The role of post-tropical cyclones for European extreme weather

虽然欧洲不直接受到北大西洋热带气旋的影响，但其中一些气旋在到达欧洲的温带过渡期间确实会折返并转变为中纬度风暴，即所谓的后热带气旋（ptc）。2017年10月，欧菲莉亚在爱尔兰登陆，造成人员伤亡，大面积破坏，经济活动中断。再保险部门现在开始意识到这类风暴的风险，风险建模社区开始将这类风暴纳入他们的模型中。虽然像欧菲莉亚这样的个别风暴受到了相当多的关注，但关于ptc在历史时期对欧洲的气候作用，以及这种作用如何随着全球变暖而变化，仍有许多未知之处。例如，单一气候模式模拟的初步证据表明，由于东北大西洋海面变暖，下个世纪可能会有更多的ptc影响欧洲（Haarsma等人，2013；Liu等人，2017）。在这个博士项目中，将解决以下研究问题：- ptc在欧洲有多特殊，即它们与到达欧洲的所有中纬度风暴在年周期中发生的地点和时间有多大不同，它们的结构是什么，以及相关的地面影响（风/波，降水）有多强？- ptc的性质如何取决于(i)原始热带气旋的性质，（ii）温带转变期间的中纬度西风气流，以及（iii）海面温度场？例如，特别强的ptc是否源自特别强的热带气旋？-最新一代的高分辨率全球气候模式如何代表ptc ?如何预测ptc在下个世纪的变化？它们会对欧洲产生更大的影响吗？不同的模式对预测的PTC变化是否一致，包括不同的大气环流变化故事线（Zappa & Shepherd, 2017）？该学生将与NCAS的高分辨率全球气候模拟小组和英国气象局密切合作，从而获得一套高分辨率气候模拟，例如目前准备的CMIP6-HighResMIP/PRIMAVERA，和/或最近可用的模拟。模式分辨率已被证明对于表示热带气旋（例如，Roberts等人，2015）和北大西洋风暴路径（Zappa等人，2013）非常重要。该项目的一个初步目标是评估模式分辨率在表示温带过渡和ptc方面的作用。对正在经历温带过渡的风暴及其对西欧的影响有了更好的了解，将有助于改善它们在再保险风险模型中的表现。Haarsma， R. J, Hazeleger， W., Severijns， C., De Vries， H., Sterl， A., Bintanja， R.等（2013）。由于全球变暖，更多的飓风将袭击西欧。地球物理学报，40(9),1783-1788。https://doi.org/10.1002/grl.50360Roberts, m.j., Vidale, p.l., Mizielinski, m.s., Demory, m.e.。， Schiemann， R., Strachan， J.等（2015）。高分辨率全球气候模式高级集合中的热带气旋。气候学报，28(2)。https://doi.org/10.1175/JCLI-D-14-00131.1 Zappa， G., Shaffrey， L. C.和Hodges， K. I.（2013）。CMIP5模式模拟北大西洋温带气旋的能力*。气候学报，26(15),5379-5396。https://doi.org/10.1175/JCLI-D-12-00501.1 Zappa， G., & Shepherd， T. G.（2017）。欧洲区域气候影响评估中大气环流变化的故事线。气候学报，30(16),6561-6577。https://doi.org/10.1175/JCLI-D-16-0807.1Liu， M., Vecchi， G. A., Smith， J. A., & Murakami， H.（2017）。北大西洋温带过渡气候学的现今模拟和21世纪预估

While Europe is not directly affected by North Atlantic tropical cyclones, some of these cyclones do recurve and transform into midlatitude storms during extratropical transition reaching Europe as so-called post-tropical cyclones (PTCs). One such storm, Ophelia, made landfall over Ireland in October 2017 and caused loss of life as well as widespread damage and shutdown of economic activity. The reinsurance sector are now becoming aware of the risk from such storms and the risk modelling community are starting to incorporate these types of storms into their models.While individual storms such as Ophelia have received considerable attention, much remains unknown about the climatological role of PTCs for Europe in the historical period, and how this role might change with global warming. There is tentative evidence from single climate model simulations, for example, that more PTCs may affect Europe in the next century due to a warmer sea surface in the northeast Atlantic (Haarsma et al., 2013; Liu et al., 2017).In this PhD project, the following research questions will be addressed:- How special are PTCs for Europe, i.e. how different are they from the entire population of midlatitude storms reaching Europe in terms of where and when they occur through the annual cycle, what their structure is, and how strong the associated surface impacts (wind/waves, precipitation) are?- How do the properties of PTCs depend on (i) the properties of the original tropical cyclone, (ii) on the midlatitude westerly flow during extratropical transition, and (iii) on the sea surface temperature field? For example, do particularly strong PTCs originate from particularly strong tropical cyclones?- How does the latest generation of high-resolution global climate models represent PTCs, and how are they predicted to change over the next century? Will more of them affect Europe, and more strongly so? Do different models agree on predicted PTC changes, including across different storylines of atmospheric circulation change (Zappa & Shepherd, 2017)?The student will work closely with the High Resolution Global Climate Modelling Group at NCAS and the Met Office, and thereby gain access to a set of high-resolution climate simulations, for example those currently prepared CMIP6-HighResMIP/PRIMAVERA, and/or more recent available simulations. Model resolution has been shown to be important for representing tropical cyclones (e.g., Roberts et al., 2015) and also the North Atlantic storm track (Zappa et al., 2013). One initial aim for this project will be to evaluate the role of model resolution for representing extratropical transition and PTCs. Having improved insights into storms undergoing extratropical transition and their impact on western Europe will lead to improvements in how well they are represented in the reinsurance risk models.Haarsma, R. J., Hazeleger, W., Severijns, C., De Vries, H., Sterl, A., Bintanja, R., et al. (2013). More hurricanes to hit western Europe due to global warming. Geophysical Research Letters, 40(9), 1783-1788. https://doi.org/10.1002/grl.50360Roberts, M. J., Vidale, P. L., Mizielinski, M. S., Demory, M.-E., Schiemann, R., Strachan, J., et al. (2015). Tropical cyclones in the UPSCALE ensemble of high-resolution global climate models. Journal of Climate, 28(2). https://doi.org/10.1175/JCLI-D-14-00131.1 Zappa, G., Shaffrey, L. C., & Hodges, K. I. (2013). The Ability of CMIP5 Models to Simulate North Atlantic Extratropical Cyclones*. Journal of Climate, 26(15), 5379-5396. https://doi.org/10.1175/JCLI-D-12-00501.1 Zappa, G., & Shepherd, T. G. (2017). Storylines of atmospheric circulation change for European regional climate impact assessment. Journal of Climate, 30(16), 6561-6577. https://doi.org/10.1175/JCLI-D-16-0807.1Liu, M., Vecchi, G. A., Smith, J. A., & Murakami, H. (2017). The Present-Day Simulation and Twenty-First-Century Projection of the Climatology of Extratropical Transition in the North Atlantic