This site was near the edge of the SHEBA home floe. Time series begins 10/11/97, ends 5/17/98.

Delaware: Three sensors. Top: Stress sensor was installed in 160-cm-thick multiyear ice, at a depth of 39.5 cm. This site was in the center of the SHEBA floe. Time series begins 10/14/97, ends 6/6/98. Bottom 1: Stress sensor was at a depth of 99 cm. Time series begins 10/14/97, ends 6/6/98. Bottom 2: Stress sensor was installed at a depth of 98 cm. Time series begins 10/14/97, ends 6/6/98.

Remote Site 9: One sensor installed in 116-cm-thick multiyear ice at a depth of 44.5 cm. This site was in the center of a floe, approximately 15 km from the SHEBA floe. Time series begins 10/2/87, ends 7/24/98. Also located at this site was a position buoy and salinity/temperature ocean profiler.

Remote Site 11: One sensor installed in 126-cm-thick multiyear ice at a depth of 44.5 cm. This site was in the center of a floe, approximately 15 km from the SHEBA floe. Time series begins 10/6/87, ends 7/25/98. Also located at this site was a position buoy and salinity/temperature ocean profiler.

 
 
                                          

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