Simultaneous Retrieval of Surface Roughness Parameters for Bare Soils From Combined Active–Passive Microwave SMAP Observations

An active–passive microwave retrieval algorithm for simultaneous determination of soil surface roughness parameters [vertical root-mean-square (RMS) height (
${s}$
) and horizontal correlation length (
${l}$
)] is presented for bare soils. The algorithm is based on active–passive microwave covariation, including the improved Integral Equation Method (I
²
EM), and is tested with global soil moisture active passive (SMAP) observations. The estimated retrieval results for
${s}$
and
${l}$
are overall consistent with values in the literature, indicating the validity of the proposed algorithm. Sensitivity analyses showed that the developed roughness retrieval algorithm is independent of permittivity for
${\varepsilon }_{s} > 10$
[-]. Furthermore, the physical model basis of this approach (I
²
EM) allows the application of different autocorrelation functions (ACF), such as Gaussian and exponential ACFs. Global roughness retrieval results confirm bare areas in deserts such as Sahara or Gobi. However, the type of ACF used within roughness parameter estimation is important. Retrieval results for the Gaussian ACF describe a rougher surface than retrieval results for the exponential ACF. No correlations were found between roughness results and the amount of precipitation or the soil texture, which could be due to the coarse spatial resolution of the SMAP data. The extension of this approach to vegetated soils is planned as an add-on study.