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Factors Affecting the Severity of European Winter Windstorms

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Factors Affecting the Severity of European Winter Windstorms Factors affecting the severity of European winter windstorms DISSERTATION zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften am Fachbereich Geowissenschaften der Freien Universit¨atBerlin vorgelegt von Thomas Moran Berlin, 1 Oktober 2019 1. Gutachter: Prof. Dr. Uwe Ulbrich, Institut f¨urMeteorologie, Freie Universit¨atBerlin 2. Gutachter: Prof. Dr. Henning Rust, Institut f¨urMeteorologie, Freie Universit¨atBerlin Datum der Disputation: 6 Juni 2020 Selbstandigkeitserklarung Hiermit erkl¨areich an Eides Statt, dass ich die vorliegende Arbeit selbstst¨andigund ohne fremde Hilfe angefertigt, keine anderen als die angegebenen Quellen und Hilfsmittel benutzt und die den benutzten Quellen w¨ortlich oder inhaltlich entnommenen Stellen als solche ken- ntlich gemacht habe. Diese Arbeit hat in gleicher oder ¨ahnlicher Form noch keiner Pr¨ufungs- beh¨orde vorgelegen. Berlin, 1. Oktober 2019 2 Abstract Every year, dozens of intense extra-tropical cyclones strike Europe with high intensity wind- fields | or windstorms | that inflict significant damage to local infrastructure and housing. In terms of reinsurance losses, windstorms are the most socioeconomically dangerous hazards affecting Europe to date. Stakeholders from both the public and private sectors are inter- ested in improving the predictability of these systems, and while substantial advancements have been made towards a better understanding of their occurrence frequency, little has been explored in terms of their potential severity. Thus, the thesis focuses on the factors affecting the severity (or footprint) of European windstorms by taking a large set of events from the MPI-ESM-LR model, and ERA-20C reanalysis data. After defining a windstorm footprint metric, windstorms are objectively matched to unique cyclones, where it is assumed the windstorm is created by its paired cyclone (the paired system is called a \storm"). These storms form the basis of the systems studied in the thesis as they provide more upstream information. Factors are explored in three core topics: i) characterising European wind- storms in terms of their parent cyclones; ii) exploring the role of various cyclonic growth factors (CGFs) on storm footprints; and iii) using a Generalized Linear Model (GLM) to quantitatively rank the importance of factors affecting these footprints. In terms of their parent cyclones, it is found that extremely high windstorm footprints are associated with deeper and faster-growing cyclones, and non-extreme systems show a weaker relationship. Strong windstorms are additionally associated with bomb cyclones. Windstorms usually form 2 days after cyclogenesis and are found to intensify in a 24-hour window centered on cyclone maximum intensification. Finally, we investigated the cyclone pathways which create high European windstorm footprints. Cyclogenesis maxima are sit- uated over the North Atlantic, with a higher density towards the West near the US East Coast. These cyclones intensify on the southwest flank of the British Isles before entering Europe with a high intensity, creating a high European footprint. Then, we study the role of five CGFs in these cyclone pathways: the jet stream, Eady Growth Rates (EGRs), potential vorticity at 850 hPa (PV850), land-sea contrasts (LSCs), and sea-surface temperature (SST) gradients. To determine whether the magnitudes of these anomalies are related to the European storm footprint, they are composited at the time and location of cyclogenesis or maximum intensification, and are compared between extremely high and low European footprint storms. We find that strong jet streams and latent heat release during the cyclone intensification phase in the East Atlantic have a large influence on the storm footprint, with some influence from these processes at the cyclogenesis phase in the West Atlantic. Lastly, we find evidence that stronger LSCs over the US East Coast are related to higher footprint windstorms, but SST gradient results disagreed between the data sets and may instead be related only to the occurrence of windstorms. Altogether, high European footprint storms are related to a few CGFs influencing cyclogenesis in the West Atlantic (e.g. LSCs and local baroclinicity), followed by other CGFs influencing intensification in the East Atlantic (e.g. PV850). The previous two sections explored how cyclone properties and CGFs influence high European footprint storms, so we utilize a GLM to quantitatively assess all of these factors which are represented as \predictors" using field means or binary conditions. Six models 3 are constructed: the first four relate to regional subsets of the storms, and the last two incorporate regional development as predictors. From the parameter estimations of each model result, we find that jet stream and PV850 anomalies at the cyclone intensification phase and West Atlantic cyclogenesis provide the most explanatory power. Moreover, a positive NAO emerges as a strong predictor in the case that storms benefit from being deflected in a northeasterly direction towards Europe (e.g. in MPI-ESM-LR data). Both LSCs and SST gradients appear to be strong predictors, but their disagreements between the two data sets suggest that further analysis is required. Overall, we determined the most likely factors (among many) influencing a high Euro- pean windstorm footprint. These are cyclones which generate in the West Atlantic thanks to baroclinicity and surface thermal gradients, a cyclone maximum intensification phase ac- companied by a transient jet anomaly and a large amount of diabatic release near the British Isles (especially during a positive NAO phase), and a windstorm genesis and intensification during this phase. 4 Zusammenfassung Jedes Jahr treffen Dutzende von intensiven extra-tropischen Zyklonen auf Europa mit hochin- tensiven Windfeldern - oder St¨urmen- die der lokalen Infrastruktur und Wohngeb¨auden / Geb¨audenerheblichen Schaden zuf¨ugen. In Bezug auf die R¨uckversicherungssch¨aden sind St¨urmedie bislang sozio¨okonomisch gef¨ahrlichsten Gefahren f¨urEuropa. Aus dem ¨offentlichen und privaten Sektor sind an einer Verbesserung der Vorhersagbarkeit dieser Sys- teme interessiert, und obwohl wesentliche Fortschritte hinsichtlich eines besseren Verst¨and- nisses ihrer Auftrittsh¨aufigkeit erzielt wurden, gibt es hinsichtlich ihrer potenziellen Schwere/Intensit¨at wenig Untersuchungen. Die Dissertation konzentriert sich daher auf die Faktoren, die die Schwere (oder den Fußabdruck) europ¨aischer St¨urmebeeinflussen, indem eine große An- zahl von Ereignissen aus MPI-ESM-LR Modell und ERA-20C-Reanalysedaten entnommen werden. Nachdem eine Windstorm-Footprint-Metrik definiert wurde, werden Windst¨urmeobjek- tiv einer Zyklone zugeordnet, wobei davon ausgegangen wird, dass der Windsturm durch seine gepaarte Zyklone erzeugt wird (das gepaarte System wird als \Sturm" bezeichnet). Diese St¨urmebilden die Grundlage f¨urdie in der Arbeit untersuchten Systeme, da sie mehr vorgelagerte Informationen liefern. Faktoren werden in drei Kernthemen untersucht: i) Charakterisierung europ¨aischer St¨urmein Bezug auf ihre Elternzyklone; ii) Untersuchung des Einflusses verschiedener Zyklonenwachstumsfaktoren (ZFs) auf die Fußabdr¨ucke von St¨urmen;und iii) Verwenden eines verallgemeinerten linearen Modells (GLM), um die Wichtigkeit von Faktoren, die diese Fußabdr¨ucke beeinflussen, quantitativ zu bewerten. In Bezug auf die ¨ubergeordneten Zyklonen wurde festgestellt, dass extrem hohe Wind- sturmspuren mit tieferen und schneller wachsenden Zyklonen verbunden sind und nicht- extreme Systeme eine schw¨achere Beziehung aufweisen. Starke St¨urmesind typischerweise auch mit Bombenzyklonen (also solchen, deren Kerndruck extrem schnell f¨allt)verbunden. St¨urmebilden sich normalerweise zwei Tage nach der Zyklogenese und verst¨arken sich in einem 24-Stunden-Fenster, das sich rund um die maximale Intensit¨atdes Zyklons zentri- ert. Schließlich wurden die Zyklonenwege von Systemen untersucht, die hohe europ¨aische Windsturmspuren erzeugen. Zyklogenese- Maxima liegen ¨uber dem Nordatlantik mit einer h¨oherenDichte nach Westen in der N¨aheder US-Ostk¨uste.Diese Zyklonen verst¨arken sich an der S¨udwestflanke der britischen Inseln, bevor sie mit hoher Intensit¨atnach Europa vordringen, wodurch ein hoher europ¨aischer Fußabdruck entsteht. Anschließend wird die Rolle von f¨unf ZFs in diesen Zyklonpfaden untersucht: Jet streams, Eady Growth Rates (EGRs), potenzielle Vorticity auf 850 hPa (PV850), Land-See-Kontraste (LSCs) und Meere- soberfl¨achentemperatur (SST). Um festzustellen, ob die Gr¨oßenordnungen dieser Anomalien mit dem europ¨aischen Sturmfußabdruck zusammenh¨angen,werden sie zum Zeitpunkt und am Ort der Zyklogenese oder der maximalen Intensivierung zusammengesetzt und zwischen St¨urmenmit extrem hohem bzw. extrem niedrigem europ¨aischen Fußabdruck verglichen. Wir finden, dass starke Jet streams und die Freisetzung latenter W¨armew¨ahrendder Zyklo- nenintensivierungsphase im Ostatlantik einen großen Einfluss auf den Fußabdruck des Sturms haben, wobei ein gewisser Einfluss von diesen Prozessen in der Zyklogenesephase im Westat- lantik ausgeht. Schließlich finden wir Hinweise darauf, dass st¨arkere LSCs ¨uber der Ostk¨uste der USA mit Windst¨urmenmit gr¨oßerer Grundfl¨ache zusammenh¨angen,die Ergebnisse des 5 SST-Gradienten jedoch nicht zwischen den Datens¨atzen¨ubereinstimmen und stattdessen m¨oglicherweise nur mit dem Auftreten von Windst¨urmenzusammenh¨angen, nicht aber mit deren Intensit¨at. Insgesamt h¨angenSt¨urmemit hohem europ¨aischen Fußabdruck mit eini-
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