Modelling of Summertime Arctic Stratus

北极是一个对气候变化特别敏感的区域：观察到的气温上升速度是世界其他地区的两倍，有强有力的证据表明北极冰盖正在变薄和消退，2007年达到创纪录的最低水平。尽管模型一致认为，北极将继续对温室气体浓度增加做出强烈反应，但它们也显示，这里的模型之间的差异比其他地方大得多，甚至无法充分再现当前的情况。因此，北极地区对持续气候变化的反应极不确定。模式在北极的表现很差，很大程度上是因为使用的参数化是从中纬度或热带地区的观测得出的；北极内部的测量很少，结果是我们对控制当地条件的过程：云动力学和微物理、辐射强迫、地表交换等的了解很差。这个PHD项目将使用气象局大涡模式和统一模式的单列版本来研究夏季北极层流云的动力和微物理过程。其目的是了解云动力学和微物理、辐射强迫和地面耦合之间的相互作用，最终目的是改善北极边界层云在气候模式中的表现。该项目将评估1)北极层流的动力学与典型的海洋层积有何不同，2)在北极低层大气独特的湿度结构中，卷吸在维持云方面起什么作用，3)云如何对不同来源的气溶胶作出反应：从表面还是从上方卷吸，因为目前尚不清楚维持北极层云所需的CCN来自哪里。这种研究至关重要，因为北极层是地表能量收支的主要控制因素，与中纬度的对应云有很大不同；如果不能在气候模式中充分代表这些云，就不可能适当地模拟北极气候。该项目与2008年夏季在北极中部进行的一项重要的国际极地年实地研究直接相关：北极夏季云-海洋研究。ASCOS方案是为了从观察的角度解决上述问题而组织的，并将为该项目提供广泛的数据集以供参考。与本研究直接相关的测量包括：气溶胶物理和化学、从表面到云顶的边界层平均和湍流结构、表面能量收支、辐射通量和云特性--这两方面都由云雷达、激光雷达、微波辐射计等遥感，以及美国航天局P3研究机的现场测量。ASCOS观测既为建立模拟研究的初始条件提供了基础，也为评估模拟的逼真度提供了一种手段。模拟研究反过来将通过允许对控制云特性的各种因素的相对重要性进行受控测试来加强观测研究。与ASCOS和IPY的链接使该项目特别及时，提供了对独特数据集的访问，并为目前由NERC资助的研究增加了重大价值。这一联系还通过与许多国际合作者的互动、参与ASCOS和国际极地年讲习班以及国际极地年之后的国际会议，提供了特殊的培训机会。主要主管(布鲁克斯)是英国对ASCOS的贡献的PI，也是方案指导小组的成员。监督小组在分析观测数据、将其整合到模型中并进行详细的过程研究方面拥有丰富的经验；气象局的合作是因为他们对解决UM的北极层层问题表现出了浓厚的兴趣。

The Arctic is a region of exceptional sensitivity to climate change: the observed rate of temperature increase is twice that of the rest of the world, and there is strong evidence for thinning and retreat of Arctic ice pack ice, which reached a record minimum during 2007. Although models agree that the Arctic will continue to have a strong response to increasing greenhouse gas concentrations, they also show much greater variability between models here than elsewhere, and fail even to reproduce current conditions adequately. The response of the Arctic to continued climate change is thus extremely uncertain. Model performance in the Arctic is poor in large part because the parameterizations used are derived from observations in mid-latitudes or the tropics; there are very few measurements within the Arctic, with the result that our understanding of the processes controlling local conditions: cloud dynamics and microphysics, radiative forcing, surface exchange, etc, is poor. This PhD project will study the dynamical and microphysical processes in summertime Arctic stratus clouds using the Met Office Large Eddy Model and a single column version of the Unified Model. The aims are to understand the interactions between the cloud dynamics and microphysics, radiative forcing, and surface coupling, with the ultimate objective of improving the representation of Arctic boundary layer clouds in climate models. The project will assess 1) how the dynamics of Arctic stratus differ from typical marine stratocumulus, 2) what the role of entrainment is in maintaining the cloud in the unique humidity structure of the Arctic lower atmosphere, 3) how the cloud responds to different sources of aerosol: from the surface or entrained from above, since it is currently unknown where the CCN required to maintain Arctic stratus derive from. Such studies are of paramount importance since Arctic stratus is the dominant controlling factor for the surface energy budget, and differ substantially from their mid-latitude counterparts; failure to adequately represent these clouds in climate models makes it impossible to properly simulate Arctic climate. The project is directly linked to a major International Polar Year (IPY) field study taking place in the central Arctic in summer 2008: the Arctic Summer Cloud-Ocean Study (ASCOS). The ASCOS programme has been organised to address the issues above from an observational stance, and will provide an extensive data set for this project to draw upon. Measurements of direct relevance to this study include: aerosol physics and chemistry, boundary layer mean and turbulence structure from the surface through cloud top, surface energy budget, radiative fluxes, and cloud properties - both remotely sensed by cloud radar, lidar, microwave radiometers, etc, and in-situ measurements from a NASA P3 research aircraft. The ASCOS observations provide both the basis for establishing initial conditions for the modelling studies, and a means of assessing the fidelity of the simulations. The modelling study will in turn enhance the observational study by allowing controlled tests of the relative importance of the various factors controlling the cloud properties. The link to ASCOS and the IPY make the project particularly timely, provides access to a unique data set, and adds significant value to current NERC funded research. This link also provides exceptional training opportunities through interaction with the many international collaborators, involvement in ASCOS and IPY workshops and international meetings on polar science following the IPY. The primary supervisor (Brooks) is PI on the UK contribution to ASCOS, and a member of the programme steering group. The supervision team has extensive experience of analysing observational data, integrating it into models, and undertaking detailed process studies; Met Office collaboration results from their strong interest in resolving the issue of Arctic stratus in the UM.