Sea Surface Salinity under rain cells: SMOS satellite and in-situ drifters observations

J. Boutin1, N. Martin1, G. Reverdin1,S. Morisset1, X. Yin1, L. Centurioni2 and N. Reul3

1 LOCEAN, Sorbone Universités, UPMC/CNRS/IRD/MNHN, Paris, France2SIO, La Jolla, CA, USA

3IFREMER, Toulon, France

Boutin et al., 2014

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?

SMOS – ARGO SSS in tropical Pacific 0.1 fresher and more variablethan 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 3932

39

Sargo

Ssm

os

Boutin et al., Ocean Science, 2013

SalinityWell Mixed

Salinity

Dep

th

Stratified with Rain

Dep

th

Salinity

Dep

th

Stratified with Evaporation

Near-Surface Salinity Schematic Diagram for L-band Radiometer

Rain-inducedfresh layer Evaporation-

inducedsalty 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

Boutin et al., EGU 2012

Vertical gradients 15cm & 45 cm depth as seen by surface drifters17 events SVP-BS / Surplas

Vertical gradients 15cm & 45 cm depth as seen by surface drifters17 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

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

Boutin et al., 2013

SATELLITE

SMOS (Soil Moisture and Ocean Salinity) SSS

ESA v5 reprocessing (available since 2010)

SSS at 1cm depth ; ~43km resolution (s~0.6) or monthly 100km averaged

(CATDS-CEC/LOCEAN_v2013 product available at www.catds.fr) ; moderate wind speed (3-

12m s-1)

Rain Rate: - RemSS: www.remss.com ; SSM/I; AMSRE; TMI; WindSat: 0.25° resolution within

[-30mn;+15mn] from SMOS SSS - 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 drifterhttp://www.locean-ipsl.upmc.fr/smos/drifters

Data & Methods

Rain effect -The closest rainy ARGO-SMOS colocation case

SSSsmos on 11/8 at 13h39= 32.9+/-0.2; N=38

SSSsmos on 16/8 at 13h44 = 33.4 +/-0.3; N=34

0

50

100

150

200

Dep

th(m

)

a) ARGO profile on 11/8 20:00 UTC

32.5 33.0 33.5 34.0 34.5 35.0SSS

2

ARGO

AMSRE 11 Aug. 20:55 ~ARGO profile

<SSSsmos 10d,100km>

ISAS

Boutin et al, 2013, Ocean ScienceRain Rate (mm/hr)0 2

SSSsmos on 11/8 at 13h39= 32.9+/-0.2; N=38

SSSsmos on 16/8 at 13h44 = 33.4 +/-0.3; N=34

0

50

100

150

200

Dep

th(m

)

a) ARGO profile on 11/8 20:00 UTC

32.5 33.0 33.5 34.0 34.5 35.0SSS

2

ARGO

AMSRE 11 Aug. 20:55 ~ARGO profile

SSMIs F17 11 Aug. 13:40 ~SMOS 1st pass <SSSsmos 10d,100km>

ISAS

Boutin et al, 2013, Ocean ScienceRain Rate (mm/hr)0

Effect of rain on ARGO & SMOS (The closest colocated case)

Norain

10 days after...

SSSsmos on 11/8 at 13h39= 32.9+/-0.2; N=38

SSSsmos on 16/8 at 13h44 = 33.4 +/-0.3; N=34

0

50

100

150

200

Dep

th(m

)

a) ARGO profile on 11/8 20:00 UTC

32.5 33.0 33.5 34.0 34.5 35.0SSS

0

ARGO

<SSSsmos 10d,100km>

ISAS

SSSsmos on 21/8 at13h49= 33.6+/-0.2; N=37

SSSsmos on 24/8 at13h33= 33.3+/-0.6; N=34

Dep

th(m

)

b) ARGO profile on 22/08 6:52 UTC

32.5 33.0 33.5 34.0 34.5 35.0SSS

0

50

100

150

200

0.7 0 Rain Rate (mm/hr) 2

0 2Rain Rate (mm/hr)

AMSRE 22 Aug. 9:40 ~ARGO profile

SSMI F15 24 Aug. 13:12 ~SMOS 2nd pass

ARGO

Norain

The impact of rain on SMOS SSSSMOS SSS - ARGO_rainfree[-2hr;+1hr] SSSSMOS SSS- ARGO SSS versus satellite RRTropical Pacific 5N-15N (July-Sept 2010)

-0.18 pss/ mm/hrr = -0.5

Boutin et al, JGR, 2014

In SW Pac :-0.22 pss/mm/hrr = -0.5

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/hrLimit of significance

Error on a priori WS=5m/s

Error on a priori WS=2m/s

Boutin et al, JGR, 2014Aquarius (atm-rouhness corrected )=> ~-0.12pss/mm/hr @7m/s, Meissner

The impact of rain on SMOS SSSSMOS 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/hrr=-0.66

Rain

y –

‘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 S45cm drifters observations?

Trajectories of drifters deployed and validated since 2010

Þ Identification of 470 ‘freshening’ (>0.4 signature) eventsÞ Colocations with 6 satellites (WindSat, TMI, SSMIs, AMSRE), at

+/-15mn => 25 matchups

The impact of rain on in situ SSS at 45cm depth

Thanks to LEGOS, LPO, ICM and SPURS-US colleagues for drifters deployments

Example of SSS decrease observed by drifter and colocated with RR within 15mn

SSM/I F16 RR21:06

-0.6

The impact of rain on in situ SSS at 45cm depth

SSS decrease between local maximum and SSS minimum

SSS decrease observed by drifter and colocated with RR within 15mn: a complicated case

SSM/I F17 RR19:24

-0.6

Sref

Smin

SSM/I F16 RR19:36

19:30

SSS decrease between local maximum and SSS minimum

Large variability of RR within 12mn and in most cases only one satellite RR pass => in order to smooth the RR variability, we take RR averaged over 9 pixels around the drifter

The impact of rain on in situ SSS at 45cm depth

Wind Speed >15m/s

Wind Speed < 3m/s

RR averaged over ~75km (mm/hr)

The impact of rain on drifter SSS at 45cm depth

Loca

l S45

cm d

ecre

ase

Moderate wind speed (3-12m/s)SSSmin-SSSref= -0.21 (+/-0.14) RRr=-0.6N=21

O Subtropics• Tropics

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

• At moderate wind speed (~7m/s), decrease of SSS associated with microwave rain rates estimated to be:• AQUARIUS (150km resolution, S1cm ) , ~0.12pss/mm/hr (Meissner et al. 2014)• SMOS (40km resolution, S1cm ) , > ~0.14 pss/mm/hr (This work) • Drifters (ponctual , S45cm ) , ~0.21 pss/mm/hr (This work)

=> the rain impact seems to decrease when increasing the pixel size – qualitatively expected from a spatially heterogeneous process like rain (NB: not only stratification matters - drifters S are deeper than satellite S!) - Is it quantitatively reasonnable? SPURS 2 experiment should help constraining the S vertical and horizontal variability and the RR horizontal variability within a satellite pixel • N.B.: the monthly difference SMOS-ISAS SSS cannot be fully explained

applying a -0.2pss/mm/hr correction on monthly SMOS SSS (locally <0.2 correction) explains less than 40% of the observed SMOS-ISAS SSS : ISAS smoothing/relaxation to climatology, SMOS artefacts (RFIs, islands ...)....