Revising Models of the Glacier-ocean Boundary Layer with Novel Laboratory Experiments

格陵兰岛和南极冰盖的融化越来越多地导致海平面上升。这种冰盖质量损失主要是由与海洋接触的冰川变薄、退缩和加速造成的。来自实地和卫星的观测表明，冰川对冰-海界面的变化很敏感，海底融化的增加很可能是由冰川下的融水排放（称为冰下融水羽流）造成的。冰川冰的融化还直接向海洋中注入大量淡水，可能导致调节全球热量输送的海洋沃茨循环发生重大变化，使冰-海洋相互作用成为气候变化和变异的一个重要潜在因素。因此，能否预测气候变化和海平面上升，从而作出适当反应，取决于我们对冰-海界面冰下融水羽流附近发生的小规模过程的了解。目前，对潜在物理学的理解还不完整;例如，不同的冰川-海洋相互作用模型可能会产生不同的融化速率，对于相同的海洋热量供应，其差异超过五倍。因此，很难评估预测模型的可靠性。该项目将利用全面的实验室实验，研究冰羽附近冰川的融化速度如何受到冰-海洋界面处的冰粗糙度、冰的几何形状、海洋湍流和海洋密度分层的影响。这些实验将用于开发新的和改进的海洋冰盖融化预测模型。该项目在现代实验流体力学和冰川学之间建立了桥梁，目标是在这两个领域取得进展。作为这项工作的一部分，两名研究生将接受跨学科培训，每年两名本科生将接受实验流体力学培训，以协助这项工作并开发自己的研究项目。该项目包括一个综合实验计划，旨在研究冰川冰在以下因素的综合影响下的融化速率：（1）冰-海洋界面附近和界面处发生的湍流，（2）周围水柱中的密度分层，（3）冰架底部拓扑结构的不规则性，以及（4）多种融水羽流的不同空间分布。实验的目的是获得冰-海洋界面附近/处的速度、密度和温度的高分辨率数据，然后将这些数据用于提高对毫米级融化过程的理解，并设计新的、更可靠的冰川演变和海平面上升的数值模型。特别是，实验流体力学中基于激光的光学技术（粒子图像速度和激光诱导荧光）将用于收集数据，实验将使用折射率匹配技术进行，以消除折射率的变化，否则可能会使测量产生偏差。实验将在一个气候受控的冷室内进行，以模拟现场条件（海洋温度为0-10摄氏度）。该项目将使用3D打印来创建不同的铸造模具，用于制作具有不同粗糙度的冰块。目标是研究冰融化速率如何作为羽流、环境海水和冰界面几何特性的函数而变化。该项目将根据实验结果，开发和测试一个新的与区域环流模型（MITgcm）耦合的整体羽流模型，该模型可用于预测冰川融化对海洋环流和海平面上升的影响。该奖项反映了NSF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。

Melt from the Greenland and Antarctic ice sheets is increasingly contributing to sea-level rise. This ice sheet mass loss is primarily driven by the thinning, retreat, and acceleration of glaciers in contact with the ocean. Observations from the field and satellites indicate that glaciers are sensitive to changes at the ice-ocean interface and that the increase in submarine melting is likely to be driven by the discharge of meltwater from underneath the glacier known as subglacial meltwater plumes. The melting of glacier ice also directly adds a large volume of freshwater into the ocean, potentially causing significant changes in the circulation of ocean waters that regulate global heat transport, making ice-ocean interactions an important potential factor in climate change and variability. The ability to predict, and hence adequately respond to, climate change and sea-level rise therefore depends on our knowledge of the small-scale processes occurring in the vicinity of subglacial meltwater plumes at the ice-ocean interface. Currently, understanding of the underlying physics is incomplete; for example, different models of glacier-ocean interaction could yield melting rates that vary over a factor of five for the same heat supply from the ocean. It is then very difficult to assess the reliability of predictive models. This project will use comprehensive laboratory experiments to study how the melt rates of glaciers in the vicinity of plumes are affected by the ice roughness, ice geometry, ocean turbulence, and ocean density stratification at the ice-ocean interface. These experiments will then be used to develop new and improved predictive models of ice-sheet melting by the ocean. This project builds bridges between modern experimental fluid mechanics and glaciology with the goal of leading to advances in both fields. As a part of this work, two graduate students will receive interdisciplinary training and each year two undergraduate students will be trained in experimental fluid mechanics to assist in this work and develop their own research projects.This project consists of a comprehensive experimental program designed for studying the melt rates of glacier ice under the combined influences of (1) turbulence occurring near and at the ice-ocean interface, (2) density stratification in the ambient water column, (3) irregularities in the bottom topology of an ice shelf, and (4) differing spatial distributions of multiple meltwater plumes. The objective of the experiments is to obtain high-resolution data of the velocity, density, and temperature near/at the ice-ocean interface, which will then be used to improve understanding of melt processes down to scales of millimeters, and to devise new, more robust numerical models of glacier evolution and sea-level rise. Specially, laser-based, optical techniques in experimental fluid mechanics (particle image velocity and laser-induced fluorescence) will be used to gather the data, and the experiments will be conducted using refractive-index matching techniques to eliminate changes in refractive indices that could otherwise bias the measurements. The experiments will be run inside a climate-controlled cold room to mimic field conditions (ocean temperature from 0-10 degrees C). The project will use 3D-printing to create different casting molds for making ice blocks with different types of roughness. The goal is to investigate how ice melt rate changes as a function of the properties of the plume, the ambient ocean water, and the geometric properties of the ice interface. Based on the experimental findings, this project will develop and test a new integral-plume-model coupled to a regional circulation model (MITgcm) that can be used to predict the effects of glacial melt on ocean circulation and sea-level rise.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.