Evaluating an Improved Parameterization of the Soil Emission in L-MEB

In the forward model [L-band microwave emission of the biosphere (L-MEB)] used in the Soil Moisture and Ocean Salinity level-2 retrieval algorithm, modeling of the roughness effects is based on a simple semiempirical approach using three main “roughness” model parameters:
$H_{R}$
,
$Q_{R}$
, and
$N_{R}$
. In many studies, the two parameters
$Q_{R}$
and
$N_{R}$
are set to zero. However, recent results in the literature showed that this is too approximate to accurately simulate the microwave emission of the rough soil surfaces at L-band. To investigate this, a reanalysis of the PORTOS-93 data set was carried out in this paper, considering a large range of roughness conditions. First, the results confirmed that
$Q_{R}$
could be set to zero. Second, a refinement of the L-MEB soil model, considering values of
$N_{R}$
for both polarizations (namely,
$N_{\rm RV}$
and
$N_{\rm RH}$
), improved the model accuracy. Furthermore, simple calibrations relating the retrieved values of the roughness model parameters
$H_{R}$
and
$(N_{\rm RH} - N_{\rm RV})$
to the standard deviation of the surface height were developed. This new calibration of L-MEB provided a good accuracy (better than 5 K) over a large range of soil roughness and moisture conditions of the PORTOS-93 data set. Conversely, the calibrations of the roughness effects based on the Choudhury approach, which is still widely used, provided unrealistic values of surface emissivities for medium or large roughness conditions.