Aerosol-Cloud Coupling And Climate Interactions in the Arctic

北极的气候变化比地球上几乎任何其他地方都快，变暖的速度是全球平均水平的两倍。这种变暖伴随着海冰的迅速融化--2007年夏季冰川面积达到创纪录的最低水平，自那以后的几年里，夏季冰川面积分别为有记录以来的第二和第三低水平--以及每年剩余的冰层变薄。北极的强烈变暖是由于几个正反馈过程，包括海冰反照率反馈(较暖的条件融化冰，降低混合冰/海洋表面的平均反射率，从而吸收更多的太阳辐射，导致冰融化和反照率进一步降低)和几个云反馈。在全球大部分地区，低云因为反射阳光而起到冷却表面的作用；在北极，高反射率的冰面降低了云的反射率，云水滴对红外辐射的吸收成为主导效应--这将把热量困在云下，使表面变暖。尽管气候模型通常显示出北极强烈的温室气体变暖效应，但它们在北极地区的分歧也比其他地区更大，从而产生了更广泛的未来气候条件。这些模型也往往不能非常准确地再现当前的北极气候条件。北极气候模型中的这种巨大不确定性主要是因为模型中对物理过程的描述不佳，以及一些独特和特别具有挑战性的条件。不确定的最大单一来源是云的表示。这些模型使用了对中纬度或热带云系的观察得出的云特性的简单表示，这些云系的条件与北极存在的条件非常不同。该项目将对云的微物理性质、边界层的垂直结构和气溶胶性质以及云上、云下和云内的太阳和红外辐射通量进行机载现场测量。它还将测量海面气溶胶的产生率和特性及其随季节和海冰覆盖范围的变化情况。这些测量将与一系列气溶胶和云过程的数值模式以及大气动力学一起用于评估海冰范围、气溶胶产量和云特性之间的相互作用。将在气象局气候模式HadGEM中开发、测试和实施适用于气候模式的对这些过程的新的和改进的描述。将测试当前MetOffice模式再现观测到的北极云和边界层特性的能力，并评估新的参数化方案的影响。最后，我们将进行一系列气候模拟，以考察未来气候将如何演变，以及北极变暖、海冰融化、气溶胶产生和云特性之间的反馈。

The climate of the Arctic is changing faster than that almost anywhere else on Earth, warming at a rate of twice the global average. This warming is accompanied by a rapid melting of the sea ice - 2007 saw a record minimum in summer ice extent, and the years since have seen the 2nd and 3rd lowest summer ice extents on record - and a thinning of the ice that remains from year to year. The strong warming in the Arctic is due to several positive feedback processes, including a sea-ice albedo feedback (warmer conditions melt ice, lowering the average reflectivity of the mixed ice/ocean surface and thus absorbing more solar radiation, leading to increased ice melt and further lowering of the albedo) and several cloud feedbacks. Over most of the globe low clouds act to cool the surface since they reflect sunlight; over the arctic the highly reflective ice surface reduces the significance of cloud reflectivity, and the absorption of infrared radiation by cloud water droplets becomes the dominant effect - this acts to trap heat below cloud, warming the surface.Although climate models generally show a strong greenhouse warming effect in the Arctic, they also disagree with each other more in the Arctic than anywhere else, producing a wider range of possible future climate conditions. The models also tend not to be able to reproduce current Arctic climate conditions very accurately. This large uncertainty in models of the Arctic climate results primarily from poor representation of physical processes within the models, and some unique and particularly challenging conditions. The largest single source of uncertainty is the representation of clouds. The models use simple representations of cloud properties that were developed from observations in mid latitude or tropical cloud systems - very different conditions from those that exist in the Arctic. This project will make airborne in situ measurements of cloud microphysical properties, the vertical structure of the boundary layer and aerosol properties, and the fluxes of solar and infra red radiation above, below, and within cloud. It will also measure the production rates and properties of aerosol at the surface and their variability with season and extent of sea ice cover. These measurements will be used, along with a range of numerical models of aerosol and cloud processes, and atmospheric dynamics to evaluate the interactions between sea ice extent, aerosol production and cloud properties. New and improved descriptions of these processes suitable for use within climate models will be developed, tested, and implemented within the MetOffice climate model HadGEM. The ability of the current MetOffice models to reproduce the observed Arctic cloud and boundary layer properties will be tested, and the impact of the new parameterization schemes evaluated.Finally we will undertake a series of climate simulations to examine how future climate will evolve, and the feedbacks between warming of the Arctic, melting of sea ice, production of aerosol, and the properties of clouds evaluated.