Uncertainties in Antarctic sea-ice thickness retrieval from ICESat

Abstract

A sensitivity study was carried out for the lowest-level elevation method to retrieve total (sea ice+snow) freeboard from Ice, Cloud and land Elevation Satellite (ICESat) elevation measurements in the Weddell Sea, Antarctica. Varying the percentage ( P ) of elevations used to approximate the instantaneous sea-surface height can cause widespread changes of a few to >10cm in the total freeboard obtained. Other input parameters have a smaller influence on the overall mean total freeboard but can cause large regional differences. These results, together with published ICESat elevation precision and accuracy, suggest that three times the mean per gridcell single-laser-shot error budget can be used as an estimate for freeboard uncertainty. Theoretical relative ice thickness uncertainty ranges between 20% and 80% for typical freeboard and snow properties. Ice thickness is computed from total freeboard using Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) snow depth data. Average ice thickness for the Weddell Sea is 1.73 0.38m for ICESat measurements from 2004 to 2006, in agreement with previous work. The mean uncertainty is 0.72 0.09m. Our comparison with data of an alternative approach, which assumes that sea-ice freeboard is zero and that total freeboard equals snow depth, reveals an average sea-ice thickness difference of 0.77m

Satellite Remote Sensing of Snow Depth on Antarctic Sea Ice: An Inter-Comparison of Two Empirical Approaches

“…We compute maps of the average Antarctic snow depth on sea ice distribution for all periods listed in Table 1 from the AMSR-E snow depth product and from the ICESat elevation measurements. The retrieval of the total freeboard is described in detailed in [16]. Here we briefly repeat a few key aspects.…”

“…We first filter and pre-process ICESat surface elevations following [15,31]. Subsequently, we apply the lowest-level elevation method as described in [16] to approximate the sea surface height and compute the total freeboard [14][15][16]. The retrieved freeboard is quite sensitive to the parameters used in the lowest-level elevation method.…”

“…Uncertainties in S are estimated via Gaussian error propagation with uncertainties of the regression lines (see [6]) and of F (see [16,20]) as input. The used coefficients are summarized in Table 3.…”

“…Note that data need to be averaged over such long periods-about 33-35 days-to obtain a sufficiently dense coverage with valid freeboard values. In addition, we computed maps of the freeboard retrieval uncertainty as described in [16]. We focus on winter (MJ) and spring (ON) periods.…”

“…We downloaded the data for ICESat measurement periods 2B to 2J from the U.S. National Snow and Ice Data Center (NSIDC): http://nsidc.org/data/gla13.html). We used this data to compute Antarctic wide distribution of the total freeboard as described in [16] at 100 km grid resolution separately for each ICESat period from 2B (February/March 2004 or FM04) to 3I (October/November 2007 or ON07) (see Table 1). Note that data need to be averaged over such long periods-about 33-35 days-to obtain a sufficiently dense coverage with valid freeboard values.…”