Canopy Reflectance Model Benchmarking: RAMI and the ROMC

Abstract

Canopy radiative transfer (RT) models simulate radiative transfer processes in the solar domain at or near the Earth’s terrestrial surface, i.e., within plant canopies and over bare soil surfaces. Such models are capable of simulating the transmitted, reflected and absorbed radiation fluxes, as well as, the angular distribution of the reflected light above vegetated surfaces in the optical domain of the solar spectrum. This directional aspect is important since the brightness with which a given canopy target appears in Earth Observation data (pixel) depends not only on the structural and spectral properties of the target area but also on the viewing and illumination geometry at the time of measurement. The quantitative interpretation of remote sensing data thus hinges more and more on the exploitation of RT models: In operational contexts they are used to pre-compute Look-Up-Tables of quantities required by the inversion algorithms, whereas, in off-line applications they may be directly inverted against the measurements to yield sets of state variables that describe the properties of the observed target. In both cases, the accuracy and reliability of the solutions to the inverse problems are determined by the performance of the RT model as well as the remote sensing instrument. Reducing the uncertainty of RT models will thus increase the quality of the information derived from remote sensing data (which is of interest to scientists and policy makers), and augment the reliability of RT model studies in forward mode (which is of interest to space agencies when testing new sensor concepts or mission strategies). This understanding has led to a series of model intercomparison projects (MIP) aiming either to document the spread of currently available simulation models, or, else to assess and benchmark the quality of their simulation results, e.g. (Bull. Am. Meteorol. Soc. 73:1962–1970, 1998; J. Geophys. Res. 106:11937–11956, 2001; J. Geophys. Res. 109:D06210, 2004; J. Geophys. Res. 110, 2005; Bull. Am. Meteorol. Soc. 86:1275–1293, 2005; J. Geophys. Res., 2007). Among these MIPs the RAdiation transfer Model Intercomparison (RAMI) activity focuses on the proper representation of the radiative processes occurring in vegetated environments in the optical domain of the solar spectrum.