Effective tree scattering and opacity at L-band

Authors: KURUM, M - National Aeronautics And Space Administration (NASA); O'NEILL, P - National Aeronautics And Space Administration (NASA); LANG, R - George Washington University; JOSEPH, A - National Aeronautics And Space Administration (NASA); Cosh, Michael; Jackson, Thomas

Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/10/2012
Publication Date: 3/15/2012
Citation: Kurum, M., O'Neill, P.E., Lang, R.H., Joseph, A.T., Cosh, M.H., Jackson, T.J. 2012. Effective tree scattering and opacity at L-band. IEEE Transactions on Geoscience and Remote Sensing. 118:1-9.

Interpretive Summary: Radiative transfer modeling was used to determine the impact of coniferous trees on L-band soil moisture remote sensing. Accounting for the vegetation attenuation is a necessary component of soil moisture remote sensing. This paper used a first-order model and truck-based microwave measurements over natural conifer stands to investigate the relationship between vegetation parameterization and its theoretical definition. Physical analysis of the scattered and emitted radiation from vegetated terrain were performed using the microwave data which were collected over natural conifer stands located in Maryland in 2008 and 2009. Vegetation opacity of coniferous trees is first obtained by three independent approaches that provide effective, measured, and theoretical estimates. The effective values are found to be smaller than but of similar magnitude to both measured and theoretical values. This implies that the opacity values retrieved by the model could be approximated by the theoretical values and it preserves its physical meaning. This results is an efficient method to link microwave observations with a parameter model, simplifying soil moisture remote sensing over forests.

Technical Abstract: This paper investigates vegetation effects at L-band by using a first-order radiative transfer (RT) model and truck-based microwave measurements over natural conifer stands to assess the performance of the tau-omega model over trees. The tau-omega model is a zero-order RT solution and it accounts for vegetation effects in terms of effective vegetation parameters (vegetation opacity and single-scattering albedo) which represent the canopy as a whole. This approach inherently ignores multiple-scattering effects and it thus has a limited validity depending on the level of scattering within the canopy. The fact that the scattering from large forest components such as branches and trunks is significant at L-band requires that zero-order vegetation parameters be evaluated (compared) with their theoretical definitions to provide better understanding of these parameters in the retrieval algorithms over trees. This paper first compares the effective vegetation opacities, computed from multi-angular pine tree brightness temperature data, against the results of two independent approaches that provide theoretical and measured optical depths. These two techniques are based on forward scattering theory and radar corner reflector measurements, respectively. The results indicate that the effective vegetation opacity values are smaller than but of similar magnitude to both radar and theoretical estimates. The effective opacity of the zero-order model is thus set equal to the theoretical opacity and an explicit expression for the effective albedo is then obtained from the zero- and first- order RT model comparison. The resultant albedo is found to be in similar magnitude with the effective albedo values, obtained from brightness temperature measurements, but of less than half of that of the theoretical calculations, which are generally around 0.5 to 0.6 for tree canopies at L-band. This reduced albedo balances the scattering darkening effect of the large theoretical albedo with a first-order multiple-scattering contribution which is a function of both vegetation and ground properties.