Characterising the Ice Shelf/Ocean Boundary Layer

全球平均海平面正以每年约3毫米的速度上升。鉴于这一变化的巨大经济和社会影响，迫切需要准确预测海平面，以便为考虑缓解和适应战略的政策制定者提供信息。南极洲和格陵兰岛冰盖的融化目前约占海平面上升的三分之一。这种融化的未来非常不确定，最糟糕的情况是冰盖对危险的海平面上升有很大的贡献。冰盖对海平面上升的最大贡献发生在海洋融化冰架(地面冰盖的浮动延伸部分)的底部时。冰在海水中的融化速度是由热量和盐分从海洋向冰的转移决定的。观测揭示了冰架下海洋中的湍流边界层，在那里强烈的混合是由上升的融水流动、海洋中的大规模环流和潮汐驱动的。边界层中的热和盐的混合会影响冰的融化速度，但其中涉及的物理过程知之甚少，在可预见的未来也不会在气候模型中得到解决。拟议的项目将提高我们对冰架/海洋边界层的理解，并改进冰架融化的表示法，用于气候模式。为了实现这些目标，我们将使用一套数值模式和最新的观测结果。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(~1立方米)旁边的小盒子海洋。这将提供对冰层附近的湍流及其与融化的相互作用的洞察。然后我们将使用大涡模拟(LES)在高分辨率(约10厘米-1米)下研究更大的体积(约1平方公里乘以100米高)。这将解决整个边界层中最大的湍流运动。这两个模型将利用最近从乔治六世和拉森C冰架(Nicholls，NE/H009205/1)的系泊地点获得的观测数据进行验证。模式结果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正海洋气候模式中使用的冰融化和垂直混合的参数。我们将在一维(垂直)模式中添加候选参数，该模式结合了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，并根据需要对其进行修改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，并与气候建模组织(包括英国气象局)分享，以改进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.