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  • 1
    Language: English
    In: Physical Review B, 2/2004, Vol.69(6)
    Description: Using a series expansion based on the flow-equation method we study the ground state energy and the elementary triplet excitations of a generalized model of crossed spin-1/2 chains starting from the limit of decoupled quadrumers. The triplet dispersion is shown to be very sensitive to the inter-quadrumer frustration, exhibiting a line of almost complete localization as well as lines of quantum phase transitions limiting the stability of the valence-bond solid phase. In the vicinity of the checkerboard-point a finite window of exchange couplings is found with a non-zero spin-gap, consistent with known results from exact diagonalization. The ground state energy is lower than that of the bare quadrumer case for all exchange couplings investigated. In the limiting situation of the fully frustrated checkerboard magnet our results agree with earlier series expansion studies.
    Keywords: Physics;
    ISSN: 1098-0121
    E-ISSN: 1550-235X
    Source: American Physical Society (APS) (via CrossRef)
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  • 2
    In: Tellus A, 03/2011
    ISSN: 0280-6495
    E-ISSN: 1600-0870
    Source: CrossRef
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  • 3
    Language: English
    In: Tellus A: Dynamic Meteorology and Oceanography, 01 January 2011, Vol.63(2), pp.263-282
    Description: A forward operator for Global Positioning System (GPS) slant total delay (STD) data and its adjoint were implemented into the Mesoscale Model version 5 (MM5) 4DVAR system to investigate its impact on quantitative precipitation forecasting (QPF)....
    Keywords: Meteorology & Climatology
    E-ISSN: 1600-0870
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  • 4
    Language: English
    In: Quarterly Journal of the Royal Meteorological Society, 2012, Vol.138(667), pp.1652-1667
    Description: The Convective and Orographically-driven Precipitation Study (COPS) carried out in summer 2007 over northeastern France and southwestern Germany provided a fairly comprehensive description of the low-troposphere water-vapour field, thanks in particular to the deployment of two airborne...
    Keywords: Sciences of the Universe ; Earth Sciences ; Meteorology ; Leandre 2 ; Wales ; Arome ; Vera ; Meteorology & Climatology
    ISSN: 0035-9009
    E-ISSN: 1477-870X
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  • 5
    In: Water Resources Research, February 2014, Vol.50(2), pp.1337-1356
    Description: Interactions between the soil, the vegetation, and the atmospheric boundary layer require close attention when predicting water fluxes in the hydrogeosystem, agricultural systems, weather, and climate. However, land‐surface schemes used in large‐scale models continue to show deficiencies in consistently simulating fluxes of water and energy from the subsurface through vegetation layers to the atmosphere. In this study, the multiphysics version of the Noah land‐surface model (Noah‐MP) was used to identify the processes, which are most crucial for a simultaneous simulation of water and heat fluxes between land surface and the lower atmosphere. Comprehensive field data sets of latent and sensible heat fluxes, ground heat flux, soil moisture, and leaf area index from two contrasting field sites in South‐West Germany are used to assess the accuracy of simulations. It is shown that an adequate representation of vegetation‐related processes is the most important control for a consistent simulation of energy and water fluxes in the soil‐plant‐atmosphere system. In particular, using a newly implemented submodule to simulate root growth dynamics has enhanced the performance of Noah‐MP. We conclude that further advances in the representation of leaf area dynamics and root/soil moisture interactions are the most promising starting points for improving the simulation of feedbacks between the subsoil, land surface and atmosphere in fully coupled hydrological and atmospheric models. Selecting different model options strongly influences accuracy of simulations The ensemble size can be reduced by constraining Noah‐MP to different data types Considering dynamics of root growth results in more accurate simulations
    Keywords: Land‐Surface Model ; Structural Uncertainty ; Root Water Uptake ; Latent And Sensible Heat ; Ground Heat Flux ; Soil Moisture
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 6
    Language: English
    In: Journal of Magnetism and Magnetic Materials, May 2004, Vol.272-276, pp.916-917
    Keywords: Quantum Magnets ; Statistical Mechanics ; Physics
    ISSN: 0304-8853
    Source: ScienceDirect Journals (Elsevier)
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  • 7
    Description: Using a series expansion based on the flow-equation method we study the ground state energy and the elementary triplet excitations of a generalized model of crossed spin-1/2 chains starting from the limit of decoupled quadrumers. The triplet dispersion is shown to be very sensitive to the inter-quadrumer frustration, exhibiting a line of almost complete localization as well as lines of quantum phase transitions limiting the stability of the valence-bond solid phase. In the vicinity of the checkerboard-point a finite window of exchange couplings is found with a non-zero spin-gap, consistent with known results from exact diagonalization. The ground state energy is lower than that of the bare quadrumer case for all exchange couplings investigated. In the limiting situation of the fully frustrated checkerboard magnet our results agree with earlier series expansion studies. Comment: 8 pages, 7 figures
    Keywords: Condensed Matter - Strongly Correlated Electrons
    Source: Cornell University
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  • 8
    Language: English
    In: Environmental Earth Sciences, 2013, Vol.69(2), pp.317-333
    Description: Sustainable water quality management requires a profound understanding of water fluxes (precipitation, run-off, recharge, etc.) and solute turnover such as retention, reaction, transformation, etc. at the catchment or landscape scale. The Water and Earth System Science competence cluster (WESS, http://www.wess.info/ ) aims at a holistic analysis of the water cycle coupled to reactive solute transport, including soil–plant–atmosphere and groundwater–surface water interactions. To facilitate exploring the impact of land-use and climate changes on water cycling and water quality, special emphasis is placed on feedbacks between the atmosphere, the land surface, and the subsurface. A major challenge lies in bridging the scales in monitoring and modeling of surface/subsurface versus atmospheric processes. The field work follows the approach of contrasting catchments, i.e. neighboring watersheds with different land use or similar watersheds with different climate. This paper introduces the featured catchments and explains methodologies of WESS by selected examples.
    Keywords: Water and solute fluxes ; Water quality ; Catchments ; Land-surface atmosphere exchange ; Processes and feedbacks ; Modeling ; Monitoring
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 9
    Language: English
    In: Meteorologische Zeitschrift, 01 December 2008, Vol.17(6), pp.867-885
    Description: This article describes the development of tools for routine 4-dimensional variational data assimilation of Global Positioning System Slant Total Delay (STD) data in the framework of the MM5 system at the Institute of Physics and Meteorology of the University of Hohenheim. The Slant Total Delay forward operator is introduced which allows model validation and the assimilation in the Message-Passing Interface environment. An experiment is conducted which highlights the importance of accurate model physics in the variational assimilation system. We demonstrate that the model minus observation statistics of STD data crucially depends on the convection scheme and the implementation of horizontal diffusion. A set of modifications to the existing non linear, tangent linear and adjoint model is presented. These include an improvement of the horizontal diffusion scheme and the implementation of the Grell cumulus convective scheme in order to eliminate the observed systematic tendency in the model minus observation statistics of the STD data and precipitation in mountainous terrain. A first assimilation experiment with the improved MM5 variational assimilation system shows promising results.
    Keywords: Meteorology & Climatology
    ISSN: 0941-2948
    E-ISSN: 1610-1227
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  • 10
    Language: English
    In: Quarterly Journal of the Royal Meteorological Society, 137(S1):3-30, 24 February 2011, Vol.137(S1)
    Description: Within the frame of the international field campaign COPS (Convective and Orographically-induced Precipitation Study), a large suite of state-of-the-art meteorological instrumentation was operated, partially combined for the first time. The COPS field phase was performed from 01 June - 31 August 2007 in a low-mountain area in southwestern Germany/eastern France covering the Vosges Mountains, the Rhine valley and the Black Forest Mountains. The collected data set covers the entire evolution of convective precipitation events in complex terrain from their initiation, to their development and mature phase up to their decay. 18 Intensive Operation Periods (IOPs) with 34 operation days and 8 additional Special Observation Periods (SOPs) were performed providing a comprehensive data set covering different forcing conditions. In this paper an overview of the COPS scientific strategy, the field phase, and its first accomplishments is given. Some highlights of the campaign are illustrated with several measurement examples. It is demonstrated that COPS provided new insight in key processes leading to convection initiation and to the modification of precipitation by orography, in the improvement of QPF by the assimilation of new observations, and in the performance of ensembles of convection permitting models in complex terrain.
    Keywords: Environmental Sciences ; Complex Terrain ; Convection ; Performance ; Atmospheric Precipitations ; Field Tests ; Climate Models ; Meteorology & Climatology ; Environmental Sciences
    ISSN: 0035-9009
    E-ISSN: 1477-870X
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