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)
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