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SMOS L2 Ocean Salinity

Level 2 Ocean Salinity

Using TEC estimated from Stokes 3

24 October 2012

ACRI-st, LOCEAN & ARGANS

SMOS+polarimetry



• TEC may be estimated from A3 assuming Stokes 3 at ground level = 0 (ATBD from ACRI-st & LOCEAN)

• TEC has a high latitudinal gradient in descending orbits with an active sun: L1c snapshot TEC may be biased at high incidence angles, giving incorrect Faraday rotation angles

• Study impact of ‘better’ A3 TEC estimation for high TEC descending orbits 1-6 May 2012: construct OTT, use OTT in L2OS

• Nominally, L2OS retrieves TEC using median of L1c TEC grid point dwell measurements as prior; prototype L2OS v600 can use A3 TEC per measurement (& convergence algorithm adjusts mean TEC bias)

• Test the A3 TEC solution...

Overview



Estimates of TBfwd TEC sensitivity

20 south 25 north

Most sensitive TB is A3 at high incidence angles



TBfwd TEC sensitivity

Most sensitive TB is A3

No TEC retrieval possible: line-of-sight perpendicular to magnetic field



• Apply OTT & perform outlier detection: only use good TB

• Select A3 measurements with high incidence angles close to the trackeg -0.025 < xi < 0.025 and 0.15 < eta < 0.2

• Compute latitude where line of sight cross altitude of 450 km

• Estimate TEC for each A3 value using the forward model & L1c TEC as prior

• Use a latitudinal slippery window to detect & remove A3 TEC outliers,estimate mean & error by latitude

A3 TEC methodology



Example TEC retrieval

Sensitivity of A3 to TEC along the swath

Coast

delta = A3 minus modelled A3 using L1c TEC



Nominal OTT



A3 TEC OTT

A3 TEC interpolated at 450km along each measurement line-of-sight



Nominal OTT std/ra



A3 TEC OTT std/ra

OTT corrected at high incidence angles in HH/VV & Stokes 3



Nominal OTT & L2OS using L1cTEC



OTT & L2OS using A3 TEC

~3% more retrievals, mostly at regions with high TEC gradient/peaks



Delta = nominal minus A3 TEC

Up to ±0.5 psu error in SSS where TEC is wrong (eg latitudes -25 & -50)



A3 TEC compared with L1c TEC

Why is L1cTEC different from A3 TEC?



L2OS retrieved TEC using A3 OTT & A3 TEC prior

Retrieved TEC shows L1c (& A3) TEC underestimated



L2OS retrieved TEC using nominal OTT & L1c TEC prior

Retrieved TEC matches A3 TEC better than L1c TEC



• OTT significantly corrected at high incidence angles in HH/VV & Stokes 3

• Up to ±0.5 psu improvement/correction of retrieved salinity in high TEC regions

• Up to 4% increase in retrievals using A3 TEC for OTT & in L2OS

• Retrieved TEC matches A3 TEC better than L1c TEC

• Next:– evaluate impact on more orbits: low TEC gradient, ascending...– evaluate impact on retrieved salinity – compare to Argo...– determine how algorithm can be integrated into L2OS processor

(OTTs are generated by forward model using L1c TEC, but this method requires OTTs to be applied before deriving A3 TEC)

Conclusions

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