Abstract
Earth observations collected by remote sensors provide unique information to our ever-growing knowledge of the terrestrial biosphere. Yet, retrieving information from remote sensing data requires sophisticated processing and demands a better understanding of the underlying physics. This paper reviews research efforts that lead to the developments of the stochastic radiative transfer equation (RTE) and the spectral invariants theory. The former simplifies the characteristics of canopy structures with a pair-correlation function so that the 3D information can be succinctly packed into a 1D equation. The latter indicates that the interactions between photons and canopy elements converge to certain invariant patterns quantifiable by a few wavelength independent parameters, which satisfy the law of energy conservation. By revealing the connections between plant structural characteristics and photon recollision probability, these developments significantly advance our understanding of the transportation of radiation within vegetation canopies. They enable a novel physically-based algorithm to simulate the “hot-spot” phenomenon of canopy bidirectional reflectance while conserving energy, a challenge known to the classic radiative transfer models. Therefore, these theoretical developments have a far-reaching influence in optical remote sensing of the biosphere.
Highlights
The past a few decades have seen rapid development in scientific research and applications that monitor and/or simulate terrestrial ecosystems with the help of remote sensing data [1]
We focus on four particular topics in the main text, including the decomposition of radiative transfer equation (RTE) into the black-soil (“BS”) and the soil (“S”) problems, the development of the stochastic RTE that efficiently packs 3D canopy features into a 1D form, the spectral invariants theory that links the solutions of the RTE at different wavelengths by a few key canopy structural parameters, and the latest effort to address the “hot-spot” problem in vegetation remote sensing
The directional illumination has an important effect on the angular signature of canopy bidirectional reflectance factor (BRF), which we review
Summary
The past a few decades have seen rapid development in scientific research and applications that monitor and/or simulate terrestrial ecosystems with the help of remote sensing data [1]. This paper intends to contribute a review of the theoretical advancements in modeling radiative transfer processes in 3D vegetation canopies It focuses on the developments of the stochastic radiative equation and the spectral invariant theory, which have been widely applied in retrieving vegetation structural information from remote sensors like MODIS and MISR (Multi-Angle Imaging Spectroradiometer) to the recent EPIC (Earth Polychromatic Imaging Camera) on the DSCOVR (Deep Space Climate Observatory) platform and the latest geostationary sensors like AHI We focus on four particular topics in the main text, including the decomposition of RTE into the black-soil (“BS”) and the soil (“S”) problems, the development of the stochastic RTE that efficiently packs 3D canopy features into a 1D form, the spectral invariants theory that links the solutions of the RTE at different wavelengths by a few key canopy structural parameters, and the latest effort to address the “hot-spot” problem in vegetation remote sensing. We invite the readers to pay more attention to these ideas rather than the mathematical details of the theory, if the latter appears to be a bit complicated at the first look
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