Sea Surface Salinity as Measured by SMOS and by Surface Autonomous Drifters: Impact of Rain

1. Sea Surface Salinity as Measured by SMOS and by Surface Autonomous Drifters:Impact of Rain J. Boutin, N. Martin, X. Yin, G. Reverdin, S. Morrisset LOCEAN, UMR CNRS/UPMC/IRD, Paris, France + collaborations with French GLOSCAL SMOS Cal/Val participants (IFREMER, Meteo-France, LEGOS) and SMOS ESA Expert Support laboratories (ICM/CSIC, LOS/IFREMER, ARGANS-st, CLS, ACRI-st)

2. Motivations => L-band Radiometry (Soil Moisture and Ocean Salinity, SMOS) • SSSsmos-SSSargo in the tropical Pacific are 0.1 lower than in the subtropical Atlantic. Could it be an effect of rain? • Skin depth of L-band radiometric signal is 1cm whereas ARGO SSS are measured at typically 5m depth: is rain effect detectable on SMOS-ARGO comparisons?

3. SMOS SSS - July 2010L1 & L2 v500 Ascending swaths(center swath, 3m/s<WS<12m/s) SMOS SSS Comparison with ARGO: SMOS averaged over 10 days-100km around ARGO Subtropical Atl N (SPURS region) SSSsmos-SSSargo=0.04+/-0.22 N=76 ARGO objective analysis (Gaillard et al.) ITCZ trop Pac N SSSsmos-SSSargo=-0.05+/-0.40 N=248

4. SMOS SSS & Rain colocation • SMOS SSS - 40km resolution retrieved from ~150 Tbs measured at various incidence angles, 2 polarisations=> SSS and ECMWF adjusted wind speed • Rain Rate deduced from SSMI F16 and F17 (RemSS version 7); 32km spatial resolution • Colocation of SMOS SSS and SSMI Rain Rate: Maximum of RR (within -5hours and +1hour to SMOS time) and falling in the SMOS ISEA grid point • Statistics of SSSsmos-SSSargo depending on Maximum RR within -5h/+1h of SMOS SSS

5. SMOS SSS - ARGO SSS < in the tropical Pacific than in the subtropical Atlantic : Rain freshening in 1 cm of the ocean surface (SMOS) with respect to ~5m (ARGO) SSMI F17 Rain Rate (mm/hr) in July 2010 Rain Rate <1mm/hr N=9419 Mean diff: 0.05 Std_Diff=0.56 Skewness=0.014 All Rain Rate N=12679 Mean diff: -0.02 Std_Diff=0.62 Skewness=-0.47 Rain Rate >1mm/hr N=3260 Mean diff: -0.23 Std_Diff=0.75 Skewness=-0.708 V500 Sun Off - OTT asc - July 2010 Boutin et al. 2011 SSSsmos-SSSargo

6. r=0.47 -0.2 / mm/hr

7. Comparison with ARGO: SMOS averaged over 10 days-100km around ARGO Subtropical Atl N SSSsmos-SSSargo=0.04+/-0.22 Npairs=76 Nsmos/argo=64 ITCZ trop Pac N All Rain: SSSsmos-SSSargo=-0.05+/-0.40 N=248 No Rain SSSsmos-SSSargo=0.06+/-0.39 Npairs=219; Nsmos/argo=28 Ascending orbits (center of orbit; OTT asc) 3<WS<12m/s SMOS SSS averaged around ARGO (+/-50km; +/-10days) V500 Sun Off - OTT asc - July 2010

8. Conclusions & Perspectives • Precision of 10days-100km SMOS SSS in a warm non rainy region ~ 0.2 (ascending orbits only in July v500) • Monthly SMOS SSS (1cm) minus ARGO Salinity (~5m) in the tropical Pacific (average over 5°N-15°N; 110°W-180°W) 0.1 lower than in the Atlantic subtropics because of rain freshening(ΔSSS/RRmax=-0.2 / mm/hr) Perspectives • Redo/extend this analysis in time with reprocessed SSS • Look at angular polarised signature of rain • Rainy drifters - SMOS SSS colocations once SMOS reprocessed archive available (challenging: need for a lot of colocated events; punctual versus integrated SSS…)

9. Small effect of rain in the atmosphere (Rayleigh scattering) at L-band (J Schulz, 2002; Wentz, 2005) In warm region, RR ~ 10mm/hr =>rayleigh scattering ~ 0.2K => SSS bias ~ 0.14pss at 10mm/hr : we see a much larger effect Wentz, 2005

10. Could it be an artefact of L-band radiometric measurements? • At L-band, weak impact of atmosphere (RR<10mm/hr); • Roughness effect on SMOS rainy measurements? Differences between ECMWF and SMOS retrieved wind speed are not correlated with RR (N.B.: 1psu  ~0.7K  ~3.5m/s)

11. Motivations (2) • Surface in situ salinity data between 15cm and 1m depth evidence salinity gradients near the surface in case of rain (TOGA COARE, Soloviev and Lucas, 1997; next slides); however these observations are punctual • Most in situ measurements (ship; ARGO) are made at a few meters depth (=> they miss near-surface gradients) • Fresh-lenses => thin surface layer isolated from the water below (stratification; air-sea exchanges) • Gas properties (e.g. CO2 solubility) depend on salinity

12. 03/02/03 28/01/03 In situ measurements :large salinity stratification in case of rain 34 60 TAO buoy 3.5°N – 95°W, Pacifique Est 1,1 pss 1,5 pss 1,4 pss 31 0 Precipitation rate (mm/hr)salinity at 1 m depthWind speed (m/s) salinity at 5 m depthsalinity at 10 m depth Hénocq et al., JAOT, 2010

13. • Pacific Gyre (SBE 37 SI) ARGO at +/-100km, +/-2days Float Autonomous drifter (~50cm depth) Example of SSS freshening in Atlantic ITCZ See poster Morrisset, et al; Reverdin et al. JGR 2011, in revision

14. SSS rain-freshening temporal evolution as seen by 60 drifters in the tropical Oceans 35.1 SSS temporal evolution after a freshening event (time 0) 34.7 4hr 15hr 0hr 10 Satellite Rain Rates (SSM/I, TMI, AMSRE (www.ssmi.com)) colocated with floats SSS 0 0hr 4hr 15hr Figure 7: Average cycle of salinity (upper panel) among 60 salinity drop events (relative to a common time of beginning of event). Individual records are shifted to a common salinity value at the initial drop time and the magnitude of the drop is adjusted to the mean drop. The average is plotted as well as individual events. The associated average reported rainfall (mm/hour) is plotted in the lower panel by 2-hour average, as well as the individual values at the exact time of reports. See poster Morrisset, et al; Reverdin et al. JGR 2011, in revision

15. Vertical gradients 15cm – 45 cm 17 events SVP-BS / Surplas SURPLAS tied to a SVP-BS drifter (CAROLS2010 cruise, Gulf of Biscay) See poster Morrisset, et al; Reverdin et al. JGR 2011, in revision