Day 4

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Paper title Exploring the opportunities of geostrophic current observations from space in the joint estimation of mean dynamic topography and geoid undulation
  1. Christian Neyers University of Bonn Speaker
  2. Moritz Borlinghaus Institute of Geodesy and Geoinformation (IGG), Universität Bonn
  3. Jan Martin Brockmann University of Bonn
Form of presentation Poster
  • A8. Ocean
    • A8.10 Ocean Doppler: Challenges and Opportunities for Future Missions of Global Ocean Surface Currents
Abstract text We undertake numerical experiments to show how observations of the geostrophic currents based on satellite data like the Sentinel-1 RVL products would influence and potentially improve the ”geodetic” (i.e. satellite-based only) estimation of the mean dynamic topography. The dynamic topography is the divergence of the sea surface from a hypothetical ocean at rest (the geoid) resulting from various “dynamic” processes. In particular, the mean dynamic topography is related to the steady state circulation in the oceans and consequently has meaning for studying global mass and heat transport. In this study we restrict ourselves to a mean model of the dynamic topography and assume a static gravity field. A purely observation driven approach is the joint estimation by means of a least-squares adjustment in which the sea surface height as measured by satellite altimetry is modelled as the sum of the geoid undulation and the dynamic topography. Supplementary to altimetric observations are gravity field solutions obtained from space missions, e.g. GRACE and/or GOCE, that are required to separate the two signals. Such an approach yields a so-called geodetic model of the dynamic topography that is independent of strictly oceanographic models that implement ocean physics. This enables its use in validation of oceanographic models as well as providing input data for combined models (“data assimilation”). A great challenge of the geodetic approach lies in the inconsistencies in spatial resolution between the different observation types. While the altimetry data boasts high resolution along-track (across-track depends on mission), the gravity field data is coarser on the order of one or two magnitudes. Thus it is difficult to separate the higher frequency signal that can be seen in the altimetry. For this to succeed it is required to introduce either higher resolution gravity data and/or a sufficiently accurate and preferably homogeneously sampling source of information for the dynamic topography, both under the premise of being satellite-only. Our hypothesis is that a huge opportunity comes with Doppler-derived surface current velocity measurements from SAR-satellites like Sentinel-1. Assuming the feasibility of reducing these observations to reflect geostrophic surface currents, these can be directly mapped to the spatial gradient of the dynamic topography. Such data points then provide exclusive information in the joint estimation, yielding a more stable separation. The presented study evaluates the potential gains that could be achieved by incorporating satellite-based measurements of the geostrophic surface currents, e.g. reduced Sentinel-1 RVL WV-mode type observations.