Greenland Traverse for Accumulation and Climate Studies (GreenTrACS) - multi-channel ground-penetrating radar 2017

The stability of the Greenland Ice Sheet is of critical interest to scientists and society(at large in the context of future sea-level rise. The extent to which the Greenland Ice Sheet will lose mass and contribute to rising sea level(in the coming decades depends on the discharge from glaciers at its edges and on the surface mass balance, which is the balance between snow accumulation and surface melt. Estimates of Greenland surface mass balance increasingly utilize climate reanalyses and high-resolution regional climate models to determine snow accumulation, surface melt and runoff/refreeze. These models show significant, and model-dependent, biases (differences from observations) along the steep edges of the Greenland Ice Sheet where the highest and most variable (in space and time) rates of accumulation and surface melt are observed. Thus, the edges of the Greenland Ice Sheet are in critical need of updated accumulation(and melt data to validate models and improve mass balance estimates. The investigators propose a traverse in the Western Greenland percolation zone over two field seasons to develop continuous in-situ snow accumulation and firn density records using ground-based radar and shallow firn cores. The research objectives include: (1) determining the patterns, in time and space, of snow accumulation in Western Greenland over the past 20-40 years; (2) evaluating surface melt refreeze and englacial meltwater storage in the Western Greenland percolation(zone over the past 20-40 years; and (3) quantifying the accumulation and surface melt biases of the most recent climate reanalysis models and their regional climate model counterparts. These data are radar echograms from the GreenTrACS multi-channel 500 MHz ground-penetrating radar operated in a multi-offset configuration. The multi-channel radar technique accurately and independently estimates density, depth, and annual snow accumulation -- between the Greenland Traverse for Accumulation and Climate Studies (GreenTrACS) firn core and snow pit sites -- by horizon velocity analysis of common midpoint radar reflections from the snow and shallow firn.

格陵兰冰盖（Greenland Ice Sheet）的稳定性，在未来海平面上升的研究背景下，是科学家与全社会均极为关注的核心议题。未来数十年内，格陵兰冰盖的物质流失规模及其对海平面上升的贡献程度，取决于其边缘冰川的径流量与表面物质平衡——即积雪积累与表面融雪之间的动态平衡关系。当前格陵兰表面物质平衡的估算工作，愈发多地采用气候再分析数据（climate reanalyses）与高分辨率区域气候模型，以确定积雪积累量、表面融雪量与径流/再冻结量。此类模型在格陵兰冰盖的陡峭边缘区域存在显著且因模型而异的偏差（与实际观测结果的差值），而该区域正是积雪积累与表面融雪速率最高、时空变异性最强的区域。因此，格陵兰冰盖的边缘区域亟需更新后的积雪积累（与融雪）观测数据，用于验证模型并优化物质平衡估算结果。研究团队提议在两个野外考察季期间，沿格陵兰西部渗流区（percolation zone）开展穿越考察，利用地面雷达与浅层粒雪芯（firn core）获取连续的原位积雪积累与粒雪密度数据记录。本研究的目标包含以下三项：（1）明确过去20至40年间格陵兰西部积雪积累的时空分布模式；（2）评估过去20至40年间格陵兰西部渗流区的表面融雪再冻结过程与冰内融水储存情况；（3）量化最新一代气候再分析模型及其配套区域气候模型在积雪积累与表面融雪方面的偏差。本次研究所用的数据，来自格陵兰积累与气候研究穿越队（GreenTrACS）所搭载的多偏移距配置多通道500 MHz探地雷达（ground-penetrating radar）采集的雷达回波图。该多通道雷达技术通过对积雪与浅层粒雪层的共中心点雷达反射波进行层速度分析，能够独立且精准地估算格陵兰积累与气候研究穿越队的粒雪芯与雪坑站点之间区域的密度、深度与年度积雪积累量。