The role of post-tropical cyclones for European extreme weather

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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