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

随着气候因温室气体增加而变暖，北冰洋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薄，9月份的最小海冰面积显著减少。随着环境的变化，北极地区的人类活动正在增加，越来越多的居民和游客在夏季利用增加的航运、离岸作业和旅游窗口。这推动了对北极地区天气、海洋和海冰状况的联合预报的需求，这是做出基于风险的决策所需的时间尺度。北极的天气预报技能低于北部中纬度地区，但原因是多方面的，还不完全清楚。我们的假设是，北极环境的某些方面没有得到很好的预测，因为由于海冰的移动，北极天气系统下的地表条件更加动态。对耦合大气、海洋和海冰的物理过程的理解是不完整的，预测的新前沿是以准确和熟练的方式对这种耦合系统进行建模。努力提高这一领域能力的中心是我们的项目合作伙伴：气象局、欧洲气象中心和大都会气象局。北极气旋是夏季影响北极环境的主要类型的危险天气系统--因此是所有人类活动的关切。它们也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“非常迅速的冰层流失事件”(VRILEs-时间尺度数天到数周)，这对耦合预报构成了重大挑战；对北极周围的斜压性(温度梯度)，影响随后的天气系统和对未来几周到一个季节的北极气候预测。我们提议的观测实验将是第一次关注夏季北极气旋，并采取必要的测量来调查海冰条件对其发展的影响。需要进行新的观测，包括与飞机轨道下的海冰或海洋表面同时测量的近地表湍流动量、热量和水汽通量，以及沿气旋尺度横断面测量的动量、热量和水汽通量。这些通量决定了地表对天气系统发展的影响。我们将于2021年夏天在挪威斯瓦尔巴特群岛执行任务，使用英国南极调查局的双水獭低空飞行飞机，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。我们的美国合作伙伴设计了一项名为ThinIce的观测实验，从对流层顶上空(10公里)的飞机上俯瞰北极气旋结构。我们的项目是为2021年夏季共同设计的，因此来自双水獭的观测将在美国上空的航空和卫星测量与地表通量和海冰的特性之间建立一座桥梁。该项目汇集了表面交换、气旋动力学和海冰物理方面的观测、建模和理论方法方面的专业知识。我们将使用新的基于理论的方法来询问预报模型的运行情况，并确定表面特性改变气旋增长的机制。新的地面和湍流数据将用于改进模式中形状阻力的参数化，该模式对于海冰运动的风强迫以及地面风的减速至关重要。这些方面将通过最先进的大气和海冰动力学模型进行探索。最后，我们将通过调查地表粗糙度增加对北极气旋的影响及其与北极温度梯度的耦合作用来结束环路。该项目的一个主要遗产将是史无前例的观测，这将使未来几十年迫切需要的环境预测模型的评估和开发成为可能。

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.