Radiation transfer model intercomparison (RAMI) exercise

The community involved in modeling radiation transfer over terrestrial surf aces designed and implemented the first phase of a radiation transfer model intercomparison (RAMI) exercise. This paper discusses the rationale and mo tivation for this endeavor, presents the intercomparison protocol as well a s the evaluation procedures, and describes the principal results. Participa nts were asked to simulate the transfer of radiation for a variety of preci sely defined terrestrial environments and illumination conditions. These we re abstractions of typical terrestrial systems and included both homogeneou s and heterogeneous scenes. The differences between the results generated b y eight different models, including both one-dimensional and three-dimensio nal approaches, were then documented and analyzed. RAMI proposed a protocol to quantitatively assess the consequences of the model discrepancies with respect to application, such as those motivating the development of physica lly based inversion procedures. This first phase of model intercomparison h as already proved useful in assessing the ability of the modeling community to generate similar radiation fields despite the large panoply of models t hat were tested. A detailed analysis of the results also permitted to ident ify apparent "outliers" and their main deficiencies. Future undertakings in this intercomparison framework must be oriented toward an expansion of RAM I into other and more complex geophysical systems as well as the focusing o n actual inverse problems.