Massentransporte und Massenverteilungen im System Erde  
 
 SPP1257 / Projekte / JIGOG 
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Project JIGOG: Surface mass redistribution from joint inversion of GPS site displacements, ocean bottom pressure models and GRACE global gravity models

The JIGOG team

Jürgen Kusche1,Roelof Rietbroek1, Jens Schröter2, Sandra-Esther Brunnabend2, Christoph Dahle3, ilias Daras3, Frank Flechtner3, Mathias Fritsche4, Reinhard Dietrich4

 

1Astronomische, Physikalische und Mathematische Geodäsie, Institut für Geodäsie und Geoinformation, Bonn

²Alfred-Wegener-Institut (AWI), Bremerhaven, Germany

3Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam , Germany

4Technische Universität Dresden, Institut für Planetare Geodäsie, Dresden, Germany

Introduction

 

Within the JIGOG project we develop a strategy to consistently combine data from the satellite mission GRACE, permanent GPS stations and simulations from FESOM (Finite Element Sea Ice-Ocean Model) with the aim of retrieving temporal mass transfers on the Earths surface. Measuring these mass redistributions is crucial as they reflect phenomena in terrestrial water storage, ice sheets, the atmosphere and the ocean. The exchange between the Earth's major mass compartments is currently largely unknown and the retrieval by single techniques are accompanied by data-specific disadvantages.

Our combination approach, using least squares techniques, allows for better accuracy, a higher temporal resolution, and it addresses data-specific problems. Our results (download) therefore contribute to the current knowledge of global mass transport on the surface of the Earth.

 

Figure 1: The redistribution of ice,water, snow and air will cause several measurable phenomena (highlighted with red text)

Mass Transport

 

The time variable gravity field of the Earth predominantly reflect changes in mass distribution in the thin outer layer, comprising terrestrial water storage (lakes/aquifers/soil/snow), the atmosphere, ice sheets/glaciers and the ocean. The dynamics in this layer strongly affect our daily life and are increasingly influenced by anthropogenic effects. Global warming causes changes in the atmospheric and oceanic dynamics, affecting rainfall and consequently terrestrial water storage, it causes melting of ice sheets with an associated sea level rise and additionally thermal expansion of the ocean. Clearly, such dynamics cannot be properly studied as isolated events but require a more global approach.

 

When water, snow and air are redistributed over the Earth's surface we see that the following effects take place:

  • The gravity field of the Earth changes, because regions with higher mass content yield a stronger gravitational attraction.
  • The surface pressure changes because the new column of atmospheric and oceanic masses assert a different weight on the surface.
  • The shape of the solid Earth changes because it deforms under the surface pressure and the change in gravity (self gravitation).

 

Measuring Mass Changes

 

The above phenomena can be observed/modelled with the following techniques:

  • The satellite mission GRACE measures small changes of the gravity field
    • The ocean model FESOM can be used to retrieve bottom pressure variations over the ocean.
    • A continuous GPS network (TU Dresden/GFZ reprocessing solution) , consisting of stations fixed to the crust, monitors the shape of the solid Earth.
    Complementary to these data sets there are several other geodetic techniques which might be used in the (near) future, such as for example satellite laser ranging (SLR) and altimetry.
Figure 2: Geocenter motion variations in mm for various combination schemes. Red: reference combination using GRACE+GPS+OBP data, blue: combination using GPS+OBP only, green: GRACE+GPS+OBP combination which excludes OBP information at higher latitudes. From Rietbroek et al. 2009

Scientific benefits of the project

  • An important reference system issue is addressed. The use of GRACE alone results in measurements relative to the Earths center of mass. Many datasets, like tide and bottom pressure gauges, altimetry, are in fact provided with respect to the center of figure of the Earth (sea floor, topography or an reference frame fixed to the topography). A consistent comparison would therefore require the incorporation of the so called geocenter motion. This motion (see figure 2), which is non neglible for many data comparisons, is solved for in the joint inversion.
  • The temporal resolution of the solutions is increased by additionally using dedicated weekly GRACE solutions from GFZ.
  • Weaknesses in the ocean model, GRACE and GPS can be investigated. For example the global adjustment allows for the joint estimation of mass bias corrections in the ocean model.
  • The combination allows for a thorough investigation in how the individual data types contribute to the joint estimate.
  • A major application will be the determination of variations of the total ocean mass (see figure 3). In conjunction with measured oceanic volume variations, from altimetry, we will be able to determine thermal expansion and thus global ocean warming with a much higher accuracy than conventional estimates can provide.

Selection of Publications, presentations and posters

 

Brunnabend S-E., J. Schröter, R. Rietbroek, J. Kusche, Ch. Dahle, F. Flechtner, M.J.F. Jansen, B. Gunter, C. Böning, R. Timmermann, Surface Mass Transport by joint inversion of modelled ocean bottom pressure, GRACE gravity models and GPS site displacements, Geodetic Week, Bremen, Germany, 30. September - 02. October 2008 (oral)

 

Dahle Ch., F. Flechtner, J. Kusche, R. Rietbroek, GFZ EIGEN-GRACE05S Weekly Gravity Field Time Series, Grace Science Team Meeting, Session A.1 (GRACE Geodesy), San Francisco, 12.-13. December 2008 (oral)

 

M. J. F. Jansen, B. C. Gunter, R. Rietbroek, C. Dahle, J. Kusche, F. Flechtner, S. E. Brunnabend, and J.̇ Schrö̈ter. Estimating sub-monthly global mass transport signals using grace, gps and obp data sets. In Stelios Mertikas, editor, Proc. of the IAG International Symposium on Gravity, Geoid and Earth Observation (GGEO2008), Chania, Crete, June 23-27, 2008. Springer, 2010.

 

Kusche J, Approximate decorrelation and non-isotropic smoothing of time-variable GRACE gravity field models, IUGG, session GS002 (Gravity Field), Perugia, 2. 7. 2007 (oral)

 

Kusche J, On the problem of combining time-variable spherical harmonic potential field data with time series of point-wise, broad-band, data, EGU 2008, Session G2 (Recent Developments in Geodetic Theory) Vienna, 14. 4. 2008 (oral)

 

J. Kusche, R. Schmidt, S. Petrovic, and R. Rietbroek. Decorrelated grace time-variable gravity solutions by gfz, and their validation using a hydrological model. Journal of Geodesy, 83(10):903, 2009, doi:10.1007/s00190-009-0308-3.

 

Rietbroek R, Surface mass estimation from GPS site displacements, modelled Ocean bottom pressure and GRACE, EGU 2008, Session G3 (GRACE Science Applications), Vienna, 17. 4. 2008 (oral)

 

Rietbroek R, Surface mass estimation from GPS site displacements, modelled ocean bottom pressure and GRACE, GGEO 2008, Session 7 (Temporal Gravity Changes and Geodynamics), Chania, 26. 6. 2008 (oral)

 

R Rietbroek, SE Brunnabend, C Dahle, J Kusche, F Flechtner, J Schröter, and R Timmermann. Changes in total ocean mass derived from grace, gps, and ocean modeling with weekly resolution. Journal Of Geophysical Research-Oceans, 114(C11004):C11004, 2009, doi:10.1029/2009JC005449.

 

R. Rietbroek, M. Fritsche, S.-E. Brunnabend, Dahle Ch., J. Kusche, J. Schrö̈ter, F. Flechtner, and R. Dietrich. The impact of reprocessed gps and grace data in joint surface loading inversions. EGU, poster session G9.1, Vienna, 2010.

Schröter J, A global finite-element sea ice and ocean model, session OS15 Model development for large- and small-scale processes in the ocean, Vienna, 17. 4. 2008 (oral)

 

Timmermann, R, Danilov, S, Schröter, J, Böning, C, Sidorenko, D, Rollenhagen, K (2009): Ocean circulation and sea ice distribution in a finite element global sea ice - ocean model, Ocean Modelling, doi:10.1016/j.ocemod.2008.10.009