MOSAiC Boundary Layer

研究北极气候的多学科漂流观测站(MOSAIC)倡议是一项主要的国际方案，其动机是在过去几十年观察到的北极气候的快速变化。这是由北极平均气温的加速上升推动的；北极变暖的速度是全球平均速度的2-3倍。最明显的变化是海冰面积的急剧减少，特别是夏季的最低值，以每十年13%的速度减少。这些快速变化是反馈过程组合的结果--最著名的是冰反照率反馈，即冰的丧失暴露了下面的陆地或海洋表面，降低了该地区的平均反照率，并允许吸收更多的太阳辐射，从而使表面变暖，促进了冰的融化。其他反馈涉及大气温度和湿度的垂直分布、云特性和大范围大气环流。虽然气候模型也显示北极变暖加剧，但它们没有再现许多观测到的变化细节；例如，它们没有再现过去10年观察到的夏季海冰最小值非常迅速的下降，而且模型之间存在很大差异。这对我们预测气候系统未来状态的能力产生了重大影响。对北极系统物理过程的描述存在多个前导阶数的缺陷，导致模式性能不佳。MOSAIC的目标是通过一种大规模的协调方法来解决这些问题，对与气候有关的许多相互依存的过程进行一整历年的同时测量。这种做法是必要的，因为北极气候系统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。观测活动将在北极星号破冰船及其周围进行，这艘破冰船将在夏季融化结束时冻结在积冰的边缘。这为冰盖内的多年冰和刚形成的冰提供了便捷的通道。将测量冰层上下两侧表面能量收支的所有组成部分，以及冰层厚度、温度、物理性质、地形和随时间变化的变形。将详细研究控制能量收支的过程，包括天气尺度强迫、云特性、湍流混合以及它们之间的相互作用。这些测量将得到一个广泛的模拟计划的补充，该计划涵盖了从小规模过程研究到全球气候模拟的所有范围。上一项试图与马赛克相提并论的研究是北极地表热量预算(SHEBA)项目，该项目在1997-1998年间的11个月里进行了一组更有限的测量。现在每年夏天发生SHEBA的地区都是无冰的，强调了自那时以来发生的变化。这项提议是对MASIC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。将部署几个遥感系统(多普勒声雷达和激光雷达)，以连续测量整个边界层的平均风廓线。对垂直湍流结构的反演将使人们能够了解云、边界层和地面之间的复杂相互作用，从而在模式中更好地描述对地面能量收支和冰融化和形成的主要控制。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。除了我们的科学之外，它们还支撑着其他小组正在开展的马赛克科学的许多方面，并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.