SMOS L2 OS ATBD - ARGANS

64
ICM-CSIC
LOCEAN/SA/CETP
IFREMER
SMOS L2 OS
Algorithm Theoretical
Baseline Document
Doc: SO-TN-ARG-GS-0007
Issue: 3 Rev: 9
Date: 25 January 2013
Page: 64
where ais the azimuth angle between wind direction (w) and the azimuthal observation
angle of radiometer look direction (r): aw-r with all these angles counted
counterclockwise, with origin on x axis (mathematical convention).
Tabulations of Th0, Th1, Th2, Tv0, Tv1, Tv2, U1, U2, V1 and V2 are provided, as functions
of incidence angle, SSS, SST, wind speed assuming a neutral atmosphere, WSn. We will
assume that Tbrough can be entirely described using WSn in future versions, depending on:
-accuracy of ECMWF WS
-possibility of deducing a reliable U* from ECMWF WS and Tair-sea
-sensitivity of Tbrough to WS/U*
we could add dependency on WS.
It is proposed to use WSn components WSx and WSy from ECMWF to initialize the
inversion. Wind speed and direction used to initialize Tb mod values are deduced from WSx
(positive eastward) and WSy (positive northward) as:
2
WSn WSx WSy
w = arctan(WSy/WSx)
2
The tabulation is provided for SSS between 30 and 39 psu, SST between 0 and 30°C, WSn
between 2 and 30 ms -1 , and an incidence angle between 0 and 68 degrees. All the Tb rough
are set to 0 for WSn=0. For the interpolation of the table, a switch allows to choose between a
linear interpolator (following the method described in TGRD) and a Hermite interpolator.
The Hermite interpolation results in a continuous theoretical error on SSS, but is
computationally heavy. The linear interpolation is faster, but it results in a discontinuity of
the theoretical error on SSS, linked to the non-linearity of the model for wind speeds of 7
m/s.
If one of the prior values of the retrieved geophysical parameters is out of the LUT range, or
if any retrieved geophysical parameters goes out of LUT range during the retrieval, different
flags (Fg_OoR_Rough_dim1, Fg_OoR_Rough_dim2, Fg_OoR_Rough_dim3,
Fg_OoR_Rough_dim4) are raised. No extrapolation is done and the boundary value is taken.
4.2.1.3. Error budget estimates (sensitivity analysis)
The advantage of retrieving WSn is that it is comparable to scatterometer derived wind speed.
4.2.2. Practical considerations
4.2.2.1. Calibration and validation
The algorithm described above was derived from L1c data v344 and the new LUT has been
implemented in L2OS v500.
4.2.2.2. Quality control and diagnostics
Values outside the min/max ranges given in the Tbrough tabulations should be deduced from
a linear extrapolation of the two edge values of the tabulations (since most of the