SMOS L2 OS ATBD - ARGANS

170
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: 170
The brightness temperature at the ground level (BOA) due to sea surface emission (at BOA
there is also the reflection of atm. signal + galactic signal etc) is as follows (steps 1 to 3):
Tb ( Tb Tb
)( 1
F)
Tb
[4.13.1]
sea
flat
rough
Tbflat is the Tb for a flat sea, as described in module 1, and Tbrough is the contribution of the
roughness of the sea, with 3 different options described in 4.2, 3 and 4. F is the fraction of sea
foam coverage that mainly depends on the wind speed (WS) and is given in module 5. Tbfoam
is the brightness temperature due to the foam and described also in module 5. If flag
Fg_ctrl_foam_MX is set to false, then F and Tbfoam must be set to 0. This will always happen
with the empirical roughness model since it takes already indirectly into account the foam
effect. The effect of foam is only appreciable for wind speeds higher than 12 m/s. However,
the terms envolving the foam contribution (F and Tbfoam) should be applied always, unless
otherwise specified through a switch (see sections 4.2 and 4.3), and it is already considered in
the definition of F and Tbfoam in module 5. When roughness models 4.2 and 4.3 are applied,
Tbrough and Tbfoam can be set to 0 according to sea surface conditions (behind a switch).
Then atmosphere and extraterrestrial sources are considered (steps 6 and 7)
Tb
BOA
atm
Tb Tb Tb ( Tb ( Tgal _ refl ) e
τ
) [4.13.2]
sea
reflected
sea
DN
where Tbreflected is the radiometric temperature from the sky and atmosphere scattered by the
surface, which is the addition of two terms; the downward emitted atmospheric radiation
(TbDN) and the brightness due to extraterrestrial sources. The extraterrestrial sources
considered here (Tgal_ref) are the hydrogen line, the cosmic and galactic contribution already
reflected on the sea surface, as explained in section 4.6 and 4.7 of this document. Tgal_ref is
multiplied by an attenuation factor due to the atmosphere, since its formulation is at top of the
atmosphere. A switch will be available to choose between galactic noise contamination 1
(section 4.6) or 2 (section 4.7) to computed the reflected noise (at the moment, the semiempirical
model will only use the galactic noise contamination 1 module). The atmospheric
contribution term (TbDN) is multiplied by , that is the reflection coefficient
To compute Tb at top of the atmosphere (without considering Faraday rotation, for practical
reasons) in the Earth reference frame (steps 8 and 9):
EARTH
atm
TbTOA TbBOA
e
τ
Tb
UP
foam
[4.14.3]
where TbUP is the atmospheric self emission direct to the antenna (but computed at TOA and
atm
with Earth reference frame) and e
τ
is the attenuation produced by the atmosphere. Section
4.9 specifies that TbUP and TbDN are very close, and considered equal to Tbatm, which is
defined in that section.
Finally to compute this temperature at the antenna reference frame, the geometrical
transformation and the ionospheric effect should be considered, following module 12, as
(step 10):
EARTH
MR Tb
ANTENNA
Tb
4 [4.13.4]
TOA