A global perspective on the urban heat island effect

全球许多地区的城市覆盖面正在扩大，预计到2050年，世界人口的70%将居住在城镇。城市热岛效应可能导致城市比周围农村地区高出6-12摄氏度，这是非常令人担忧的，因为与热浪相关的死亡人数增加，气候变化将加剧这一问题。正如许多用于全球天气和气候预测的模型一样，欧洲中期天气预报中心(ECMWF)的模型目前没有城市地区，但它们正在被引入，以期在未来几年以9公里的分辨率运行。这包括修改地表粗糙度和水的可获得性，以及严格处理城市地区主要要素(街道、墙壁和屋顶)的湍流和辐射交换。这个博士项目是参与这项激动人心的工作的绝佳机会，从发展城市地区建模的基础科学，到在ECMWF系统中实施和利用这一新能力，以回答全球范围内的重要科学问题。拟议的城市地区3D辐射交换的快速处理方法处于辐射传递建模科学的前沿，但还需要开发和评估几个额外的功能。一个潜在的优势是，只需要很少的参数来描述城市区域的形态。一项重要的任务将是检查现有的全球数据集，以确定适用于全球不同城市形态的适当值。随着全球范围内城市地区的加入，将有能力运行一个完全互动的全球天气预报模型。这将使S能够比以往更全面地解决关于城市地区的影响和不同规模城市的重要性的大量新问题。博士项目将由三个部分组成，涉及一些关键问题：1.如何以最佳方式表示城市地区的3D辐射传递？一个很好的起点将是最近开发的一种新方法“Spartacus”，该方法用于有效地表示太阳和热红外辐射与随机分布物体的3D相互作用，并已成功应用于云和植被。学生身份将解决需要哪些变化才能使其适应建筑的问题？将树木和公园融入城市景观对辐射有何影响？这种方法与更详细(且计算成本更高)的辐射模型的基准计算相匹配的程度如何？2.ECMWF模型中的新城市处理方法对近地表变量(例如，气温和地表热湿通量)的预测有多好？将根据来自世界各地许多不同城市地区的现有观测结果进行系统评估，并可能还使用对皮肤温度的卫星观测。通过重新运行ECMWF模型的全球地表模块(快速)，学生将研究模型中城市区域处理的改进如何提高相对于观测的性能。3.然后可以使用完整的模型提出许多有趣的科学问题。例如，全球城市地区对天气有什么影响？影响是否显著在文献中是一个有争议的话题，但学生将能够利用现有的ECMWF软件工具来确定城市下游的预测是否得到改善。未来城市覆盖率的增加和可能的缓解努力，如建设更绿色的城市，将产生什么影响？通过修改模型使用的城市属性的全球地图，将有可能探索城市热岛效应在未来可能发生的变化

The areal coverage of urban areas is expanding in many parts of the globe, with 70% of the world's population projected to live in towns and cities by 2050. The urban heat island effect can cause cities to be 6-12C warmer than the surrounding rural areas, which is of great concern due to the increased fatalities associated with heatwaves, an issue that will be exacerbated by climate change.As in many models used for global weather and climate prediction, urban areas are currently absent from the model of the European Centre for Medium-Range Weather Forecasts (ECMWF), but they are in the process of being introduced with a view to becoming operational at 9-km resolution in the next few years. This involves modification of the surface roughness and water availability, and a rigorous treatment of the turbulent and radiative exchanges at the principal elements of an urban area (the streets, walls and roofs). This PhD project is an excellent opportunity to be involved in this exciting work, from development of the underpinning science of modelling urban areas, through to the implementation and exploitation of this new capability in the ECMWF system to answer important scientific questions on a global scale.The proposed approach to the fast treatment of 3D radiative exchanges in urban areas is at the forefront of the science of radiative transfer modelling but will require several additional features to be developed and evaluated. A potential advantage will be that very few parameters will be required to describe the morphology of urban areas. An important task will be to examine existing global datasets to determine appropriate values to use for different urban forms globally.With urban areas added in on a global scale, there will be the capacity to run a fully interactive global weather forecast model. This will enable s a large number of new questions about the impact of urban areas, and the importance of cities of different sizes, to be addressed more comprehensively than has been done to date.The PhD project would have three components involving a number of key questions:1.How can we optimally represent 3D radiative transfer in urban areas? A good starting point will be a new method "SPARTACUS" that has recently been developed for efficiently representing the 3D interaction of solar and thermal-infrared radiation with randomly distributed objects and has been applied successfully to both clouds and vegetation. The studentship will address what changes will be needed to adapt it for buildings? What is the impact on radiation of incorporating trees and parks into the urban landscape? How well does this approach match benchmark calculations from much more detailed (and computationally expensive) radiation models?2.How well does the new urban treatment in the ECMWF model predict near-surface variables (e.g. air temperature and surface fluxes of heat and moisture)? A systematic evaluation will be performed against existing observations from a number of contrasting city areas from around the world, and possibly also using satellite observations of skin temperature. By re-running the global land-surface module (which is fast) of the ECMWF model, the student will investigate how improvements in the treatment of urban areas in the model can improve performance relative to observations.3.Many intriguing scientific questions could then be asked using the full model. For example, What is the impact of urban areas globally on the weather? Whether or not the impact is significant is a topic of controversy in the literature, but the student will be able to exploit existing ECMWF software tools to determine whether forecasts are improved downstream of cities. What will be the impact of future increases in urban cover and possible mitigation efforts such as building greener cities? By modifying the global maps of urban properties used by the model it'll be possible to explore how the urban heat island effect could change in future