USING A MULTI-YEAR DATA ARCHIVE OF ERS SAR IMAGERY FOR THE MONITORING OF FIRN LINE POSITIONS AND ABLATION PATTERNS ON THE KING GEORGE ISLAND ICE CAP (ANTARCTICA)
USING A MULTI-YEAR DATA ARCHIVE OF ERS SAR IMAGERY FOR THE MONITORING OF FIRN LINE POSITIONS AND ABLATION PATTERNS ON THE KING GEORGE ISLAND ICE CAP (ANTARCTICA)
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使用 ERS SAR 图像的多年数据档案来监测乔治王岛冰盖（南极洲）的 FIRN 线位置和消融模式
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
F. Rau
中科院分区：
文献类型：
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作者：
M. Braun;F. Rau
多年SAR数据用于研究南极洲乔治王岛冰盖上积雪融化模式的季节动态。整个冰盖的积雪在夏季变得潮湿，然而，在最高海拔地区，可以观察到频繁的再冻结和融化循环。裸露的冰区形成于冰盖的最低海拔处。这些发现与能量平衡研究和探地雷达调查的结果非常吻合。中间海拔的反向散射值在消融季节结束时显着上升。这一事实归因于在消融季节结束时潮湿的变质雪盖上的表面粗糙度增加。研究了阈值对确定夏季和冬季图像以及上升和下降轨道中裸冰雷达区域范围的影响。因此，裸冰雷达区域的边界已使用夏季图像 –14 dB 和冬季图像 –6 dB 的阈值进行绘制。可以看出，–12 dB 和 –8 dB 的阈值分别不会显着改变该雷达冰川区的范围。连续几年绘制了芬恩线海拔高度。这条线的年际变化较小，海拔高度约为 200 米。被观察到。雪松线的最大高度记录于1996/97质量平衡年，海拔高度约为250米。将这些值与之前测量的平衡线位置进行比较表明，自 20 世纪 70 年代以来，平衡线高度发生了向上移动。引言 南极半岛和邻近的南设得兰群岛已被确定为具有显着变暖趋势的地区（例如 King 1994、Smith 等人 1996、Skvarca 等人 1998）。在南极半岛北部和南设得兰群岛，冰雪融化是小冰川和冰盖的重要质量平衡参数（Bintanja 1995，Braun 和 Schneider 2000，Braun 等人 2001-a）。此外，融水已被确定为该地区冰架不稳定和大规模崩解的可能来源（Hulbe 1997）。在乔治王岛本身，一些作者（例如 Braun 和 Goßmann 正在出版，Park 等人 1998，Simões 等人 1998，Wunderle 1996）观察到了冰川的长期退缩，并将其归因于冰川质量平衡的变化。因此，需要更详细的冰川质量平衡参数（例如消融模式和冰线位置）的空间分布信息，a）作为冰川融化模型的输入和验证参数，b）改进对短期冰川质量平衡变化的监测，c）提高冰川质量平衡变化长期预测的准确性。由于位于智利奥希金斯基地的德国南极接收站的运行，现在可以使用 ERS-1/2 SAR 图像的多年数据记录来进行南极半岛的冰川学研究。该时间序列从 1991 年开始，能够监测乔治王岛南极冰盖的季节性演变和年际变化，Braun 等人已经在单年中证明了这一点。 （2000）和旺德尔（1996）。 EARSeL-SIG-Workshop Land Ice and Snow，德累斯顿/FRG，2000 年 6 月 16 – 17 日 EARSeL eProceedings No. 1 282 在这项研究中，我们利用来自乔治王岛的 ERS SAR 数据来补充基于微气象地面观测的冰川融化模型，以观察积雪动态和冰盖上的雪线位置的年际变化，以及支持综合探地雷达测量的解释（Pfender 1999）。研究地点 乔治王岛面积约 1250 平方公里，是南设得兰群岛中最大的岛屿。岛上93%的面积被冰雪覆盖。岛上的主要冰盖范围在海平面至海拔约 700 米之间。 （图1）。该岛 30% 以上的海拔范围位于海拔 250 米以下。乔治王岛西北部普遍存在低坡度，而陡峭的峡湾状入口与南侧冰盖光滑的表面形态相交。该岛地形的详细轮廓可以在 Simões 等人的著作中找到。 （1999）和布劳恩等人。 （2001-b）。由于位于南半球西风区，乔治王岛属于高度海洋性气候。别林斯高晋站的年平均气温（1944-1999）为 – 2.9 °C。在整个夏季，低海拔地区的气温都记录在正值。冬季融雪事件也频繁发生（Rachlewicz 1997）。一般来说，来自北方向的暖湿气团的平流导致最高的融雪率（Braun et al. 2001-a）。图1：乔治王岛示意图。感兴趣区域 (AOI) 的位置用数字表示。由于表面形态光滑，冰盖西北部的 SAR 图像中地形引起的畸变很小。此外，使用卫星遥感技术在中等倾斜的表面上比在陡坡上更容易检测到消融区域大小的变化（Bindschadler 1998）。因此，本研究将集中在乔治王岛的这一地区。 EARSeL-SIG-Workshop Land Ice and Snow，德累斯顿/西德，2000 年 6 月 16 - 17 日 EARSeL eProceedings No. 1 283 数据库和数据处理 在本次调查中，使用来自上升和下降轨道的数据对 40 个 ERS-1/2 场景进行了分析。图像涵盖 1992 年 7 月至 1999 年 11 月期间（图 2）。数据准备是使用 ESA SAR 工具箱中的处理链实现的。使用基于 Laur 等人的算法计算所有图像的反向散射值。 （1998）。在此过程中，包括对模数转换期间损耗的补偿以及对复制品功率变化的校正。随后，使用 1998 年 2 月 18 日的下降轨道主图像和 1997 年 7 月 15 日的上升轨道图像对这些图像进行了共同配准。 0.0 3.0 6.0 9.0 12.0 15.0 18.0 1991/92 ERS-1/2 SAR 图像数量
Multi-year SAR data is used to study the seasonal dynamic of the snow melt patterns on the ice cap of King George Island, Antarctica. The snow cover of the entire ice cap becomes wet during summer months, however, in the highest elevations frequent refreeze and melt cycles are observed. Bare ice areas form in the lowest elevations of the ice cap. These findings are in good agreements with energy balance studies and results of a ground penetrating radar survey. Backscatter values in intermediate elevations show a marked rise at the end of the ablation season. This fact is attributed to an increasing surface roughness on a wet metamorphosed snow cover at the end of the ablation season. The influence of thresholds on the determination of the extent of the bare ice radar zone in summer and winter imagery as well as in ascending and descending orbit is investigated. As a consequence, the border of the bare ice radar zone have been mapped using thresholds of –14 dB in summer and –6 dB in winter imagery. It could be shown, that thresholds of –12 dB and –8 dB, respectively, do not change significantly the extent of this radar glacier zone. Firn line altitudes were mapped for several consecutive years. A low inter-annual variation of this line with elevations of about 200 m a.s.l. was observed. The maximum altitude of the firn line was recorded in the mass balance year 1996/97with about 250 m a.s.l. Comparing these values to equilibrium line positions of previous measurements indicate an upward shift in the equilibrium line altitude since the 1970’s. INTRODUCTION The Antarctic Peninsula and the adjacent South Shetland Islands have been identified as a region with a significant warming trend (e.g. King 1994, Smith et al. 1996, Skvarca et al. 1998). On the northern Antarctic Peninsula and the South Shetland Islands, snow and ice melt are important mass balance parameters of the small glaciers and ice caps (Bintanja 1995, Braun and Schneider 2000, Braun et al. 2001-a). Moreover, melt water has been identified as a probable source for the destabilisation and spectacular disintegration of the ice shelves in that region (Hulbe 1997). On King George Island itself, extended glacier retreat has been observed by several authors (e.g. Braun and Goßmann in press, Park et al. 1998, Simões et al. 1998, Wunderle 1996) and attributed to changes in glacier mass balance. Therefore, more detailed spatially distributed information on glacier mass balance parameters such as ablation patterns and firn line positions are required, a) as input and verification parameters for glacier melt models, b) to improve the monitoring of short-term glacier mass balance changes, c) to increase the accuracy of long-term predictions of glacier mass balance changes. Due to the operation of the German Antarctic Receiving Station at the Chilean base O’Higgins, it is now possible to use a multi-year data record of ERS-1/2 SAR imagery for glaciological studies on the Antarctic Peninsula. This time series, starting in 1991, enables the monitoring of the seasonal evolution and inter-annual variation of ablation patterns on the Antarctic ice cap of King George Island as already demonstrated for single years by Braun et al. (2000) and Wunderle (1996). Proceedings of EARSeL-SIG-Workshop Land Ice and Snow, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 282 In this study, we utilize ERS SAR data from King George Island to complement glacier melt modelling based on micro-meteorological ground observations, to observe inter-annual variations of the snow cover dynamics and firn line positions on the ice cap and to support the interpretation of a comprehensive ground penetrating radar survey (Pfender 1999). STUDY SITE With about 1250 km2, King George Island is the largest of the South Shetland Islands. 93 % of the island are ice covered. The islands major ice cap ranges between sea level and about 700 m a.s.l. (Figure 1). More than 30 percent of the island is located in an elevation range below 250 m a.s.l. Low gradient slopes prevail on the north-western part of King George Island, whereas steep fjordlike inlets intersect the smooth surface morphology of the ice cap on the southern side. A detailed outline of the island’s topography can be found in Simões et al. (1999) and Braun et al. (2001-b). Due to its location in the southern hemisphere west-wind zone, King George Island is subject to a highly maritime climate. The mean annual air temperature (1944-1999) at Bellingshausen Station is – 2.9 °C. Positive air temperatures are recorded throughout all summer months at lower elevations. Snow melt events also occur frequently during winter (Rachlewicz 1997). Generally, advection of warm humid air masses from northerly directions lead to the highest snowmelt rates (Braun et al. 2001-a). Figure 1: Sketch map of King George Island. Locations of the areas of interest (AOIs) are indicated with numbers As a consequence of the smooth surface morphology, relief-induced distortions in the SAR imagery are small on the north-western part of the ice cap. Furthermore, changes in the size of the ablation area using satellite remote sensing are more easily detected on moderately inclined surfaces than on steep slopes (Bindschadler 1998). Therefore, the present study will focus on this area of King George Island. Proceedings of EARSeL-SIG-Workshop Land Ice and Snow, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 283 DATABASE AND DATA PROCESSING For the present investigation, 40 ERS-1/2 scenes were analysed using data from both ascending and descending orbits. The imagery covers the time period between July 1992 and November 1999 (Figure 2). Data preparation was realized using processing chains in the ESA SAR Toolbox. From all images backscatter values were calculated using the algorithm based on Laur et al. (1998). In this procedure a compensation for losses during the analogue-digital conversion and a correction for the replica power variation was included. Subsequently, the images were co-registered using a master image from 18 February 1998 for the descending and an image from 15 July 1997 for the ascending orbit. 0.0 3.0 6.0 9.0 12.0 15.0 18.0 Number of ERS-1/2 SAR images 1991/92