Dependence of SMOS/MIRAS brightness temperatures on wind speed: sea surface effect and latitudinal biases 

Xiaobin Yin, Jacqueline Boutin

LOCEAN

Yin et al., IEEE TGRS 2012

Wind-induced sea surface emissivity with respect to the ECMWF wind speed: emissivity of H polarization at 0° incidence angle (a) and emissivity of V polarization at 0° incidence angle.

At 0° incidence angle, wind-induced sea surface emissivities (EBw) extracted from SMOS L1C TB v344 in August 2010 are different in H and V pol. Why?

Along track in the FOV (+-20km) and in front of Nadir

SMOS ascending Tbs (2010, both L1c real-time v3 and reprocessing v5): Tbs along track (~ no mixing of polarization) in the Southern Pacific (far from land) from 50°S to 0°N. L2 measurement discrimination except resolution.1. 50 ascending orbits in July (low galactic noise and orbital dynamics in Tp7)2. 50 ascending orbits in August (low galactic noise and orbital dynamics in Tp7)3. 50 ascending orbits in December (high orbital dynamics in Tp7)

SMOS data used

Incidence angles from 0° to 60°

Modeling of SMOS Tbs• Tb = Tbatm↑ + Rsea (Tbatm↓ + Tbsky) exp(-atm) + Tbsea exp(-atm)

Ocean

Atmosphere

Tbsea= (Tbflat+Tbrough) (1-F) + F Tbfoam

=Tbflat+Tbwind

Tbwind derived from SMOS Tbs after correcting for all other effects

Tbsea=esea SST

esea =eflat + ewind

Rsea =1- esea

Wind induced components from the SMOS TB

method

For each month (July, August, December 2010) and each version of TB (v3.4 and V 5.0), Tbwind derived from SMOS TB are binned every1)5° incidence angle2)2° latitude3)1 m/s

Tbwind are related with incidence angle, wind speed, latitude, month and version of L1.

We then check the latitudinal profile of Tbwind

Latitudinal biases of Thwind at 7m/s

L1C v504

July

We observe latitudinal drifts of TBw at given WS and incidence angle θ.

TBwf

Latitudinal biases of Tvwind at 7m/sL1C v504

July

TBwf

Differences between fitted v3.4 and v5.0 TBwf at 0°S and that at 50°S at 7ms-1 i.e.ΔTBw (WS, θ) = TBwf(WS, θ, 0S) - TBwf(WS, θ, 50S)versus incidence angle (circles and points correspond to significant linear correlations). The AFFOV is shown by shading. Number of measurements are shown by dashed lines

ΔTHwind

ΔTVwind

V3.4 V5.0

Seasonal differences are larger than the differences among different versions

No. of measurements at 30° incidence angle vs WS and latitude+ latitudinal profile of WS

July

December

August

Differences between TBw (averaged between 50°S and 0°S) and Yin et al., 2012 model versus wind speed at 5°, 15°, 25°, 35°, 45°, 55° in H polarization (left) and in V polarization (right).

H polarization V polarization

The slopes of the TBw-WS curves changes with seasons and versions of L1C TB.•Problems of galactic noise model at 55°incidence angle.(Gourrion et al., 2013 SMOS- Aquarius workshop)

Can uncertainties in forward models explain the latitudinal biases in TBw?

TB of flat sea surface simulated with permittivity models of Lang et al. (2010), Meissner and Wentz (2004), Blanch and Aguasca (2004) and Klein and Swift (1977) model are different. But the latitudinal differences are less than 0.2 K and the relationship with respect to latitude is non-linear at different incidence angles.

Latitudinal profiles of TB differences between three models and KS model.

Can uncertainties in forward models explain the latitudinal biases in TBw?

The latitudinal differences in the modeled scattered galactic signals (TBgal) are less than 0.05K below 30° incidence angle. At incidence angle above 45°, large latitudinal gradient of TBgal can be seen only between 0°S and 15°S; the differences of TBgal between 15S and 50S are less than 0.1K. Uncertainties in permittivity models and scattered galactic signals can not explain the latitudinal biases in TBw.

Latitudinal profile of scattered galactic signal w.r.t 0N at different incidence angles in July

Conclusions and Perspectives

1. Latitudinal drifts in TBw deduced from SMOS TB of v3 and v5 are observed, especially at low incidence angles in EAFFOV and at large incidence angle above 50°in the front of the FOV.

2. Inaccuracies in modeling of Tbgal, Tbflat, Tbatm and Faraday rotation can not explain the latitudinal drifts in TBw.

3. Empirical estimate of TBw versus WS from SMOS TB is dependent on various seasons and on the TB versions.

SMOS SSS of model 1 (LOCEAN’s) and model 2 (IFREMER’s) are similar and are lower than ARGO OI SSS between 10°S and 20°S, and between 40°S and 55°S, whereas SMOS SSS of model 3 (ICM’s) is higher than ARGO OI SSS between 10°S and 20°S, and between 40°S and 55°S

(a) SSS model 1 (b) SSS model 2

North-south profile of differences between SMOS SSS and ARGO OI SSS in the eastern Pacific OceanMOS SSS within +/-300km from the swath center of the pass over the eastern Pacific on 6th of August, 2010 between 13h03m07s and 13h56m26s are used and averaged over 0.25° bin in latitude.

(c) SSS model 3

dTbwind/dSSS and dTbwind/dSST are small and negligible(<0.02K at 60° incidence angle and the magnitude increase with incidence angle)

dTbwind/dSSS and dTbwind/dSST are small and negligible(<0.02K at 60° incidence angle and the magnitude increase with incidence angle)

Total drifts from 55S to 0S decrease in L1c v504H

pol.V pol.

Incidence angle

Incidence angle

Derivatives of sea surface brightness temperatures with respect to water temperature versus incidence angle can not explain the angular trend nor the magnitude in TB drifts.

July Aug

Dec.

Total drifts from 55S to 0S are close in July and August and are different to the value in December

Orbital dynamics of Tp7 are close in July and August and are different to the value in December