Collaborative Research: Understanding the controls on spatial and temporal variability in ice discharge using a Greenland-wide ice sheet model

据观察，海平面正在以越来越快的速度上升。在过去的几十年里，一个重要的贡献来自山脉冰川，但格陵兰冰盖的贡献预计将在不久的将来占据主导地位。这一贡献以融化的水和冰山的形式输送到海洋，融化在格陵兰周围的峡湾和沿海海洋中。这些贡献在空间上有所不同。这项拟议的工作将建立一个格陵兰对海平面上升的贡献模型，使用观测数据约束该模型，并根据未来气候情景估计贡献。该项目将通过为两名研究生的培训提供支持，为STEM劳动力发展做出贡献。它还将为一名初学者在其职业生涯的形成阶段提供支持。它将通过维护和增强开源并行冰盖模型(PISM)代码为社区使用而贡献社区资源。已经确定了对出口冰川系统动态的四种可能的控制：1)表层以下海水变暖和/或冰下径流增加可能会增加冰川-峡湾界面的海底冰融化；2)刚性海冰和冰川混杂(海冰和冰山的混合物)可能会抑制崩解，允许末端前进；3)末端相对于冰下地形的位置(例如，过深或冰床)影响退缩速度；以往对冰盖对海平面上升贡献的模拟由于模型和观测数据的空间分辨率不足而受到限制，这使得全冰盖模拟无法真实地捕捉到出口冰川的流动。到目前为止，结果要么来自区域模型，要么来自高度理想化的流线模型，这些模型被放大到冰盖尺度。冰盖模拟的最新进展，以及高分辨率冰下地形的可用性，现在使人们能够在整个冰盖范围的模拟中解决单个出口冰川流动的问题。该项目将使用开源并行冰盖模式(PISM)的框架，单向耦合到新的高分辨率大气和海洋后播。这将为在逐个冰川的基础上评估以下问题提供一个试验台：1)四种控制措施目前对出口冰川流量和冰流量的相对影响是什么；2)21世纪冰流量大幅增加的可能性是什么；3)什么条件会导致大的变化(例如，海洋变暖的时空分布、地表径流增加)；以及4)需要什么观测来支持格陵兰冰盖海洋观测系统来捕捉大变化的强迫或开始？与现有的遥感和现场观测相比较，包括但不限于地表速度、地表高程和质量变化的时间序列，将作为成功的衡量标准。然后，在现有的大气-海洋预测的推动下，将对格陵兰冰盖21世纪的演变进行模拟，以提供对未来冰排放的现实估计。

Sea level is observed to be rising at an increasing rate. A significant contribution during the past decades has been from mountain glaciers, but the contribution from the Greenland Ice Sheet is anticipated to become dominant in the near future. This contribution is delivered to the ocean as both meltwater and icebergs that melt in the fjords and coastal ocean around Greenland. These contributions vary spatially. The proposed work will develop a model of Greenland's contribution to sea level rise, constrain the model using observed data, and estimate contributions based on scenarios of future climate.The project will contribute to STEM workforce development by providing support for the training of two graduate students. It will also provide support for a beginning investigator during the formative years of his career. It will contribute to the community resources by maintaining and enhancing the open source Parallel Ice Sheet Model (PISM) code for community use.Four possible controls on outlet glacier systems dynamics have been identified: 1) Warming subsurface ocean water and/or increased subglacial runoff may increase submarine ice melting at the glacier-fjord interface;2) Rigid sea ice and ice mélange (a mixture of sea ice and icebergs) may suppress calving, allowing for terminus advance; 3) The terminus position relative to subglacial topography (e.g., over-deepenings or sills) influences rates of retreat; and 4) Changes in the resistive stress caused by contact with the fjord walls and/or glacier bed can lead to terminus advance, retreat, and/or thinning.Previous simulations of ice sheet contributions to sea level rise have been limited by the insufficient spatial resolution of models and observational data, which prevented whole-ice sheet simulations to faithfully capture outlet glacier flow. Results to date are either obtained from regional models or from highly idealized flow line models that were upscaled to ice-sheet scale. Recent advances in ice sheet modeling, and the availability of high-resolution subglacial topography, now allow one to resolve individual outlet glacier flow in ice sheet-wide simulations. This project will use the framework of the open-source Parallel Ice Sheet Model (PISM), uni-directionally coupled to new high-resolution hindcasts of the atmosphere and ocean. This will provide a test bed for assessing, on a glacier-by-glacier basis: 1) what is the relative present-day influence of the four controls on outlet glacier flow and ice discharge; 2) what is the potential for a substantial increase in 21st century ice discharge; 3) what conditions would precipitate large changes (e.g., spatio-temporal distribution of ocean warming, enhanced surface runoff); and 4) what observations are required in support of a Greenland Ice Sheet Ocean Observing System to capture the forcing or onset of large changes? Comparison to available remotely-sensed and in-situ observations, including, but not limited to, time-series of surface velocities, surface elevation, and mass changes will serve as metrics of success. Simulations of the 21st century evolution of the Greenland Ice Sheet will then be performed, forced by available atmosphere-ocean projections, to provide realistic estimates of future ice discharge.