ASCOS Analysis - surface-cloud coupling in the arctic boundary layer

北极是一个对气候变化极端敏感的地区。观测表明，它的温度上升速度是世界其他地区的两倍。模型显示，这种强烈反应将持续下去；然而，他们也显示出这里比世界上任何其他地方都更大的不确定性。快速变化和高度不确定性的结合使得提高北极的预测能力成为当务之急。气候对温室气体日益增强的气候强迫的强烈响应被认为是几个重要反馈过程的结果；例如冰反照率反馈，以及一些与云相关的反馈。在夏季的几个月里，北极的层云既广泛又持久。它们对地表辐射收支的影响，因而对总能量收支的影响，与世界其他地方不同：海冰和低云具有非常相似的反照率，因此在冰上，云对地表太阳辐射收支的影响很小，而长波（红外）过程往往占主导地位。与世界上其他地方不同的是，低云使地表变暖而不是变冷。由于液滴大小分布不同，云本身的辐射特性也与低纬度地区不同。北极的气溶胶浓度是地球上最低的；由于广泛的云和雾，距离强大陆气溶胶源很远，以及海冰使海洋源最小化，导致高沉积。气溶胶提供了云滴形成的核，所以低数值意味着云滴的数量更少，因此比中纬度层中通常发现的要大。不同的水滴大小分布意味着北极层与中纬度层具有不同的辐射特性：这是模式不能很好地反映其影响的原因之一。云性质的微小变化，无论是气溶胶可利用性、热力学结构还是湍流过程的变化，都可能对其辐射性质产生重大影响。气候模式内必须以简单形式参数化小尺度过程；这些参数化必须基于测量。大多数使用的测量都来自中纬度地区或热带地区；因此，从它们得到的参数化不一定适用于北极条件。在北极进行测量的难度和费用意味着很少有可用的方法来测试现有的参数化或开发新的，更合适的参数化。ASCOS野外活动实现了在北极中部进行的最广泛和最广泛的测量之一，旨在解决是什么控制了北极云的特性的问题。该提案将使用独特的ASCOS数据集来研究地表与云之间的相互作用，这些相互作用控制着热量、水和气溶胶的交换；还有来自自由对流层的空气在云顶的交换。它将研究基本过程，并检查它们在气候模型中的表现情况，确定参数化失败的地方，并提出更适合北极的替代方法。这项研究将利用对低层大气结构、湍流混合、气溶胶特性、云特性和辐射通量的详细原位和遥感测量来研究基本过程。这些将与大涡模拟模型相结合，以提供对三维相互作用的更深入的了解，并允许进行微扰实验来研究不同参数的相对重要性。最后，Met Office统一模型将用于研究这些过程的参数化如何执行，并发现其中的弱点或失败。最终，这项工作将导致改进的参数化和对未来气候更准确的预测。

The Arctic is a region of extreme sensitivity to climate change. Observations show its temperature to be increasing at twice the rate of the rest of the world. Models suggest this strong response will continue; however they also show a greater uncertainty here than for anywhere else in the world. The combination of rapid change and high uncertainty make improving predictive capability in the Arctic a matter of urgency. The strong climate response to increasing climate forcing by greenhouse gases is believed to be a result of several important feedback processes; for example the ice-albedo feedback, and several cloud-related feedbacks. Arctic stratus cloud is both extensive and long lived during the summer months. Their impact on the surface radiative budget, and hence on total energy budget, differs from that elsewhere in the world: sea ice and low clouds have very similar albedo, so that over ice the cloud has little impact on the solar radiation budget at the surface, and longwave (infra red) processes tend to dominate. Unlike anywhere else in the world, low cloud acts to warm the surface rather than cool it. The radiative properties of the clouds themselves also differ from lower latitudes because of different droplet size distributions. The Arctic has the lowest aerosol concentrations of anywhere on earth; a result of high deposition due to extensive cloud and fog, the great distance from strong continental aerosol sources, and the sea ice, which minimises marine sources. Aerosol provide the nuclei upon which cloud droplets form, so the low numbers mean the clouds have a smaller number of drops, which are consequently larger than typically found in mid-latitude stratus. The different drop-size distribution means Arctic stratus has different radiative properties than mid-latitude stratus: one of the reasons models represent their effects poorly. Small changes to cloud properties, whether from changes in aerosol availability, thermodynamic structure, or turbulent processes, can have a significant impact on their radiative properties. Small-scale processes must be parameterised in a simple form within climate models; these parameterisations must be based on measurement. Most measurements used have been from mid-latitudes or the tropics; thus the parameterizations derived from them are not necessarily appropriate for Arctic conditions. The difficulty and expense of making measurements in the Arctic has meant there are very few available against which to test the existing parameterisations or with which to develop new, more appropriate ones. The ASCOS field campaign achieved one of the most extensive and wide-ranging sets of measurements ever made in the central Arctic, and is intended to address the questions of what controls the properties of Arctic cloud. This proposal will use the unique ASCOS data set to study the interactions between the surface and the cloud that control the exchange of heat, water, and aerosol; and also the exchanges across cloud top with air from the free troposphere. It will study both the fundamental processes and examine how well they are represented within climate models, identify where parameterisations are failing, and propose alternative approaches more appropriate to the Arctic. The study will draw upon detailed in-situ and remote sensing measurements of lower-atmosphere structure, turbulent mixing, aerosol properties, cloud properties, and radiative fluxes to study the fundamental processes. These will be combined with large eddy simulation modelling to provide deeper insight into the 3-dimensional interactions, and allow perturbation experiments to be undertaken to study the relative importance of different parameters. Finally, the Met Office Unified Model will be used to study how parameterisations of these processes perform, and to discover weaknesses or failings within them. Ultimately this work will lead to improved parameterizations and more accurate predictions of future climate.