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

北极是一个对气候变化极为敏感的地区。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，这种强烈的反应将继续下去;然而，它们也显示出比世界其他任何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提高北极地区的预测能力成为当务之急。气候对温室气体造成的气候强迫的强烈反应被认为是几个重要反馈过程的结果;例如冰-冰反馈和几个与云有关的反馈。北极层云在夏季的几个月里既广泛又长寿。它们对地表辐射收支的影响，进而对总能量收支的影响，与世界上其他地方的不同：海冰和低云有非常相似的辐射收支，因此在冰上，云对地表的太阳辐射收支几乎没有影响，长波（红外）过程往往占主导地位。与世界上其他地方不同，低云的作用是使表面变暖而不是冷却。由于水滴大小分布不同，低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;这是由于广泛的云和雾造成的高沉积，远离强大的大陆气溶胶来源，以及海冰，使海洋来源最小化。气溶胶提供了云滴形成的核心，因此低数字意味着云具有较少的水滴数量，因此比中纬度层云中通常发现的要大。不同的水滴大小分布意味着北极层云与中纬度层云具有不同的辐射特性：这是模型表现其效果不佳的原因之一。云特性的微小变化，无论是来自气溶胶可用性、热力学结构还是湍流过程的变化，都可能对其辐射特性产生重大影响。小尺度过程必须在气候模型中以简单的形式参数化;这些参数化必须以测量为基础。所使用的大多数测量数据来自中纬度或热带地区;因此，从中得出的参数化不一定适合北极的条件。在北极进行测量的难度和费用意味着很少有可用的参数来测试现有的参数化或开发新的，更合适的参数化。ASCOS实地活动取得了北极中部有史以来最广泛和范围最广的一组测量结果，旨在解决控制北极云特性的问题。该提案将使用独特的ASCOS数据集来研究控制热量，水和气溶胶交换的表面和云之间的相互作用;以及云顶与自由对流层空气的交换。它将研究这两个基本过程，并检查它们在气候模型中的表现，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现场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，以提供更深入的了解三维相互作用，并允许扰动实验进行研究不同参数的相对重要性。最后，气象局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故障。最终，这项工作将导致改进参数化和更准确地预测未来气候。

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.