Collaborative Research: How fast do tidewater glaciers melt? Quantifying the processes that control boundary layer transport across the ice-ocean interface

合作研究：潮水冰川融化的速度有多快？

• 批准号: 2023269
• 负责人: David Sutherland
• 金额: $ 25.32万
• 依托单位: University of Oregon Eugene
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-10-01 至 2024-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2023269&HistoricalAwards=false
• 关键词: Collaborative; Research; How; fast; do; Collaborative; Research; How; fast; do

Sea-level rise will affect millions of people in coastal communities within the next several decades. Accurate predictions of how quickly it will rise is challenging because it depends on many different processes and how these processes interact with and feedback on each other. One process that may play a surprisingly large role is the effect of small swirls and eddies (only a few feet across) of warm water that control the rate of ice melt at the near-vertical cliff faces of the world’s marine-terminating (tidewater) glaciers. At these glaciers, ice flows directly into the ocean and melts underwater or calves icebergs. Melting of the ice produces freshwater that flows out near the ocean surface and drives a return flow that draws in deep warmer ocean water toward the glacier. According to current theory, increasing the rate of ice melt increases the strength at which warmer ocean water is pulled in towards the ice face, which further enhances the melting. The details of this process - particularly the small-scale dynamics near the ice face - have never been measured because the calving ice cliffs are too dangerous to make measurements. Here we propose to use a highly specialized underwater robot (a remotely operated vehicle, or “ROV”) with state-of-the-art optical and acoustic instruments to observe the melt rate and the processes that control it. One of the novel aspects is the use of “melt stakes” - 6 ft long rods that will be driven into the glacier face by the ROV and monitored continuously to determine the melt processes. These stakes then provide a frame of reference for our ROV to make a suite of detailed measurements of the shape of the glacier face, the dynamics of the currents adjacent to it, and how the ice-water interface evolves. At the same time, we will observe the local ocean environment in the fjord - the currents, salinity and temperature - which are the main ingredients we need to predict ice melt in larger-scale and climate models. Our analyses will combine field data with a high-resolution fluid-flow model that recreates the conditions along the ice with realistic water properties. The combination of model and data will be used to refine our melt predictions and verify these directly using our observed measurements. At the end of the project, we will be able to extend our results to estimate how much melt is occurring for tidewater glaciers around the globe, and how this may change in time. Beyond this importance to society and the scientific community, this grant provides broader impacts across several levels: (1) mentorship and support for two early career women (2) support for three graduate students in interdisciplinary ice-ocean studies, (3) experiential opportunities, funding, and mentorship for 45 senior-year undergraduate students, whose capstone projects will directly contribute to this project while being supervised by our gender and culturally diverse team of engineers and technical staff, (4) classroom experiments showing buoyancy and convection to engage K-12 students and the general public, and (5) two teams of high-school women will additionally be involved and make observations through Girls in Icy Fjords expeditions.Melting at the ice-ocean interface of marine-terminating glaciers influences the rate of mass loss from the world's ice sheets. In addition to contributing to sea-level rise, details of the melt process dictate the depth at which fresh meltwater enters the ocean (which in turn affects ocean circulation on a variety of scales) and alters calving rates. Existing theory suggests that the rate of submarine melting along these ice faces is set by the strength of subglacial discharge. However, recent observations find unexpectedly high melt rates over broad sections of glacier termini, even outside discharge plume areas. The observed order of magnitude discrepancies between observed and predicted melt rates suggests the presence of energetic dynamics elsewhere along the ice face that drive near-ice turbulent flows. We hypothesize that this discrepancy arises from differences in the rate-controlling physics within the boundary layers. Current turbulent transfer coefficients were derived from stable boundary layers. Yet on vertical glacier ice faces, boundary layers have strong buoyant forcing and marginal stability that likely produce dynamics not captured by laboratory or idealized models. Because buoyant meltwater fluxes provide kinetic energy for near-boundary outer flows -- and because enhancement of those flows leads to enhanced melting -- there is potential for strong positive feedbacks in the dynamics. As a result, small errors in the melt parameters or the parameterization functional form can have significant consequences to the total melt calculation. No studies have yet to make observations immediately next to near-vertical ice faces, or measure melt dynamics with the resolution necessary to investigate these dynamical feedbacks. This grant supports the development of a first-of-its-kind network of coordinated underwater acoustic, optical and in-situ unmanned sensors to be deployed at LeConte Glacier, Alaska. Using methods that meld glaciology, oceanography, and robotics, these systems will collect the first geophysical observations of the turbulent boundary layer at a near-vertical glacier face. Specifically, we will directly measure velocity, salinity and temperature through a buoyancy-forced near-vertical boundary layer and relate these to observations of the subsurface ice morphology (e.g., slope, roughness) across several spatial scales. By combining these data with high-resolution realistic simulations, we will characterize the dominant contributions to boundary layer turbulence and explicitly relate these to local melt rates. Our ultimate goal is to determine what parameters need to be measured (e.g., fjord u,T,S) over what time and space scales, as well as what assumptions can be made in order to connect dynamics from the small-scale ice interface to the large-scale ocean and glacier forcing. This grant builds an observational capacity that does not exist at present. Measurements will span a sufficient range of the parameter space (in ocean temperature, velocity variance and ice morphology) for us and others to test existing and advance new melt models that underlie many ice-ocean community models.This award is co-funded by the Arctic Natural Sciences Program and the Physical Oceanography Program.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.

在未来几十年中，海平面上升将影响沿海社区的数百万人。准确地预测它的速度是挑战，因为它取决于许多不同的过程以及这些过程如何相互作用和反馈。一个可能发挥令人惊讶的重要作用的过程是，温水的小漩涡和涡流（只有几英尺）控制着世界海洋末端（Tidewater）冰川的近垂直悬崖面融化的速度。在这些冰川上，冰直接流入海洋，并在水下或犊牛冰山融化。冰的融化会产生淡水，该淡水在海面附近流出，并驱动回流，使冰川进入更温暖的海水。根据当前的理论，增加冰融化的速率会增加较温暖的海水向冰面拉进的强度，从而进一步增强了融化。此过程的细节 - 尤其是冰面附近的小规模动力学 - 从未被测量过，因为产犊冰悬崖太危险了，无法进行测量。在这里，我们建议使用具有最先进的光学和声学仪器的高度专业化的水下机器人（远程操作的车辆或“ ROV”）来观察熔体速率及其控制的过程。新颖的方面之一是使用“熔体赌注” -6英尺长的杆，该杆将被ROV驱动到冰川面中并连续监测以确定熔体过程。然后，这些赌注为我们的ROV提供了参考框架，以对冰川面的形状，与之相邻的电流的动力学以及冰水界面的演变进行详细测量。同时，我们将观察峡湾中的当地海洋环境 - 电流，盐度和温度 - 这是我们需要预测较大规模和攀爬模型中冰融化的主要成分。我们的分析将将现场数据与高分辨率流体流量模型相结合，该模型以逼真的水性质重新创建沿冰的条件。模型和数据的组合将用于完善我们的熔体预测，并使用我们观察到的测量值直接验证这些预测。在项目结束时，我们将能够扩展结果，以估计全球潮水冰川发生了多少融化，以及时间如何改变。除了对社会和科学界的重要性之外，这项赠款还提供了多个层面的更广泛的影响：（1）对两个早期职业妇女的指导和支持（2）在跨学科的冰基研究中为三名研究生提供支持，（3）经验体验机会，资金，资金，资金，资金和精神训练，对45名高年级的研究员，他们的工程阶层将在我们的工程中直接贡献我们的工程阶层，并在我们的工程中受到了四个班级的贡献（我们的工程师四个工程（我们的工程），并在45个班级中构成了工程的养育，并且（我们的工程四）在45级中，（实验显示了与K-12学生和公众互动的浮力和联系，（5）将参与其中的两支高中女性团队，并通过冰冷的峡湾探险中的女孩进行观察。在海洋末期冰川的冰山界面上融化，会影响世界冰纸从世界冰纸中造成的大规模损失。除了促进海平面上升之外，熔体过程的细节还决定了新鲜融合水进入海洋的深度（又影响了各种尺度上的海洋循环），并改变了产犊率。现有理论表明，沿着这些冰面融化的海底融化速率是由冰山下排放的强度确定的。然而，最近的观察结果发现，即使是冰川末端，即使是外排羽毛，也出乎意料的高熔体速率。观察到的数量级差异和预测的熔体速率之间存在差异，这表明沿着冰面的其他地方存在能量动力学，从而驱动接近冰的湍流流动。我们假设这种差异源于边界层内速率控制物理的差异。当前的湍流转移系数来自稳定的边界层。然而，在垂直冰川冰面上​​，边界层具有强大的强迫和边际稳定性，可能会产生实验室或理想化模型未捕获的动力学。由于浮力熔融通量为近似外部流动提供了动能 - 并且这些流动的增强会导致熔融增强 - 因此动态中有强烈的积极反馈。结果，熔体参数或参数化功能形式的小误差可能会对总熔体计算产生重大影响。尚无研究能够在近垂直冰面旁边立即进行观察，或者通过研究这些动态反馈所需的分辨率来测量熔体动力学。这项赠款支持开发在阿拉斯加Leconte Glacier的首次协调的水下声学，光学和原位无人传感器。使用融合冰川学，海洋学和机器人技术的方法，这些系统将收集近垂直冰川面部湍流边界层的第一个地球物理观测。具体而言，我们将通过浮力近垂直边界层直接测量速度，盐度和温度，并将其与几个空间尺度上的地下冰形态（例如，斜率，粗糙度）的观察联系起来。通过将这些数据与高分辨率逼真的模拟相结合，我们将表征对边界层湍流的主要贡献，并将其与局部熔体速率明确相关。我们的最终目标是确定在什么时间和空间尺度上需要测量哪些参数（例如，峡湾U，T，S），以及可以做出哪些假设，以连接从小规模冰界面到大型海洋和冰川强迫的动态。该赠款建立了目前不存在的观察能力。 Measurements will span a sufficient range of the parameter space (in ocean temperature, velocity variance and ice morphology) for us and others to test existing and advance new melt models that underlie many ice-ocean community models.This award is co-funded by the Arctic Natural Sciences Program and the Physical Oceanography Program.This award reflects NSF's statutory mission and has been deemed precious of support through evaluation using the Foundation's intellectual merit and broader影响审查标准。