Feuchte-Transport aus der atmosphärischen Grenzschicht

Betreuer: Stephan Pfahl, Svetlana Botsyun

Beschreibung: Mischung von Wasserdampf zwischen der atmosphärischen Grenzschicht und der freien Atmosphäre ist ein wichtiger Prozess im Zusammenhang mit der Bildung von Wolken und Niederschlag, ist aber schwierig zu quantifizieren. In dieser Arbeit sollen solche Mischungsprozesse auf der globalen Skala mit Hilfe von Modell-Simulationen und numerischen Tracern (siehe Winschall et al., 2012) abgeschätzt werden. Außerdem werden Wettersysteme identifiziert, die zu intensiver Mischung führen.

Voraussetzungen: Voraussetzungen für diese Arbeit sind Interesse an numerischer Modellierung und Programmierung in Fortran sowie einer Skriptsprache (R, Python).

Literatur:

• Boutle, I.A., Belcher, S.E. and Plant, R.S. (2011): Moisture transport in midlatitude cyclones. Q. J. R. Meteorol. Soc. 137, 360-373, https://doi.org/10.1002/qj.783.

• Winschall, A., Pfahl, S. Sodemann, H. and Wernli, H. (2012): Impact of North Atlantic evaporation hot spots on southern Alpine heavy precipitation events. Q. J. R. Meteorol. Soc. 138, 1245-1258, https://doi.org/10.1002/qj.987.

Analyse der Unsicherheiten bei der Berechnung von Trajektorien

Betreuer: Ingo Kirchner, Stephan Pfahl

Beschreibung: Die offline Berechnung von Trajektorien beruht auf einem vorliegenden Stroemungsfeld. Die raeumliche und zeitliche Aufloesung dieses Stroemungsfeldes verursacht Unsicherheiten. Zur objektiven Abschaetzung dieser Fehler soll auf der Grundlage der ERA5 Daten untersucht werden, welchen Einfluss die zeitliche und raeumliche Aufloesung des Stroemungsfeldes auf den Verlauf der Trajektorien haben. Dabei werden folgende Nebenbedingungen betrachtet:
- Wetterlage
- Region
- Hoehenschicht
- Laufzeit
Die Untersuchung soll sich auf typische Wetterlagen fuer Europa im Zeitraum 1979 bis 2019 beziehen. Zur Beurteilung des Fehlers wird der raeumliche Abstand der Trajektorien bei gleicher Laufzeit herangezogen. Verglichen werden Trajektorien, die zeitgleich an den gleichen Raumpunkten ansetzen, jedoch unterschiedlich aufgeloeste Stroemungsfelder verwenden. Zur Berechung der Trajektorien wird LAGRANTO benutzt.

Literatur:

• Sprenger, M. and Wernli, H. (2015): The LAGRANTO Lagrangian analysis tool - version 2.0, Geosci. Model Dev., 8, 2569–2586, doi:10.5194/gmd-8-2569-2015.

• Wernli, H. and Davies, H. C. (1997): A Lagrangian-based analysis of extratropical cyclones. I: The method and some applications. Q. J. R. Meteorol. Soc., 123, 467–489. doi:10.1002/qj.49712353811.

• Objektive Wetterlagenklassen nach P. Bissolli: https://www.dwd.de/DE/leistungen/klimastatusbericht/publikationen/ksb2003_pdf/06g_2003.html

Abschätzung der "precipitation efficiency" aus Reanalyse-Daten

Betreuer: Stephan Pfahl

Beschreibung:Die "precipitation efficiency" beschreibt das Verhältnis aus in der Atmosphäre kondensierendem Wasser (Wolkenbildung) und Niederschlag am Boden. Sie hängt von einer Reihe von mikrophysikalischen Prozessen ab (Tropfenwachstum, Verdunstung von Niederschlag unterhalb der Wolke, ...). Eine genaue Bestimmung der precipitation efficiency ist wichtig, um z.B. Auswirkungen der Klimaerwärmung auf den Wasserkreislauf besser zu verstehen. In dieser Arbeit soll diese Größe mit Hilfe von Reanalyse-Daten abgeschätzt werden, wobei ein theoretischer Ansatz zur Berechnung der Kondensationsrate aus Wind und Temperatur (nach Pfahl et al., 2017) verwendet wird.

Voraussetzungen: Voraussetzungen für diese Arbeit sind Interesse am atmosphärischen Wasserkreislauf sowie Grundkenntnisse in der Skriptsprache R.

Literatur: 

• Li, R.L., Studholme, J.H.P., Fedorov, A.V. and Storelvmo, T. (2022): Precipitation efficiency constraint on climate change. Nature Clim. Change 12, 642-648, https://doi.org/10.1038/s41558-022-01400-x.

• Pfahl, S., O’Gorman, P.A. and Fischer, E.M. (2017): Understanding the regional pattern of projected future changes in extreme precipitation. Nature Clim Change 7, 423-427, https://doi.org/10.1038/nclimate3287.

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.

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.

Effects of uplift of the Tian Shan Mountains on Asian climate and stable water isotopes in precipitation

Betreuer: Svetlana Botsyun, Stephan Pfahl

Beschreibung: Stable hydrogen and oxygen isotopes in water vapour and precipitation are widely used as tracers to improve our understanding of the hydrological cycle on a variety of time scales, from hourly variations to changes over millions of years (Botsyun et al., 2019; Pfahl and Wernli, 2008). Stable water isotope proxy data (e.g., lake carbonates in which paleoprecipiation isotope values are preserved) are commonly used for paleoclimate reconstructions. However, mountain uplift affects both the climate and the preserved isotope signal (Botsyun et al., 2016, 2019). Therefore, reconstructing the influence of mountain elevation on stable water isotopes is critical for proper interpretation of paleoclimate data. Interactions between mid-latitude westerlies and the Tian Shan Mountains significantly impact climate patterns in Asia today (Sha et al., 2018) and Tian Shan uplift is hypothesised to play an important role in driving Asian climate change since the last ~ 30 Ma (Wang et al., 2020). Presently, Central Asia receives recycled, enriched with heavy isotopes moisture that has been mainly transported across Eurasia by the westerlies (Caves et al., 2015), but for the time when the Tian Shan Mountains were lower, the isotopic signature across the region remains enigmatic. Our preliminary analysis shows that Tian Shan uplift impact westerly and monsoon circulation in the region that results in precipitation and precipitation δ¹⁸O changes

Vorgehen: This work will focus on analysing completed model simulations rather than creating new model runs. We propose using a high-resolution (∼0.75°) isotope-enabled general circulation model (ECHAM5-wiso). Experiments are conducted with boundary conditions for the pre-industrial (year 1850) and varied elevation of Tian Shan. Student will be asked to analyse climatological mid-latitude circulation change and associated surface hydroclimate change. For the M.Sc. – Lagrangian analysis tool “LAGRANTO'' (Sprenger and Wernli, 2015) will be applies to trace moisture trajectories changes for various Tian Shan elevation scenarios.

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 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.
• Caves, J. K., Winnick, M. J., Graham, S. A., Sjostrom, D. J., Mulch, A. and Chamberlain, C. P.: Role of the westerlies in Central Asia climate over the Cenozoic, Earth Planet. Sci. Lett., 428, 33–43, 2015.
• 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.
• Sha, Y., Shi, Z., Liu, X., An, Z., Li, X. and Chang, H.: Role of the Tian Shan Mountains and Pamir Plateau in increasing spatiotemporal differentiation of precipitation over interior Asia, J. Clim., 31(19), 8141–8162, 2018.
• Sprenger, M. and Wernli, H.: The LAGRANTO Lagrangian analysis tool–version 2.0, Geosci. Model Dev., 8(8), 2569–2586, 2015.
• Wang, X., Carrapa, B., Sun, Y., Dettman, D. L., Chapman, J. B., Caves Rugenstein, J. K., Clementz, M. T., DeCelles, P. G., Wang, M., Chen, J., Quade, J., Wang, F., Li, Z., Oimuhammadzoda, I., Gadoev, M., Lohmann, G., Zhang, X. and Chen, F.: The role of the westerlies and orography in Asian hydroclimate since the late Oligocene, Geology, 48(7), 728–732, doi:10.1130/G47400.1, 2020.