@phdthesis{Sommerfeld2015,
  author    = {Sommerfeld, Anja},
  title     = {Quantification of internal variability of the arctic summer atmosphere based on HIRHAM5 ensemble simulations},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-85347},
  school      = {Universit{\"a}t Potsdam},
  pages     = {VII, 110, vi},
  year      = {2015},
  abstract  = {The non-linear behaviour of the atmospheric dynamics is not well understood and makes the evaluation and usage of regional climate models (RCMs) difficult. Due to these non-linearities, chaos and internal variability (IV) within the RCMs are induced, leading to a sensitivity of RCMs to their initial conditions (IC). The IV is the ability of RCMs to realise different solutions of simulations that differ in their IC, but have the same lower and lateral boundary conditions (LBC), hence can be defined as the across-member spread between the ensemble members. For the investigation of the IV and the dynamical and diabatic contributions generating the IV four ensembles of RCM simulations are performed with the atmospheric regional model HIRHAM5. The integration area is the Arctic and each ensemble consists of 20 members. The ensembles cover the time period from July to September for the years 2006, 2007, 2009 and 2012. The ensemble members have the same LBC and differ in their IC only. The different IC are arranged by an initialisation time that shifts successively by six hours. Within each ensemble the first simulation starts on 1st July at 00 UTC and the last simulation starts on 5th July at 18 UTC and each simulation runs until 30th September. The analysed time period ranges from 6th July to 30th September, the time period that is covered by all ensemble members. The model runs without any nudging to allow a free development of each simulation to get the full internal variability within the HIRHAM5. As a measure of the model generated IV, the across-member standard deviation and the across-member variance is used and the dynamical and diabatic processes influencing the IV are estimated by applying a diagnostic budget study for the IV tendency of the potential temperature developed by Nikiema and Laprise [2010] and Nikiema and Laprise [2011]. The diagnostic budget study is based on the first law of thermodynamics for potential temperature and the mass-continuity equation. The resulting budget equation reveals seven contributions to the potential temperature IV tendency. As a first study, this work analyses the IV within the HIRHAM5. Therefore, atmospheric circulation parameters and the potential temperature for all four ensemble years are investigated. Similar to previous studies, the IV fluctuates strongly in time. Further, due to the fact that all ensemble members are forced with the same LBC, the IV depends on the vertical level within the troposphere, with high values in the lower troposphere and at 500 hPa and low values in the upper troposphere and at the surface. By the same reason, the spatial distribution shows low values of IV at the boundaries of the model domain. The diagnostic budget study for the IV tendency of potential temperature reveals that the seven contributions fluctuate in time like the IV. However, the individual terms reach different absolute magnitudes. The budget study identifies the horizontal and vertical 'baroclinic' terms as the main contributors to the IV tendency, with the horizontal 'baroclinic' term producing and the vertical 'baroclinic' term reducing the IV. The other terms fluctuate around zero, because they are small in general or are balanced due to the domain average. The comparison of the results obtained for the four different ensembles (summers 2006, 2007, 2009 and 2012) reveals that on average the findings for each ensemble are quite similar concerning the magnitude and the general pattern of IV and its contributions. However, near the surface a weaker IV is produced with decreasing sea ice extent. This is caused by a smaller impact of the horizontal 'baroclinic' term over some regions and by the changing diabatic processes, particularly a more intense reducing tendency of the IV due to condensative heating. However, it has to be emphasised that the behaviour of the IV and its dynamical and diabatic contributions are influenced mainly by complex atmospheric feedbacks and large-scale processes and not by the sea ice distribution. Additionally, a comparison with a second RCM covering the Arctic and using the same LBCs and IC is performed. For both models very similar results concerning the IV and its dynamical and diabatic contributions are found. Hence, this investigation leads to the conclusion that the IV is a natural phenomenon and is independent from the applied RCM.},
  language  = {en}
}
@phdthesis{Sander2015,
  author    = {Sander, Andreas Alexander Christoph},
  title     = {Radiatively driven winds of hot stars},
  school      = {Universit{\"a}t Potsdam},
  pages     = {153},
  year      = {2015},
  language  = {en}
}
@phdthesis{Bittermann2015,
  author    = {Bittermann, Klaus},
  title     = {Semi-empirical sea-level modelling},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-93881},
  school      = {Universit{\"a}t Potsdam},
  pages     = {v, 88},
  year      = {2015},
  abstract  = {Semi-empirical sea-level models (SEMs) exploit physically motivated empirical relationships between global sea level and certain drivers, in the following global mean temperature. This model class evolved as a supplement to process-based models (Rahmstorf (2007)) which were unable to fully represent all relevant processes. They thus failed to capture past sea-level change (Rahmstorf et al. (2012)) and were thought likely to underestimate future sea-level rise. Semi-empirical models were found to be a fast and useful tool for exploring the uncertainties in future sea-level rise, consistently giving significantly higher projections than process-based models. In the following different aspects of semi-empirical sea-level modelling have been studied. Models were first validated using various data sets of global sea level and temperature. SEMs were then used on the glacier contribution to sea level, and to infer past global temperature from sea-level data via inverse modelling. Periods studied encompass the instrumental period, covered by tide gauges (starting 1700 CE (Common Era) in Amsterdam) and satellites (first launched in 1992 CE), the era from 1000 BCE (before CE) to present, and the full length of the Holocene (using proxy data). Accordingly different data, model formulations and implementations have been used. It could be shown in Bittermann et al. (2013) that SEMs correctly predict 20th century sea-level when calibrated with data until 1900 CE. SEMs also turned out to give better predictions than the Intergovernmental Panel on Climate Change (IPCC) 4th assessment report (AR4, IPCC (2007)) models, for the period from 1961-2003 CE. With the first multi-proxy reconstruction of global sea-level as input, estimate of the human-induced component of modern sea-level change and projections of future sea-level rise were calculated (Kopp et al. (2016)). It turned out with 90\% confidence that more than 40 \% of the observed 20th century sea-level rise is indeed anthropogenic. With the new semi-empirical and IPCC (2013) 5th assessment report (AR5) projections the gap between SEM and process-based model projections closes, giving higher credibility to both. Combining all scenarios, from strong mitigation to business as usual, a global sea-level rise of 28-131 cm relative to 2000 CE, is projected with 90\% confidence. The decision for a low carbon pathway could halve the expected global sea-level rise by 2100 CE. Present day temperature and thus sea level are driven by the globally acting greenhouse-gas forcing. Unlike that, the Milankovich forcing, acting on Holocene timescales, results mainly in a northern-hemisphere temperature change. Therefore a semi-empirical model can be driven with northernhemisphere temperatures, which makes it possible to model the main subcomponent of sea-level change over this period. It showed that an additional positive constant rate of the order of the estimated Antarctic sea-level contribution is then required to explain the sea-level evolution over the Holocene. Thus the global sea level, following the climatic optimum, can be interpreted as the sum of a temperature induced sea-level drop and a positive long-term contribution, likely an ongoing response to deglaciation coming from Antarctica.},
  language  = {en}
}
@phdthesis{Maerten2015,
  author    = {Maerten, Lena},
  title     = {Spectroscopic perspectives on ultrafast coupling phenomena in perovskite oxides},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-77623},
  school      = {Universit{\"a}t Potsdam},
  year      = {2015},
  abstract  = {In this thesis, I study ultrafast dynamics in perovskite oxides using time resolved broadband spectroscopy. I focus on the observation of coherent phonon propagation by time resolved Brillouin scattering: following the excition of metal transducer films with a femtosecond infrared pump pulse, coherent phonon dynamics in the GHz frequency range are triggered. Their propagation is monitored using a delayed white light probe pulse. The technique is illustrated on various thin films and multilayered samples. I apply the technique to investigate the linear and nonlinear acoustic response in bulk SrTiO_3, which displays a ferroelastic phase transition from a cubic to a tetragonal structural phase at T_a=105 K. In the linear regime, I observe a coupling of the observed acoustic phonon mode to the softening optic modes describing the phase transition. In the nonlinear regime, I find a giant slowing down of the sound velocity in the low temperature phase that is only observable for a strain amplitude exceeding the tetragonality of the material. It is attributed to a coupling of the high frequency phonons to ferroelastic domain walls in the material. I propose a new mechanism for the coupling of strain waves to the domain walls that is only effective for high amplitude strain. A detailed study of the phonon attenuation across a wide temperature range shows that the phonon attenuation at low temperatures is influenced by the domain configuration, which is determined by interface strain. Preliminary measurements on magnetic-ferroelectric multilayers reveal that the excitation fluence needs to be carefully controlled when dynamics at phase transitions are studied.},
  language  = {en}
}
@phdthesis{Vlasov2015,
  author    = {Vlasov, Vladimir},
  title     = {Synchronization of oscillatory networks in terms of global variables},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-78182},
  school      = {Universit{\"a}t Potsdam},
  pages     = {82},
  year      = {2015},
  abstract  = {Synchronization of large ensembles of oscillators is an omnipresent phenomenon observed in different fields of science like physics, engineering, life sciences, etc. The most simple setup is that of globally coupled phase oscillators, where all the oscillators contribute to a global field which acts on all oscillators. This formulation of the problem was pioneered by Winfree and Kuramoto. Such a setup gives a possibility for the analysis of these systems in terms of global variables. In this work we describe nontrivial collective dynamics in oscillator populations coupled via mean fields in terms of global variables. We consider problems which cannot be directly reduced to standard Kuramoto and Winfree models. In the first part of the thesis we adopt a method introduced by Watanabe and Strogatz. The main idea is that the system of identical oscillators of particular type can be described by a low-dimensional system of global equations. This approach enables us to perform a complete analytical analysis for a special but vast set of initial conditions. Furthermore, we show how the approach can be expanded for some nonidentical systems. We apply the Watanabe-Strogatz approach to arrays of Josephson junctions and systems of identical phase oscillators with leader-type coupling. In the next parts of the thesis we consider the self-consistent mean-field theory method that can be applied to general nonidentical globally coupled systems of oscillators both with or without noise. For considered systems a regime, where the global field rotates uniformly, is the most important one. With the help of this approach such solutions of the self-consistency equation for an arbitrary distribution of frequencies and coupling parameters can be found analytically in the parametric form, both for noise-free and noisy cases. We apply this method to deterministic Kuramoto-type model with generic coupling and an ensemble of spatially distributed oscillators with leader-type coupling. Furthermore, with the proposed self-consistent approach we fully characterize rotating wave solutions of noisy Kuramoto-type model with generic coupling and an ensemble of noisy oscillators with bi-harmonic coupling. Whenever possible, a complete analysis of global dynamics is performed and compared with direct numerical simulations of large populations.},
  language  = {en}
}
@phdthesis{Hainich2015,
  author    = {Hainich, Rainer},
  title     = {The Wolf-Rayet stars of the nitrogen sequence in environments of different metallicities},
  school      = {Universit{\"a}t Potsdam},
  pages     = {161},
  year      = {2015},
  language  = {en}
}