Sea surface roughness and foam signature onto SMOS brightness temperature and salinity

X. Yin1, J. Boutin1, N. Martin1, J. Vergely2, P. Spurgeon3

(1) LOCEAN / IPSL, Paris, France(2) ACRI-ST, Sophia Antipolis, France(3) ARGANS Ltd, Plymouth, England

South Pacific: SSS=0.09+/-0.56N=160

ITCZ trop Pac N:SSS=-0.18+/-0.44N=236

South Indian:SSS=0.11+/-0.46N=50

Subtropical Atl N:SSS=0.1+/-0.35N=59

Colocation radius: +/-5days; +/-50km

Motivation: SMOS Model 1 - ARGO SSS (3-31 August 2010; asc orbits)

versus wind speed (center of orbit)

Pre-launch model 1 (2 scale model + wave spectrum of Durden & Vesecky *2, no foam)

Adjustment of some parameters of roughness and foam modeling

100.225log ( /2)23 *

0*

1.25( )

kku

S k a kg

Roughness model 1:Omnidirectional wave spectrum Durden & Vesecky,1985 :

Foam coverage (from Monahan & O'Muircheartaigh 1986): 10 exp( 0.0861 )cF bU T

a0? Original publication: a0=0.004; DV2, a0=0.008

b? c? original publication: b=1.95×10-5, c=2.55 ; ΔT =Tsea-Tair (neglected in this first step study); in first SMOS SSS1 processing, F=0: no foam.

Foam emissivity (Stogryn, 1972): assumed to be correct

~0.2K/m/s

Dinnat et al., IJRS, 2002, Radio Science, 2003

At 15°C, a 0.1K Tb variation can be generated by :

-0.2pss SSS variation or

- 0.5m/s wind speed variation

10m equivalent neutral wind speed (m/s)

Nadir

Th_30°

DV2

3m/s<U<7m/sWave spectrum parameter

a0 (prior=0.004 – 0.008)

8m/s<U<17m/sFoam coverage model

b, c

Model parameters fitting

Wind induced component of emissivity extracted from SMOS data corrected from flat sea emission, atmospheric effects, galactic noise, Faraday rotation

100.225log ( /2)23 *

0*

1.25( )

kku

S k a kg

10 exp( 0.0861 )cF bU T

Incidence angle (°)Radiometric accuracy Along track in the AFFOV

0

5K

0

5K

SMOS Tbs: Tbs along track (~ no mixing of polarization) in the AFFOV (good radiometric accuracy) from 19 ascending orbits in August (low galactic noise) in the South Pacific (far from land) from 50°S to 0°N – Inciden

ce angles from 20° to 55°

SMOS data used in the fit

(20-55° in step of 5°)

19 half orbits of SMOS in the southern Pacific. 2 sources of wind speeds considered: SMOS-ECMWF and/or SSMI from RemSS:

1. ECMWF WS2. ECMWF WS with the differences between ECMWF and SSMI WS restricted to

be less than 2 ms-1

3. SSMI WS collocated at +-0.5h +-50km,100km away from rain

Wind Speed data

Fitted parameters

Wave spectrum

a0

Foam coverage

b

Foam coverage exponent

c

M1(ECMWF; N~237500) 0.0050 4.862.42×10-8

M2 (ECMWF only if ECMWF-SSMI WS <2m/s; N~127000)

0.0062 5.672.20×10-9

M3 (SSMI; N~137000) 0.0070 5.512.90×10-9

Pre-launch 0.008 - -

Comparison of SMOS-foam coverage model with existing parametrizations

(all ECMWF-SMOS WS)

(ECMWF only if WS consistent with SSM/I)(SSM/I)

Relatively small impact of foam (thin passive foam has probably a small effect)

Pre-launch model 1

The original model

SMOS measured and simulated emissivity versus ECMWF wi

nd speed H & V and various incidence angles in

AFFOV

H pol. 20° V pol. 20°

H pol. 30° V pol. 30°

H pol. 40° V pol. 40°

H pol. 50° V pol. 50°

H pol. 55° V pol. 55°

ECMWF WS ECMWF WS

1. The tropical Southern Pacific ocean (20°S10°S- 140°W110°W) far away from continent and island characterized by relative stable moderate wind speed and high SST; mean (standard deviation) of SST and SSS are 24.5 (1.0) °C and 36.2 (0.3) pss

2. The high latitude Southern Pacific ocean (50°S45°S- 180°W100°W) characterized by very variable wind speed and low SST; mean (standard deviation) of SST and SSS are 9,8 (1.8) °C and 34,4 (0.2) pss

Comparison with ARGO SSS

SMOS SSS retrieved with the pre-launch model 1

SMOS SSS with the new model M1

in red for the tropical Southern Pacific and in green for the high latitude Southern Pacific

Comparison with ARGO measurementsAugust; ascending orbits

Less biases than pre-launch model at high wind speed but still large scatter: We trust more retrievals between 3 and 12m/s

Comparisons w.r.t WOA05 climatology (similar to ARGO analysed map)

prelaunch model 1 (DV2)

New parameters for roughness and foam coverage

Monthly averages,170 Ascending orbits, Pacific Ocean 100W-140W,August 2010

SSS North-South profile,SMOS SSS <300km

Comparisons w.r.t WOA05 climatology (similar to ARGO analysed map) 1m/s

SMOS SSS <300km ECMWF-SMOS WS

SMOS SSS anomaly in the east equatorial Pacific is related to WS correction1 month (170 orbits in Pacific Ocean)

Monthly wind speed

ECMWFSSMI radiometric WS (RSS monthly product)(SSMI 19/22/37 GHz)

SMOS retrieved WS

Radiometer wind speeds lower than ECMWF in that region because of strong surface currents

Summary1. SMOS data evidence that Tb(ws) is non linear

2. New paramters of DV spectrum and foam coverage model are obtained by fitting to SMOS data

3. Preliminary validation of our new model shows decrease of mean retrieved SSS biases in wind speed range of 3-17 ms-1 with respect to the results with the pre-launch model.

4. SMOS retrieved wind speed is partly (but not entirely) corrected for inconsistencies between ECMWF wind speeds and true wind speeds => it remains flaws in SMOS SSS when there are large WS discrepancies => when looking at SMOS SSS anomalies; check first the consistency between ECMWF and SMOS retrieved WS!

Future1. Repeat the same work with L1C v500 and a larger number of data.

2. Find a tradeoff between giving more freedom to the WS in the retrieval and not degrade SSS

Zone Southern PacificLatitude 50S-40S 20S-10SLongitude 180W-100W 140W-110WSST (°C) 9.8 24.5σSST (°C) 1.8 1.0SSS (pss) 34.4 36.2σSSS (pss) 0.2 0.3

pre-launch model 1Wind speed (ms-1) 3-12 12-20 3-12No. of collocations 11714 4060 4556mean(SSSsmos-SSSargo) (pss) 0.08499 -0.89729 0.08477σ (SSSsmos-SSSargo) (pss) 1.06832 1.37645 0.62445median(SSSsmos-SSSargo) (pss) 0.1 -0.82515 0.0905

New model M1No. of collocations 11718 4150 4551mean(SSSsmos-SSSargo) (pss) -0.04846 -0.40917 -0.06231σ (SSSsmos-SSSargo) (pss) 1.0703 1.50764 0.63529median(SSSsmos-SSSargo) (pss) -0.0268 -0.5132 -0.055

Statistics of the SMOS SSS collocated with the ARGO at +/-5days and +/-50km during August ascending passes

EH at 0 deg incidence angle

EV at 0 deg incidence angle