CAREER: Taking Process-based Models to the Field to Understand the Possibility and Implication of an Internal Shear Band Forming in Ice Flowing over Rough Topography

南极洲和格陵兰岛的冰盖通过嵌入相对静止的冰的狭窄排水通道失去了绝大部分冰块。这些排水路线内外冰速度的巨大差异与赛车和行人之间的速度差异相当，这就引出了一个问题，为什么整个冰盖上性质相对相似的冰表现如此不同。迄今为止最令人信服的解释是，快速移动的冰已经与下面的基岩失去了直接接触，并向海洋滑动，而缓慢移动的冰仍然与床相连，并通过内部变形移动。尽管这个解释可能很有说服力，但它并没有说明导致快速移动的冰柱滑动发生在哪里，但这个位置对于评估滑动以及相关的冰运动如何响应气候变化至关重要。如果在冰盖底部的冰和岩石相遇处发生滑动，来自变暖海洋的水渗透可能会直接影响冰的速度。如果滑动发生在靠近冰盖底部但在冰本身内部，这种反馈可能不会那么直接。该研究计划与一个合作教育项目相结合，旨在增加来自历史上处于不利地位的社区的高中生对极地科学的参与，这些社区在不久的将来容易受到海平面上升的影响。CAREER计划的目标是通过开发一套定制的数值模型来了解在粗糙地形上流动的冰中形成内部剪切带的可能性和影响，这些模型将作为冰川学社区使用和进一步开发的开源工具。研究小组假设，在非空化极限条件下，在地形粗糙的硬床上流动的冰可能会在地形高点上方的冰中经历足够的应变局部化，从而通过蠕变不稳定形成内部剪切带，尽管随着冰变薄和加速，平流冷却。这个内部剪切带的变形是高度局部化的，让人想起滑动，突出表明流动和滑动可能是由日益局部化的冰川变形谱连接起来的末端成员。该团队将通过三个步骤来验证其假设：(1)确定具有理想地形的地层内部剪切带形成所需的无量纲条件；(2)对具有更现实的分形地层地形的地层进行概括，并将结果与现有的现场观测结果进行比较；(3)对格陵兰岛和南极洲可能形成内部剪切带的地方进行大陆尺度的评估。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Ice sheets in Antarctica and Greenland lose the vast majority of their ice mass through narrow drainage routes that are embedded into comparatively stagnant ice. The sharp difference in ice speed inside and outside of these drainage routes is comparable in magnitude to the speed difference between a race car and a pedestrian, begging the question how ice with relatively similar properties across the entirety of the ice sheet behaves so differently. The most convincing explanation to date is that rapidly moving ice has lost direct contact with the bedrock below and is slipping towards the ocean while slowly moving ice remains connected to the bed and is moving through internal deformation. Compelling as this explanation might be, it does not address where in the ice column the slip occurs that enables fast motion, but this location is essential for assessing how sliding, and the associated ice motion, responds to climate change. If slip occurs where ice and rock meet at the base of the ice sheet, water infiltration from a warming ocean could directly affect ice speed. If slip occurs close to the base of the ice sheet but within the ice itself, this feedback would likely be less immediate. The research plan is integrated with a collaborative educational program that aims to increase participation in polar science for high-school students from historically disadvantaged communities that are prone to be impacted by sea-level rise in the near future.The goal of this CAREER proposal is to understand the possibility and implications of an internal shear band forming in ice flowing over rough topography by developing a suite of customized numerical models that will be available as open-source tools to the glaciological community for usage and further development. The team hypothesizes that ice flowing over a hard bed with rough topography in the non-cavitating limit may experience sufficient strain localization in the ice just above topographic highs to form an internal shear band through creep instability despite advective cooling as the ice thins and accelerates. The deformation in this internal shear band is highly localized and reminiscent of sliding, highlighting that flow and sliding may be end members that are connected by a spectrum of increasingly localized englacial deformation. The team will test its hypothesis through a three-step process by (1) identifying the nondimensional conditions that are required for internal-shear band formation for a bed with an idealized topography, (2) generalizing insights to beds with more realistic, fractal bed topography and comparing results to existing field observations, and (3) producing continental-scale assessments of where an internal shear band could form for both Greenland and Antarctica.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.