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停滞的冰中的狭窄排水路线失去了绝大多数的冰块。这些排水路线内外冰层速度的巨大差异，在大小上相当于赛车和行人之间的速度差异，这让人不禁要问，在整个冰盖上，性质相对相似的冰如何表现得如此不同。迄今为止最令人信服的解释是，快速移动的冰与下面的基岩失去了直接联系，正在滑向海洋，而缓慢移动的冰仍然与海床相连，并通过内部变形移动。尽管这一解释可能令人信服，但它并没有解决冰柱中导致快速运动的滑移发生在哪里，但这个位置对于评估滑移和相关的冰运动对气候变化的反应是必不可少的。如果冰和岩石在冰盖底部相遇处发生滑动，变暖的海洋中的水渗透可能会直接影响冰川的速度。如果滑移发生在靠近冰盖底部但在冰层内部，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。这项职业计划的目标是通过开发一套定制的数值模型，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，这些模型将作为开源工具提供给冰川学社区使用和进一步发展。该团队假设，流经坚硬河床的冰可能会在地形高度略高于地形高度的地方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，尽管随着冰层变薄和加速平流冷却，但冰层的地形崎岖。这种内部剪切带中的变形是高度局部化的，使人联想到滑动，突出了流动和滑动可能是通过一系列日益局部化的冰川变形连接在一起的末端成员。该团队将通过三个步骤来验证其假设：(1)确定具有理想化地形的床层形成内部剪切带所需的无量纲条件，(2)将洞察推广到具有更真实、分形床层地形的床层，并将结果与现有的现场观测进行比较，(3)对格陵兰和南极洲的内部剪切带形成的位置进行大陆范围的评估。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.