Blocking und Persistenz

Betreuer: Stephan Pfahl

Beschreibung: Blockierende Hochdrucklagen sind häufig mit verschiedenen Typen von Extremwetterereignissen wie Hitze, Kälte oder Trockenheit verknüpft. Ein wichtiger Faktor für die Wetterwirksamkeit von solchen Lagen ist dabei deren Persistenz. Allerdings wurde der Zusammenhang zwischen Blockierungen und Persistenz von bodennahen Variablen (Temperatur, Niederschlag) bisher nicht explizit quantifiziert, und es ist deshalb schwierig, die relative Bedeutung dieses Zusammenhangs im Vergleich zu anderen Prozessen einzuschätzen. In dieser Arbeit soll daher, basierend auf ERA5 Reanalyse-Daten, die Verbindung zwischen blockierenden Hochdrucklagen und einfachen Persistenzmaßen, wie der Autokorrelation und der Amplitude von Schwankungen von Tag zu Tag der bodennahen Temperatur, untersucht werden.

Voraussetzungen: Interesse an atmosphärischer Dynamik und Statistik. Grundkenntnisse in einer Skriptsprache (R, Python), die zur Datenauswertung verwendet werden kann, sind von Vorteil, können aber auch während der Arbeit erworben werden.

Literatur:

• Kautz, L.-A., Martius, O., Pfahl, S., Pinto, J. G., Ramos, A. M., Sousa, P. M., and Woollings, T. (2022): Atmospheric blocking and weather extremes over the Euro-Atlantic sector – a review. Weather Clim. Dynam. 3, 305-336, doi:10.5194/wcd-3-305-2022.
• Li, J. and Thompson, D.W.J (2021). Widespread changes in surface temperature persistence under climate change. Nature 599, 425-430, doi:10.1038/s41586-021-03943-z.

Der Zusammenhang zwischen Fronten und extremen Niederschlägen

Betreuer: Stephan Pfahl, George Pacey, Florian Ruff

Beschreibung: Extreme Niederschläge treten häufig in Zusammenhang mit Fronten auf. Es ist allerdings nicht klar, wie dabei der genaue räumliche Zusammenhang aussieht, also ob z.B. mehr extreme Niederschläge in der pre-frontalen Luftmasse oder direkt an der Front auftreten. In dieser Arbeit soll dieser Zusammenhang klimatologisch untersucht werden, in dem ein Datensatz zu Kaltfronten in Europa während der warmen Jahreszeit mit Niederschlagsdaten kombiniert wird.

Voraussetzungen: Interesse an Klimatologie und meteorologischen Extremereignissen. Grundkenntnisse in einer Skriptsprache (R, Python), die zur Datenauswertung verwendet werden kann, sind von Vorteil, können aber auch während der Arbeit erworben werden.

Literatur:

• Catto J. L. and Pfahl S. (2013). The importance of fronts for extreme precipitation. J. Geophys. Res. 118, 10791–10801, doi:10.1002/jgrd.50852.

Effects of sea surface temperature change on the global distribution of stable water isotopes

Betreuer: Svetlana Botsyun, Stephan Pfahl

Beschreibung: Stable hydrogen and oxygen isotopes in water vapor (δDv, δ¹⁸Ov) and precipitation (δDp, δ¹⁸Op) are widely used as tracers to improve our understanding of the hydrologic cycle on various time scales, from hourly variations to changes over millions of years (Botsyun et al., 2019; Pfahl and Wernli, 2008). Isotope fractionation is strongly dependent on temperature (Gat, 1996). Moreover, global atmospheric dynamics affecting stable water isotopes (Botsyun et al., 2016) is largely controlled by surface temperature. In this study, we aim to isolate the effects of sea surface temperature (SST) change (both increase and decrease) on stable water isotopes. Preliminary results show that the magnitude of isotopic change for the case of a 5°C decreased SST is > 8‰ and is most pronounced over continents, particularly over mountainous regions.

Vorgehen: In this work, we will first analyze the completed model simulations. Two sensitivity experiments are performed with a high-resolution (∼0.75°) isotope-enabled general circulation model (ECHAM5-wiso). In these simulations, the SST is changed (increased and decreased) by 5°C. All other boundary conditions are kept as pre-industrial (year 1850). In the first step, student will be asked to analyze the global climate and associated isotope changes caused by the shift in SST. In the second step (or at M. Sc. level), additional ECHAM5-wiso experiments will be designed to test the impact of model resolution on the SST-isotope link.

Voraussetzungen: Requirements for this project are an interest in stable water isotopes and paleoclimate, and a willingness to communicate in English (one of the supervisors is not a German speaker). Nevertheless, the final report can be written in German. Experience with scripting languages (Python, Ferret) would be helpful, but can be acquired/improved during the project.

Literatur:

• Botsyun, S., Sepulchre, P., Risi, C. and Donnadieu, Y.: Impacts of Tibetan Plateau uplift on atmospheric dynamics and associated precipitation δ18O, Clim. Past, 12(6), 1401–1420, doi:10.5194/cp-12-1401-2016, 2016.
• Botsyun, S., Sepulchre, P., Donnadieu, Y., Risi, C., Licht, A. and Caves Rugenstein, J. K.: Revised paleoaltimetry data show low Tibetan Plateau elevation during the Eocene, Science (80-. )., 363(6430), eaaq1436, doi:10.1126/science.aaq1436, 2019.
• Gat, J. R.: Oxygen and hydrogen isotopes in the hydrologic cycle, Annu. Rev. Earth Planet. Sci., 24(1), 225–262, 1996.
• Pfahl, S. and Wernli, H.: Air parcel trajectory analysis of stable isotopes in water vapor in the eastern Mediterranean, J. Geophys. Res. Atmos., 113(20), 1–16, doi:10.1029/2008JD009839, 2008.

Zukünftige Änderungen in der Häufigkeit von Zyklonen

Betreuer: Stephan Pfahl

Beschreibung: Zyklonen, also Tiefdruckgebiete, sind ein wesentlicher Faktor für die Variabilität von Wetter und Klima in den mittleren Breiten. Zukünftige Änderungen in der Häufigkeit von Zyklonen sind allerdings mit erheblichen Unsicherheiten behaftet, was sich in Unterschieden zwischen Projektionen von verschiedenen Klimamodellen widerspiegelt. In dieser Arbeit sollen verschiedene Modelle aus dem CMIP6-Projekt bezüglich solcher Änderungen verglichen und andere Faktoren (wie Änderungen im Jet oder der Baroklinität) untersucht werden, die die Unterschiede zwischen den Modellen erklären könnten.

Voraussetzungen: Voraussetzungen für diese Arbeit sind Interesse an atmosphärischer Dynamik und Klimamodellen. Grundkenntnisse in einer Skriptsprache (R, Python), die zur Datenauswertung verwendet werden kann, sind von Vorteil, können aber auch während der Arbeit erworben werden.

Literatur:

• Priestley, M. D. K. and Catto, J. L. (2022): Future changes in the extratropical storm tracks and cyclone intensity, wind speed, and structure. Weather Clim. Dynam. 3, 337-360, doi:10.5194/wcd-3-337-2022.

Circumglobal teleconnection pattern in warm and cold climate

Betreuer: Svetlana Botsyun, Stephan Pfahl

Beschreibung: Large-scale atmospheric circulation in the extratropics fluctuates in its own dynamics, involving nonlinear interaction among eddies with various horizontal scales and a wide range of time scales. The circumglobal teleconnection (CGT) is a major teleconnection pattern in the northern hemisphere in boreal summer and is characterized by several geographically-anchored action centers along the upper-tropospheric westerly jet stream (Ding and Wang, 2005). Alternative names for the CGT also exists in literature, such as the “Silk Road pattern”, “European wave train” for the Eurasian segments, and the “Tokyo–Chicago express” for the Pacific segment. The CGT has important impacts on the mid-latitude temperature, precipitation, and extreme weather (Bothe et al., 2011). In addition, the CGT is closely related to the monsoonal circulation in the tropics (Saeed et al., 2011). For example, there is a chain of interactions between the CGT and the Indian monsoon rainfall variability: one interaction involves the wave train that develops over the North Atlantic and modulates the monsoon rainfall activity, and another interaction involves anomalous monsoon heating influencing the downstream development of the CGT (Ding and Wang, 2005). For the paleo case, the changes in CGT under different climatic forcings, its connection to the Asian monsoons, and its impact on surface climate are not clear. In this project, we suggest exploring changes in CGT for the Pliocene (3 Ma) and the Last Glacial Maximum (LGM, 21 ka) compared to pre-industrial. The Pliocene is considered as a past analogue of future warm climate (Haywood et al., 2016) and the LGM is an example of a colder-than-present climate (Braconnot et al., 2007). Possible research questions:

1. How do slowly varying components of the climate, such as sea surface temperature (SST), greenhouse gas concentrations, ice sheets, etc. affect large-scale circulation variability and the westerly jet stream?
2. How do changes in the North Atlantic pressure gradient affect the CGT?
3. What is the relationship between the surface climate in Asia and the GCT in warm and cold climates?

Vorgehen: We propose to investigate changes in CGT using General Circulation model (GCM) ECHAM5 experiments with boundary conditions for pre-industrial (year 1850), the Pliocene (~3 Ma), and the Last Glacial Maximum (LGM, ~21 ka). Emphasis is on analyzing completed model simulations rather than creating new model runs. Student will be asked to analyze climatological midlatitude circulation in the Northern Hemisphere based on, for example, high-level geopotential height and winds anomalies. For MSc thesis, a temporal variation of the Northern Hemisphere pressure systems will be studied using the empirical orthogonal function (EOF) analysis.

Voraussetzungen: Requirements for this project are an interest in global atmospheric circulation and paleoclimate and a willingness to communicate in English (one of the supervisors is not a German speaker). Nevertheless, the final report can be written in German. Experience with scripting languages (Python, Ferret, ncl) would be helpful, but can be acquired during the project.

Literatur:

• Bothe, O., Fraedrich, K. and Zhu, X.: Large-scale circulations and Tibetan Plateau summer drought and wetness in a high-resolution climate model, Int. J. Climatol., 31(6), 832–846, doi:10.1002/joc.2124, 2011.

• Braconnot, P., Otto-Bliesner, B., Harrison, S., Joussaume, S., Peterchmitt, J.-Y. J.-Y., Abe-Ouchi, A., Crucifix, M., Driesschaert, E., Fichefet, T., Hewitt, C. D. and others: Results of PMIP2 coupled simulations of the Mid-Holocene and Last Glacial Maximum–Part 1: experiments and large-scale features, Clim. Past, 3(2), 261–277 [online] Available from: http://www.clim-past.net/3/279/2007/cp-3-279-2007-relations.html, 2007.

• Ding, Q. and Wang, B.: Circumglobal teleconnection in the Northern Hemisphere summer, J. Clim., 18(17), 3483–3505, 2005.

• Haywood, A. M., Dowsett, H. J., Dolan, A. M., Rowley, D., Abe-Ouchi, A., Otto-Bliesner, B., Chandler, M. A., Hunter, S. J., Lunt, D. J. and Pound, M.: The Pliocene model intercomparison project (PlioMIP) phase 2: scientific objectives and experimental design, Clim. Past, 12(3), 663–675, 2016.

• Saeed, S., Müller, W. A., Hagemann, S. and Jacob, D.: Circumglobal wave train and the summer monsoon over northwestern India and Pakistan: The explicit role of the surface heat low, Clim. Dyn., 37(5), 1045–1060, doi:10.1007/s00382-010-0888-x, 2011.

Synoptic analysis of convection

Betreuer: Stephan Pfahl, George Pacey

Beschreibung: Many strong convective precipitation events leading to flash floods in Germany occur at or near a cold front (e.g., Fink et al, 2009). The scale interactions between the frontal circulation and smaller-scale convective processes drive the dynamics of these convective storms. In this project, one particular case study in August 2013 that led to flash flooding, severe wind and hail reports across parts of Germany (including Berlin, see also the figure below) will be analyzed in detail. Combining radar and reanalysis data the role of various environmental factors will be investigated that may have favored the occurrence of convection (e.g., Doswell III et al., 1996). Prerequisites for this project are an interest in atmospheric convection and synoptic analyses as well as the willingness to communicate in English, as one of the supervisors does not speak German. The thesis may still be written in German. First experience with a scripting language (such as python) that will be used for data analysis can be helpful, but may also be obtained during the project.

Literatur: 

• Doswell III, C. A., H. E. Brooks, and R. A. Maddox, 1996. Flash flood forecasting: An ingredients-based methodology. Wea. Forecasting11, doi:10.1175/1520-0434%281996%29011%3C0560%3AFFFAIB%3E2.0.CO%3B2. 
• Fink, A. H., T. Brücher, V. Ermert, A. Krüger, and J. G. Pinto, 2009: The European storm Kyrill in January 2007: synoptic evolution, meteorological impacts and some considerations with respect to climate change. Nat. Hazards Earth Syst. Sci. 9, 405-423, doi:10.5194/nhess-9-405-2009.