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You searched for +publisher:"ETH Zürich" +contributor:("Pfahl, Stephan"). Showing records 1 – 3 of 3 total matches.

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ETH Zürich

1. Büeler, Dominik. Potential Vorticity Diagnostics to Quantify Effects of Latent Heating in Extratropical Cyclones: Methodology and Application to Idealized Climate Change Simulations.

Degree: 2017, ETH Zürich

Extratropical cyclones develop due to baroclinic instability, but their intensification is often substantially amplified by diabatic processes, most importantly latent heating (LH) through cloud formation. Although this amplification is well understood for individual cyclones, there is still need for a systematic and quantitative investigation of how LH affects cyclone intensification in different, particularly warmer and moister climates predicted for the future. In this thesis, we thus introduce a simple diagnostic method to quantify the contribution of LH to cyclone intensification within the potential vorticity (PV) framework. The two leading terms in the PV tendency equation, diabatic PV modification and vertical advection, are used to derive a diagnostic equation to explicitly calculate the fraction of a cyclone's positive lower-tropospheric PV anomaly caused by LH. The strength of this anomaly is strongly coupled to cyclone intensity and the associated impacts in terms of surface weather. To evaluate the performance of the diagnostic, sensitivity simulations of 12 Northern Hemisphere cyclones with artificially modified LH are carried out with a numerical weather prediction model. Based on these simulations, we demonstrate that the PV diagnostic captures the mean sensitivity of the cyclones' PV structure to LH as well as parts of the large case-to-case variability, which is a particular benefit for climatological applications. The simple and purely diagnostic characteristics of the PV diagnostic allow for a versatile application to cyclones in weather and climate model output as well as in reanalysis data, independent of their spatial and temporal resolution.It is still unclear how enhanced LH in a warmer and moister climate will affect cyclones, mainly because its intensifying effect is expected to partly counteract the effects of further changes in the meridional temperature gradient and static stability. We thus use our PV diagnostic to study the role of LH for cyclones in different idealized climate change experiments. In a first step, we perform high-resolution surrogate climate change simulations of a set of moderate and intense Northern Hemisphere cyclones, in which temperatures are spatially homogeneously increased by 4 K, keeping relative humidity constant and thus increasing specific humidity. With our PV diagnostic we can demonstrate that enhanced LH associated with the higher moisture content is the main driver for an increase in intensity and impacts of all cyclones: it amplifies their positive lower-tropospheric PV anomaly by intensifying diabatic PV generation. This amplification leads to a stronger intensification of the cyclones by enhancing cyclonic circulation in the lower troposphere, which, in most cases, results in higher cyclone intensities, stronger wind gusts at the surface, and, in combination with the higher moisture content, more precipitation. This correlation is most robust from an average perspective, but also the strong case-to-case variability of the changes can partly be… Advisors/Committee Members: Wernli, Heini, id_orcid0000-0001-9674-4837, Pfahl, Stephan, Ulbrich, Uwe.

Subjects/Keywords: Atmospheric dynamics; Climate change; Extratropical cyclones; Latent heating; Potential vorticity; Diabatic processes; Meteorology; Wind storms; Winter storms; Potential vorticity diagnostics; Diagnostic tools; Idealized climate change simulations; Surrogate climate change simulations; Idealized GCM simulations; Numerical weather prediction models; Atmospheric physics

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APA (6th Edition):

Büeler, D. (2017). Potential Vorticity Diagnostics to Quantify Effects of Latent Heating in Extratropical Cyclones: Methodology and Application to Idealized Climate Change Simulations. (Doctoral Dissertation). ETH Zürich. Retrieved from http://hdl.handle.net/20.500.11850/250887

Chicago Manual of Style (16th Edition):

Büeler, Dominik. “Potential Vorticity Diagnostics to Quantify Effects of Latent Heating in Extratropical Cyclones: Methodology and Application to Idealized Climate Change Simulations.” 2017. Doctoral Dissertation, ETH Zürich. Accessed November 14, 2019. http://hdl.handle.net/20.500.11850/250887.

MLA Handbook (7th Edition):

Büeler, Dominik. “Potential Vorticity Diagnostics to Quantify Effects of Latent Heating in Extratropical Cyclones: Methodology and Application to Idealized Climate Change Simulations.” 2017. Web. 14 Nov 2019.

Vancouver:

Büeler D. Potential Vorticity Diagnostics to Quantify Effects of Latent Heating in Extratropical Cyclones: Methodology and Application to Idealized Climate Change Simulations. [Internet] [Doctoral dissertation]. ETH Zürich; 2017. [cited 2019 Nov 14]. Available from: http://hdl.handle.net/20.500.11850/250887.

Council of Science Editors:

Büeler D. Potential Vorticity Diagnostics to Quantify Effects of Latent Heating in Extratropical Cyclones: Methodology and Application to Idealized Climate Change Simulations. [Doctoral Dissertation]. ETH Zürich; 2017. Available from: http://hdl.handle.net/20.500.11850/250887


ETH Zürich

2. 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 · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

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 November 14, 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. 14 Nov 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 Nov 14]. 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


ETH Zürich

3. Dütsch, Marina Lara. Stable water isotope fractionation processes in weather systems and their influence on isotopic variability on different time scales.

Degree: 2016, ETH Zürich

Stable water isotopes are naturally available tracers of moist processes in the atmosphere. Due to their different mass and symmetry they experience fractionation during phase transitions and thereby record information about the condensation and evaporation history of air parcels. They can be used to study sources and transport of atmospheric moisture or as climate proxies to reconstruct past temperature changes from measurements in paleoarchives. The isotopic composition of atmospheric moisture varies largely on different time scales. However, the dynamical and physical processes influencing the isotopic variability are complex and not completely understood so far, in particular on short (hourly to daily) time scales. The application of numerical models is a useful way of studying these processes. They provide the full four-dimensional structure of the isotope fields and can be used for sensitivity tests to quantify the role of specific mechanisms for isotopic variability. In this thesis, atmospheric stable water isotope processes are investigated with the help of different numerical model simulations. The aim is to improve our understanding of the hourly to seasonal variations of δ2H and deuterium excess in water vapour and precipitation, with a focus on Europe. In the first part of the thesis, a simple Rayleigh model simulating the isotopic composition of air parcels during moist adiabatic ascent is used to study the impact of the nonequilibrium effect, the temperature effect, and the nonlinear effect of the δ scale on deuterium excess. The δ scale effect is important especially in depleted air parcels, for which it can change the sign of the deuterium excess in the remaining vapour from negative to positive. In this case the deuterium excess to a large extent reflects an artefact of its own definition, which overwrites both the nonequilibrium and the temperature effect. We propose an alternative definition of the deuterium excess that solves this problem, as it is based on the logarithmic scale and therefore not affected by the nonlinearity of the δ scale. In the second part of the thesis, stable water isotopes are simulated in an idealised extratropical cyclone using the isotope-enabled version of the COSMO model (COSMOiso). A set of experiments with differing initial conditions of δ2H in vapour and partly deactivated isotopic fractionation allows quantifying the relative roles of cloud fractionation and vertical and horizontal advection for the simulated δ2H signals associated with the cyclone and its fronts. Horizontal transport determines the large-scale pattern of δ2H in both vapour and precipitation, while fractionation and vertical transport are more important on a smaller scale, near the fronts. During the passage of the cold front fractionation leads to a V-shaped pattern of δ2H in precipitation and vapour, which is, for vapour, superimposed on a gradual decrease caused by the arrival of colder air masses. Finally, in the third part of the thesis, realistic COSMOiso simulations are performed over… Advisors/Committee Members: Wernli, Heini, id_orcid0000-0001-9674-4837, Pfahl, Stephan, Sodemann, Harald, Werner, Martin.

Subjects/Keywords: Stable water isotopes; Fractionation; Water cycle; Weather systems; Extratropical cyclones

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Dütsch, M. L. (2016). Stable water isotope fractionation processes in weather systems and their influence on isotopic variability on different time scales. (Doctoral Dissertation). ETH Zürich. Retrieved from http://hdl.handle.net/20.500.11850/58

Chicago Manual of Style (16th Edition):

Dütsch, Marina Lara. “Stable water isotope fractionation processes in weather systems and their influence on isotopic variability on different time scales.” 2016. Doctoral Dissertation, ETH Zürich. Accessed November 14, 2019. http://hdl.handle.net/20.500.11850/58.

MLA Handbook (7th Edition):

Dütsch, Marina Lara. “Stable water isotope fractionation processes in weather systems and their influence on isotopic variability on different time scales.” 2016. Web. 14 Nov 2019.

Vancouver:

Dütsch ML. Stable water isotope fractionation processes in weather systems and their influence on isotopic variability on different time scales. [Internet] [Doctoral dissertation]. ETH Zürich; 2016. [cited 2019 Nov 14]. Available from: http://hdl.handle.net/20.500.11850/58.

Council of Science Editors:

Dütsch ML. Stable water isotope fractionation processes in weather systems and their influence on isotopic variability on different time scales. [Doctoral Dissertation]. ETH Zürich; 2016. Available from: http://hdl.handle.net/20.500.11850/58

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