Antarctic Grounding Line Migration from ICESat-2 Altimetry

有关研究领域的重要性：接地线标志着接地冰盖和漂浮冰架之间的边界。接地线位置是衡量冰盖不平衡和稳定性的重要指标。即使接地线上的冰流量或冰层厚度的微小变化也会直接改变冰盖的冰消失率，从而影响整个南极对海平面上升的贡献。在南极洲西部，这一区域位于一个固有的不稳定的逆行床坡上，温暖的海水推动了高冰流率，卫星观测显示，接地线以每年超过1公里的速度后退。1除了这种持续的动态退缩之外，还有一个记录不多的短期变化，原因是海潮暂时将搁浅的冰从床上抬起。这种潮汐迁徙可以在几个小时内达到几公里，并可能掩盖长期的行为。2项目概述：该项目提供了一个在气候和空间科学的交界处开展工作的令人兴奋的机会，为研究气候变化的影响和影响的国际努力作出了重要贡献。在这个博士学位中，你将与世界领先的地球观测专家密切合作，更好地了解南极冰盖的变化。虽然不可能直接观测冰盖表面下的冰床界面，但可以通过测量海洋潮汐引起的浮动冰架的垂直位移，利用卫星遥感监测接地线的位置。该项目将利用ICESat-2激光测高数据绘制目前接地线位置的变化图，监测南极冰盖最脆弱部分的稳定性，并调查海洋潮汐驱动的迁移如何影响冰的行为。ICESat-2卫星于2018年发射，提供了前所未有的空间分辨率和对南极洲的覆盖。它的激光高度计被分成六束以10千赫的频率传输，这使得每隔70厘米就有一束光脉冲照射到冰盖表面。这颗卫星是专门为准确测量地形陡峭地区(即接地线附近)的交叉轨道坡度而设计的。其他卫星包括哨兵一号和TerraSAR-X对接地线位置的合成孔径雷达干涉测量将与这些测量结果相辅相成。利用历史和新卫星任务的能力，并聚焦于冰盖最敏感的部分，该项目将提供新的洞察，以驱动南极冰架速度变化的基本过程，并允许以更高的置信度观察当前接地线退缩的速度。学生将做什么？学生将分析来自ICESat-2的新的激光测高数据，跨越2018年以后，建立在以前的高度计(ICESat和Cryosat-2)的接地线测绘研究的基础上。雷达干涉测量和ICESat(2003-2009年)将用于确定接地线位置的长期变化，而重复的ICESat-2测量将用于确定短期变化。接地线的水平运动将与海洋潮汐模型相关联，以确定接地线迁移如何与潮汐幅度、当地冰层特性和海床坡度有关。

Importance of the area of research concerned:Grounding lines mark the boundary between the grounded ice sheet and floating ice shelves. Grounding line location is a critical indicator of ice sheet imbalance and stability. Even small changes in ice flow or ice thickness at the grounding line can directly alter the rate of ice loss from the ice sheet, impacting the overall Antarctic contribution to sea level rise. In West Antarctica, a region situated on an inherently unstable retrograde bed slope where high rates of ice loss have been driven by warm ocean waters, satellite observations show that grounding lines have retreated at a rate of over one kilometre per year.1On top of this sustained dynamic retreat, there is also a poorly documented short-term variability caused by ocean tides temporarily lifting the grounded ice off the bed. This tidal migration can be up to several kilometres over a few hours and can obscure the long-term behaviour. It is also thought to increase overall glacier velocity through reduced drag at the bed.2Project summary:This project offers an exciting opportunity to work at the interface of climate and space science, making an important contribution to international efforts to study the effects and impact of climate change. In this PhD, you will work closely with world-leading Earth observation experts to better understand change on the Antarctic Ice Sheet. Whilst it is impossible to make a direct observation of the ice-bed interface beneath the ice sheet surface, the grounding line position can be monitored using satellite remote sensing by measuring vertical displacement of the floating ice shelf caused by ocean tides3. This project will make use of ICESat-2 laser altimetry data to map present day change in the grounding line position, monitor stability of the most vulnerable sectors of the Antarctic ice sheet, and investigate how migration driven by ocean tides impacts ice behaviour. The ICESat-2 satellite launched in 2018, and provides unprecedented spatial resolution and coverage of Antarctica. Its laser altimeter is split into six beams transmitting at 10 kHz, which allows a pulse of light to hit the ice sheet surface every 70 cm. The satellite has been specifically designed to allow accurate measurement of cross-track slopes in regions of steep topography (i.e. near grounding lines). These altimetry measurements will be complimented by synthetic aperture radar interferometric measurements of the grounding line location from other satellites including Sentinel-1 and TerraSAR-X.Harnessing the capability of both historical and new satellite missions, and focusing on one of the most sensitive parts of the ice sheet, the project will provide new insight into fundamental processes driving velocity variation in Antarctic ice shelves and allow current rates of grounding line retreat to be observed with higher confidence.What will the student do?The student will analyze new laser altimetry data from ICESat-2 spanning the period from 2018 onwards, building on grounding line mapping studies from previous altimeters (ICESat and Cryosat-2). Radar interferometry and ICESat (2003-2009) will be used to identify long-term change in the grounding line location, while repeat ICESat-2 measurements will be used to identify short-term variability. Horizontal movement of the grounding line will be correlated with ocean tide models to identify how grounding line migration is related to tidal amplitude, local ice properties and bed slope.