Impact of potential climate change on plant available soil water and percolation in the Upper Danube basin

The soil root zone of the land surface provides plants with water for transpiration and therefore biomass production and its excess water percolates downwards and ultimately recharges the groundwater aquifers. Within the project GLOWA-Danube regional scale impacts of climate change on the water cycle are investigated. Potential changes in the water cycle based on climate scenarios for 2011 to 2060 are simulated with the decision support system DANUBIA that integrates models of natural as well as social sciences. This article presents the results of DANUBIA driven by an ensemble of 12 climates scenarios generated with a stochastic climate simulator regarding the future state of soil moisture and groundwater recharge in the Upper Danube basin

Spectral energy distribution of super-Eddington flows

Spectral properties of super-Eddington accretion flows are investigated by means of a parallel line-of-sight calculation. The subjacent model, taken from two-dimensional radiation hydrodynamic simulations by Ohsuga et al. (2005), consists of a disc accretion region and an extended atmosphere with high velocity outflows. The non-gray radiative transfer equation is solved, including relativistic effects, by applying the FLD approximation. The calculated spectrum is composed of a thermal, blackbody-like emission from the disc which depends sensitively on the inclination angle, and of high energy X-ray and gamma-ray emission from the atmosphere. We find mild beaming effects in the thermal radiation for small inclination angles. If we compare the face-on case with the edge-on case, the average photon energy is larger by a factor of ~1.7 due mainly to Doppler boosting, while the photon number density is larger by a factor of ~3.7 due mainly to anisotropic matter distribution around the central black hole. This gives an explanation for the observed X-ray temperatures of ULXs which are too high to be explained in the framework of intermediate-mass black holes. While the main features of the thermal spectral component are consistent with more detailed calculations of slim accretion discs, the atmosphere induces major changes in the high-energy part, which cannot be reproduced by existing models. In order to interpret observational data properly, simple approaches like the Eddington-Barbier approximation cannot be applied.Comment: 10 pages, 8 figures, accepted for publication in MNRA

Black hole accretion: theoretical limits and observational implications

Recently, the issue of the role of the Eddington limit in accretion discs became a matter of debate. While the classical (spherical) Eddington limit is certainly an over-simplification, it is not really clear how to treat it in a flattened structure like an accretion disc. We calculate the critical accretion rates and resulting disc luminosities for various disc models corresponding to the classical Eddington limit by equating the attractive and repulsive forces locally. We also discuss the observational appearance of such highly accreting systems by analyzing their spectral energy distributions. Our calculations indicate that the allowed mass accretion rates differ considerably from what one expects by applying the Eddington limit in its classical form, while the luminosities only weakly exceed their classical equivalent. Depending on the orientation of the disc relative to the observer, mild relativistic beaming turns out to have an important influence on the disc spectra. Thus, possible super-Eddington accretion, combined with mild relativistic beaming, supports the idea that ultraluminous X-ray sources host stellar mass black holes and accounts partially for the observed high temperatures of these objects.Comment: to appear in "Black Holes: from Stars to Galaxies" Proceedings IAU Symp. No. 238, eds. V. Karas & G. Matt; 4 pages, 2 figures, needs iaus.cl

Black hole accretion disks : sources of viscosity and signatures of super-Eddington accretion

We study the role of convection in black hole accretion flows. We investigate the influence of convection on the energy transport as well as the effect of convective turbulence on the disk’s viscosity. The results reveal that convection supports the radiative energy transport efficiently in massless disks, while it can turn into a negative feedback if self-gravity becomes important. Convective turbulence adds significantly to the total viscosity, but cannot account for it on its own. In the second part, we study the spectral energy distribution of super-Eddington accretion flows onto a black hole, based on 2D RHD simulation data. We model the continuum emission as well as the iron K line emission and absorption features with a ray-tracing radiative transfer code. We find that mild relativistic beaming effects become important, leading to super-Eddington luminosities for face-on seen disks. We confirm the diagnostic power of the iron K lines on the accretion process in the inner disk region, finding a strong correlation between the central black hole mass and the ratio of the Kβ to the Kα lines. We also detect a trend of line broadening for edge-on seen disks

Towards convection-resolving, global atmospheric simulations with the Model for Prediction Across Scales (MPAS) v3.1: an extreme scaling experiment

The Model for Prediction Across Scales (MPAS) is a novel set of Earth system simulation components and consists of an atmospheric model, an ocean model and a land-ice model. Its distinct features are the use of unstructured Voronoi meshes and C-grid discretisation to address shortcomings of global models on regular grids and the use of limited area models nested in a forcing data set, with respect to parallel scalability, numerical accuracy and physical consistency. This concept allows one to include the feedback of regional land use information on weather and climate at local and global scales in a consistent way, which is impossible to achieve with traditional limited area modelling approaches. Here, we present an in-depth evaluation of MPAS with regards to technical aspects of performing model runs and scalability for three medium-size meshes on four different high-performance computing (HPC) sites with different architectures and compilers.We uncover model limitations and identify new aspects for the model optimisation that are introduced by the use of unstructured Voronoi meshes.We further demonstrate the model performance of MPAS in terms of ist capability to reproduce the dynamics of the West African monsoon (WAM) and its associated precipitation in a pilot study. Constrained by available computational resources, we compare 11-month runs for two meshes with observations and a reference simulation from the Weather Research and Forecasting (WRF) model. We show that MPAS can reproduce the atmospheric dynamics on global and local scales in this experiment, but identify a precipitation excess for the West African region. Finally, we conduct extreme scaling tests on a global 3 km mesh with more than 65 million horizontal grid cells on up to half a million cores. We discuss necessary modifications of the model code to improve its parallel performance in general and specific to the HPC environment. We confirm good scaling (70% parallel efficiency or better) of the MPAS model and provide numbers on the computational requirements for experiments with the 3 km mesh. In doing so, we show that global, convection-resolving atmospheric simulations with MPAS are within reach of current and next generations of high-end computing facilities