Sensitivity of CryoSat-2 Arctic sea-ice freeboard and thickness on radar-waveform interpretation
Sensitivity of CryoSat-2 Arctic sea-ice freeboard and thickness on radar-waveform interpretation
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DOI：
10.5194/tc-8-1607-2014
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发表时间：
2014-01-01
期刊：
影响因子：
5.2
通讯作者：
Davidson, M.
中科院分区：
文献类型：
--
作者：
Ricker, R.;Hendricks, S.;Davidson, M.
在量化全球尺度北极冰体积减少的背景下，CryoSat-2卫星于2010年发射，配备了K-u波段合成孔径雷达高度计SIRAL（合成孔径干涉雷达高度计），我们使用它来获得海冰干舷，定义为冰面高于海平面的高度。在开阔水域和冰面上进行精确的CryoSat-2距离测量是实现干舷到厚度转换所需精度的必要条件。除了实际海面高度的不确定性和对冰雪特性的有限了解外，雷达后向散射的组成和雷达回波的解释也至关重要。这在选择用于跟踪主散射水平和为每个CryoSat-2测量分配距离估计的回调算法方面产生了影响。在本研究中，我们应用了一种回溯算法，阈值分别为雷达回波功率第一次最大值的40,50和80%，跨越了当前文献中使用的值范围。通过使用选定的回撤器和机载验证测量的额外结果，我们评估了海冰干舷和更高水平产品的不确定性，这些不确定性仅由回撤器阈值的选择引起，与冰雪特性相关的不确定性无关。研究表明，回归阈值的选择对海冰干舷和厚度估计值的大小有显著影响，但对这些参数的空间分布影响较小。具体来说，考虑到2013年3月的第一年冰（多年冰），我们发现40%阈值的平均雷达干舷值为0.121米（0.265米），50%阈值为0.086米（0.203米），80%阈值为0.024米（0.092米）。研究表明，干舷和厚度不确定性的主要来源是雷达回波的选择和雷达脉冲在雪层中的未知穿透以及表面粗糙度的影响。这些不确定性可能导致干舷偏差大约为0.06-0.12米。此外，从2013年3月到2013年11月，在格陵兰岛和加拿大北部的多年海冰区域，我们获得了0.02-0.15m的干舷显著上升。由于这是不可能的，这就产生了一个假设，即根据雪荷载的季节性特性，将来有必要应用不同的回调阈值。
In the context of quantifying Arctic ice-volume decrease at global scale, the CryoSat-2 satellite was launched in 2010 and is equipped with the K-u band synthetic aperture radar altimeter SIRAL (Synthetic Aperture Interferometric Radar Altimeter), which we use to derive sea-ice freeboard defined as the height of the ice surface above the sea level. Accurate CryoSat-2 range measurements over open water and the ice surface of the order of centimetres are necessary to achieve the required accuracy of the freeboard-to-thickness conversion. Besides uncertainties of the actual sea-surface height and limited knowledge of ice and snow properties, the composition of radar backscatter and therefore the interpretation of radar echoes is crucial. This has consequences in the selection of retracker algorithms which are used to track the main scattering horizon and assign a range estimate to each CryoSat-2 measurement. In this study we apply a retracker algorithm with thresholds of 40, 50 and 80% of the first maximum of radar echo power, spanning the range of values used in the current literature. By using the selected retrackers and additionally results from airborne validation measurements, we evaluate the uncertainties of sea-ice freeboard and higher-level products that arise from the choice of the retracker threshold only, independent of the uncertainties related to snow and ice properties. Our study shows that the choice of retracker thresholds does have a significant impact on magnitudes of estimates of sea-ice freeboard and thickness, but that the spatial distributions of these parameters are less affected. Specifically we find mean radar freeboard values of 0.121m (0.265 m) for the 40% threshold, 0.086m (0.203 m) for the 50% threshold and 0.024m (0.092 m) for the 80% threshold, considering first-year ice (multiyear ice) in March 2013. We show that the main source of freeboard and thickness uncertainty results from the choice of the retracker and the unknown penetration of the radar pulse into the snow layer in conjunction with surface roughness effects. These uncertainties can cause a freeboard bias of roughly 0.06-0.12 m. Furthermore we obtain a significant rise of 0.02-0.15m of freeboard from March 2013 to November 2013 in the area for multiyear sea ice north of Greenland and Canada. Since this is unlikely, it gives rise to the assumption that applying different retracker thresholds depending on seasonal properties of the snow load is necessary in the future.