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

Candidate carriers and synthetic spectra of the 21- and 30-mu protoplanetary nebular bands

Computational chemistry is used here to determine the vibrational line spectrum of several candidate molecules. It is shown that the thiourea functional group, associated with various carbonaceous structures (mainly compact and linear aromatic clusters), is able to mimic the 21-$\mu$m band emitted by a number of proto-planetary nebulae. The combination of nitrogen and sulphur in thiourea is the essential source of emission in this model: the band disappears if these species are replaced by carbon. The astronomical 21-$\mu$m feature extends redward to merge with another prominent band peaking between 25 and 30 $\mu$m, also known as the 30-$\mu$m band. It is found that the latter can be modelled by the combined spectra of aliphatic chains, made of CH$_{2}$ groups, oxygen bridges and OH groups, which provide the 30-$\mu$m emission. The absence of oxygen all but extinguishes the 30-$\mu$m emission. The emission between the 21- and 30-$\mu$m bands is provided mainly by thiourea attached to linear aromatic clusters. The chemical software reveals that the essential role of the heteroatoms N, S and O stems from their large electronic charge. It also allows to determine the type of atomic vibration responsible for the different lines of each structure, which helps selecting the most relevant structures. A total of 22 structures have been selected here, but their list is far from being exhaustive; they are only intended as examples of 3 generic classes. When background dust emission is added, model spectra are obtained, which are able to satisfactorily reproduce recent observations of proto-planetary nebulae. The relative numbers of atomic species used in this model are typically H:C:O:N:S=53:36:8:2:1.Comment: 9 pages, 14 figure

Evolution of the Solar Nebula. IX. Gradients in the Spatial Heterogeneity of the Short-Lived Radioisotopes 60^{60}Fe and 26^{26}Al and the Stable Oxygen Isotopes

Short-lived radioisotopes (SLRI) such as $^{60}$Fe and $^{26}$Al were likely injected into the solar nebula in a spatially and temporally heterogeneous manner. Marginally gravitationally unstable (MGU) disks, of the type required to form gas giant planets, are capable of rapid homogenization of isotopic heterogeneity as well as of rapid radial transport of dust grains and gases throughout a protoplanetary disk. Two different types of new models of a MGU disk in orbit around a solar-mass protostar are presented. The first set has variations in the number of terms in the spherical harmonic solution for the gravitational potential, effectively studying the effect of varying the spatial resolution of the gravitational torques responsible for MGU disk evolution. The second set explores the effects of varying the initial minimum value of the Toomre $Q$ stability parameter, from values of 1.4 to 2.5, i.e., toward increasingly less unstable disks. The new models show that the basic results are largely independent of both sets of variations. MGU disk models robustly result in rapid mixing of initially highly heterogeneous distributions of SLRIs to levels of $\sim$ 10% in both the inner ($$ 10 AU) disk regions, and to even lower levels ($\sim$ 2%) in intermediate regions, where gravitational torques are most effective at mixing. These gradients should have cosmochemical implications for the distribution of SLRIs and stable oxygen isotopes contained in planetesimals (e.g., comets) formed in the giant planet region ($\sim$ 5 to $\sim$ 10 AU) compared to those formed elsewhere.Comment: 37 pages, 1 table, 19 figures, ApJ accepte

Feedback of observed interannual vegetation change: a regional climate model analysis for the West African monsoon

West Africa is a hot spot region for land–atmosphere coupling where atmospheric conditions and convective rainfall can strongly depend on surface characteristics. To investigate the effect of natural interannual vegetation changes on the West African monsoon precipitation, we implement satellite-derived dynamical datasets for vegetation fraction (VF), albedo and leaf area index into the Weather Research and Forecasting model. Two sets of 4-member ensembles with dynamic and static land surface description are used to extract vegetation-related changes in the interannual difference between August–September 2009 and 2010. The observed vegetation patterns retain a significant long-term memory of preceding rainfall patterns of at least 2 months. The interannual vegetation changes exhibit the strongest effect on latent heat fluxes and associated surface temperatures. We find a decrease (increase) of rainy hours over regions with higher (lower) VF during the day and the opposite during the night. The probability that maximum precipitation is shifted to nighttime (daytime) over higher (lower) VF is 12 % higher than by chance. We attribute this behaviour to horizontal circulations driven by differential heating. Over more vegetated regions, the divergence of moist air together with lower sensible heat fluxes hinders the initiation of deep convection during the day. During the night, mature convective systems cause an increase in the number of rainy hours over these regions. We identify this feedback in both water- and energy-limited regions of West Africa. The inclusion of observed dynamical surface information improved the spatial distribution of modelled rainfall in the Sahel with respect to observations, illustrating the potential of satellite data as a boundary constraint for atmospheric models

The WASCAL high-resolution regional climate simulation ensemble for West Africa: concept, dissemination and assessment

Climate change and constant population growth pose severe challenges to 21st century rural Africa. Within the framework of the West African Science Service Center on Climate Change and Adapted Land Use (WASCAL), an ensemble of high-resolution regional climate change scenarios for the greater West African region is provided to support the development of effective adaptation and mitigation measures. This contribution presents the overall concept of the WASCAL regional climate simulations, as well as detailed information on the experimental design, and provides information on the format and dissemination of the available data. All data are made available to the public at the CERA long-term archive of the German Climate Computing Center (DKRZ) with a subset available at the PANGAEA Data Publisher for Earth & Environmental Science portal (https://doi.pangaea.de/10.1594/PANGAEA.880512). A brief assessment of the data are presented to provide guidance for future users. Regional climate projections are generated at high (12 km) and intermediate (60 km) resolution using the Weather Research and Forecasting Model (WRF). The simulations cover the validation period 1980–2010 and the two future periods 2020–2050 and 2070–2100. A brief comparison to observations and two climate change scenarios from the Coordinated Regional Downscaling Experiment (CORDEX) initiative is presented to provide guidance on the data set to future users and to assess their climate change signal. Under the RCP4.5 (Representative Concentration Pathway 4.5) scenario, the results suggest an increase in temperature by 1.5 °C at the coast of Guinea and by up to 3 °C in the northern Sahel by the end of the 21st century, in line with existing climate projections for the region. They also project an increase in precipitation by up to 300 mm per year along the coast of Guinea, by up to 150 mm per year in the Soudano region adjacent in the north and almost no change in precipitation in the Sahel. This stands in contrast to existing regional climate projections, which predict increasingly drier conditions. The high spatial and temporal resolution of the data, the extensive list of output variables, the large computational domain and the long time periods covered make this data set a unique resource for follow-up analyses and impact modelling studies over the greater West African region. The comprehensive documentation and standardisation of the data facilitate and encourage their use within and outside of the WASCAL community