不连续植被二向性反射的几何光学与辐射传输一体化综合模型初探

A Hybrid Geometric Optical and Radiative Transfer Approach for Modeling BRDF of Discontinuous Canopies

本文论述作者对不连续植被冠层与辐射间相互作用机理研究的最新进展,提出了建立不连续植被二向性反射分布函数的几何光学与辐射传输一体化综合模型的设想和基本理论。该综合模型的建立以Li-Strahler间隙概率模型为基础,在大尺度上用纯几何光学模型反映植被的结构特征,在适于辐射传输模型的尺度上则以冠层内的间隙概率处理叶丛的截获辐射与散射。本文详细说明了冠层内任意高度的间隙率模型,也介绍了多次散射的建模方法。

This paper presents new ideas and recent progress in our joint effort to model the interaction mechanism of radiant flux with discontinuous canopies. At the scale of a small volume in which leaves can be regraded as homogeneously distributed, some recently develop-ped RT (Radiative Transfer) theories are the btst to model the phenomenon by using descriptors such as leaf scattering characteristics, leaf size, volume density of leaf area, etc. However, at the scale of a stand or TM pixel, GO (Geometric Optical) models catch the basic features of such stands under sunlight, i.e., foliage are clustered into crowns and crowns cast shadows. Hence in present practi ce, simple pure GO models are the only models which can be applied for natural discontinuous canopies.However, pure GO models require signatures of sunlit and shaded crown surface and background as known parameters under given situation or to be determined in situ. This has been proven a major restricting factor in applications and model inversion. And it is also a drawback that these signatures have not been related to leaf descriptors, wavelength, and sky conditions.In this paper, we proposed an integrated GO-RT approach to model the radiation climate in a discontinuous canopy. A key link in this approcah is gap probability (Pgap) model which we developped earlier. Gap probability, on one hand, can be obtained through pure GO model which reflects the structure at the stand scale. On the other hand, gap probability within crown is closely related to the pro cess that radiation collides and is scattered by foliage at the scale of RT models. Hence it becomes a natural link between two kinds of models best at the corresponding scales.In Li and Straheler (1988), Pgap was modeled at the ground only. In this paper, the way to model Pgap at any height within canopies is developed. Vertical distributions of sunlit crown surface and within-crown pathlength distributions for different canopy structures are demonstrated. Since the penetration depth of direct sunlight into canopies play an important role in many fields other than remote sensing, we present here the results at this early stage of our modeling work to share with our colleagues in China for comments and possible validations. The way to model multiple scattering is also suggested in this paper. The concept of 'Qpenness(Kopen) distribution' is introduced, and the (Kopen) at ground has been applied to shrub sites in HAPEX-Ⅱ-Sahel experiment and got good results. (Franklin et al. submitted to Agricultural and Forest Meteor., Feb., 1993). The approach is still under development, though the initial results are encouraging.