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Above Snow Vegetation Effects on Wideband Autocorrelation Radiometry

The concept of wideband autocorrelation radiometry has recently been proposed and applied for the remote sensing of snow and lake ice. Such instruments measure the microwave emission from a scene of snow and ice over a wide and low frequency band where volume scattering within the snow/ice layer is negligible. Experiments have demonstrated that such wideband brightness temperature spectra can show oscillatory features. These features arise from coherent interference among the direct upward emission and its replicas from multiple reflections, are related to the layer thickness of snow or ice, and can be affected by interface roughness or any above snow vegetation canopy. The latter is therefore an important factor affecting the application of wideband autocorrelation radiometry in terrestrial snow remote sensing. In this paper, we analyze the effects of an above snow vegetation layer on brightness temperature spectra, particularly the possible decay of wave coherence arising from volume scattering in the vegetation canopy. In our analysis, the snow layer is assumed to be flat, and its upward emission and surface reflectivities are modeled by a fully coherent model, while the volume scattering from the vegetation layer is described by an incoherent radiative transfer model. The solution to the radiative transfer equation is obtained through an interative approach accounting for multiple scattering effects. The angular and polarization coupling arising from volume scattering and the emission contributed by the vegetation layer all cause smoothing of oscillatory patterns in the observed brightness temperature spectra.