Simulating field-aligned diffusion of a cosmic ray gas     

Andrew P. Snodin  Axel Brandenburg  Antony J. Mee  Anvar Shukurov 《Monthly notices of the Royal Astronomical Society》2006,373(2):643-652

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Geochemistry of drill core samples from Yaxcopoil‐1, Chicxulub impact crater,Mexico     

Ralf T. Schmitt  Axel Wittmann  Dieter Stffler 《Meteoritics & planetary science》2004,39(6):979-1001

Abstract— The chemical composition of suevites, displaced Cretaceous target rocks, and impact‐generated dikes within these rocks from the Yaxcopoil‐1 (Yax‐1) drill core, Chicxulub impact crater, Mexico, is reported and compared with the data from the Yucatán 6 (Y6) samples. Within the six suevite subunits of Yax‐1, four units with different chemical compositions can be distinguished: a) upper/lower sorted and upper suevite (depth of 795–846 m); b) middle suevite (depth of 846–861 m); c) brecciated impact melt rock (depth of 861–885 m); and d) lower suevite (depth of 885–895 m). The suevite sequence (a), (b), and (d) display an increase of the CaO content and a decrease of the silicate basement component from top to bottom. In contrast, the suevite of Y6 shows an inverse trend. The different distances of the Yax‐1 and Y6 drilling sites from the crater center (～60, and ～47 km, respectively) lead to different suevite sequences. Within the Cretaceous rocks of Yax‐1, a suevitic dike (depth of ～916 m) does not display chemical differences when compared with the suevite, while an impact melt rock dike (depth of ～1348 m) is significantly enriched in immobile elements. A clastic breccia dike (depth of ～1316 m) is dominated by material derived locally from the host rock, while the silicate‐rich component is similar to that found in the suevite. Significant enrichments of the K₂O content were observed in the Yax‐1 suevite and the impact‐generated dikes. All impactites of Yax‐1 and Y6 are mixtures of a crystalline basement and a carbonate component from the sedimentary cover. An anhydrite component in the impactites is missing (Yax‐1) or negligible (Y6).  相似文献   

Rainfall‐runoff modelling for assessing impacts of climate and land‐use change     

Axel Bronstert 《水文研究》2004,18(3):567-570

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On the relation between the Sun's (geocentric) angular diameter and the observed (topocentric) ‘drift time’     

Axel D. Wittmann  Heinz Neckel 《Solar physics》1996,163(1):1-5

It is pointed out that the relation between the Sun's geocentric diameter and the so-called drift time observed on the Earth's surface does not depend on topocentric data (distance, declination, parallax, refraction), but only on the geocentric values of distance, declination, and variation of right ascension.  相似文献   

Decadal predictability of soil water,vegetation, and wildfire frequency over North America     

Yoshimitsu Chikamoto  Axel Timmermann  Samantha Stevenson  Pedro DiNezio  Sally Langford 《Climate Dynamics》2015,45(7-8):2213-2235

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Comparison of hydrodynamic models of different complexities to model floods with emergency storage areas     

Chandranath Chatterjee  Saskia Förster  Axel Bronstert 《水文研究》2008,22(24):4695-4709

A flood emergency storage area (polder) is used to reduce the flood peak in the main river and hence, protect downstream areas from being inundated. In this study, the effectiveness of a proposed flood emergency storage area at the middle Elbe River, Germany in reducing the flood peaks is investigated using hydrodynamic modelling. The flow to the polders is controlled by adjustable gates. The extreme flood event of August 2002 is used for the study. A fully hydrodynamic 1D model and a coupled 1D–2D model are applied to simulate the flooding and emptying processes in the polders and flow in the Elbe River. The results obtained from the 1D and 1D–2D models are compared with respect to the peak water level reductions in the Elbe River and flow processes in the polders during their filling and emptying. The computational time, storage space requirements and modelling effort for the two models are also compared. It is concluded that a 1D model may be used to study the water level and discharge reductions in the main river while a 1D-2D model may be used when the study of flow dynamics in the polder is of particular interest. Further, a detailed sensitivity analysis of the 1D and 1D–2D models is carried out with respect to Manning's n values, DEMs of different resolutions, number of cross-sections used and the gate opening time as well as gate opening/closing duration. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

The effects of spiral arms on the multi-phase ISM     

Anvar Shukurov  Graeme R. Sarson  Åke Nordlund  Boris Gudiksen  Axel Brandenburg 《Astrophysics and Space Science》2004,289(3-4):319-322

Statistical parameters of the ISM driven by thermal energy injectionsfrom supernova explosions have been obtained from 3D, nonlinear,magnetohydrodynamic, shearing-box simulations for spiral arm andinterarm regions. The density scale height obtained for the interarm regionsis 50% larger than within the spiral arms because of thehigher gas temperature. The filling factorof the hot gas is also significantly larger between the armsand depends sensitively on magnetic field strength.  相似文献   

Potentials and constraints of different types of soil moisture observations for flood simulations in headwater catchments     

Axel Bronstert  Benjamin Creutzfeldt  Thomas Graeff  Irena Hajnsek  Maik Heistermann  Sibylle Itzerott  Thomas Jagdhuber  David Kneis  Erika Lück  Dominik Reusser  Erwin Zehe 《Natural Hazards》2012,60(3):879-914

Flood generation in mountainous headwater catchments is governed by rainfall intensities, by the spatial distribution of rainfall and by the state of the catchment prior to the rainfall, e.g. by the spatial pattern of the soil moisture, groundwater conditions and possibly snow. The work presented here explores the limits and potentials of measuring soil moisture with different methods and in different scales and their potential use for flood simulation. These measurements were obtained in 2007 and 2008 within a comprehensive multi-scale experiment in the Weisseritz headwater catchment in the Ore-Mountains, Germany. The following technologies have been applied jointly thermogravimetric method, frequency domain reflectometry (FDR) sensors, spatial time domain reflectometry (STDR) cluster, ground-penetrating radar (GPR), airborne polarimetric synthetic aperture radar (polarimetric SAR) and advanced synthetic aperture radar (ASAR) based on the satellite Envisat. We present exemplary soil measurement results, with spatial scales ranging from point scale, via hillslope and field scale, to the catchment scale. Only the spatial TDR cluster was able to record continuous data. The other methods are limited to the date of over-flights (airplane and satellite) or measurement campaigns on the ground. For possible use in flood simulation, the observation of soil moisture at multiple scales has to be combined with suitable hydrological modelling, using the hydrological model WaSiM-ETH. Therefore, several simulation experiments have been conducted in order to test both the usability of the recorded soil moisture data and the suitability of a distributed hydrological model to make use of this information. The measurement results show that airborne-based and satellite-based systems in particular provide information on the near-surface spatial distribution. However, there are still a variety of limitations, such as the need for parallel ground measurements (Envisat ASAR), uncertainties in polarimetric decomposition techniques (polarimetric SAR), very limited information from remote sensing methods about vegetated surfaces and the non-availability of continuous measurements. The model experiments showed the importance of soil moisture as an initial condition for physically based flood modelling. However, the observed moisture data reflect the surface or near-surface soil moisture only. Hence, only saturated overland flow might be related to these data. Other flood generation processes influenced by catchment wetness in the subsurface such as subsurface storm flow or quick groundwater drainage cannot be assessed by these data. One has to acknowledge that, in spite of innovative measuring techniques on all spatial scales, soil moisture data for entire vegetated catchments are still today not operationally available. Therefore, observations of soil moisture should primarily be used to improve the quality of continuous, distributed hydrological catchment models that simulate the spatial distribution of moisture internally. Thus, when and where soil moisture data are available, they should be compared with their simulated equivalents in order to improve the parameter estimates and possibly the structure of the hydrological model.  相似文献   

Effects on geodetic VLBI measurements due to polarization leakage in S/X receivers     

Alessandra Bertarini  Alan L. Roy  Brian Corey  R. Craig Walker  Walter Alef  Axel Nothnagel 《Journal of Geodesy》2011,85(10):715-721

Geodetic very long baseline interferometry (VLBI) delivers time series of station positions and Earth orientation parameters. These series offer a viable and precise way to study Earth crustal and core dynamics and to support space navigation. Their accuracy is degraded by instrumental errors, of which polarization leakage is considered to be one of the largest that is not yet being addressed. Its effect on the data can be corrected, provided one knows the leakage characteristics of the receivers. For this purpose, we designed a VLBI session to measure the polarization leakage at 15 geodetic and very long baseline array stations over the frequency range from 8,212.99 to 8,932.99 MHz. We describe the polarization leakage measurements and the algorithm that was implemented to correct for its effect on the geodetic delay observables. Subsequently, we applied the correction for polarization leakage to the same data that were used to determine the leakage and checked for the resulting improvement. From the measured leakage terms, one would expect polarization leakage to affect the group delay measurements in 90% of the cases by 1.6 ps or less. This proved to be below the statistical noise in our single VLBI session, and hence, an improvement from the correction could not be detected. Applying this analysis in the context of VLBI2010, we provide a specification for the allowable polarization leakage to achieve the target submillimetre accuracy.  相似文献   

Regime-dependent evaluation of accumulated precipitation in COSMO     

Tom Akkermans  Tim B?hme  Matthias Demuzere  Susanne Crewell  Christoph Selbach  Thorsten Reinhardt  Axel Seifert  Felix Ament  Nicole P. M. van Lipzig 《Theoretical and Applied Climatology》2012,108(1-2):39-52

Regime-dependent evaluation is a relatively new approach to assess model performance. It consists of classifying the model biases according to a discrete number of regimes and evaluating model output within each regime. In this paper, the regimes are firstly defined by the large-scale atmospheric circulation, based on the objective Jenkinson-Collison classification technique which distinguishes synoptic patterns by strength, direction and vorticity of the geostrophic flow. Eight directional and two vorticity circulation regimes (circulation types) are specified. In this way, it is possible to quantify the model performance for cases with for example westerly winds only, or with cyclonic circulation only. A second regime classification is based on temperature, which allows for detection of temperature-dependent model performance. Modelled accumulated precipitation (mm/6?h) is evaluated with rain gauges for the years 2007 and 2008. Two variants of the COSMO model are evaluated: a fine-resolution version (2.8?km, COSMO-DE) and a coarse-resolution version (7?km, COSMO-EU). In COSMO-EU, a windward/leeward effect becomes visible since circulation is related to dominant wind direction, hence to windward and lee side of orography. In COSMO-DE, no circulation dependent but a height-related bias is identified and further explored, making use of temperature-dependent evaluation which unveils a positive model bias related to solid precipitation.  相似文献