1. New improvements on the Dynamic Atmospheric Corrections L. Carrère, F. Lefèvre, F. Briol, J. Dorandeu (CLS), L. Roblou, E. Jeansou, G. Jan (Noveltis), F. Lyard (LEGOS/CNRS) Tides/HF Aliases splinter session - Hobart, March 2007

2. 2 Fabien LEFEVRE Introduction Ocean response to wind and pressure => Dynamic Atmospheric Correction (DAC): –Combined correction –IB for low frequencies –MOG2D barotropic model for high frequencies (forced by wind and pressure) –Cutoff 20 days –Distributed In the GDRs Improvements: new MOG2D model: –New finite element mesh: high resolution –New bathymetry –New run configuration –Validation against tide gauges Comparative results: –With DAC high resolution (DAC_HR) –Vs. IB correction –Vs. DAC low resolution (DAC_LR) –Altimetry variance reduction

3. 3 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Low resolution MOG2D finite element mesh ~100.000 nodes Resolution: ~400 km in deep ocean and ~20 km in shallow water

4. 4 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE High resolution MOG2D finite element mesh ~300.000 nodes - resolution: ~150 km in deep ocean and ~15 km in shallow water Gradients of topography are better represented Semi-enclosed seas: Baltic Sea, Black Sea, Persian Gulf and Red Sea

5. 5 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE New bathymetry Composite bathymetry: –GEBCO gridone –Digital data for southern seas (Genco, Lyard and Le Provost, The Oceanic Tides in the South Atlantic Ocean, Annales Geophysicae, 1994) Interpolated on the high resolution finite element mesh

6. 6 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Preliminary validation of the high resolution MOG2D model Along coastlines With 112 tide gauges: –from the GLOSS/CLIVAR network –filtered from tides (diurnal, semi-diurnal and long period) –Over 2003 Residual variance: –at tide gauges sites –for high frequencies 2-20 days Variance (cm²) ObsObs-IB Obs- MOG2D_LR Obs- MOG2D_HR Global Ocean32.417.57.06.5 High Latitudes57.2228.79.99.0 Low Latitudes8.46.84.2

7. 7 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Preliminary validation of the high resolution MOG2D model Gain obtained with the MOG2D correction for: –Low Resolution model –High Resolution model compared to IB correction at the global scale Gain/IB (%)MOG2D_LRMOG2D_HR Global Ocean60.0%62.8% High Latitudes65.5%68.7% Low Latitudes37.6%38.5%

8. 8 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE New DAC Dynamic Atmospheric Correction: –Combined correction –IB for low frequencies (LF) –MOG2D barotropic model for high frequencies (HF) forced by wind and pressure –Cutoff 20 days DAC_LR: –IB for LF –MOG2D_LR for HF DAC_HR: –IB for LF –MOG2D_HR for HF DAC interpolation on 1/4° Cartesian grids

9. 9 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Crossover differences Variance reduction at crossovers: Between IB and DAC_HR: –Improvement = 13.2 cm² Between DAC_LR and DAC_HR –Improvement = 2.2 cm² DAC_HR performs: –Worse in blue areas –Better in red yellow/areas Main differences in high latitudes! -25 cm²025 cm² -25 cm²025 cm²

10. 10 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Crossover differences on focused areas Variance reduction at crossovers Differences between IB and MOG2D_HR In the Baltic Sea In the Black Sea Main differences in high latitudes! -25 cm²025 cm² -25 cm²025 cm²

11. 11 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Crossover differences: Jason-1 cycles Global ocean SSH without atmospheric correction at crossovers: red SSH with IB correction: green SSH with DAC_LR: blue SSH with DAC_HR: purple

12. 12 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Crossover differences: low latitude/high latitude Low latitudes –Between -30°S and +30°N –Effects at the equator and between tropics High latitudes: –Between -90°S and -30°S –Between +30°N and +90°N

13. 13 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Crossover differences: deep ocean / Shallow waters Deep ocean: –Depths higher than 1000 meters Shallow waters –Depths lower than 1000 meters

14. 14 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Numerical results: variance reduction at Jason-1 crossovers Variance (cm²) COCO-IBCO-DAC_LRCO-DAC_HR Global Ocean 172.962.852.848.7 High Latitudes 197.462.851.548.9 Low Latitudes 39.335.133.333 Deep Ocean155.746.740.939.5 Shallow Water 243.4148.5107.7100.4

15. 15 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Numerical results: gain in variance compared to IB Gain / IB (%)CO-DAC_LRCO-DAC_HR Global Ocean15.9%22.4% High Latitudes18%22.1% Low Latitudes5.1%6% Deep Ocean12.3%15.5% Shallow Water27.4%32.4%

16. 16 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Along track differences -15 cm²015 cm²

17. 17 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Summary New high resolution MOG2D model New high resolution Dynamic Atmospheric Corrections (DAC_HR) Gain in variance at global scale: –IB/DAC_LR: 15.8% –IB:DAC_HR: 22.8% The ground segment processing chains at CLS are now ready for providing new DAC_HR with MOG2D_HR (MOG2D_HR HF + IB LF ) in an operational context (Jason-1 GDRs…) Perspectives: 1.Use of T-UGOm 2.Assimilation 3.…

18. 18 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE I would to personally thank everybody in Australia and in particular in Tasmania for their wonderful welcome in their beautiful country

19. 19 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE

20. 20 Tides/HF Aliases splinter session - Hobart, March 2007 Fabien LEFEVRE Sensitive study on run configurations Shear dragU0 Internal wave Chézydt Test10.1500.0025180/90 s Test20.1“50““ Test3=run0.01500.0025180/90 s Test40.01“50““ Test50.001“0““ Test60.001“50““ Test80.01“0“90/45 s Test9““00.005180/90 s Test10”1000.0025“ Test11“150““ Test12“200““ 6 months period tests => comparison results against TG quite similar