Fig. 2: Radiometric angular response from deciduous Paulownia trees is plotted. The red, blue, black, and green curves trace the simulated values of four models while the squares and triangles represent v- and h-polarized measured data collected in April, 2007, respectively. L-Band (1.4 GHz) Brightness Temperature from Vegetated Landscapes: Comparison of Approximate Techniques Mehmet Kurum and Peggy O’Neill, NASA GSFC, Code Fig. 1: ComRAD microwave instrument system deployed over a stand of Paulownia trees. Four approximate physical microwave radiometry models, based on radiative transfer (RT) theory, is presented to calculate brightness temperatures from a forest canopy. These models are based on zero-order and first-order RT solutions with different choice of small parameters. As seen from Fig. 2, the first-order scattering solution (τ-ω-Ω model) captures the angular and polarization behavior of the data, which were collected by the ComRAD microwave instrument over deciduous trees as seen in Fig. 1. The τ-ω-Ω model balances the scattering albedo darkening effect with a single scattering contribution for vegetation canopies (with large scatterers). The scattering in tree canopies thus takes place as a combination of reduction due to albedo and addition due to the single scattering.

Name: Mehmet Kurum and Peggy O’Neill, NASA GSFC Phone: References: M.Kurum, R. H. Lang, P. O’Neill, A Joseph, T. Jackson, and M. Cosh, “A First-Order Radiative Transfer Model for Microwave Radiometry of Forest Canopies at L- band”, Accepted for Publication, IEEE Transcation on Geoscience and Remote Sensing, M. Kurum, R. H. Lang, P. O’Neill, “L-Band Brightness Temperature from Forest: Approximate Techniques”, presented, Progress in Electromagnetics Research Symposium (PIERS), Cambridge, USA, on 5-8 July, 2010 (invited). Data Source: NASA’s Terrestrial Hydrological Program has funded a three-year field experiment to measure the L band microwave response to soil moisture (SM) under different types of small to medium tree canopies. The project was a collaboration between GSFC, the George Washington University, and USDA. The truck- mounted ComRAD radar/radiometer instrument system was used to obtain microwave data over deciduous and coniferous trees coincident with measurements of soil and vegetation properties. Technical Description of Image: Iterative techniques to solve Radiative Transfer (RT) equations have been extensively applied to microwave remote sensing of earth for more than three decades. The classic books of this subject include Ishimaru, Tsang et al., Ulaby et al., and Fung. Iterative solutions of RT equations are described for active and/or passive problems of random medium. The volume scattering coefficient is usually considered small in these works and as a result the first-order solutions for active and/or passive problems refer to a first order solution in volume scattering coefficient. In this study, a similar approach with a different choice of small parameter is adapted to compute forest emission. Basically, the scattering source function of RT equations is interpreted as a perturbation to the non- scattering RT equations as oppose to volume scattering coefficient. This perturbation technique is known as the method of “successive orders of scattering”. The first order scattering solution is named as τ-ω-Ω model. Fig. 2 compares four approximate RT-based microwave radiometry models, including first-order scattering solution, against the experimental data collected over deciduous Paulownia trees by ComRAD microwave system. These models are (1) zero-order albedo expansion (a0 - non-scattering), (2) first-order albedo solution [a1- equivalent to Peak technique utilizing the active solution obtained from the Distorted Born Approximation (DBA)], (3) τ-ω model (s0 - zero-order scattering approximation to RT equations), (4) τ-ω-Ω model (s1- first order scattering approximation to the RT equations). These models are physically-based and treat vegetation as a layer of discrete scatterers over a rough surface. Vegetation components within the canopy are represented by canonical shapes such as dielectric discs and cylinders. Scientific Significance: Despite the progress that has been made in the development of global soil moisture (SM) retrieval algorithms, accurately correcting for with respect to effects of vegetation scattering and attenuation over a wide range of vegetation canopies remains one of the ongoing challenges. The current baseline retrieval algorithms for ESA’s SMOS mission and candidate retrieval algorithms for NASA’s SMAP are based on an easily implemented but theoretically simple zero- order RT approach (τ-ω model) model. The model includes components from the soil and vegetation, but vegetation scattering is not represented properly. This approach essentially places a limit on the density of the vegetation through which SM can be accurately retrieved. As the simulation results indicate, the τ-ω model will need modification to enable accurate characterization of vegetation parameters when applied over moderately to densely vegetated landscapes. More scattering terms (at least up to first-order at L-band) should be included in the RT solutions for forest canopies due to the large size of the tree canopy components o wavelength. This study could lead eventually to better characterization of dense vegetation in view of spaceborne SM retrieval algorithms. Relevance for future science and relationship to Decadal Survey: The τ-ω-Ω model can be attractive for routine microwave SM retrieval since the formula relating terrain emission is physically-based, takes canopy scattering into account properly, and requires few parameters. The new parameter Ω depends on polarization and incidence angle and mostly SM independent. This model could potentially overcome the vegetation scattering limitation and thus could be used with SMAP and SMOS data to increase the accuracy and reliability of SM products over moderately to densely vegetated landscapes.