Collaborative Research: Subglacial Water Intrusion in Greenland

A full understanding of the flow and dynamics of an ice sheet will require knowledge of the state of its subglacial interface. While the topography of the Greenland Ice Sheet (GrIS) bed has been studied by radio echo sounding, its state in terms of melting and freezing, which itself affects its future evolution, has been inferred from numerical models, rather than from direct evidence. This project is for analysis of existing radar echo data gathered under the Program for Arctic Regional Climate Assessment (PARCA) and Center for Remote Sensing of Ice (CReSIS) programs, with the objective of generating direct evidence of the extent of subglacial water in Greenland and providing the glaciological community with maps of its extent. Previous echo analysis has found the variability of the received signal too great to extract the component due to variation of the bed material. Now, a method has been developed, validated and reported, based on further analysis of radio echo signal intensities, which allows the state of the bed to be determined reliably. This work will apply this method to all the available radio echo data, to generate maps of the extent of basal melting throughout Greenland. The procedure relies on a more precise assessment of radio echoes from the bed of the ice sheet than has previously been provided. It allows the variability of echo intensity to be reduced, and the variation of the bed material to be detected. It has been tested with data from a number of flights over Greenland during the 1999 field season, and is validated by tests of self-consistency, of echo intensity statistics, of interface smoothness, and of the relation between measured surface and interface slopes. The team will process data from flights undertaken between 1998 and 2005. Appropriate tests of self-consistency and statistical behavior will be applied. The presence or absence of water will be determined throughout. Results will be interpreted and maps compiled illustrating these results. The continuity and topographical context of each major water feature will be investigated and compared.

要全面认知冰盖的流动与动力学特性，必须掌握其冰下界面的状态信息。尽管格陵兰冰盖（Greenland Ice Sheet, GrIS）床层地形已通过无线电回声测深完成研究，但其融冻状态（该状态本身会对冰盖未来演化产生影响）此前仅通过数值模型推演获得，而非直接观测证据。本项目将对北极区域气候评估计划（Program for Arctic Regional Climate Assessment, PARCA）与冰遥感中心（Center for Remote Sensing of Ice, CReSIS）所采集的现有雷达回波数据展开分析，旨在获取格陵兰冰下水体分布范围的直接观测证据，并为冰川学界提供冰下水体分布制图成果。过往的回波分析发现，接收信号的变异性过高，无法提取由床层物质变化所引发的信号分量。如今，研究人员基于对无线电回声信号强度的进一步分析，开发、验证并发表了一种可可靠反演冰床状态的方法。本研究将把该方法应用于全部可用的无线电回声数据，以生成覆盖全格陵兰的冰底融区分布地图。该分析流程相较此前的评估方案，对冰盖床层无线电回波的解析精度更高：它可降低回波强度的变异性，同时能够识别床层物质的变化。该方法已借助1999年野外作业期间多次格陵兰飞行观测数据完成测试，并通过了自一致性检验、回波强度统计特性检验、界面平滑度检验以及实测地表与界面坡度相关性检验的验证。研究团队将处理1998年至2005年间的所有飞行观测数据，并采用适配的自一致性与统计特性检验方法，对全域冰下水体的存在与否进行判定。后续将对研究结果开展解读，并编制相关分布地图；同时还将对每一处主要冰下水体的连续性与地形背景展开调研与对比分析。