global validation of grace gravity measurements by in-situ and modelled ocean bottom pressure...
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Global Validation of GRACE Gravity Measurements by in-situ and modelled
Ocean Bottom Pressure
GSTM/SPP 1257Potsdam, 15.10.2007
C. Böning, A. Macrander, R. Timmermann, O. Boebel, J. Schröter
AWI Bremerhaven
PIES Deployment in Southern Ocean, RV Polarstern, 2006
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
0GFZ RL04 GSM+GAD [RMS]
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 wavesmay 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
Validation of GRACE with OBP
Data used in this study:
I. GRACE
• different data centres: CSR, GFZ, JPL, GRGS, ITG
• different releasesRL01 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
0GFZ RL04 GSM+GAD [RMS]
Validation of GRACE with OBP
Data used in this study:
II. in-situ OBP data
Global OBP database at AWIcontains:
• 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
0GFZ RL04 GSM+GAD [RMS]
• 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
Framstrait at 79°N300 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 = 0.55RL04 GSM+GAC r = 0.71RL04 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
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
Filtering GRACE data
Issues of Gaussian filtering methods due tosymmetrie of Gauss function:
•adds land signal to oceanic data
•ignores ocean circulation pattern
FESOM simulations indicate that OBP anomalies arecoherent over a certain area which corresponds tobottom topography
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
OBP cross-validation FESOM/PIES/GRACE
PIES: in-situ data 2002-2005FESOM: simulations 2002-2005, patch filteredGRACE: 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)
NOAA
Comparison of Gauss and coherence-patch filtered data
GFZ RL04GSM+GAD
AWIPOL/CNES/IFREMER
AWI
NOAA
POL
AWIPOL/CNES/IFREMER
MOVE
RAPID
AWI
POL
Correlations GFZ RL04 (750 km Gauss)/in situ OBP
Correlations GFZ RL04 (patch filtered)/in situ
•Correlation ofGRACE and in situ data indicates animprovement due tothe new filteringmethod
•At many locationscorrelation increasesby 0.1-0.2
•Improvement atPOL Array in DrakePassage, butcorrelation stillnegative
RAPID
MOVE
GFZ RL04GSM+GAD
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: [email protected], [email protected]