Wegener Center for Climate and Global Change University of Graz Scientific Report No. 55-2013 Tropopause Characteristics from GPS Radio Occultation Data Therese Rieckh October 2013 kindly supported by The Wegener Center for Climate and Global Change combines as an interdisciplinary, internationally oriented research institute 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. (more information at www.wegcenter.at) The present report is the result of a Master Thesis work completed in June 2013. Alfred Wegener (1880-1930), after whom the Wegener Center is named, was founding holder of the University of Graz Geophysics Chair (1924- 1930). In his work in the fields of geophysics, meteorology, and climatology he was a brilliant scientist and scholar, thinking and acting in an interdisciplinary way, 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 © 2013 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 978-3-9503608-2-0 October 2013 (edited version of June 2013 thesis) Contact: Therese Rieckh [email protected] Wegener Center for Climate and Global Change University of Graz Brandhofgasse 5 A-8010 Graz, Austria www.wegcenter.at Diploma Thesis to obtain the degree Magistra Rerum Naturalium at the School of Natural Sciences, University of Graz Tropopause Characteristics from GPS Radio Occultation Data Therese Rieckh June 2013 Supervisor: Assoz.-Prof. Mag. Dr. rer. nat. Ulrich Foelsche Co-Supervisor: Mag. Dr. rer. nat. Barbara Scherllin-Pirscher Wegener Center for Climate and Global Change (WEGC) and Inst. for Geophysics, Astrophysics, and Meteorology/Inst. of Physics (IGAM/IP) University of Graz Abstract The tropopause is a transition layer in which the atmosphere changes from tropospheric to stratospheric conditions. Tropopause characteristics contain a wide range of infor- mation. Tropopause altitude, for example, has been suggested to be an indicator of climate change, as it is sensitive to air temperature changes of both troposphere and stratosphere. In this thesis, a general overview of the tropopause is given, and its role in the atmo- sphere is discussed. This work focuses on the thermal tropopause, and interpretation and implementation of this definition are explained in detail. To compute tropopause parameters from temperature profiles, a high vertical reso- lution of the data set is required. This work is based on GPS radio occultation data, which do not only fulfill the criterion of high vertical resolution, but also feature charac- teristics like high accuracy and precision, all-weather capability, and long-term stability. In addition, global coverage of radio occultation measurements allows to study spatial variations thoroughly. In this thesis, lapse rate tropopause and cold point tropopause altitude and tem- perature are investigated. Spatiotemporal characteristics are discussed on the basis of individual profiles. High tropical altitudes and low extratropical altitudes (low tropical temperatures and high extratropical temperatures) form two well-defined regions. Longi- tudinal variations due to land/sea coverage and orography occur mainly in the northern hemisphere. The annual cycle is investigated for different latitude bands on the basis of monthly averaged data. Both amplitude and phase vary strongly with latitude. Double and triple tropopause occurrence as well as their characteristics are discussed. A trend study using multiple regression analysis is performed for the time series from September 2001 to December 2010. iii Zusammenfassung Die Tropopause bildet die Übergangszone von Troposphäre zu Stratosphäre. Charak- teristika der Tropopause beinhalten Information über den Zustand der Atmosphäre. Die Tropopausenhöhe etwa reagiert auf Temperaturänderungen in Troposphäre und Stratosphäre und kann als Indikator für Klimaänderungen betrachtet werden. In dieser Arbeit wird eine Einführung in die Tropopause gegeben sowie ihre Rolle in der Atmosphäre diskutiert. Der Fokus liegt auf der Interpretation und Implementierung der thermischen Tropopause. Tropopausenparameter werden von Temperaturprofilen berechnet, wobei eine hohe vertikale Auflösung Voraussetzung ist. Die Radiookkulationsmethode liefert hochaufge- löste Temperaturprofile von hoher Genauigkeit und globaler Abdeckung, und ist zusätz- lich kalibrationsfrei und wetterunabhängig. Diese Eigenschaften machen genaue Unter- suchungen räumlicher Variationen möglich. In dieser Arbeit werden die Tropopausenparameter Höhe und Temperatur behandelt. Zeitliche sowie breiten- und längenabhängige Tropopausencharakteristika werden auf der Basis von Einzelprofilen untersucht. Hohe tropische Tropopausenhöhen und niedrige ex- tratropische Tropopausenhöhen bzw. niedrige tropische Tropopausentemperaturen und hohe extratropische Tropopausentemperaturen sind klar unterscheidbar. Längenab- hängige Variationen kommen vor allem in der Nordhemisphäre aufgrund der Land- und Meerverteilung vor. Der Jahresgang verschiedener geographischer Breiten wird mithilfe zonaler Monatsmittelwerte diskutiert und zeigt starke Unterschiede in Amplitude und Phase. Die Häufigkeit bzw. Charakteristika von zweiten und dritten Tropopausen in einem Temperaturprofil werden für verschiedene Breiten und Jahreszeiten analysiert. Die Zeitreihe von September 2001 bis Dezember 2010 ermöglicht eine Studie bezüglich der Trends von Tropopausenhöhe und -temperatur. v Acknowledgments First and foremost, I would like to thank my supervisor Ulrich Foelsche for the oppor- tunity to write my thesis at the Wegener Center for Climate and Global Change and for the financial support provided by the research grant from the Austrian Science Fund (FWF) through his project BENCHCLIM. I am grateful for his encouragement to take part in national and international conferences and present my work there. Also, I would like to express my gratitude to the Wegener Center for the financial support that helped me to join these conferences, and for the excellent infrastructure, knowledge base and motivating scientific environment. Sincere thanks to my co-supervisor Barbara Scherllin-Pirscher, who has the amazing skill of bringing clarity in times of confusion and helped me to keep a clear head. Her ideas, support and fair critique during the last months were much appreciated, as was her help in proofreading this thesis. I wish to offer particular thanks to Florian Ladstädter for his endless enthusiasm in so many different fields. He was a constant source of motivation and I really appreciate his willingness to invest so much time and effort into sharing his knowledge. I also wish to thank him for proofreading this thesis. I thank my office mates for the enjoyable and fun work atmosphere. Special thanks to Veronika Proschek for her IT-related support and her encouragement. I am very lucky to have met a number of people during the course of my studies, who, through the many hours of solving integrals, have become close friends. I will be finishing my studies at the Wegener Center with three of them and I thank them for their great company during the recent months. Finally, I would like to express thanks to my parents. Apart from their financial backing these past years, they have always encouraged me to choose my own path and have never unduly pushed me, but rather reminded me to do what is best for me. Thank you both so very much for supporting not only my academic pursuits, but also all manner of non-academic activities, allowing me to be the most well-rounded person I can be. vii Contents Acronyms xi 1 Introduction 1 2 The radio occultation technique 5 2.1 Measuring technique and retrieval . .5 2.2 Occultation Processing System at the WEGC . .8 2.3 Radio occultation missions . .8 2.4 Radio occultation characteristics . .8 3 The tropopause 11 3.1 Different tropopause definitions . 12 3.1.1 The thermal tropopause . 12 3.1.2 Other tropopause definitions . 15 3.2 Tropopause retrieval . 16 4 Temporal and spatial tropopause characteristics 21 4.1 Temperature profiles and the location of the tropopause . 21 4.2 Latitudinal perspective . 30 4.2.1 Tropopause altitude versus latitude . 30 4.2.2 Tropopause temperature versus latitude . 32 4.3 Longitudinal perspective . 35 4.3.1 Altitude of the first and second tropopause .