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Doctoral Thesis Dissertation Die approbierte Originalversion dieser Dissertation ist in der Hauptbibliothek der Technischen Universität Wien aufgestellt und zugänglich. http://www.ub.tuwien.ac.at The approved original version of this thesis is available at the main library of the Vienna University of Technology. http://www.ub.tuwien.ac.at/eng Doctoral Thesis ATMOSPHERIC CYCLONE TRACKS AND LARGE SCALE PRECIPITATION UNDER CLIMATE CHANGE CONDITIONS submitted in satisfaction of the requirements for the degree of Doctor of Science of the Vienna University of Technology, Faculty of Civil Engineering Dissertation ATMOSPHÄRISCHE ZUGBAHNEN VON TIEFDRUCKGEBIETEN UND GROSSFLÄCHIGER NIEDERSCHLAG IM KLIMAWANDEL ausgeführt zum Zwecke der Erlangung des akademischen Grades eines Doktors der technischen Wissenschaften eingereicht an der Technischen Universität Wien, Fakultät für Bauingenieurwesen von Mag. Michael Hofstätter Matrikelnummer 0300993 Propst‐Führer‐Str. 42/8, AT‐3100 Sankt Pölten Gutachter: Univ. Prof. Dipl.‐Ing. Dr. techn. Günter Blöschl Institut für Wasserbau und Ingenieurhydrologie Technische Universität Wien Karlsplatz 13/222, A‐1040 Wien Gutachter: Univ. Prof. Dr. rer. nat. Bodo Ahrens Institut für Atmosphäre und Umwelt Goethe‐Universität Frankfurt am Main Altenhöferallee 1, D‐60438 Frankfurt am Main Gutachter: Univ. Prof. Dr.‐Ing. Bruno Merz Institut für Erd‐ und Umweltwissenschaften Universität Potsdam Telegrafenberg, D‐14473 Potsdam Wien, November 2018 ________________________ Parts of the present work have been published or are under review in peer reviewed journals: Hofstätter, M., & Blöschl, G. Vb cyclones sychronized with the North Atlantic‐/Arctic Oscillation. Journal of Geophysical Research: Atmospheres. (under review). Hofstätter, M., Lexer, A., Homann, M., & Blöschl, G. (2018). Large‐scale heavy precipitation over central Europe and the role of atmospheric cyclone track types. International Journal of Climatology, 38, e497–e517. doi.org/10.1002/joc.5386. Hofstätter, M., Chimani, B., Lexer, A., & Blöschl, G. (2016). A new classification scheme of European cyclone tracks with relevance to precipitation. Water Resources Research, 52(9), 7086–7104. doi.org/10.1002/2016WR019146. Hofstätter, M., & Chimani, B. (2012). Van Bebber’s cyclone tracks at 700 hPa in the Eastern Alps for 1961‐2002 and their comparison to Circulation Type Classifications. Meteorologische Zeitschrift, 21(5), 459–473. doi.org/10.1127/0941‐2948/2012/0473. Meteosat VISSR Mid‐IR/Water Vapour 5.7‐7.1µm 11/08/2002‐12:00 On the 11th of August 2002 a major Vb cyclone hit Europe. Excessive large scale precipitation led to devastating floods in many Central European countries. Satellite image provide by: NERC Satellite Receiving Station, Dundee University, Scotland, http://www.sat.dundee.ac.uk/ i ii Kurzfassung Exzessiver, großflächiger Niederschlag führt zu einem hohen Hochwasserrisiko und ist daher von eminenter Bedeutung für monetäre und infrastrukturelle Schäden sowie die Gefährdung von Leben. Ziel dieser Dissertation ist es, den Zusammenhang zwischen dem Auftreten von Zugbahnen von Tiefdruckgebieten und der Charakteristik von mittleren und extremen Niederschlägen in Mitteleuropa besser zu verstehen. Die Eigenschaften und Variabilität von Vb‐Zugbahnen werden insbesondere untersucht. Zunächst wird eine einfache Tracking‐routine auf ERA‐40 Daten angewandt, um Vb‐artige Zugbahnen im Zeitraum 1961‐2002 zu bestimmen. Nach der Erstellung einer einfachen Klimatologie für diese Zugbahnen wird die Eignung von 83 Zirkulationsklassifikationen evaluiert, Vb‐Ereignisse durch möglichst wenige Zirkulationsklassen zu erklären. Nachfolgend wird eine komplexere Tracking‐routine implementiert und für den Zweck dieser Studie erweitert. Eine neuartige Klassifikation der Zugbahntypen von Tiefdruckgebieten über Europa, samt deren klimatologischen Eigenschaften, wird basierend auf JRA‐55 Daten für den Zeitraum 1959‐2015 präsentiert. Für diese Zugbahntypen erfolgen eine Auswertung des langjährigen Niederschlages und des Starkniederschlages für Teilgebiete Mitteleuropas, sowie ein Vergleich mit einer Starkniederschlag‐Chronologie für Flusseinzugsgebiete. Schließlich wird die zeitliche Variabilität der Vb‐Häufigkeit unter dem Gesichtspunkt der Häufungseigenschaften und zeitlichen Änderungen der Ankunftsraten untersucht, sowie ein Zusammenhang mit der großräumigen atmosphärischen Zirkulation hergestellt. Gemäß der Zugbahnanalyse auf Meeresniveau bzw. 700hPa geopotentieller Höhe erreichen etwa 108 bzw. 76 Tiefdruckgebiete pro Jahr Mitteleuropa. Davon sind 25% Atlantiktiefs, 24% entstehen direkt über Mitteleuropa und 22% südlich der Alpen. Andere Zugbahntypen sind weniger häufig (Mittelmeertiefs 12%, Polare 7%, Vb 5%, Kontinentale 2%, Ostalpine 2% und Subtropische 1%). Die Niederschlagsmengen in den Regionen Mitteleuropas variieren erheblich zwischen den Zugbahntypen, wobei bei den Typen Atlantik und Vb die größten und bei den Typen Kontinental und Polar die geringsten Werte auftreten. Die Saisonalität des Niederschlags kann gut durch die Häufigkeit starker Tiefs der unterschiedlichen Typen erklärt werden. Um Starkniederschläge im Sommer und Herbst vollständig zu erklären, sind aber weitere Faktoren wie die Lufttemperatur und die Temperatur der umliegenden Ozeane, notwendig. Vb Tiefs ergeben mit Abstand die größten Niederschlagsmengen zu allen Jahreszeiten und sind am häufigsten mit Starkniederschlägen verbunden. Vb zeigt die größte Starkniederschlagsrelevanz in den östlichen Regionen Mitteleuropas, wohingegen winterliche Atlantiktiefs in den westlichen von größerer Bedeutung sind. So konnten 30% der 50 stärksten Niederschlagsereignisse dem Typ Vb zugeordnet werden, aber nur 15% den Typen Atlantik und Mittelmeertief. Bei der Beurteilung des Gesamtrisikos von Starkniederschlägen sind neben dem Typ Vb auch andere Zugbahnen von Bedeutung, die wesentlich häufiger auftreten. So wurden zwischen 60 % (Tschechien) und 88% (W‐Deutschland) der 50 stärksten Niederschlagsereignisse (1961‐2006) durch andere Zugbahntypen verursacht. An den zehn extremsten Ereignissen in der Region Tschechien/Ostösterreich waren allerdings in acht Fällen Vb Tiefs ursächlich beteiligt. iii Die Gründe für die enorme Hochwasserrelevanz von Vb Tiefs sind: Starke Vb Tiefs sind in allen Jahreszeiten gleichmäßig häufig; die Verweildauer in Mitteleuropa ist aufgrund der kreisförmigen Zugbahn im Mittel um einen Tag länger; und die Starkniederschlagmengen sind um 40% bis 90% größer als bei anderen Typen. Am häufigsten treten Vb Tiefs Anfang April und Ende November auf. In bestimmten Perioden kommen Vb Tiefs gehäuft (in Clustern) vor, bevorzugt wenn die NAO und AO Zirkulationsindices in einer negativen Phase sind. Im Gegensatz dazu war die Vb Häufigkeit zwischen 1988 und 1997, einer ausgeprägt positiven Phase der NAO und AO Indices, besonders niedrig. Während des Auftretens eines Vb Tiefs ist in der Regel ein signifikanter Einbruch der NAO und AO Indices zu sehen, welcher fast drei Wochen andauert. Da dieser Zustand das Entstehen von neuen Vb Tiefs im westlichen Mittelmeer begünstigt, wird dieser Zusammenhang als NAAO– Vb+ Rückkoppelung identifiziert. Abschließend zeigen die Ergebnisse, dass die Häufigkeit von Vb Tiefs in den 1960er Jahren signifikant hoch war, sich aber danach auf einem etwa gleichbleibenden Niveau bewegt hat. Das in dieser Arbeit geschaffene vertiefte Verständnis des Verhaltens von Tiefdruckgebieten als atmosphärische Auslöser großräumiger Starkniederschlags‐ereignisse, soll nicht nur den Wissensstand der Hydrometeorologie erweitern, sondern auch Entscheidungsträger des Hochwasser‐Risikomanagement in Hinblick auf Klimavariabilität und Klimawandel unterstützen. iv Abstract Excessive large scale precipitation entails a high risk of flooding and is thus of particular significance for infrastructural damage, financial loss and threat to human life. The aim of this thesis is to understand the relationship between European cyclone track types and the characteristics of mean and heavy precipitation in of Central Europe. Specifically, the characteristics and variability of Vb cyclones are investigated in depth. In a first step, a tracking routine of atmospheric cyclones is used to identify Vb and congeneric track types using ERA‐40 data for the period 1961‐2002. The climatologic characteristics of these types are analyzed and the ability of 83 circulation type classifications to discriminate between days with and without Vb cyclones is evaluated. Second, a more complex tracking routine is implemented and refined for the purposes of this study. A new classification of Central European track types, including their climatological characteristics, is presented on basis of the JRA‐55 data for the period 1959‐2015. These track types are related to long term precipitation totals as well as to Central European heavy precipitation records of selected river basins. Finally, the temporal characteristics of Vb cyclone occurrence is analyzed in depth with a focus on dispersion characteristics and temporal changes of the arrival rates as well as on the relationship with the large scale atmospheric circulation. About 108 and 76 cyclone tracks per year are identified at sea level pressure and 700hPa geopotential height, respectively. About 25% are Atlantic type cyclones, 24% emerge directly over Central Europe and another 22% originate from the lee of the Alps. The other types are less frequent (Mediterranean 12%, Polar 7%, Vb 5%, Continental 2%, Eastern Alpine 2% and Subtropical 1%). Precipitation differs significantly between cyclones, with Atlantic
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