An anisotropic flat index (AFX) to derive BRDF archetypes from MODIS

Spectral vegetation indices can be generalized as a function of surface reflectance with respect to wavelength. However, there is significant information on vegetation structure embedded in the anisotropic effects of the target. In this study, we describe and characterize a new vegetation index, the Anisotropic Flat Index (AFX) that captures this anisotropic scattering information and can be derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) bidirectional reflectance distribution function (BRDF) product suite (MOD43A1). The AFX is created by normalization of net scattering magnitude (obtained from volumetric and geometric-optical scattering) with the isotropic scattering. The AFX summarizes the variability of basic dome-bowl anisotropic reflectance patterns of the terrestrial surface. A classification scheme for BRDF typology is created based on AFX archetypes that capture characteristic BRDF shape types. This study fully characterizes AFX in a number of steps. First, sensitivity to random noise and observation geometries is explored by comparing the AFX with other variables derived from field measurements that comprehensively sample the viewing hemisphere. Second, AFX is compared with normalized difference vegetation index (NDVI) values using field measurements from many ground campaigns, as well as global MODIS observations from EOS Land Validation Core Sites (LVCS). Third, a BRDF typology is developed by classification of an a priori database of BRDF archetypes from field measurements, and from MODIS observations that cover the full range of vegetation types from grasslands to closed forest (MCD43A). Fourth, the response of AFX to the parameter variability of canopy architectures and background optical properties for three vegetation types with discontinuous woody canopies is investigated through the use of a 5-Scale BRDF model simulation. Finally, global BRDF archetypes are mapped and discussed through the use of a global high-quality MODIS BRDF/albedo gap filled product (MCD43GF). The results show that the AFX summarizes BRDF archetypes and provides additional information on vegetation structure and other anisotropic reflectance characteristics of the land surface.