Abstract
To enhance the capability of three-dimensional (3D) radiative transfer models at the kilometer scale (km-scale), the radiosity applicable to porous individual objects (RAPID) model has been upgraded to RAPID3. The major innovation is that the homogeneous porous object concept (HOMOBJ) used for a tree crown scale is extended to a heterogeneous porous object (HETOBJ) for a forest plot scale. Correspondingly, the radiosity-graphics-combined method has been extended from HOMOBJ to HETOBJ, including the random dynamic projection algorithm, the updated modules of view factors, the single scattering estimation, the multiple scattering solutions, and the bidirectional reflectance factor (BRF) calculations. Five cases of the third radiation transfer model intercomparison (RAMI-3) have been used to verify RAPID3 by the RAMI-3 online checker. Seven scenes with different degrees of topography (valleys and hills) at 500 m size have also been simulated. Using a personal computer (CPU 2.5 GHz, memory 4 GB), the computation time of BRF at 500 m is only approximately 13 min per scene. The mean root mean square error is 0.015. RAPID3 simulated the enhanced contrast of BRF between backward and forward directions due to topography. RAPID3 has been integrated into the free RAPID platform, which should be very useful for the remote sensing community. In addition, the HETOBJ concept may also be useful for the speedup of ray tracing models.
Highlights
Modeling radiative transfer (RT) of vegetation is a fundamental problem of land surface remote sensing
In mountainous areas, forest bidirectional reflectance distribution function (BRDF) and albedo are significantly affected by the heterogeneous terrain and complex vegetation composition [9,10]
To simulate the combined effects of heterogeneous terrain and complex vegetation composition on moderate resolution imaging spectroradiometer (MODIS) or multiangle imaging spectroradiometer (MISR) BRDF products, RT models are expected to contend with a domain size greater than 0.5 km
Summary
Modeling radiative transfer (RT) of vegetation is a fundamental problem of land surface remote sensing. To simulate the combined effects of heterogeneous terrain and complex vegetation composition on MODIS or MISR BRDF products, RT models are expected to contend with a domain (or scene) size greater than 0.5 km. The GO models have a key role in describing generalized BRDF features, and can simulate canopy reflectance reasonably well in many cases They are heavily reliant on the validity of various sets of assumptions and simplifications, and are limited to relatively simple heterogeneous scenes on flat or constant slope terrains. Complaints of computer simulation models (both ray tracing and radiosity) have been discussed because of their low computation efficiency from considering too much of the 3D details in the explicit descriptions of location, orientation, size, and shape of each scatterer This limitation may hamper their expansion at the km-scale.
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