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Global Validation of GRACE Gravity Measurements by in-situ and modelled Ocean Bottom Pressure GSTM/SPP 1257 Potsdam, 15.10.2007 C. Böning, A. Macrander, R. Timmermann, O. Boebel, J. Schröter AWI Bremerhaven PIES Deployment in Southern Ocean, RV Polarstern, 2006
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Ground-truth validation of GRACE with OBP GRACE GFZ RL04 GSM+GAD RMS variability [dbar of water equivalent]. GRACE: do2-50, 750 km Gauss filter. OBP ground truth sites GRACE measures mass variability on Earth large hydrological cycle over continents much smaller signal over oceans (0.01 … 0.05 dbar) Does GRACE capture real oceanic variability? Here: Global comparison with in-situ and modelled data of Ocean Bottom Pressure (OBP) GRACE: monthly variability [RMS / dbar] AWI POL/CNES/ IFREMER MOVE RAPID AWI NOAA POL 0.15 0.10 0.05 0 GFZ RL04 GSM+GAD [RMS]
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Ocean Bottom Pressure (OBP)p = ∫ g dz Vertical integral of oceanic + atmospheric mass In-situ observations by instruments deployed at the sea floor Short term variability ≤ O(1 dbar) tides, planetary waves may cause aliasing in GRACE data, corrected by GAC/GAD model Monthly variability (0.01 to 0.05 dbar) water mass changes, geostrophic ocean currents Ocean Bottom Pressure OBP timeseries in AWI ACC array (PIES ANT 7-1). OBP [dbar] at 45°S 7°E: unfiltered data, 7 days LP – 4677 dbar – 4676 dbar
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Validation of GRACE with OBP Data used in this study: I. GRACE different data centres: CSR, GFZ, JPL, GRGS, ITG different releases RL01 to RL04 different products: GSM monthly geoid + GAC/D monthly average of de-aliasing model spatial smoothing: degree/order 2 – 50, 750 km Gauss filter or newly developed patch filtering GRACE GFZ RL04 GSM+GAD RMS variability [dbar of water equivalent]. GRACE: do2-50, 750 km Gauss filter. OBP ground truth sites GRACE: monthly variability [RMS / dbar] AWI POL/CNES/ IFREMER MOVE RAPID AWI NOAA POL 0.15 0.10 0.05 0 GFZ RL04 GSM+GAD [RMS]
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Validation of GRACE with OBP Data used in this study: II. in-situ OBP data Global OBP database at AWI contains: dedicated GRACE-arrays: ACC (AWI), MOVE (IFM-GEOMAR/SIO) oceanographic OBP-sections: Drake Passage (POL/CNES), Kerguelen (POL/IFREMER), Framstrait (AWI), RAPID (NOC) tsunami warning system: DART (NOAA) … further contributions appreciated GRACE GFZ RL04 GSM+GAD RMS variability [dbar of water equivalent]. GRACE: do2-50, 750 km Gauss filter. OBP ground truth sites GRACE: monthly variability [RMS / dbar] AWI POL/CNES/ IFREMER MOVE RAPID AWI NOAA POL 0.15 0.10 0.05 0 GFZ RL04 GSM+GAD [RMS]
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OBP variability of about ± 0.03 dbar OBP anomalies strongly related to barotropic velocity anomalies High pressure anomalies → anticyclonic currents Low pressure anomalies → cyclonic currents Data used in this study: III. FESOM Finite Element Sea Ice Ocean Model hydrostatic primitive equation OGCM with sea ice coupling 1.5° horizontal resolution 26 z-levels atmospheric forcing: usually NCEP/NCAR daily reanalysis 1958-2005 no restoring Validation of GRACE with OBP
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→large-scale GRACE data agree well with in-situ point measurements →Correlation levels GFZ3 GSM+GAC r ≤ 0.7; further improved with RL04 GRACE vs. in-situ OBP: ACC array Black: In-situ data: AWI PIES ACC array 2002 – 2005 Grey: GRACE GFZ3 GSM+GAC Antarctic Circumpolar Current (ACC) 2-D PIES array since 2006 deployed by AWI to capture large-scale OBP variability further extension planned high OBP variability O(0.05 dbar) ANT 7 [44°S 7°E] ANT 11 [50°S 1°E] 2 PIES 2002 – 2005 data available 6 PIES 2005/6 – 2010 +3 PIES 2008 – 2010
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Framstrait at 79°N 300 km from Greenland Ice Shield PIES data from AWI since 2003 Monthly OBP variability O(0.05 dbar) GRACE captures real oceanic variability: → correlation improvements by recent releases, e.g. GFZ: RL03 GSM+GAC r = 0.55 RL04 GSM+GAC r = 0.71 RL04 GSM+GAD r = 0.76 →best agreement of all GRACE products: GRGS (10day time axis; r = 0.80) →GAC, GAD de-aliasing models alone do not show observed variability, actual GRACE measurements (GSM) necessary GRACE vs. in-situ OBP: Framstrait 79°N Arctic Blue: In-situ data: Framstrait PIES F8 – 1-3 [A. Beszczynska-Möller, AWI] Other colours: GRACE do2-50, 750 km Gauss filter
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GRACE vs. in-situ OBP: MOVE, Tropical Atlantic Blue: In-situ data: MOVE PIES V404 1-5 [J. Karstensen, IFM-GEOMAR] Other colours: GRACE do2-50, 750 km Gauss filter Tropical Atlantic at 16°N in-situ OBP variability small O(0.02 dbar) GRACE strongly overestimates variability unrealistic annual cycle in all GRACE products [ GFZ, CSR, GRGS, ITG, JPL ] → see also: Poster of U. Neumann et al. this afternoon
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GRACE vs. in-situ OBP: MOVE, Tropical Atlantic Blue: In-situ data: MOVE PIES V404 1-5 [J. Karstensen, IFM-GEOMAR] Other colours: GRACE do2-50, 750 km Gauss filter Tropical Atlantic at 16°N in-situ OBP variability small O(0.02 dbar) GRACE variability too large small improvements RL03 → RL04 variability underestimated by GAD model; GSM adds mostly noise (change!!!) unrealistic annual cycle in all GRACE products [ GFZ, CSR, GRGS, ITG, JPL ] →see also: Poster of U. Neumann et al. this afternoon
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Filtering GRACE data Issues of Gaussian filtering methods due to symmetrie of Gauss function: adds land signal to oceanic data ignores ocean circulation pattern FESOM simulations indicate that OBP anomalies are coherent over a certain area which corresponds to bottom topography
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Finite Element Sea Ice Ocean Model FEOM/FENA, Kivman et al. (AWI) The ocean component: FEOM hydrostatic primitive equation OGCM (Danilov et al. 2004) free surface isopycnic diffusivity, Gent&McWilliams 1990 parameterization Smagorinski viscosities vertical diffusivity/viscosity: a v (Ri,Monin-Obukhov-length) (Timmermann and Beckmann, 2004)
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Finite Element Sea Ice Ocean Model The sea-ice component: FESIM (based on Danilov and Yakovlev, 2003) thermodynamics: surface energy balance incl. conductive heat flux (Parkinson&Washington, 1979) prognostic snow layer incl. snow- ice-conversion, but no internal heat storage momentum balance ( „dynamics“ ): elastic-viscous-plastic rheology (Hunke/Dukowicz, 1997) The ocean component: FEOM hydrostatic primitive equation OGCM (Danilov et al. 2004) free surface isopycnic diffusivity, Gent&McWilliams 1990 parameterization Smagorinski viscosities vertical diffusivity/viscosity: a v (Ri,Monin-Obukhov-length) (Timmermann and Beckmann, 2004) conforming grid, linear base functions
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Spatial coherence of OBP OBP anomalies of 2002-2005 from 50-yr FESOM simulation 4-months high pass filter to subtract dominant seasonal cycle Cross-correlation of time series at one point with time series at all other points Cut-off at correlation <0.7 and radius 20º filter data by weighting with correlation coefficients ANT 7 ANT 11
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OBP cross-validation FESOM/PIES/GRACE PIES: in-situ data 2002-2005 FESOM: simulations 2002-2005, patch filtered GRACE: GFZ RL04, d/o 2-50, patch filtered increase in correlation of 0.1-0.2 FESOM reproduces seasonal cycle AWI ACC array: PIES ANT 7, ANT 11 (more to come)
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OBP cross-validation FESOM/PIES/GRACE Carmen Böning white line: correlation = 0.7 monthly mean anomalies
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OBP cross-validation FESOM/PIES/GRACE Carmen Böning white line: correlation = 0.7 3-month running mean anomalies
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NOAA Comparison of Gauss and coherence-patch filtered data GFZ RL04 GSM+GAD AWI POL/CNES/ IFREMER AWI NOAA POL AWI POL/CNES/ IFREMER MOVE RAPID AWI POL Correlations GFZ RL04 (750 km Gauss)/in situ OBP Correlations GFZ RL04 (patch filtered)/in situ Correlation of GRACE and in situ data indicates an improvement due to the new filtering method At many locations correlation increases by 0.1-0.2 Improvement at POL Array in Drake Passage, but correlation still negative RAPID MOVE GFZ RL04 GSM+GAD
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Example: AWI ACC Array Amplitude is underestimated in the first time period... but overestimated by the Gauss filtered timeseries in 2004 Characteristics are better reproduced when the coherence patch had been applied Correlation increase ~0.1 in situ (blue), 500km Gauss filter (red dashed), patch applied (red solid)
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GFZ RL03 vs. GFZ RL04 GFZ RL04 GSM+GAD Improvements in certain areas in the Atlantic Small deterioration especially close to the coast of North America GFZ RL03 GSM+GAC AWI POL/CNES/ IFREMER MOVE RAPID AWI NOAA POL AWI POL/CNES/ IFREMER MOVE RAPID AWI NOAA POL Correlations GFZ RL04 (patches applied)/in situ Correlations GFZ RL03 (patches applied)/in situ
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Example: AWI Framstrait Array Better agreement in amplitude and phase for GFZ RL04 in comparison to RL03 improvement of ~0.1 in correlation in situ (blue), GFZ RL03 (red dashed), GFZ RL04 (red solid)
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Conclusions Recent GRACE releases capture real oceanic OBP variability with r = 0.8 … 0.9 at some locations (all at high latitudes) Improvements by recent GAC, GAD de-aliasing models, but actual GRACE measurements (GSM fields) necessary to capture real variability Improvements RL03→RL04 and GAC→GAD Improvements by using the patch filtering method OBP database at AWI now available upon request Contact: Carmen.Boening@awi.de, Andreas.Macrander@awi.de
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