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Estimating Ice Thickness in Polynyas with Multiple Sensors

by Ben Holt, Jet Propulsion Laboratory, and Seelye Martin, University of Washington

Polynyas are recurring regions of open water and thin ice within the polar sea ice cover, having intermittent openings and closings during winter. The large heat and salt fl uxes related to the rapid ice formation and subsequent release of brine during polynya openings play key roles in coupling atmospheric heat loss with sea ice mass balance and oceanic circulation, particularly in the formation and maintenance of deep water layers in polar oceans.

We have applied an innovative algorithm to estimate thin ice thickness within arctic polynyas, using data from both the 37 GHz channel of the Special Sensor Microwave/Imager (SSM/I) and the 36GHz Advanced Microwave Scanning Radiometer-E (AMSRE). The V/H polarization ratio of SSM/I data is sensitive to ice thickness from 0 to 10 cm, the thickness range with the largest heat flux from the ocean to the atmosphere. The algorithm enables daily observations using the extensive passive microwave time series and avoids discrepancies found in previous results that make use of ice concentration estimates.

Using same-day data taken over an extensive polynya found in the Chukchi Sea, a multisensor comparison is illustrated at the left. Panel A of the comparison reveals ice thicknesses up to 15 centimeters within the polynya region, based on the results from an ice-thickness algorithm using Advanced Very High Resolution Radiometer (AVHRR) data.

Panel B illustrates pixel-by-pixel values of the 25-km-resolution SSM/I data while panel C displays the resulting SSM/I ice thickness estimates. These two panels show, in turn, a good relationship to the AVHRR-derived thickness results at the top.

Data from the 36GHz AMSR-E sensor (not shown), with its 12.5km resolution, reveals even more detail than the SSM/I results, which further improves the estimate of polynya extent, especially closer to land where the largest heat flux occurs

Additionally, the RADARSAT-1 data in panel D provides an accurate and independent visual corroboration of the derived polynya extent.

Results from another study of the polynya, performed over 12 winters, demonstrate that the calculated heat losses are consistent with two years of temperature field data and overwinter salinity. Th e graph below shows how those results compare with numerical and satellite estimates of ice production. Although our results of ice production per unit area are smaller, the size of the 12-winter polynya study area was larger, so that our ice production estimates are of the same order.

The importance of our results is that the salt flux from the polynya is distributed over a much larger area than a model. This change in the salt distribution with distance from the coast will aff ect the off shore length scale of the oceanic response. We are currently extending our analysis to include the entire Arctic Ocean and the Ross Sea in Antarctica.

For more detailed information about the evolving development of this algorithm, see the following references:

Drucker, R., S. Martin, and R. Moritz (2003), Observations of ice thickness and frazil ice in the St. Lawrence Island polynya from satellite imagery, upward looking sonar, and salinity/temperature moorings, J. Geophys. Res., 108(C5), 3149, doi:10.1029/2001JC001213

Martin, S., R. Drucker, R. Kwok, and B. Holt (2004), Estimation of the thin ice thickness and heat flux for the Chukchi Sea Alaskan coast polynya from SSM/I data, 1990-2001, J. Geophys. Res., 109, C10012, doi:10.1029/2004JC002428

Martin, S., R. Drucker, R. Kwok, and B. Holt (2005), Improvements in the estimates of ice thickness and production in the Chukchi Sea polynyas derived from AMSR-E, Geophys. Res. Lett., 32, L05505, doi:10.1029/2004GL022013.

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