Description: The ability of any satellite gravity mission concept to monitor mass transport processes in the Earth system is typically tested well ahead of its implementation by means of various simulation studies. Those studies often extend from the simulation of realistic orbits and instrumental data all the way down to the retrieval of global gravity field solution time-series. Basic requirement for all these simulations are realistic representations of the spatio-temporal mass variability in the different sub-systems of the Earth, as a source model for the orbit computations. For such simulations, a suitable source model is required to represent (i) high-frequency (i.e., subdaily to weekly) mass variability in the atmosphere and oceans, in order to realistically include the effects of temporal aliasing due to non-tidal high-frequency mass variability into the retrieved gravity fields. In parallel, (ii) low-frequency (i.e., monthly to interannual) variability needs to be modelled with realistic amplitudes, particularly at small spatial scales, in order to assess to what extent a new mission concept might provide further insight into physical processes currently not observable. The new source model documented here attempts to fulfil both requirements: Based on ECMWF’s recent atmospheric reanalysis ERA-Interim and corresponding simulations from numerical models of the other Earth system components, it offers spherical harmonic coefficients of the time-variable global gravity field due to mass variability in atmosphere, oceans, the terrestrial hydrosphere including the ice-sheets and glaciers, as well as the solid Earth. Simulated features range from sub-daily to multiyear periods with a spatial resolution of spherical harmonics degree and order 180 over a period of 12 years. In addition to the source model, a de-aliasing model for atmospheric and oceanic high-frequency variability with augmented systematic and random noise is required for a realistic simulation of the gravity field retrieval process, whose necessary error characteristics are discussed. The documentation of the updated ESA Earth System Model (updated ESM) for gravity mission simulation studies is organized as follows: The characteristics of the updated ESM along with some basic validation is presented in Volume 1. A detailed comparison to the original ESA ESM (Gruber et al., 2011) is provided in Volume 2, while Volume 3 contains the description of a strategy to derive realistic errors for the de-aliasing model of high-frequency mass variability in atmosphere and ocean.

Description: The ability of any satellite gravity mission concept to monitor mass transport processes in the Earth system is typically tested well ahead of its implementation by means of various simulation studies. Those studies often extend from the simulation of realistic orbits and instrumental data all the way down to the retrieval of global gravity field solution time-series. Basic requirement for all these simulations are realistic representations of the spatio-temporal mass variability in the different sub-systems of the Earth, as a source model for the orbit computations. For such simulations, a suitable source model is required to represent (i) high-frequency (i.e., sub-daily to weekly) mass variability in the atmosphere and oceans, in order to realistically include the effects of temporal aliasing due to non-tidal high-frequency mass variability into the retrieved gravity fields. In parallel, (ii) low-frequency (i.e., monthly to interannual) variability needs to be modelled with realistic amplitudes, particularly at small spatial scales, in order to assess to what extent a new mission concept might provide further insight into physical processes currently not observable. The new source model documented here attempts to fulfil both requirements: Based on ECMWF’s recent atmospheric reanalysis ERA-Interim and corresponding simulations from numerical models of the other Earth system components, it offers spherical harmonic coefficients of the time-variable global gravity field due to mass variability in atmosphere, oceans, the terrestrial hydrosphere including the ice-sheets and glaciers, as well as the solid Earth. Simulated features range from sub-daily to multiyear periods with a spatial resolution of spherical harmonics degree and order 180 over a period of 12 years. In addition to the source model, a de-aliasing model for atmospheric and oceanic high-frequency variability with augmented systematic and random noise is required for a realistic simulation of the gravity field retrieval process, whose necessary error characteristics are discussed. The documentation is organized as follows: The characteristics of the updated ESM along with some basic validation are presented in Volume 1 of this report (Dobslaw et al., 2014). A detailed comparison to the original ESA ESM (Gruber et al., 2011) is provided in Volume 2 (Bergmann-Wolf et al., 2014), while Volume 3 (Forootan et al., 2014) contains a description of the strategy to derive a realistically noisy de-aliasing model for the high-frequency mass variability in atmosphere and oceans. The files of the updated ESA Earth System Model for gravity mission simulation studies are accessible at DOI:10.5880/GFZ.1.3.2014.001.

Description: The ability of any satellite gravity mission concept to monitor mass transport processes in the Earth system is typically tested well ahead of its implementation by means of various simulation studies. Those studies often extend from the simulation of realistic orbits and instrumental data all the way down to the retrieval of global gravity field solution time-series. Basic requirement for all these simulations are realistic representations of the spatio-temporal mass variability in the different sub-systems of the Earth, as a source model for the orbit computations. For such simulations, a suitable source model is required to represent (i) high-frequency (i.e., sub-daily to weekly) mass variability in the atmosphere and oceans, in order to realistically include the effects of temporal aliasing due to non-tidal high-frequency mass variability into the retrieved gravity fields. In parallel, (ii) low-frequency (i.e., monthly to interannual) variability needs to be modelled with realistic amplitudes, particularly at small spatial scales, in order to assess to what extent a new mission concept might provide further insight into physical processes currently not observable. The new source model documented here attempts to fulfil both requirements: Based on ECMWF’s recent atmospheric reanalysis ERA-Interim and corresponding simulations from numerical models of the other Earth system components, it offers spherical harmonic coefficients of the time-variable global gravity field due to mass variability in atmosphere, oceans, the terrestrial hydrosphere including the ice-sheets and glaciers, as well as the solid Earth. Simulated features range from sub-daily to multiyear periods with a spatial resolution of spherical harmonics degree and order 180 over a period of 12 years. In addition to the source model, a de-aliasing model for atmospheric and oceanic high-frequency variability with augmented systematic and random noise is required for a realistic simulation of the gravity field retrieval process, whose necessary error characteristics are discussed. The documentation is organized as follows: The characteristics of the updated ESM along with some basic validation are presented in Volume 1 of this report (Dobslaw et al., 2014). A detailed comparison to the original ESA ESM (Gruber et al., 2011) is provided in Volume 2 (Bergmann-Wolf et al., 2014), while Volume 3 (Forootan et al., 2014) contains a description of the strategy to derive a realistically noisy de-aliasing model for the high-frequency mass variability in atmosphere and oceans. The files of the updated ESA Earth System Model for gravity mission simulation studies are accessible at DOI:10.5880/GFZ.1.3.2014.001.

Description: Recently released global gravity field models generated solely from CHAMP and GRACE satellite observations allow with an unprecedented accuracy and resolution the recovery of the mean sea surface topography from the difference between an altimetry-based mean sea surface height model and the gravity model's derived geoid. Here the CHAMP EIGEN-2 gravity field model, and the first GFZ GRACE gravity model, EIGEN-GRACE01S, are used. The mean sea surface height model has been compiled from four years'; worth of TOPEX altimeter data. To evaluate the accuracy and resolution limits of the CHAMP and GRACE geoids for the envisaged application, a low pass filter in the spatial domain with different cut-off wavelengths has been applied to the geoid and sea surface data before subtraction. The minimum wavelength, where noisy and erroneous features in the recovered sea surface topography are minimised, can be interpreted as an indicator for the best suited common spatial resolution. The EIGEN-2 model's geoid has been tested to have a resolution of 1800 km, which corresponds to a truncation degree of l = 22 in terms of spherical harmonics. Using the EIGEN-GRACE01S model, the resolution could be extended to 1000 km (l = 40). These boundaries can be attributed to the geoid's error, exceeding 2 cm in case of the CHAMP model, and in case of the GRACE model to spurious systematic signals increasing with increasing spherical harmonic degree. The calculated sea surface topography models have been used to derive absolute geostrophic sea surface velocities. An error propagation shows that the requirement of 1 cm/s for geoid induced velocity errors is fulfilled at the given resolutions for all latitudes excluding a narrow equatorial band. Maximum geostrophic velocities are derived in the 1000 km-resolution model for the Kuroshio region with 40 cm/s, and for the Gulf Stream east off Cape Hatteras with 25 cm/s.

Description: Ziel dieser Arbeit ist es, ein baroklines, global definiertes Ozeanmodell so für den operationellen Betrieb vorzubereiten, dass kurzperiodische Massen- und Meereshöhenvariationen mit geringer zeitlicher Verzögerung simuliert werden können. Dabei sind insbesondere die Anforderungen der GRACE-Prozessierung an die Modelldaten zu berücksichtigen und gleichzeitig die Verwendbarkeit der Simulationsergebnisse für die Korrektur von Altimeterbeobachtungen vorzubereiten. Weiterhin sind die simulierten Daten zur Verifikation der GRACE-Massenanomalien über den Ozeanen heranzuziehen, um Abschätzungen über die Qualität der monatlichen Schwerefelder treffen zu können und die Interpretation dieser neuartigen Beobachtungen zu unterstützen. Dazu wird in dieser Arbeit das globale Ozeanmodell für Zirkulation und Gezeiten (OMCT; Thomas, 2002) verwendet, das speziell an die hier vorliegenden Fragestellungen auf kurzen und mittleren Zeitskalen angepasst wurde (Kapitel 2). Um eine Vergleichbarkeit der Simulationsergebnisse im Zeitbereich zu ermöglichen, wird das Modell mit in Kapitel 3 beschriebenen realistischen Atmosphärendaten des Europäischen Wetterzentrums ECMWF angetrieben.