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ETH Zürich

1. Graf, Pascal. The effect of below-cloud processes on short-term variations of stable water isotopes in surface precipitation.

Degree: 2017, ETH Zürich

The atmospheric water cycle is a key component of our climate system and a better understanding of the involved processes is crucial to estimate the impacts of a changing climate on, e.g., the spatial distribution of extreme precipitation. In the mid-latitudes, over 80% of precipitation extremes are associated with extratropical cyclones and their occurrence or absence controls the frequency and severity of floods and droughts. Phase-change processes like condensation and rain evaporation affect the dynamics of extratropical cyclones and their impact by contributing latent heat to the cyclone intensification, and by significantly reducing the surface precipitation amount, respectively. A widely applied method to passively trace the atmospheric water cycle with a focus on phase-change processes is to consider stable water isotopes, because these naturally available tracers record the condensation and evaporation history of atmospheric water vapour and precipitation. The aim of this thesis is to identify the driving mechanisms of isotopic variability on the sub-event time scale and draw conclusions for the dynamics of the investigated events. This is done by performing parallel high-frequency measurements of stable water isotopes in near-surface vapour and precipitation for selected rain events and compare these data to simulations with a below-cloud interaction model fed with meteorological observations. In the first part of this thesis, the techniques to measure the isotopic composition of surface vapour and rain are outlined. In-situ vapour isotope measurements of δ2H, δ18O and hence d-excess are performed with two cavity ring-down laser spectrometers (Picarro L1115-i and L2130-i). Precipitation is sampled in short intervals (∼ 10 min) during several rain events and analysed for its isotopic composition. Supporting meteorological observations are taken from radiosondes, a disdrometer, a micro rain radar and two X-band radars. An overview of the data is presented, which was collected during two measurement campaigns in Switzerland. The second part introduces the below-cloud interaction model, which simulates the changing isotopic composition of a single falling hydrometeor. The model is used to enhance the conceptual understanding of below-cloud processes and test the sensitivity of the isotopic signal of rain to temperature, relative humidity, formation height, the formation mechanism, and to the isotopic composition of the source vapour. Variables that determine the initial isotopic composition of rain mostly affect large hydrometeors and intense rain. In contrast, small hydrometeors and weak rain are mostly affected by variables that determine the strength of below-cloud processes. These processes are limited by the height of the melting layer and weak if the melting layer is low. The driving mechanisms of isotopic variability of selected frontal rain events are analysed in the third part of this thesis. The evolution of the isotopic difference between rain and near-surface vapour of three cold frontal… Advisors/Committee Members: Wernli, Heini, id_orcid0000-0001-9674-4837, Sodemann, Harald, Pfahl, Stephan, Risi, Camille.

Subjects/Keywords: PRECIPITATIONS (METEOROLOGY); Stable water isotopes; precipitation evaporation; below-cloud processes; MODELLRECHNUNG UND SIMULATION IN DEN UMWELTWISSENSCHAFTEN; Atmospheric dynamics; Water Cycle; Weather systems; COLD FRONTS (METEOROLOGY)

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APA (6th Edition):

Graf, P. (2017). The effect of below-cloud processes on short-term variations of stable water isotopes in surface precipitation. (Doctoral Dissertation). ETH Zürich. Retrieved from http://hdl.handle.net/20.500.11850/266387

Chicago Manual of Style (16th Edition):

Graf, Pascal. “The effect of below-cloud processes on short-term variations of stable water isotopes in surface precipitation.” 2017. Doctoral Dissertation, ETH Zürich. Accessed October 18, 2019. http://hdl.handle.net/20.500.11850/266387.

MLA Handbook (7th Edition):

Graf, Pascal. “The effect of below-cloud processes on short-term variations of stable water isotopes in surface precipitation.” 2017. Web. 18 Oct 2019.

Vancouver:

Graf P. The effect of below-cloud processes on short-term variations of stable water isotopes in surface precipitation. [Internet] [Doctoral dissertation]. ETH Zürich; 2017. [cited 2019 Oct 18]. Available from: http://hdl.handle.net/20.500.11850/266387.

Council of Science Editors:

Graf P. The effect of below-cloud processes on short-term variations of stable water isotopes in surface precipitation. [Doctoral Dissertation]. ETH Zürich; 2017. Available from: http://hdl.handle.net/20.500.11850/266387

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