Turbulent Exchange in the Arctic Boundary Layer

北极是地球上对气候变化最敏感的地区之一。观测显示，它的变暖速度是世界其他地区的两倍。气候模型表明，这是由于对温室气体增加的反应比低纬度地区更强烈;然而，模型预测也显示北极地区的不确定性最大，甚至难以重现当前的条件。模型性能差的主要原因是未能充分代表冰上方浅层大气边界层的过程：表面湍流热湿交换，边界层的垂直结构，低层层云的特性，以及太阳和红外辐射通量。我们对这些过程的理解不佳是由于缺乏现场测量来研究它们/由于位置偏远和在冰上工作困难。进行这种测量是必要的，因为北极边界层和云的性质和行为与中低纬度测量良好的环境中的性质和行为有很大不同。该项目将对北极大气层从地表到云层的垂直结构进行一套独特的现场测量，重点是动量、热量和水分的湍流交换以及地表和云层之间的耦合。这样的测量以前没有在北极中部进行过，远离大陆或开阔海洋的影响。偏远的位置使这一地区无法进入飞机，垂直剖面通过边界层，并在云中的测量，因此，将与新的湍流包进行系留气球。这些测量与通过合作小组提供的各种辐射计、激光雷达和雷达系统对低层大气和云特性进行的连续遥感测量相结合，将提供了解北极特有的边界层和云过程的手段，并重新确定它们在气候模型中的表示。该项目是一个规模大得多的国际测量方案-北极夏季云-海洋研究-的一个组成部分，该方案将于2008年夏季在北极中部进行，由瑞典“奥登”号破冰船执行，并由斯德哥尔摩大学协调。从7月初靠近极点开始，这艘船将停泊在一块稳定的浮冰上，并将与冰层一起漂流，直到9月。一些仪器将安装在远离船舶直接影响的冰上桅杆上;其他仪器将在船舶甲板或集装箱实验室操作。ASCOS的合作者将进行广泛的其他测量：气溶胶物理和化学特性;大气气相化学;海洋上层结构;以及海洋化学和生物学。该方案是高度跨学科的，旨在阐明气候系统不同部分之间的联系，例如海洋生物来源产生的气溶胶粒子，它们的生长和变化成为能够作为云凝结核的粒子，以及这些粒子通过湍流输送过程向上输送到云层。所有参与者之间的密切合作和数据共享是方案设计的一个固有部分。

The Arctic is one of the most sensitive regions on Earth to climate change. Observations show it to be warming at twice the rate of the rest of the world. Climate models suggest this is due to a stronger response to increases in greenhouse gases than at lower latitudes; however the model predictions also show the greatest uncertainty in the Artic, and have difficulty even reproducing current conditions. Poor model performance results primarily from a failure to adequately represent processes in the shallow atmospheric boundary layer over the ice: surface turbulent exchanges of heat and moisture, the vertical structure of the boundary layer, the properties of low-level stratus clouds, and thus the solar and infra-red radiative fluxes. Our poor understanding of these processes is a result of a lack of in-situ measurements with which to study them / a consequence of the remote location and difficulty of working over the ice. It is necessary that such measurements be made because the properties and behaviour of the Arctic boundary layer and cloud differ significantly from those in well-measured environments in mid and low latitudes. This project will make a unique set of in-situ measurements of the vertical structure of the Arctic atmosphere from the surface through the cloud layer focusing on the turbulent exchanges of momentum, heat, and moisture and the coupling between the surface and cloud layers. Such measurements have not previously been made over the central Arctic, away from the influence of continental landmasses or the open ocean. The remote location makes this region inaccessible to aircraft; the vertical profiles through the boundary layer, and measurements in cloud will thus be made with novel turbulence package carried by a tethered balloon. These measurements / in conjunction with continuous remote-sensing measurements of the lower atmosphere and cloud properties via a variety of radiometers, lidar, and radar systems provided by collaborating groups / will provide the means to understand boundary-layer and cloud processes unique to the Arctic, and to redefine their representation within climate models. The project is an integral part of a much larger international measurement programme / the Arctic Summer Cloud-Ocean Study (ASCOS) / that will take place in the central Arctic during the summer of 2008, operating from the Swedish icebreaker Oden, and coordinated by the Stockholm University. Starting close to the pole in early July the ship will be moored to a stable ice floe and will drift with the ice pack until September. Some instrumentation will be sited on masts erected on the ice away from the immediate influence of the ship; others will operate from the deck of the ship or container labs. Collaborators on ASCOS will make a wide range of other measurements: aerosol physical and chemical properties; atmospheric gas-phase chemistry; upper ocean structure; and marine chemistry and biology. The programme is highly interdisciplinary, and designed to elucidate the links between different parts of the climate system, such as the generation of aerosol particles from biological sources in the ocean, their growth and modification into particles capable of acting as cloud condensation nuclei, and the transport of those particle up into the cloud layer by turbulent transport processes. Close collaboration and sharing of data between all participants is an inherent part of the programme design.