One-dimensional models of radiation transfer in heterogeneous canopies: a review, re-evaluation, and improved model

Abstract

Abstract. Despite recent advances in the development of detailed plant radiative transfer models, large-scale canopy models generally still rely on simplified one-dimensional (1-D) radiation models based on assumptions of horizontal homogeneity, including dynamic ecosystem models, crop models, and global circulation models. In an attempt to incorporate the effects of vegetation heterogeneity or “clumping” within these simple models, an empirical clumping factor, commonly denoted by the symbol Ω, is often used to effectively reduce the overall leaf area density and/or index value that is fed into the model. While the simplicity of this approach makes it attractive, Ω cannot in general be readily estimated for a particular canopy architecture and instead requires radiation interception data in order to invert for Ω. Numerous simplified geometric models have been previously proposed, but their inherent assumptions are difficult to evaluate due to the challenge of validating heterogeneous canopy models based on field data because of the high uncertainty in radiative flux measurements and geometric inputs. This work provides a critical review of the origin and theory of models for radiation interception in heterogeneous canopies and an objective comparison of their performance. Rather than evaluating their performance using field data, where uncertainty in the measured model inputs and outputs can be comparable to the uncertainty in the model itself, the models were evaluated by comparing against simulated data generated by a three-dimensional leaf-resolving model in which the exact inputs are known. A new model is proposed that generalizes existing theory and is shown to perform very well across a wide range of canopy types and ground cover fractions.

A parameterization of sub-grid topographical effects on solar radiation in the E3SM Land Model (version 1.0): implementation and evaluation over the Tibetan Plateau

“…The inclusion of sub-grid topographic parameterizations for solar radiation in ELM improves the representations of surface energy balance to some degree, but many shortcomings in ELM's existing radiative transfer modeling scheme limit the potential for further improving the ELM simulations. The 1D two-stream approximation method used in ELM represents the vegetation canopy as a homogeneous "big leaf" (Yuan et al, 2017) and neglects the vertical multilayer structure (Bonan et al, 2018) and the horizontal leaf clumping (Bailey et al, 2020;Braghiere et al, 2020;. In the snow-covered regions, the ELM parameterizations for the effects of snow impurities (i.e., black carbon and dust mixing) on light scattering and absorption processes need to be refined to account for internal mixing and nonspherical shapes of snow grains (Dang et al, 2019;He et al, 2018).…”

“…ELM also represents the snow hydrological processes including snowfall accumulation, melting, refreezing, compaction, aging, water transfer across layers, etc. New features in ELM to better represent land surface processes include an updated representation of soil hydrology, improved treatment of ecosystem carbon dynamics, a novel topography-based sub-grid spatial structure, and an irrigation scheme constrained by water management (Bisht et al, 2018;Tang and Riley, 2018;Tesfa and Leung, 2017;Zhou et al, 2020).…”

A Parameterization of Sub-grid Topographical Effects on Solar Radiation in the E3SM Land Model (Version 1.0): Implementation and Evaluation Over the Tibetan Plateau

“…The inclusion of sub-grid topographic parameterizations in ELM improves the representations of surface energy balance to some degree, but many shortcomings in ELM's existing radiative transfer modeling scheme limit the potential for further improving the ELM simulations. The 1D two stream approximation method used in ELM represents the vegetation canopy as a homogeneous "big leaf" (Yuan et al, 2017) and neglects the vertical multi-layer structure and the horizontal leaf clumping (Bailey et al, 2020;Braghiere et al, 2020;Li et al, 2019a). In the snow-covered regions, the ELM parameterizations for the effects of snow impurities (i.e., black carbon and dust mixing) on light scattering and absorption https://doi.org/10.5194/gmd-2021-55 Preprint.…”