A Multiple Scattering Model of Brightness Temperatures for Soil Moisture Retrieval in Forests and Comparison with SMAPVEX12 Data

Abstract:

Microwave remote sensing of the vegetated land surface is essential, and the number of microwave radiometers, including AMSR2, SMOS, SMAP, NASA-ISRO, and NISAR, is growing rapidly. The sensitivity of brightness temperature signatures to land surface features are affected strongly by the sensing wavelength, as well as the landscape structure and soil moisture conditions. A physical model of the vegetated land surface is required to interpret and synergistically utilize data from these missions.

Passive microwave remote sensing and retrieval algorithms of satellite missions have been based on the tau-omega model over the years. The tau-omega model does not include the effects of multiple scattering. The effective scattering albedo of the tau-omega model, estimated empirically for specific vegetation types, is generally much smaller than the physical scattering albedo. It remains questionable whether the tau-omega model applies for moderate to large scattering albedo and optical thickness. The new physical model discussed in this paper is derived from the radiative transfer theory. In this method, the scattering albedo and the optical thickness are calculated physically. The solution to the radiative transfer equations are computed iteratively to include higher order scattering which represents the multiple scattering for thick vegetated land surfaces.

This paper evaluates the effects of multiple scattering for a layer of trees on top of soil by solving the vector radiative transfer equations with an iterative method to high orders numerically. The dependences of brightness temperatures on soil moistures and volumetric water contents (VWC) of the trees are examined and compared to the SMAPVEX12 campaign data. Results on the brightness temperatures of the Aspen trees will be presented and discussed.