MOSAiC Boundary Layer

北极气候研究多学科漂流观测站（MOSAiC）倡议是一个主要的国际项目，其动机是在过去几十年中观测到的北极气候的快速变化。这是由北极平均温度的加速上升所驱动的；它的变暖速度是全球平均速度的2-3倍。最明显的变化是海冰范围的急剧减少，特别是夏季最小值的减少，以每十年13%的速度减少。这些快速变化是一系列反馈过程共同作用的结果——其中最著名的是冰反照率反馈，即冰的减少暴露了下面的陆地或海洋表面，降低了区域平均反照率，使更多的太阳辐射被吸收，从而使地表变暖，加速了冰的融化。其他反馈与大气温度和湿度、云特性和大尺度大气环流的垂直剖面有关。虽然气候模型也显示北极变暖加剧，但它们并没有重现许多观测到的变化细节；例如，它们没有重现过去10年观测到的夏季海冰最小值的快速下降，而且模型之间存在很大差异。这对我们预测未来气候系统状态的能力有重大影响。较差的模型性能是由于其对北极系统中物理过程的表示存在多个领先阶缺陷。MOSAiC旨在通过大规模协调的方法解决这些问题，在整个日历年期间同时测量与气候有关的许多相互依存的过程。这种方法是必要的，因为北极气候系统的不同部分之间有很强的联系和反馈，而且许多过程具有很强的季节性。观测活动将在“极地之星”号破冰船上及其周围进行，该船将在夏季融化结束时被冻结在浮冰的边缘。这样就可以随时接触到包内的多年冰和包外新形成的冰。测量将包括冰面上下两侧表面能量收支的所有组成部分，以及冰的厚度、温度、物理性质、地形和随时间的变形。将详细研究控制能量收支的过程，包括天气尺度强迫、云性质、湍流混合以及它们之间的相互作用。这些测量将由一个广泛的模拟方案加以补充，该方案涵盖从小规模过程研究到全球气候模拟的全部范围。最后一个试图与MOSAiC相比的研究是北极表面热收支（SHEBA）项目，该项目在1997-1998年的11个月里进行了一组有限得多的测量。“示巴”发生的地方现在每年夏天都不结冰，这凸显了自那以后发生的变化。这一建议是对MOSAiC大气科学部分的核心贡献。我们将在测量活动期间对低层大气平均和湍流动力学进行详细测量。几个遥感系统（多普勒雷达和激光雷达）将被部署来连续测量整个边界层的平均风廓线。垂直湍流结构的检索将使云、边界层和地表之间复杂的相互作用得以理解，从而使模型更好地表示地表能量收支和冰融化和形成的主要控制因素。这种对边界层垂直动态结构的详细测量在北极将是独一无二的，没有可比的数据集存在。除了我们的科学之外，它们还支撑着马赛克科学的许多方面，这些科学正在由其他团体进行，并且被马赛克大气科学协调员认为是必不可少的。

The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) initiative is a major international programme motivated by the rapid changes in Arctic climate observed over the last few decades. This is driven by an accelerated rise in the mean temperature of the Arctic; which is warming at 2-3 times the mean global rate. The most visible change is the dramatic reduction in sea ice extent, particularly of the summer minimum, which is decreasing at a rate of 13% per decade. These rapid changes are the result of a combination of feedback processes - the best known is the ice albedo feedback, whereby the loss of ice exposes the land or sea surface beneath, lowering the area mean albedo and allowing more solar radiation to be absorbed, which warms the surface and enhances ice melt. Other feedbacks relate to the vertical profiles of atmospheric temperature and humidity, cloud properties, and large-scale atmospheric circulation.While climate models also show enhanced warming in the Arctic, they do not reproduce many of the observed details of the change; for example they do not reproduce the very rapid decline in the summer sea ice minimum observed over the last 10 years, and there are big differences between models. This has a significant impact on our ability to predict the future state of climate system. Poor model performance results from multiple leading-order deficiencies in their representation of physical processes in the Arctic system. MOSAiC aims to address these through a large-scale coordinated approach, making simultaneous measurements of the many interdependent processes relevant to climate over a full calendar year. This approach is necessary because of the strong linkages and feedbacks between different parts of the Arctic climate system and the strong seasonality in many processes. The observational campaign will take place on, and around, the icebreaker Polarstern, which will be frozen in at the edge of the pack ice at the end of the summer melt. This provides ready access to both multi-year ice within the pack and to freshly forming ice just outside it. Measurements will be made of all components of the surface energy budget on both the upper and lower sides of the ice, along with ice thickness, temperature, physical properties, topography, and deformation over time. The processes controlling the energy budget, including synoptic-scale forcing, cloud properties, turbulent mixing, and the interactions between them, will be studied in detail. The measurements will be complemented by an extensive modelling programme, spanning a full range from small scale process studies up to global climate modelling.The last study to attempt anything remotely comparable to MOSAiC was the Surface Heat Budget of the Arctic (SHEBA) project, which undertook a much more limited set of measurements over 11 months in 1997-1998. The area where SHEBA took place is now ice-free every summer, emphasising the changes that have taken place since then.This proposal is a core contribution to the atmospheric science component of MOSAiC. We will make detailed measurements of lower atmosphere mean and turbulent dynamics for the duration of the measurement campaign. Several remote sensing systems (Doppler sodar and lidar) will be deployed to make continuous measurements mean wind profiles throughout the boundary layer. Retrievals of the vertical turbulence structure will allow the complex interactions between clouds, the boundary layer, and the surface to be understood and thus the primary controls on the surface energy budget and ice melt and formation to be better represented in models. Such detailed measurements of the vertical dynamic structure of the boundary layer will be unique in the Arctic, no comparable data set exists. In addition to our science they underpin many aspects of MOSAiC science being undertaken by other groups, and are considered essential by the MOSAiC atmospheric science coordinators.