Stable water isotope fractionation processes in weather systems and their influence on isotopic variability on different time scales
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2016
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Doctoral Thesis
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Abstract
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 Europe for the years 2002 – 2011, with input data from two different general circulation models (IsoGSM and ECHAMwiso). The nested COSMOiso simulations show an improved performance compared to the general circulation models, underlining the added value of simulating stable water isotopes with high resolution numerical models. Linear correlations of δ2H and deuterium excess with meteorological variables highlight the important role of temperature and relative humidity for the variability of δ2H and deuterium excess, respectively. Furthermore, a new Lagrangian method for quantifying the impact of different processes on isotopes along trajectories shows that evaporation from the ocean, evaporation from land, and mixing with moister air are the most important processes determining climatological mean δ2H and deuterium excess at low levels, while cloud formation processes are important for their variability.
In summary, this thesis applies different numerical model simulations to disentangle the complex interactions of meteorological processes involving isotopic fractionation, and provides new insight into how they shape the isotopic composition of water vapour and precipitation. This is a step towards a better understanding of the mechanisms responsible for isotopic variations in atmospheric moisture and towards the overall aim of using stable water isotopes as tracers of key processes in the atmospheric water cycle.
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Examiner: Wernli, Heini
Examiner : Pfahl, Stephan
Examiner : Sodemann, Harald
Examiner : Werner, Martin
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ETH Zurich
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Subject
Stable water isotopes; Fractionation; Water cycle; Weather systems; Extratropical cyclones
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03854 - Wernli, Johann Heinrich / Wernli, Johann Heinrich
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Is variant form of: 10.1002/2016GL068600