Characterising and Interpreting FLuxes Over Sea-ice (CANDIFLOS)

大气和地球表面之间的相互作用是由湍流通量调节的--湍流通量是在两者之间传输动量、热量、水分和微量气体的混沌混合。湍流混合的尺度从毫米到数百米-远远小于数值模式的网格尺度；这种亚网格尺度的过程必须用更简单的模式变量来参数化(或表示)，如平均风速、气温、湿度等。这些参数化是通过直接测量通量本身(这是一项非常困难和昂贵的工作)，然后确定它们与更简单(也更容易测量)的变量(如风速、温度等)的经验关系来开发的。本建议旨在解决长期以来海冰上湍流通量的参数化问题。北极和南极海冰的偏远位置和恶劣条件意味着，对通量的直接测量很少，模型必须依赖于在较低纬度制定的参数，例如在海洋无冰地区。海冰上出现的非常不同的情况--高度的空间变异性，冰中铅的冰和开阔水域之间的强烈温度对比，以及冬季强烈稳定的大气条件--意味着在较低纬度制定的参数化往往不合适，模型往往在描述表面通量方面做得很差。与过去35年左右的卫星观测相比，当前一代模型无法反映海冰面积的平均变化，甚至在几个月前对海冰面积的季节性预测往往非常不准确。海冰的增长和融化受到地表能量收支的控制，而冰和大气之间的湍流通量是这一预算的关键组成部分。太阳辐射通量和地面辐射通量控制着大气收支，而湍流通量控制着大气边界层结构，并影响着边界层云的发展，而边界层云是辐射通量的主要控制因素。因此，所有这些通量都是相互关联的，因此，如果不能正确地表示空气-冰湍流通量，就会通过它们对云的影响而对地表辐射平衡产生连锁影响。因此，湍流通量的准确表示对于准确预测天气、海冰和气候系统至关重要。从短期来看，最近北极海冰的减少，以及随之而来的北极商业活动(航运、旅游、石化开采等)的增加，意味着迫切需要在从天到季的时间尺度上对天气、海冰和其他环境因素进行准确的业务预报。要实现这一点，需要更好地表示控制大气边界层和其中的云特性的地面交换过程，并有助于地面能量收支，从而导致冰融化/冻结和冰流。在过去5年中，在开发控制地面通量的物理过程的理论模型方面取得了重大进展，例如在冰缘、脊线、融化池塘和冰/水温度对比处的形成阻力。然而，需要现场测量来测试这些参数并评估它们的性能。该项目将利用最近两次(2014年和2016年)北冰洋巡航期间进行的一组非常广泛的表面通量和海冰测量，总共18周，以开发最先进的动量、热量和水蒸气参数，并调整到现实世界的条件。我们将在气象局统一模式中实施这些参数化，并在一系列时间尺度上评估它们对大气和气候系统的影响。

Interactions between the atmosphere and the surface of the planet are mediated by turbulent fluxes - chaotic mixing that transports momentum, heat, moisture, and trace gases between the two. Turbulent mixing spans scales from millimetres to hundreds of meters - much smaller than the grid scale of numerical models; such sub-grid-scale processes must be parameterized (or represented) in terms of simpler model variables such as mean wind speed, air temperature, humidity, etc. These parameterizations are developed by making direct measurements of the fluxes themselves (a very difficult and expensive undertaking) and then determining their empirical relationships with the simpler (and more easily measured) variables such as wind speed, temperatures etc.This proposal aims to address long-standing issues with the parameterization of turbulent fluxes over sea ice. The remote location and harsh conditions of both Arctic and Antarctic sea ice means that very few direct measurements of the fluxes have ever been made, and models must rely on parameterizations developed at lower latitudes, e.g. over ice-free areas of the oceans. The very different conditions that occur over sea ice - a high degree of spatial variability, strong temperature contrasts between the ice and open water in leads in the ice, and strongly stable atmospheric conditions in winter - mean that the parameterizations developed at lower latitudes are often not appropriate, and models tend to do a poor job of representing the surface fluxes. The current generation of models fails to represent the mean changes in sea ice extent compared to satellite observations over the last 35 years or so, and produce often wildly inaccurate seasonal forecasts of ice extent even just a few months in advance.The growth and melt of sea ice is controlled by the surface energy budget, and the turbulent fluxes between the ice and the atmosphere are a critical component of that budget. The solar and terrestrial radiation fluxes dominate the budget, but the turbulent fluxes control the atmospheric boundary-layer structure, and influence the development of boundary-layer clouds which are the dominant control on the radiation fluxes. So all of these fluxes are inter-linked and consequently a failure to properly represent the air-ice turbulent fluxes has a knock-on influence on the surface radiation balance through their impact on clouds. An accurate representation of turbulent fluxes is thus essential for accurate predictions of weather, sea ice and the climate system.On short timescales the recent reduction of Arctic sea ice, and the accompanying increase in commercial activity in the Arctic (shipping, tourism, petrochemical extraction, etc) means that there is an urgent need for accurate operational forecasts of weather, sea ice and other environmental factors on timescales from days to seasons. Delivering these will require a much improved representation of the surface exchange processes that control the atmospheric boundary layer and properties of clouds within it, and contribute to the surface energy budget, and hence ice melt/freeze, and ice drift.Significant progress has been made over the last 5 years in developing theoretical models of the physical processes that control the surface fluxes, such as form drag at ice edges, ridges, melt ponds, and ice/water temperature contrasts. However there is a need for in situ measurements to test these parameterizations and to evaluate their performance.This project will utilise a very extensive set of surface flux and sea-ice measurements made during two recent (2014 and 2016) cruises in the Arctic Ocean, totalling 18 weeks, to develop state-of-the-art parameterizations for momentum, heat, and water vapour that are tuned to real-world conditions. We will implement these parameterizations within the Met Office Unified Model, and evaluate their impact on the atmosphere, and on the climate system, over a range of timescales.