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

海平面上升将在未来几十年内影响到沿海社区的数百万人。准确预测它将以多快的速度上升是一项挑战，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。有一个过程可能起着惊人的巨大作用，那就是小漩涡和小漩涡（只有几英尺宽）的温水的影响，它们控制着世界海洋尽头（潮汐）冰川近乎垂直的悬崖表面的冰融化速度。在这些冰川上，冰直接流入海洋，在水下融化或形成冰山。冰的融化产生了淡水，这些淡水从靠近海洋表面的地方流出，并推动了一股回流，将深层温暖的海水吸引到冰川。根据目前的理论，冰融化速度的增加会增加温暖的海水被拉向冰面的强度，从而进一步加速融化。这一过程的细节——尤其是冰面附近的小规模动态——从未被测量过，因为冰崖的崩解太危险，无法进行测量。在这里，我们建议使用高度专业化的水下机器人（远程操作车辆，或“ROV”）与最先进的光学和声学仪器来观察融化速度和控制它的过程。其中一个新颖的方面是使用“融化桩”——一种6英尺长的杆，将由ROV插入冰川表面，并持续监测融化过程。然后，这些木桩为我们的ROV提供了一个参考框架，以便对冰川表面的形状、附近水流的动态以及冰-水界面的演变进行一系列详细测量。与此同时，我们将观察峡湾当地的海洋环境——洋流、盐度和温度——这是我们在更大规模和气候模型中预测冰融化所需的主要成分。我们的分析将结合现场数据和高分辨率流体流动模型，该模型将重现具有真实水特性的冰面条件。模型和数据的结合将用于改进我们的融化预测，并直接使用我们的观测测量来验证这些预测。在项目结束时，我们将能够扩展我们的结果，以估计全球有多少潮汐冰川正在融化，以及这种情况如何随时间变化。除了对社会和科学界的重要性之外，这项资助在几个层面上提供了更广泛的影响：(1)为两名早期职业女性提供指导和支持；(2)为三名跨学科冰海研究的研究生提供支持；(3)为45名大四本科生提供体验机会、资助和指导，这些本科生的毕业项目将直接为本项目做出贡献，同时由我们性别和文化多元化的工程师和技术人员团队进行监督。(4)向K-12学生和公众展示浮力和对流的课堂实验；(5)另外两组高中女生将参与并通过“冰雪峡湾女孩”考察活动进行观察。海洋冰川在冰海界面的融化影响着世界冰盖的质量损失率。除了导致海平面上升外，融化过程的细节还决定了新鲜融水进入海洋的深度（这反过来又在各种尺度上影响海洋环流），并改变了冰解速率。现有的理论认为，沿着这些冰面的海底融化速度是由冰下流量的强度决定的。然而，最近的观测发现，在冰川末端的大部分地区，甚至在排放羽流区域之外，融化速度出乎意料地高。观测到的融化速度和预测的融化速度之间的数量级差异表明，沿着冰面的其他地方存在着驱动近冰湍流的能量动力学。我们假设这种差异是由边界层内速率控制物理的差异引起的。湍流传递系数由稳定边界层导出。然而，在垂直的冰川冰面上，边界层具有强大的浮力和边缘稳定性，可能会产生实验室或理想模型无法捕捉到的动力学。由于浮力融水通量为接近边界的外部流动提供动能，并且由于这些流动的增强导致融化的增强，因此在动力学中有可能出现强烈的正反馈。因此，熔体参数或参数化函数形式中的小误差可能对总熔体计算产生重大影响。目前还没有研究直接在接近垂直的冰面附近进行观测，也没有研究以研究这些动态反馈所需的分辨率测量融化动力学。这笔拨款将支持在阿拉斯加LeConte冰川部署首个协调水声、光学和原位无人传感器网络的开发。使用冰川学、海洋学和机器人技术相结合的方法，这些系统将收集近垂直冰川表面湍流边界层的首次地球物理观测数据。具体来说，我们将通过浮力强迫的近垂直边界层直接测量速度、盐度和温度，并将这些与跨几个空间尺度的地下冰形态观测（例如，坡度、粗糙度）联系起来。通过将这些数据与高分辨率的真实模拟相结合，我们将描述边界层湍流的主要贡献，并明确地将这些与局部融化速率联系起来。我们的最终目标是确定需要在什么时间和空间尺度上测量哪些参数（例如，峡湾u，T，S），以及可以做出哪些假设，以便将小尺度冰界面的动力学与大尺度海洋和冰川强迫联系起来。这笔赠款建立了目前尚不存在的观测能力。测量将跨越足够范围的参数空间（海洋温度，速度变化和冰形态），以便我们和其他人测试现有的和推进的新融化模型，这些模型是许多冰-海洋群落模型的基础。该奖项由北极自然科学项目和物理海洋学项目共同资助。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。

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.