Errors on SMOS retrieved SSS and their dependency to a priori wind speed

1. Errors on SMOS retrieved SSS and their dependency to a priori wind speed  X. Yin1, J. Boutin1, J. Vergely2, P. Spurgeon3, and F. Gaillard4 1. LOCEAN 2. ACRI 3. ARGANS 4. IFREMER

2. SMOS SSS map using ISAS optimum interpolation 1. SSS from L2OS v5.5 reprocessing, August 2010 (“SSSOP”) 2. In Situ Analysis System (ISAS) SSS

3. Why do we need a better estimation of wind speed? 1. ECMWF WS as prior and fixed σWS = 0m/s 2. ECMWF WS as prior and tuned σWS = 2m/s (L2OS OP) 3. SSMI WS as prior and tuned σWS = 2m/s

4. Why do we need a better estimation of wind speed? ECMWF WS v38r2 – v37r2 Retrieved WS1 v38r2 – v37r2 1m/s 1m/s -1m/s -1m/s Retrieved SSS1 v38r2 – v37r2 The differences between two versions of ECMWF are partially but not totally reduced by the L2OS retrievals. The difference between two versions of retrieved WS and that of SSS are well collocated, and the correlation coefficient between the two is 0.859. Errors in ECMWF WS can lead to errors in SSS, since retrieved WS can not reduce the errors in WS a priori totally with the L2OS retrieval scheme. 0.2psu -0.2psu

5. Introduction The retrievals are based on the Levenberg and Marquardt iterative convergence method. The first guessed geophysical inputs (SSS, SST, WS and TEC) are adjusted so as to minimize a so called “cost function” χ2 expressed by • Can we reduce the biases in retrieved wind speed, if • Relax the error on a priori WS(σWS): 2m/s -> 5m/s (“OP, 5m/s”) • Two step scheme.

6. Introduction • Two step scheme, objective: to reduce the biases in the retrieved SSS and WS without increasing noise in the SSS retrievals. • 1st step: a priori ECWMF WS with increasing error of a priori WS (σWS) to 5m/s -> retrieved WS -> 2D spatial median filtering (50 km radius close to SMOS resolution) -> smoothed WS • Objective: to relax the dependency of error/bias in retrieved WS to a priori estimate (ECMWF), and to reduce noise in retrieved WS used as a priori estimate for the next step. • 2) 2st step: smoothed WS from 1st step (instead of ECMWF) used as a priori estimate with (σWS) set back to be 2 m/s -> retrieved SSS and WS • Note: • for both steps, errors of a priori SSS, SST and TEC are the same as in the operational L2OS processor, i.e. 100psu, 1°C and 10 tecu.

7. Data 1. One ascending orbit in April 2013, with two versions of ECMWF WS(v38r2 and v37r2) uses as a priori estimate. Can the retrieved WS and SSS converge, with two different versions of a priori WS? 2. One ascending orbit in Aug, 2010 in the eastern equatorial pacific ocean, where we found large WS biases between ECWMF and SSMI. Comparisons among SMOS retrieved WS, ECWMF WS and SSMI WS, and comparisons of SSS. 3. All ascending orbits in Aug. 2010: performance over the global ocean Radiometer wind speeds lower than ECMWF WS in the eastern equatorial pacific ocean because of strong surface currents, but still higher than SSMI WS -> positive anomalies in retrieved SSS compared with ISAS SSS SMOS operational retrieved WS ECMWF WS SSMI WS rSSS - ISAS

8. Results Can the retrieved WS converge to the same value with the two step scheme, using two different versions of a priori ECMWF WS? YES with some exceptions! Two step, rWS1 v38r2 – v37r2 Operational, rWS1 v38r2 – v37r2 1m/s 1m/s -1m/s -1m/s Exceptions: 1) too large differences between two a priori WS; or 2) RFI

9. Results Can the retrieved SSS converge to the same value with the two step scheme, using two different versions of a priori ECMWF WS? YES with some exceptions! Operational, rSSS1 v38r2 – v37r2 Two step, rSSS1 v38r2 – v37r2 0.2psu 0.2psu -0.2psu -0.2psu Exceptions: 1) too large differences between two a priori WS; 2) RFI

10. TEST: comparisons of different methods 3S-2N Wind speed Salinity

11. Monthly maps of std of SSS for each 0.5 * 0.5 grid Operational SSS Two-step SSS diff With mask: abs(diff) > 0.2 No mask Two-step scheme enhances problems near the coastal and RFI regions.

12. Monthly maps in August 2010 No mask With mask

13. Monthly maps in August 2010 Differences in SSS Differences in std of SSS Std of SSS is higher if we only increase error on a priori WS.

14. retrieved WS rWSOP - WSECMWF rWStwostep - WSECMWF Problems near the coastal and RFI regions

15. Conclusions • The two step scheme enhances the capability of retrieving WS using multi-angular MIRAS TB. • The retrieved SSS and WS converge to the same value with the two step scheme, regardless of different versions of ECMWF WS used as a priori estimates. • The retrieved WS with the two step scheme are closer to SSMI WS in Eastern Equatorial Pacific than the L2OS OP WS. • The retrieved SSS with the two step scheme are closer to in-situ SSS in Eastern Equatorial Pacific than the L2OS OP SSS. • Compared with L2OS OP retrievals, the two step scheme does not increase the noise in retrieved SSS in the open ocean at low and moderate latitude. • The two step scheme enhances problems near the coastal and RFI contaminated regions.

16. TEST: comparisons of different methods • Tests (one orbit in 2010/08/06,13h-14h): • L2OS, σWS = 2 m/s • Two step: 1) σWS = 5 m/s + WS smoothing; 2) σWS = 2 m/s • Two step: 1) σWS = 5m/s + no WS smoothing; 2) σWS = 2 m/s • L2 OS, σWS = 5 m/s 3S-2N

17. Monthly SSS3 maps in August 2010 No mask

18. Variances of retrieved parameters