ALBERT

All Library Books, journals and Electronic Records Telegrafenberg

X
feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    Electronic Resource
    Electronic Resource
    Horizontal and temporal variability of surface currents in the “Lübecker Bucht”, as measured by radar (1988)
    Springer
    Ocean dynamics 41 (1988), S. 57-74 
    Details
    ISSN: 1616-7228
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Description / Table of Contents: Zusammenfassung Die mit zwei landgestützten Radarstationen (im Dekameterbereich) gemessenen Oberflächenströmungen werden dargestellt. Das Experiment in der Lübecker Bucht (westliche Ostsee) dauerte vom 14. bis 23. April 1983. Karten der zweidimensionalen Strömungen liegen alle 2 Stunden vor. Die Diskussion der Datenverarbeitung konzentriert sich auf eine Methode, die die Bestimmung horizontaler Ableitungen zuläßt. Es werden sowohl Karten der Strömungsvektoren als auch Zeitserien gezeigt, die mit Daten von Strommessern aus verschiedenen Tiefen verglichen werden. Das Verhältnis von Strömungs- zu Windgeschwindigkeit wird für die beiden horizontalen Komponenten an verschiedenen Orten des Meßgebietes berechnet. Divergenz und Rotation werden als Zeitserien dargestellt, und es werden die Beträge der linearen (lokalen) und nichtlinearen (advektiven) Beschleunigung verglichen.
    Abstract: Résumé Les résultats de mesures de courants par 2 stations radar (haute fréquence) installées à terre sont présentés. L'expérimentation fut menée dans la Lübecker Bucht (ouest de la Baltique) du 14 au 23 avril 1983. Des champs 2D de courants furent établis toutes les 2 heures. La discussion sur le traitement des données se concentre sur une méthode permettant la détermination des dérives horizontales. Les cartes de vitesses de courants mesurées à l'aide des radars présentées sous forme de séries temporelles (time séries) sont comparées avec des mesures de courants par courantomètres obtenues à différentes profondeurs. Le rapport courant/vitesse du vent est déterminé en différents points de la zone de mesure pour les deux composantes horizontales. Des séries temporelles donnant la divergence et le rotationnel sont présentées et on donne également une comparaison entre les accélérations linéaires et non linéaires.
    Notes: Summary Surface currents, as measured by two land-based HF (high frequency) radar stations, are presented. The experiment was performed in the Lübecker Bucht (western Baltic Sea) from April 14 to 23, 1983. Maps of two-dimensional currents were obtained every 2 h. The data processing is discussed with emphasis to a method allowing the determination of horizontal derivatives. Maps of the remotely measured current velocities are shown as well as time series, which are compared with data from current meters at different depths. The ratio of current-to-wind speed is determined for both horizontal components at different positions within the measuring area. Time series of divergence and curl are presented, and the magnitude of the linear (local) and nonlinear (advective) acceleration are compared.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02226274
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 2
    facet.materialart.
    Unknown
    Measurements of Surface Currents in the German Bight utilizing High Frequency Radars (2017)
    PANGAEA
    In:  EPIC3Bremerhaven, PANGAEA
    Details
    Publication Date: 2017-07-19
    Repository Name: EPIC Alfred Wegener Institut
    Type: PANGAEA Documentation , notRev
    Format: application/pdf
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    PAPER (OA)
  • 3
    facet.materialart.
    Unknown
    High-Frequency Radars: Beamforming Calibrations Using Ships as Reflectors* (2013)
    American Meteorological Society
    Details
    Publication Date: 2013-03-01
    Description: Linear array antennas and beamforming techniques offer some advantages compared to direction finding using squared arrays. The azimuthal resolution depends on the number of antenna elements and their spacing. Assuming an ideal beam pattern and no amplitude taper across the aperture, 16 antennas in a linear array spaced at half the electromagnetic wavelength theoretically provide a beam resolution of 3.5° normal to the array, and up to twice that when the beam is steered within an azimuthal range of 60° from the direction normal to the array. However, miscalibrated phases among antenna elements, cables, and receivers (e.g., caused by service activities without recalibration) can cause errors in the beam-steering direction and distortions of the beam pattern, resulting in unreliable ocean surface current and wave estimations. The present work uses opportunistic ship echoes randomly received by oceanographic high-frequency radars to correct an unusual case of severe phase differences between receiver channels, leading to a dramatic improvement of the surface current patterns. The method proposed allows for simplified calibrations of phases to account for hardware-related changes without the need to conduct the regular calibration procedure and can be applied during postprocessing of datasets acquired with insufficient calibration.
    Print ISSN: 0739-0572
    Electronic ISSN: 1520-0426
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    PAPER CURRENT
  • 4
    facet.materialart.
    Unknown
    Blending Surface Currents from HF Radar Observations and Numerical Modeling: Tidal Hindcasts and Forecasts (2015)
    American Meteorological Society
    Details
    Publication Date: 2015-02-01
    Description: An observation network operating three Wellen Radars (WERAs) in the German Bight, which are part of the Coastal Observing System for Northern and Arctic Seas (COSYNA), is presented in detail. Major consideration is given to expanding the patchy observations over the entire German Bight on a 1-km grid and producing state estimates at intratidal scales, and 6- and 12-h forecasts. This was achieved with the help of the proposed spatiotemporal optimal interpolation (STOI) method, which efficiently uses observations and simulations from a free model run within an analysis window of one or two tidal cycles. In this way the method maximizes the use of available observations and can be considered as a step toward the “best surface current estimate.” The performance of the analysis was investigated based on the achieved reduction of the misfit between model and observations. The complex dynamics of the study domain was illustrated based on the spatial and temporal changes of tidal ellipses for the M2 and M4 constituents from HF radar observations. It was demonstrated that blending observations and numerical modeling facilitates physical interpretation of processes such as the nonlinear distortion of the Kelvin wave in the coastal zone and in particular in front of the Elbe and Weser estuaries. Comparisons with in situ data acquired outside the area covered by the HF radar demonstrated that the analysis method is able to propagate the HF radar information to larger spatial scales.
    Print ISSN: 0739-0572
    Electronic ISSN: 1520-0426
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    PAPER CURRENT
  • 5
    facet.materialart.
    Unknown
    Tidal currents in the northwestern Adriatic: High-frequency radio observations and numerical model predictions (2007)
    American Geophysical Union
    Details
    Publication Date: 2007-03-27
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
    Published by American Geophysical Union
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    PAPER CURRENT
  • 6
    facet.materialart.
    Unknown
    Ensemble smoother for optimizing tidal boundary conditions by assimilation of high-frequency radar surface currents – application to the German Bight (2009)
    Copernicus
    Details
    Publication Date: 2009-10-26
    Description: High-Frequency (HF) radars measure the ocean currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift. Sequential assimilation methods updating the model state have been proven successful to correct the density-driven currents by assimilation of observations such as sea surface height, sea surface temperature and in-situ profiles. However, the situation is different for tides in coastal models since these are not generated within the domain, but are rather propagated inside the domain through the boundary conditions. For improving the modeled tidal variability it is therefore not sufficient to update the model state via data assimilation without updating the boundary conditions. The optimization of boundary conditions to match observations inside the domain is traditionally achieved through variational assimilation methods. In this work we present an ensemble smoother to improve the tidal boundary values so that the model represents more closely the observed currents. To create an ensemble of dynamically realistic boundary conditions, a cost function is formulated which is directly related to the probability of each perturbation. This cost function ensures that the perturbations are spatially smooth and that the structure of the perturbations satisfies approximately the harmonic linearized shallow water equations. Based on those perturbations an ensemble simulation is carried out using the full three-dimension General Estuarine Ocean Model (GETM). Optimized boundary values are obtained using all observations within the assimilation period using the covariances of the ensemble simulation.
    Print ISSN: 1812-0806
    Electronic ISSN: 1812-0822
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    PAPER CURRENT
  • 7
    facet.materialart.
    Unknown
    Ensemble perturbation smoother for optimizing tidal boundary conditions by assimilation of High-Frequency radar surface currents – application to the German Bight (2010)
    Copernicus
    Details
    Publication Date: 2010-02-04
    Description: High-Frequency (HF) radars measure the ocean surface currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift. Sequential assimilation methods updating the model state have been proven successful to correct the density-driven currents by assimilation of observations such as sea surface height, sea surface temperature and in-situ profiles. However, the situation is different for tides in coastal models since these are not generated within the domain, but are rather propagated inside the domain through the boundary conditions. For improving the modeled tidal variability it is therefore not sufficient to update the model state via data assimilation without updating the boundary conditions. The optimization of boundary conditions to match observations inside the domain is traditionally achieved through variational assimilation methods. In this work we present an ensemble smoother to improve the tidal boundary values so that the model represents more closely the observed currents. To create an ensemble of dynamically realistic boundary conditions, a cost function is formulated which is directly related to the probability of each boundary condition perturbation. This cost function ensures that the boundary condition perturbations are spatially smooth and that the structure of the perturbations satisfies approximately the harmonic linearized shallow water equations. Based on those perturbations an ensemble simulation is carried out using the full three-dimensional General Estuarine Ocean Model (GETM). Optimized boundary values are obtained by assimilating all observations using the covariances of the ensemble simulation.
    Print ISSN: 1812-0784
    Electronic ISSN: 1812-0792
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    PAPER CURRENT
  • 8
    facet.materialart.
    Unknown
    The Surface Expression of Semidiurnal Internal Tides near a Strong Source at Hawaii. Part I: Observations and Numerical Predictions* (2010)
    American Meteorological Society
    Details
    Publication Date: 2010-06-01
    Description: Observations of semidiurnal currents from high-frequency radio Doppler current meters and moored acoustic Doppler current profilers (ADCPs) in the Kauai Channel, Hawaii, are described and compared with two primitive equation numerical models of the tides. The Kauai Channel, separating the islands of Oahu and Kauai, is a site of strong internal tide generation by the barotropic tides flowing over Kaena Ridge, the subsurface extension of Oahu. The nature and impacts of internal tide generation in the Kauai Channel were intensively studied during the 2002–03 near-field component of the Hawaii Ocean Mixing Experiment. Comparisons of observed coherent (i.e., phase locked to the astronomical forcing) M2 and S2 surface currents with model predictions show good agreement for the phases, indicating propagation of internal tides away from the ridge. Although the predicted M2 and S2 surface currents are similar (except for their magnitudes), as expected for internal waves at periods closer to each other (12.4 and 12 h, respectively) than to the inertial period (33 h), the observed M2 and S2 surface currents differ significantly. The S2 kinetic energy pattern resembles the predicted pattern. In contrast, the observed structure and magnitude of the more important M2 kinetic energy pattern differs significantly from the model predictions. The models predict a band of enhanced M2 surface kinetic energy 30–40 km from the ridge axis, corresponding to the first surface reflection of internal tide beams generated on the ridge flanks. The beams are clearly observed by the moored ADCPs, albeit with weaker amplitudes than predicted. Observations at the surface show an area of enhanced kinetic energy that is 10–20 km farther away from the ridge than predicted, with weaker magnitude. Observed M2 surface currents also exhibit apparent seasonal variability, with magnitudes weaker in spring 2003 than in fall 2002. Complex-demodulated semidiurnal currents exhibit significant temporal variability in amplitude and phase, not only because of the interference between semidiurnal constituents (e.g., the spring–neap cycle) but also on shorter and irregular time scales. The result is that ∼20% of semidiurnal energy is incoherent with astronomical forcing. Furthermore, the temporal variability is not spatially coherent; the spatial patterns of semidiurnal kinetic energy resemble those predicted by the numerical models during the strongest spring tides but differ from them at other times. As a result, M2 and S2 kinetic energy patterns phase locked to the astronomical forcing differ from each other. Some features of the observed spatial pattern and amplitude modulations can be qualitatively reproduced by a simple analytical model of the effects of homogeneous barotropic background currents on internal tide beams.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    PAPER CURRENT
  • 9
    facet.materialart.
    Unknown
    The Surface Expression of Semidiurnal Internal Tides near a Strong Source at Hawaii. Part II: Interactions with Mesoscale Currents* (2010)
    American Meteorological Society
    Details
    Publication Date: 2010-06-01
    Description: Observations of semidiurnal surface currents in the Kauai Channel, Hawaii, are interpreted in the light of the interaction of internal tides with energetic surface-intensified mesoscale currents. The impacts on internal tide propagation of a cyclone of 55-km diameter and ∼100-m vertical decay scale, as well as of vorticity waves of ∼100-km wavelength and 100–200-m vertical decay scales, are investigated using 3D ray tracing. The Doppler-shifted intrinsic frequency is assumed to satisfy the classic hydrostatic internal wave dispersion relation, using the local buoyancy frequency associated with the background currents through thermal-wind or gradient-wind balance. The M2 internal tide rays with initial horizontal wavelength of 50 km and vertical wavelength of O(1000 m) are propagated from possible generation locations at critical topographic slopes through idealized mesoscale currents approximating the observed currents. Despite the lack of scale separation between the internal waves and background state, which is required by the ray-tracing approximation, the results are qualitatively consistent with observations: the cyclone causes the energy of internal tide rays propagating through its core to increase near the surface (up to a factor of 15), with surfacing time delayed by up to 5 h (∼150° phase lag), and the vorticity waves enhance or reduce the energy near the surface, depending on their phase. These examples illustrate the fact that, even close to their generation location, semidiurnal internal tides can become incoherent with astronomical forcing because of the presence of mesoscale variability. Internal tide energy is mainly affected by refraction through the inhomogeneous buoyancy frequency field, with Doppler shifting playing a secondary but not negligible role, inducing energy transfers between the internal tides and background currents. Furthermore, the vertical wavelength can be reduced by a factor of 6 near the surface in the presence of the cyclone, which, combined with the energy amplification, leads to increased vertical shear within the internal tide rays, with implications for internal wave-induced mixing in the ocean.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    PAPER CURRENT
  • 10
    facet.materialart.
    Unknown
    HF radar observations of small-scale surface current variability in the Straits of Florida (2009)
    American Geophysical Union
    Details
    Publication Date: 2009-08-05
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
    Published by American Geophysical Union
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    PAPER CURRENT
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...