Arctic Summer-time Cyclones: Dynamics and Sea-Ice Interaction

随着温室气体的增加，气候变暖，北极海冰的分布发生了巨大变化，夏季北冰洋盆地大部分地区的海冰变得越来越薄，9月份海冰面积最小值显著减少。随着环境的变化，北极地区的人类活动正在增加，更多的居民和游客利用夏季增加的航运，海上作业和旅游窗口。这推动了对整个北极地区天气、海洋和海冰状态的联合预测的需求，这些预测需要在做出基于风险的决策所需的时间尺度上进行。北极地区的天气预报技能低于北方中纬度地区，但原因是多方面的，并不完全清楚。我们的假设是，北极环境的某些方面没有得到很好的预测，因为北极天气系统下的表面条件由于海冰的运动而更加动态。对耦合大气、海洋和海冰的物理过程的理解是不完整的，预测的新领域是用逼真和技巧模拟这种耦合系统。致力于提高这一领域能力的中心是我们的项目合作伙伴：气象局、ECMWF和挪威气象局。北极气旋是夏季影响北极环境的主要危险天气系统，因此是所有人类活动的关注点。它们还可能对北极环境产生重大影响：特别是对海冰运动，有时会导致“非常快速的冰损失事件”（VRILEs -时间尺度从天到星期），这对耦合预报提出了重大挑战;斜压性（温度梯度）围绕北极，影响随后的天气系统和未来几周到一个季节的北极气候预报。我们提出的观测实验将是第一个重点对夏季北极气旋进行监测，并进行必要的测量，以调查海冰状况对气旋发展的影响。需要进行新的观测，包括对飞机航迹下的海冰或海洋表面以及沿着气旋尺度断面同时测量的湍流近地表动量、热量和水分通量。这些通量决定了地表对天气系统发展的影响。我们将于2021年夏天在斯瓦尔巴特群岛（挪威）进行操作，使用英国南极调查局的Twin Otter低空飞行飞机，配备通过红外和激光雷达遥感测量飞行高度的湍流和表面特性的设备。我们的美国合作伙伴设计了一个名为THINICE的观测实验，从对流层顶（10公里）上方飞行的飞机上向下观察北极气旋结构。我们的项目是为2021年夏季共同设计的，因此双水獭的观测将在美国空中和卫星测量与地面通量和海冰下面的特性之间形成桥梁。该项目汇集了观测，建模和理论方法的专业知识，以表面交换，气旋动力学和海冰物理学。我们将使用新的理论为基础的方法来询问预测模型，因为他们运行和确定的机制，通过表面特性改变气旋的增长。新的表面和湍流数据将用于改进模型中形状阻力的参数化，形状阻力是海冰运动的风强迫以及减速表面风的核心。将利用最先进的大气和海冰动力学模型探讨这些方面。最后，我们将通过调查增加的表面粗糙度对北极气旋的影响及其与北极温度梯度的耦合相互作用来结束循环。该项目的一个主要遗产将是前所未有的观测，这将使急需的评估和开发环境预测模型的未来几十年。

As climate has warmed in response to increasing greenhouse gases, the distribution of Arctic sea ice has changed dramatically, becoming thinner over large portions of the Arctic Ocean basin in summer with a prominent reduction of the September minimum in sea ice extent. Human activity is increasing within the Arctic as the environment changes, with more residents and visitors making use of the increased window for shipping, offshore operations and tourism during summer. This has driven demand for coupled forecasts of weather, ocean and sea-ice state across the Arctic on the timescales needed to make risk-based decisions. Weather forecast skill for the Arctic is lower than for northern mid-latitudes, but the reasons why are multi-faceted and not fully known. Our hypothesis is that some aspects of the Arctic environment are not well forecast because the surface conditions beneath Arctic weather systems are more dynamic due to the movement of sea ice. Understanding of the physical processes that couple the atmosphere, ocean and sea ice is incomplete and the new frontier in prediction is to model this coupled system with fidelity and skill. Centres striving to improve capability in this area are our project partners: the Met Office, ECMWF and Met Norway.Arctic cyclones are the dominant type of hazardous weather system affecting the Arctic environment in summer - thus a concern for all human activities. They can also have critical impacts on the Arctic environment: in particular on sea-ice movement, sometimes resulting in 'Very Rapid Ice Loss Events' (VRILEs - timescale days to weeks) which present a major challenge to coupled forecasts; and on the baroclinicity (temperature gradients) around the Arctic, influencing subsequent weather systems and forecasts of Arctic climate from weeks out to a season ahead.Our proposed observational experiment will be the first focusing on summer-time Arctic cyclones and taking the measurements required to investigate the influence of sea-ice conditions on their development. New observations are needed comprising of turbulent near-surface fluxes of momentum, heat and moisture measured simultaneously with the sea ice or ocean surface beneath the aircraft track and along cyclone-scale transects. These fluxes dictate the impact of the surface on the development of weather systems. We will operate from Svalbard (Norway) in summer 2021, using the British Antarctic Survey's Twin Otter low-flying aircraft equipped to measure turbulence at flight level and the surface properties through infrared and lidar remote sensing.Our US partners, have designed an observational experiment, called THINICE, looking downwards on Arctic cyclone structure from an aircraft flying above the tropopause (10 km). Our projects are co-designed for summer 2021 so that the observations from the Twin Otter will form a bridge between US airborne and satellite measurements above and the properties of the surface fluxes and sea ice beneath.The project brings together expertise in observations, modelling and theoretical approaches to surface exchange, cyclone dynamics and sea-ice physics. We will use novel theoretically-based approaches to interrogate forecast models as they run and determine the mechanisms through which the surface properties alter cyclone growth. The new surface and turbulence data will be used to improve the parametrization of form drag in models that is central to wind forcing of sea-ice motion as well as decelerating surface winds. These aspects will be explored with state-of-the-art atmosphere and sea-ice dynamics models. Finally, we will close the loop through investigation of the effects of increased surface roughness on Arctic cyclones and their coupled interaction with Arctic temperature gradients. A major legacy of the project will be the unprecedented observations that will enable much needed evaluation and development of environmental forecast models for decades to come.