ADVANCE (Aerosol-cloud-climate interactions derived from Degassing VolcANiC Eruptions)

影响气候的人为排放不仅仅局限于温室气体。二氧化硫（SO2）和其他污染物形成大气气溶胶，散射和吸收阳光，影响云的特性，调节地球-大气能量平衡。人为排放的气溶胶对气候的冷却起到了重要作用，但量化不足，从而抵消了人为排放温室气体造成的全球变暖。气溶胶-气候影响的不确定性主要是气溶胶-云相互作用（ACI）的不确定性，这种相互作用是通过气溶胶作为云凝结核（CCN）而起作用的，它增加了云滴数浓度（CDNC），同时减少了云滴的大小，并随后影响降雨的形成，这可能改变云的整体物理性质。因此，这影响了气候敏感性（每单位气候强迫的气候响应）的不确定性，因为分别具有强/弱气溶胶冷却效应和高/低气候敏感性的气候模式都能够代表全球平均温度的历史记录。在全球平均基础上，按质量和数量计算，最重要的人为气溶胶是硫酸盐气溶胶，这是由燃烧化石燃料每年排放约100Tg的二氧化硫产生的，但由于工业过程的性质，这些羽状气溶胶是准连续排放的，这意味着没有二氧化硫存在的气候不存在简单的“控制”状态。迄今为止，开/关摄动/控制观测仅限于对船舶轨迹的观测，但这些特征的空间尺度远低于天气预报模式或用于未来气候预测的气候模式的分辨率。这种情况在2014年发生了戏剧性的变化，2014-2015年冰岛Holuhraun发生了巨大的裂缝喷发，这是冰岛自1783-17849年拉基火山喷发以来最大的喷发脱气事件。holuhrun火山喷发的二氧化硫峰值排放量高达全球排放量的三分之一，在整个北大西洋形成了大量的二氧化硫和硫酸盐气溶胶。实际上，冰岛成为了一个重要的全球/区域污染源，在一个原本未受污染的环境中，云应该最容易受到气溶胶排放的影响。因此，Holuhraun火山喷发为研究人为排放的二氧化硫和由此产生的硫酸盐气溶胶对ACI的影响提供了一个极好的类比。我们的研究将全面评估Holuhraun气溶胶羽流对云、降水、能量平衡和关键天气和气候变量的影响。观测分析将扩展到我们的试点研究之外，包括高质量的地表站点。将使用两种不同的气候模型；HadGEM3，这是气象局统一模型和ECHAM6-HAM的最新版本，由汉堡MPI开发。选择这些模型是因为它们在ACI方面产生了截然不同的反应；总体而言，ECHAM6-HAM产生的ACI影响远强于HadGEM3。此外，英国气象局统一模式框架意味着低分辨率气候模式和高分辨率数值天气预报模式的底层物理本质上是相同的，这在天气/气候研究中是独一无二的。在高分辨率数值天气预报版本中，对流的参数化可以关闭，亚网格尺度的过程可以明确地表示。因此，可以评估参数化方案的选择和方案内变量的离散值的影响。这项研究为ACI和气候敏感性提供了新的见解，为我们改善未来的天气和气候模型和模拟带来了巨大的进步。

Anthropogenic emissions that affect climate are not just confined to greenhouse gases. Sulfur dioxide (SO2) and other pollutants form atmospheric aerosols that scatter and absorb sunlight, and influence the properties of clouds, modulating the Earth-atmosphere energy balance. Anthropogenic emissions of aerosols exert a significant, but poorly quantified, cooling of climate that acts to counterbalance the global warming from anthropogenic emissions of greenhouse gases. Uncertainties in aerosol-climate impacts are dominated by uncertainties in aerosol-cloud interactions (ACI) which operates through aerosols acting as cloud-condensation nuclei (CCN) which increases the cloud droplet number concentration (CDNC) while reducing the size of cloud droplets and subsequently impact rain formation which may change the overall physical properties of clouds. This consequently impacts the uncertainty in climate sensitivity (the climate response per unit climate forcing) because climate models with a strong/weak aerosol cooling effect and a high/low climate sensitivity respectively are both able to represent the historic record of global mean temperatures. On a global mean basis, the most significant anthropogenic aerosol by mass and number is sulphate aerosol resulting from the ~100Tg per year emissions of sulphur dioxide from burning of fossil fuels, but these plumes are emitted quasi-continuously owing to the nature of industrial processes, meaning that there is no simple 'control' state of the climate where sulphur dioxide is not present. On/off perturbation/control observations have, to date, been limited to observations of ship tracks but the spatial scales of such features are far less than the resolution of the weather forecast models or of the climate models that are used in future climate projections. This situation changed dramatically in 2014 with the occurrence of the huge fissure eruption at Holuhraun in 2014-2015 in Iceland, which was the largest effusive degassing event from Iceland since the eruption of Laki in 1783-17849. The eruption at Holuhraun emitted sulphur dioxide at a peak rate of up to 1/3 of global emissions, creating a massive plume of sulphur dioxide and sulphate aerosols across the entire North Atlantic. In effect, Iceland became a significant global/regional pollution source in an otherwise unpolluted environment where clouds should be most susceptible to aerosol emissions. Thus, the eruption at Holuhraun created an excellent analogy for studying the impacts of anthropogenic emissions of sulphur dioxide and the resulting sulphate aerosol on ACI.Our research will comprehensively evaluate impacts of the Holuhraun aerosol plume on clouds, precipitation, the energy balance, and key weather and climate variables. Observational analysis will be extended beyond that of our pilot study to include high quality surface sites. Two different climate models will be used; HadGEM3, which is the most up to date version of the Met Office Unified model and ECHAM6-HAM, developed by MPI Hamburg. These models are chosen because they produce radically different responses in terms of ACI; ECHAM6-HAM produces far stronger ACI impacts overall than HadGEM3. Additionally, the UK Met Office Unified Model framework means that the underlying physics is essentially identical in low-resolution climate models and high-resolution numerical weather predication models, a feature that is unique in weather/climate research. In the high resolution numerical weather prediction version, parameterisations of convection can be turned off and sub-gridscale processes can be explicitly represented. Thus the impacts of choices of parameterisation schemes and discrete values of variables within the schemes may be evaluated. The research promises new insights into ACI and climate sensitivity promising us great strides improving weather and climate models and simulations of the future.