Characterising the Ice Shelf/Ocean Boundary Layer

全球平均海平面每年上升约3毫米。鉴于这一变化对经济和社会的巨大影响，迫切需要准确的海平面预测，以便为考虑减缓和适应战略的决策者提供信息。目前，南极洲和格陵兰岛冰盖的融化造成了大约三分之一的海平面上升。这种融化的未来是高度不确定的，最坏的情况包括大量的冰盖导致危险的海平面上升。冰盖对海平面上升的最大贡献发生在海洋融化冰架底部（固定冰盖的浮动延伸部分）的时候。海水中冰的融化速度是由热量和盐从海洋向冰的转移决定的。观测显示，冰架下的海洋有一个湍流边界层，在那里，不断上升的融水、海洋的大规模环流和潮汐驱动着剧烈的混合。边界层中热量和盐的混合影响冰的融化速度，但人们对所涉及的物理过程知之甚少，在可预见的未来，气候模式无法解决这一问题。拟议的项目将提高我们对冰架/海洋边界层的理解，并开发用于气候模式的改进的冰架融化表示。为了实现这些目标，我们将使用一套数值模型和最新的观测结果。我们将从直接数值模拟（DNS）开始，以超高分辨率（~1毫米）模拟冰架旁边的小盒海洋（~1立方米）。这将有助于深入了解冰附近的湍流及其与融化的相互作用。然后，我们将使用大涡模拟（LES）以高分辨率（~10厘米- 1米）研究更大的体积（~1平方公里面积× 100米高度）。这将解决整个边界层中最大的湍流运动。这两种模型都将使用最近在George VI和Larsen C冰架系泊点获得的观测结果进行验证（Nicholls, NE/H009205/1）。模型结果反过来将有助于解释和理解观测结果。我们将使用这些数值模式来设计和校准用于海洋气候模式的冰融化和垂直混合的参数化。我们将把候选参数化添加到一维（垂直）模型中，该模型包含许多流行的海洋混合方案，并直接针对DNS和LES结果进行测试。我们将从现有的参数化开始，并根据需要修改它们以匹配高分辨率模型。成功的参数化将在英国海洋模型（NEMO）中实施，并与气候建模小组（包括英国气象局）共享，以提高对海平面上升的预测。

Global average sea level is rising by approximately 3 millimetres per year. Given the huge economic and societal impacts of this change, accurate forecasts of sea level are urgently needed to inform policymakers considering mitigation and adaptation strategies. Melting of the ice sheets of Antarctica and Greenland currently contributes about one third of sea level rise. The future of this melting is highly uncertain, and the worst-case scenario involves a substantial ice-sheet contribution to dangerous sea-level rise. The largest contribution to sea level rise from ice sheets occurs when the ocean melts the base of ice shelves (floating extensions of the grounded ice sheet). The melt rate of ice in seawater is determined by the transfer of heat and salt from the ocean towards the ice. Observations reveal a turbulent boundary layer in the ocean beneath ice shelves, where vigorous mixing is driven by the flow of rising meltwater, large-scale circulation in the ocean, and tides. Mixing of heat and salt in the boundary layer influences the ice melt rate, but the physical processes involved are poorly understood and will not be resolved in climate models for the foreseeable future. The proposed project will improve our understanding of the ice shelf/ocean boundary layer and develop improved representations of ice-shelf melting for use in climate models.To achieve these aims we will use a suite of numerical models and the latest observations. We will start with direct numerical simulations (DNS) to model a small box of ocean next to an ice shelf (~1 cubic metre) at ultra-high resolution (~1 millimetre). This will provide insight into the turbulence near the ice and its interaction with melting. We will then use large-eddy simulations (LES) to study a larger volume (~1 square kilometre in area by 100 metres height) at high resolution (~10 centimetres - 1 metre). This will resolve the largest turbulent motions in the whole boundary layer. Both models will be validated using recent observations obtained from mooring sites at the George VI and Larsen C ice shelves (Nicholls, NE/H009205/1). The model results will in turn help interpret and understand the observations.We will use these numerical models to devise and calibrate parameterisations for ice melting and vertical mixing for use in ocean climate models. We will add candidate parameterisations to a one-dimensional (vertical) model that incorporates many popular ocean mixing schemes, and test them directly against the DNS and LES results. We will begin with existing parameterisations and modify them as needed to match the high resolution models. The successful parameterisations will be implemented in the UK ocean model (NEMO) and shared with climate modelling groups (including the Met Office) to improve predictions of sea-level rise.