High Resolution Wind Field Modelling Over Complex Topography: Analysis and Future Scenarios
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Wegener Center for Climate and Global Change University of Graz Scientific Report No. 32-2010 High resolution wind field modelling over complex topography: analysis and future scenarios Heimo Truhetz April 2010 The Wegener Center for Climate and Global Change combines as an interdisciplinary, internationally oriented research center the competences of the University of Graz in the research area „Climate, Environmental and Global Change“. It brings together, in a dedicated building close to the University central campus, research teams and scientists from fields such as geo- and climate physics, meteorology, economics, geography, and regional sciences. At the same time close links exist and are further developed with many cooperation partners, both nationally and internationally. The research interests extend from monitoring, analysis, modeling and prediction of climate and environmental change via climate impact research to the analysis of the human dimensions of these changes, i.e., the role of humans in causing and being effected by climate and environmental change as well as in adaptation and mitigation. The present report is the result of a PhD thesis work completed in March 2010. Alfred Wegener (1880-1930), after whom the Wegener Center is named, was founding holder of the University of Graz Geophysics Chair (1924- 1930) and was in his work in the fields of geophysics, meteorology, and climatology a brilliant, interdisciplinary thinking and acting scientist and scholar, far ahead of his time with this style. The way of his ground-breaking research on continental drift is a shining role model — his sketch on the relationship of the continents based on traces of an ice age about 300 million years ago (left) as basis for the Wegener Center Logo is thus a continuous encouragement to explore equally innovative scientific ways: paths emerge in that we walk them (Motto of the Wegener Center). Wegener Center Verlag • Graz, Austria © 2010 All Rights Reserved. Selected use of individual figures, tables or parts of text is permitted for non-commercial purposes, provided this report is correctly and clearly cited as the source. Publisher contact for any interests beyond such use: [email protected]. ISBN 978-3-9502940-0-2 April 2010 Contact: Heimo Truhetz. [email protected] Wegener Center for Climate and Global Change University of Graz Leechgasse 25 A-8010 Graz, Austria www.wegcenter.at High resolution wind field modelling over complex topography: analysis and future scenarios Dissertation zur Erlangung des akademischen Grades Doktor der Naturwissenschaften vorgelegt von Mag. Heimo Truhetz an der Naturwissenschaftlichen Fakultät der Karl-Franzens-Universität Graz Betreuer: Univ.-Prof. Mag. Dr. Gottfried Kirchengast Mitbetreuer: Mag. Dr. Andreas Gobiet Wegener Zentrum für Klima und Globalen Wandel und Institutsbereich Geophysik, Astrophysik und Meteorologie Institut für Physik Karl-Franzens-Universität Graz Graz, März 2010 Acknowledgments I wish to express my gratitude and respect to the supervisor of this PhD thesis, Prof. Gottfried Kirchengast, for his excellent scientific advice. In his position as foun- der and head of the Wegener Center (WegCenter) he built up the proper open-minded surrounding conditions for writing this thesis. I am deeply indebted to the co-supervisor, Dr. Andreas Gobiet, for his excellent, in- spiring and time-consuming scientific and personal support. Although he was occupied by building up the Regional and Local Climate Modelling and Analysis Research Group (ReLoClim) at the WegCenter and establishing ReLoClim as an internationally reco- gnised group, he had time enough for long-lasting discussions over the last years that encouraged me and helped me to improve the quality of this thesis and its underlying analyses. The thesis’s investigations required a huge amount of computational resources. This demand was covered by the high-performance computing facilities of the European Cen- tre for Medium-Range Weather Forecasts (ECMWF), the Information Service (ZID) of the University of Graz, and the Institute for Geophysics, Astrophysics, and Meteorology (IGAM) / Institute of Physics of the University of Graz. Special thanks are expressed to the system administrators, Wim de Geeter (WegCenter), Carsten Maass (ECMWF), Ing. Roland Maderbacher (IGAM), and Dr. Ursula Winkler (ZID) for their professional and unbureaucratic support and to the Austrian Central Institute for Meteorology and Geodynamics (ZAMG) for offering parts of their computing time allocated at the ECM- WF. Many thanks are also expressed to Dr. Georg Mayr, Institute for Meteorology at the University of Innsbruck, and ZAMG for the provision of high-quality observational data. I gratefully acknowledge the financial funding from the Austrian Research Centers systems research GmbH (ARC-sys) via the project “Research for Climate Protection: Model Run Evaluation (reclip:more)” and from WegCenter’s startup support for Re- LoClim in the beginning of my doctoral programme. Furthermore, I really appreciate ReLoClim’s financial support over the last years, since apart from reclip:more this PhD thesis stood outside of any project-related funding. My thanks also go to my colleagues at WegCenter, particularly to the members of ReLoClim, for fruitful discussions and support. Due to its interdisciplinary composition, WegCenter provides an inspiring scientific and personal environment, I am proud to be able to contribute to. I wish to thank my room mates, Dr. Michael Borsche, Mag. Bettina Lackner, Mag. Susanne Schweitzer, and Mag. Martin Suklitsch for the excellent working atmosphere flavoured with a whiff of humour. Finally, my heartfelt thanks go to my parents and close friends for constantly accom- panying me with their openness and honesty through these years. Abstract / Zusammenfassung The present thesis focuses on investigating and improving a hybrid dynamic-diagnostic downscaling method for near surface wind (based on the dynamic PSU/NCAR model MM5 and the diagnostic California Meteorological model CALMET), which enables to provide wind climatologies over complex topography on the 100 m scale. A second objective is to conduct climate simulations and to identify climatological main pro- cesses affecting the long-term behaviour of averaged near surface wind conditions in the European Alpine region and the Vienna Basin under increasing greenhouse-gas concentrations. Questions of wind gusts and extremes are set aside. Several variants of the method were applied (driven by the reanalysis dataset ERA- 40) in two study regions, the Hohe Tauern region and the Vienna Basin. Comparisons to observations show that the steady-state flow concept and the orographic speed-up effect are the most dominant climatological mechanisms. The quality of the modelled climatologies is mostly affected by the ability to capture synoptic- and regional-scale processes. In the Vienna Basin the steady-state flow concept is more valid and leads to an enhanced systematic overestimation of wind speed (biases from 0.9 m/s to 2.1 m/s). The method has been successfully applied to the output of the climate model ECHAM5 (periods 1981 to 1990 and 2041 to 2050, IPCC scenario IS92a). The climate change signals show decreasing annual wind speeds (up to 20 .8 %, i. e., 1.2 m/s, in the inner Alps) and it can be concluded that a) changes of− synoptic- and− large-scale processes are affecting gradient-forced synoptic- and regional-scale air flows, particu- larly during DJF, b) the orographic speed-up effect amplifies changes of wind speeds in mountainous areas, and c) interactions between the atmosphere and the earth’s surface lead to regionally varying climate change effects. There exists strong evidence for a robust model-independent reduction of wind speed during MAM and JJA in the inner Alps. Abstract / Zusammenfassung vi Das Ziel der vorliegenden Arbeit ist die Untersuchung einer kombinierten dynamischen-diagnostischen downscaling Methode für bodennahen Wind (basierend auf dem dynamischen Modell MM5 und dem diagnostischen Modell CALMET), welche es ermöglicht Windklimatologien in komplexer Topographie auf der 100 m Skala zu er- stellen. Ein zweites Ziel ist die Durchführung von Klimasimulationen zur Identifikation klimatologischer Prozesse, die das Langzeitverhalten von mittleren Windverhältnissen in den Europäischen Alpen und dem Wiener Becken bei steigenden Treibhausgaskon- zentrationen bestimmen. Böen und Extremereignisse werden nicht betrachtet. Mehrere Varianten der Methode (angetrieben vom Re-Analysedatensatz ERA-40) wurden in den Hohen Tauern und dem Wiener Becken eingesetzt. Vergleiche zu Be- obachtungsdaten zeigen, dass die Konzepte der Stationären Strömung und der Strö- mungsüberhöhung die dominantesten Klimamechanismen sind. Die Qualität der Wind- klimatologien wird hauptsächlich von der Fähigkeit, synoptische und regionale Prozesse abzubilden, beeinflusst. Im Wiener Becken hat die Stationäre Strömung mehr Validität und führt zu einer verstärkten systematischen Überschätzung der Windgeschwindigkei- ten (Bias zw. 0,9 m/s und 2,1 m/s). Die Methode wurde erfolgreich auf eine Klimasimulation des Modells ECHAM5 (Pe- rioden 1981 bis 1990 und 2041 bis 2050, IPCC Szenario IS92a) angewandt. Die Klima- änderungssignale zeigen rückläufige Jahreswindgeschwindigkeiten (bis zu 20 .8 %, das sind 1.2 m/s, in den Alpen) und es kann gefolgert werden, dass a) Änderungen− der synoptischen− und regionalen Prozesse gradientenbedingte Strömungen beeinflussen, ins- besondere im Winter, b) die Strömungsüberhöhung Änderungen