SMOS L2 OS ATBD - ARGANS

158
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: 158
1. Carry out the integrations as indicated in equations (6). The estimated necessary altitude
range ZM are 20 km for O2, 10 km for H2O; the necessary resolution along the vertical is
better than 100 m.
2. Tabulate the GAS and TbGAS as functions of some parameters (e.g. surface atmospheric
temperature T0, the surface pressure P0, some parameter describing the structure of the
temperature profile, surface humidity,…) and then interpolate from these tables.
3. Build empirical laws to compute GAS and TbGAS.
The most efficient (and physically meaningful) way to do this consists in writing the
emission of each component as the product of optical thickness by an equivalent layer
(physical) temperature, which is conveniently defined by its difference DT with the surface
air temperature T0:
TbGAS = (T0 - DTGAS) GAS
For dry atmosphere, a quadratic fit to results obtained using the whole radiative transfer
computation has been found necessary:
O2 = 10 -6 × (k0_tau_O2 + kT0_tau_O2 × T0 + kP0_tau_O2 × P0 +
kT02_tau_O2 × T0 2 + kP02_tau_O2 × P0 2 + kT0P0_tau_O2 × T0 × P0) / cos()
DTO2 = k0_DT_O2 + kT0_DT_O2 × T0 + kP0_DT_O2 × P0 + kT02_DT_O2 ×
T0 2 + kP02_DT_O2 × P0 2 + kT0P0_DT_O2 × T0 × P0
where
is the incidence angle (radian)
T0 is the near surface air temperature (Kelvin)
P0 is the surface pressure (hectoPascal)
O2 is obtained in neper; DTO2 is obtained in Kelvin
For the water vapour contribution, a linear fit is found adequate:
H2O = 10 -6 × (k0_tau_H2O + k1_tau_H2O × P0 + k2_tau_H2O × WVC) / cos()
H2O = max(H2O, 0)
DTH2O = k0_DT_H2O + k1_DT_H2O × P0 + k2_DT_H2O × WVC (11)
where
WVC is the total precipitable water vapour content (kg m -2 ), available from ECMWF
data.
H2O is obtained in neper; DTH2O is obtained in Kelvin
From values obtained for the DT and quantities:
TbO2 and TbH2O are obtained using eq. (7) respectively for O2 and H2O;
Atmospheric term Tbatm and atm are obtained using eq. (5a) and (5b).
The numerical values for coefficients in eq. (8), (9), (10) and (11) are supplied in TGRD.
These formulas were first written for land. For sea, the required accuracy is better than 0.05
K and this could be achieved using the same formulas but restricting the surface pressure
range to [900 1100] hPa.
(7)
(8)
(9)
(10)