1. Sea Surface Salinity under rain cells: SMOS satellite and in-situ drifters observations J. Boutin 1, N. Martin 1, G. Reverdin 1,S. Morisset 1, X. Yin 1, L. Centurioni 2 and N. Reul 3 1 LOCEAN, Sorbone Universités, UPMC/CNRS/IRD/MNHN, Paris, France 2 SIO, La Jolla, CA, USA 3 IFREMER, Toulon, France Boutin et al., 2014

2. The impact of rain on SMOS SSS? SMOS SSS lower than ARGO optimal Interpolated SSS maps in rainy regions (e.g. ITCZ, SPCZ..) : what part of this difference explainable by rain stratification/intermittency?

3. SMOS – ARGO SSS in tropical Pacific 0.1 fresher and more variable than in subtrop Atlantic; if SMOS rainy measurements are removed, std_diff in ITCZ and SPURS becomes the same => rain effect in ITCZ Ascending + Descending orbits Mean(std) SMOS-ARGO SSS -0.12 (0.46) 45°S-45°N -0.02 (0.25) Subtrop. Atlantic -0.15 (0.30) N.E. Trop. Pacific SMOS - ARGO (Jul-Sep 2010) SMOS SSS averaged within +/-50km & +/- 5days around ARGO SSS 32 39 32 39 Sargo Ssmos Boutin et al., Ocean Science, 2013

4. Salinity Well Mixed Salinity Depth Stratified with Rain Depth Salinity Depth Stratified with Evaporation Near-Surface Salinity Schematic Diagram for L-band Radiometer Rain-induced fresh layer Evaporation- induced salty layer Satellite L-band radiometric salinity at depth range from 1 to 10 cm Salinity measured by in situ sensors/platforms at depth below 1 m Schematic Diagram made by the SISS working group

5. Boutin et al., EGU 2012 Vertical gradients 15cm & 45 cm depth as seen by surface drifters 17 events SVP-BS / Surplas Vertical gradients 15cm & 45 cm depth as seen by surface drifters 17 events SVP-BS / Surplas SURPLAS tied to a SVP-BS drifter (CAROLS2010 cruise, Gulf of Biscay) Reverdin et al. JGR 2012 SVP Surplas French and Spanish SSS drifters 2007-2012 http://www.locean-ipsl.upmc.fr/smos/drifters/ Freshening event +4h

6. Question How reliable is the rain induced SSS variability measured by SMOS? Can we confidently use satellite SSS for studying the influence of rain on sea surface (~1cm) salinity? SMOS SSS (color) & SSM/I rain rate (isolines) Train-Tsmos =0.5h Boutin et al. JGR 2014 in press

7. Boutin et al., 2013 SATELLITE SMOS (Soil Moisture and Ocean Salinity) SSS ESA v5 reprocessing (available since 2010) SSS at 1cm depth ; ~43km resolution (  ~0.6) or monthly 100km averaged (CATDS- CEC/LOCEAN_v2013 product available at www.catds.fr) ; moderate wind speed (3-12m s -1 )www.catds.fr Rain Rate: -RemSS: www.remss.com ; SSM/I; AMSRE; TMI; WindSat: 0.25° resolution within [-30mn;+15mn] from SMOS SSSwww.remss.com -TRMM 3B42 rain rates (within [-2hr;+1hr] from ARGO to identify‘ARGO rainfree’) IN SITU SSS ARGO INDIVIDUAL PROFILES ‘SSS’ between 10m and 4m depth; Colocation with SMOS within +/-5days, +/-50km CORIOLIS GDAAC: http://www.coriolis.eu.org ARGO + TSG OPTIMAL INTERPOLATED SSS MAPS (ISAS) Monthly maps from In-Situ Analysis System v6 http://wwz.ifremer.fr/lpo/SO-Argo/Products/Global-Ocean-T-S SEA SURFACE AUTONOMOUS DRIFTER SSS Upper S at 45cm depth; Pacific Gyre drifter http://www.locean-ipsl.upmc.fr/smos/drifters Data & Methods

8. Rain effect -The closest rainy ARGO-SMOS colocation case 2 ARGO AMSRE 11 Aug. 20:55 ~ARGO profile ISAS Boutin et al, 2013, Ocean Science Rain Rate (mm/hr) 0 2

9. 2 ARGO AMSRE 11 Aug. 20:55 ~ARGO profile SSMIs F17 11 Aug. 13:40 ~SMOS 1st pass ISAS Boutin et al, 2013, Ocean Science Rain Rate (mm/hr) 0 Effect of rain on ARGO & SMOS (The closest colocated case) Norain

10. 10 days after... 0 ARGO ISAS 0.7 0 Rain Rate (mm/hr) 2 02 AMSRE 22 Aug. 9:40 ~ARGO profile SSMI F15 24 Aug. 13:12 ~SMOS 2nd pass ARGO Norain

11. The impact of rain on SMOS SSS SMOS SSS - ARGO_rainfree[-2hr;+1hr] SSS SMOS SSS- ARGO SSS versus satellite RR Tropical Pacific 5N-15N (July-Sept 2010) -0.18 pss/ mm/hr r = -0.5 Boutin et al, JGR, 2014 In SW Pac : -0.22 pss/mm/hr r = -0.5

12. The impact of rain on SMOS SSS ~-0.2pss/mm/hr : Roughness, Atmosphere or Salinity effect? SMOS SSS retrieved with 5m/s error on prior wind speed (instead of 2m/s error) More on SMOS retrieved wind speed: Yin et al. RSE 2013 => Roughness contribution is at the limit of detection by SMOS ~ -0.01pss/mm/hr Atmospheric contribution Rayleigh approximation (e.g., Peichl et al., 2004; Wentz, 2005) ~ - 0.03pss/mm/hr => Fresher 1cm SSS linked to rain: at least -0.14 pss/mm/hr -0.01 pss/ mm/hr Limit of significance Error on a priori WS=5m/s Error on a priori WS=2m/s Boutin et al, JGR, 2014 Similar order of magnitude on Aquarius data, Meissner & Wentz, 2014

13. The impact of rain on SMOS SSS SMOS SSS spatial variations SMOS SSS (color) & SSM/I rain rate (isolines) Train-Tsmos =0.5h Rainy SSS – ‘Rain-free’ SSS ~ -0.19 RR ‘Rain-free’ SSS: average of SSS colocated with RR=0 in a radius of 150km around rainy pixels Slope=-0.19pss/mm/hr r=-0.66 Rainy – ‘rain-free’ SSS Rain Rate (mm/hr) 26 August 2012 Similar dependency as in SMOS-ARGO comparisons: After roughness & atmospheric effect correction => SSSrain – SSSrain_free : at least -0.15 pss/mm/hr How does this compare with S 45cm drifters observations?

14. Trajectories of drifters deployed and validated since 2010  Identification of 470 ‘freshening’ (>0.4 signature) events  Colocations with 6 satellites (WindSat, TMI, SSMIs, AMSRE), at +/-15mn => 25 matchups The impact of rain on in situ SSS at 45cm depth

15. Example of SSS decrease observed by drifter and colocated with RR within 15mn SSM/I F16 RR 21:06 -0.6 The impact of rain on in situ SSS at 45cm depth SSS decrease between local maximum and SSS minimum

16. Wind Speed >15m/s Wind Speed < 3m/s RR averaged over ~75km (mm/hr) The impact of rain on drifter SSS at 45cm depth Local S 45cm decrease Moderate wind speed (3-12m/s) SSSmin-SSSref= -0.21 (+/-0.14) RR r=-0.6 N=21 O Subtropics Tropics

17. Summary Validating satellite SSS under rain cell is a big issue : difficult to colocate in situ & satellite measurements (S, RR, U...) at relevant time/space scales Large variability of SMOS SSS in rainy regions is in reasonnable agreement with drifters S measurements (45cm depth) : SMOS + SSM/I RR: Rain signature on S 1cm at least -0.14 pss/mm/hr (3-12m/s) Drifters : Rain signature on S 45cm : -0.21 (+/-0.14) pss/mm/hr (3-12m/s) However, the monthly difference SMOS-ISAS SSS cannot be fully explained applying a -0.2pss/mm/hr correction on SMOS SSS lead to a smaller than observed SMOS-ISAS averaged difference (locally <0.2, <40% of the observed difference) : need to look in more details into ISAS smoothing/relaxation to climatology, SMOS artefacts (RFIs, islands...) => SPURS 2 experiment should help constraining the S vertical and horizontal variability within a satellite pixel in rainy areas => In situ reference for satellite SSS!

18. Satellite & In Situ Salinity (SISS) Working Group: Understanding Stratification and Sub-Footprint Processes Co-Chairs: J. Boutin (LOCEAN/CNRS), Yi Chao (RSSI) Web page: siss.locean-ipsl.upmc.fr Next meeting: AGU Fall meeting (if you’d like to attend, thanks for contacting us: jb@locean-ipsl.upmc.fr, ychao001@gmail.com)