The fourth radiation transfer model intercomparison (RAMI‐IV): Proficiency testing of canopy reflectance models with ISO‐13528

Abstract

[1] The radiation transfer model intercomparison (RAMI) activity aims at assessing the reliability of physics-based radiative transfer (RT) models under controlled experimental conditions. RAMI focuses on computer simulation models that mimic the interactions of radiation with plant canopies. These models are increasingly used in the development of satellite retrieval algorithms for terrestrial essential climate variables (ECVs). Rather than applying ad hoc performance metrics, RAMI-IV makes use of existing ISO standards to enhance the rigor of its protocols evaluating the quality of RT models. ISO-13528 was developed "to determine the performance of individual laboratories for specific tests or measurements." More specifically, it aims to guarantee that measurement results fall within specified tolerance criteria from a known reference. Of particular interest to RAMI is that ISO-13528 provides guidelines for comparisons where the true value of the target quantity is unknown. In those cases, "truth" must be replaced by a reliable "conventional reference value" to enable absolute performance tests. This contribution will show, for the first time, how the ISO-13528 standard developed by the chemical and physical measurement communities can be applied to proficiency testing of computer simulation models.Step by step, the pre-screening of data, the identification of reference solutions, and the choice of proficiency statistics will be discussed and illustrated with simulation results from the RAMI-IV "abstract canopy" scenarios. Detailed performance statistics of the participating RT models will be provided and the role of the accuracy of the reference solutions as well as the choice of the tolerance criteria will be highlighted.

Emulation of Leaf, Canopy and Atmosphere Radiative Transfer Models for Fast Global Sensitivity Analysis

“…A diversity of canopy RTMs have been developed over the last three decades with varying degrees of complexity. Gradual improvement in RTMs accuracy, yet in complexity too, have diversified RTMs from simple turbid medium RTMs towards advanced Monte Carlo RTMs that allow for explicit 3D representations of complex canopy architectures (e.g., see the RAMI exercises [12,13] for a meticulous comparison). This evolution has resulted in an increase in the computational requirements to run the model, which bears implications towards practical applications.…”

Global leaf trait estimates biased due to plasticity in the shade

“…ref. 32) along with radiative transfer modelling 33 offer promising paths forward. Properly accounting for plasticity in studies of leaf functional trait relationships is necessary to improve our understanding of trait variation and the nature of economic tradeoffs within the whole plant economic spectrum 1 .…”

Extracting Leaf Area Index by Sunlit Foliage Component from Downward-Looking Digital Photography under Clear-Sky Conditions

“…The 3D realistic structural scenes can account for the location, orientation, size, and shape of every individual leaf within a canopy, and thus have the advantage of knowing exactly the true values of each parameter, such as LAI, which provides the objective ground truth for the validation of various indirect LAI measurement methods [37,51]. In fact, the 3D realistic structural scene based approaches have been widely used as the "surrogate truth" in the RAdiative transfer Model Intercomparison (RAMI) exercise conducted by the Joint Research Center, European Commission [52]. It should be noted that the following analysis was based on synthetic images by the 3D scene instead of real images.…”