Context

1. Quality of real time altimeter productsimpact of the delay G.Larnicol, G. Dibarboure, C.Boone, CLS Space Oceanography DivisionAnanda Pascual, IMEDEA(CSIC-UIB), SpainPierre-Yves Le Traon, IFREMER, Brest, France

2. Context Overview of SSALTO/DUACS products Global products Regional products: Mediterranean sea: standard processing refined for Med sea European Shelves: Products dedicated to regional applications provision of tide, inverse barometer, HF signals information Sampling : Along-track Gridded fields Content : sea level anomaly absolute sea level (MDT+SLA)

3. Context Research activities (science review) Global products Regional products: Mediterranean sea: standard processing refined for Med sea European Shelves: Products dedicated to regional applications provision of tide, inverse barometer, HF signals information (Ducet et al. 2000) Sampling : Along-track Gridded fields Content : sea level anomaly absolute sea level

4. Context Operational activities (Dombrosky review) Global products Regional products: Mediterranean sea: standard processing refined for Med sea European Shelves: Products dedicated to regional applications provision of tide, inverse barometer, HF signals information Sampling : Along-track Gridded fields Content : sea level anomaly absolute sea level

5. Assess the quality of NRT altimeter observing system • Main sources of errors in near real time: • Orbit error (POE  MOE) • minor error thanks to the long wave length • error correction procedure

6. Assess the quality of NRT altimeter observing system • Main sources of errors in near real time • Orbit error (POE  MOE) • minor error thanks to the long wave length • error correction procedure • Data availability • delay in the data delivery • number of missions available June 21, 2006 Theory real life !!

7. Délay IGDR Dec 06 Jan 06 Anomalie IGDR IGDR Nominal Délai > 15j : désactivation automatique d’une mission Livraison Météo (4 jours de retard sur toutes les missions) Jason : SHM + Problème lecture mémoire GFO : Problème batteries + WVR ENVISAT : USO + SideB Assess the quality of NRT altimeter observing system • Main sources of errors in near real time • Orbit error (POE  MOE) • minor error thanks to the long wave length • error correction procedure • Data availability • delay in the data delivery • number of missions available

8. Assess the quality of NRT altimeter observing system • Main sources of errors in near real time • Orbit error (POE  MOE) • minor error thanks to the long wave length • error correction procedure • Data availability • delay in the data delivery • number of missions available • Time window used for data selection in • the mapping since we need maps as recent • as possible in NRT

9. Assess the quality of NRT altimeter observing system • Main sources of errors in near real time • Orbit error (POE  MOE) • minor error thanks to the long wave length • error correction procedure • Data availability • delay in the data delivery • number of missions available • Time window used for data selection in • the mapping since we need maps as recent • as possible in NRT • Data and data processing • AVISO products • Period where 4 altimeters are available • Oct 2002- Oct 2003

10. 0 5 10 (cm) Impact of the satellite configuration (DT analysis) Presented last year in Venice Pascual et al. (GRL, 2006) - 2 DT satellite configuration is a minimum to provide a relatively good description of Mesoscale activity - 4 satellites configuration improved this description and could be indispensable for specific area (Med for instance) 0 10 20 (cm) rms of sea level anomaly (SLA) estimated with 4 altimetric missions. rms of SLA differences between 4 and 2 satellites

11. 0cm 10cm Degradation of the NRT products (versus DT) RMS of the differences between delayed and real time SLA Estimation done with 4 satellites configuration

12. Comparison with tide gauge data Comparison with drifters data Mean square differences between drifter and altimeter velocities (AVISO+ Ekman= SURCOUF) Units are % of drifter variance. Mean square differences between tide gauge and altimeter sea level. Units are % of tide gauge variance. DAC = IB+MOG2D (Dynamic Atmos. Cor.)

13. Performance loss due to delay in the IGDR delivery RMS of differences between “optimal” DT maps with degraded NRT maps (as a function of the number of days of delivery delay) • On going work : tested for Indian ocean and Gulf-stream region • Simulated NRT : DT with delay • The NRT maps are compared to “optimal” DT maps. • Results are the same for all areas and only the base variance is different. • Impact of the delay shows linear trend 2 missions1 unavailable 3 missions2 unavailable 3 missions1 unavailable 4 missionsBest NRT available

14. Performance loss due to delay in the IGDR delivery • Definition of a performance Indicator for NRT applications • The linear trend is used to define a NRT perfor-mance indicator. The RMS of the DT-NRT difference (additional NRT error) is normalized by the best and the poorest NRT results one could obtain in a nominal scenario.

15. Performance loss due to delay in the IGDR delivery • Performance of 3 satellites conf = 72% • Absence of GFO  loss of 25% performance • For a two satellite configuration, there is a 5% error increase per day of missing data • Only 4% for a three satellite configuration. • 10% error increase with delay of 2 satellite Best NRT map (4 sat) 3 satellites 2 satellites Poorest NRT map (1sat)

16. Conclusions • 2 DT satellites is the minimum to observe the mesoscale signal • 4 NRT satellites give same results than 2 DT satellites  Need to have a at least 3 but preferably 4 satellites for operational applications • Data gaps and delivery delay cause a significant loss of accuracy on NRT maps. The IGDR delivery delay is critical for NRT applications, especially with fewer satellites Being able to process Real Time (<24h) altimeter products with an IGDR-like accuracy could improve the NRT accuracy by up to 25%  Ongoing work on DUACS • Continue the development of performance indicator. Refined the estimation by characterising the loss or the delay of Jason (reference mission) • Improvements not only come from additional missions and reduction of delay. It is also important to continue to improve the processing (geophysical corrections, dynamic atmospheric correction,etc…)