基于光谱不变量理论的植被反照率模型

Albedo is an essential parameter in the land surface energy budget and climate change. The multiple scattering among different components mainly caused the complexity of the albedo mechanism. In particular, the vegetation BRDF is diverse because of the multiple interactions of photons within the canopy-soil system. To describe the variation of surface BRDF, the common albedo model is based on a statistical method, which extropolate the BRDF model using multi-angle observations and then calculate albedo by angular integration. The reliance on multi-angle dataset confines the application of the model on high spatio-temporal resolution data. To meet the demand of multi-scale vegetation albedo products, this program expects to develop a physical and analytical albedo model for continuous and noncontinuous vegetation. The model calculates the vegetation albedo directly from the muti-scattering mechanism of canopy-soil system based on the law of energy conservation. Therefore, the model explains the mechanism of vegetation albedo and helps to quantitatively analyze the influence of the reflection performance of leaves and soil on vegetation albedo. The canopy spectral invariants theory provides the parameterization method of the interaction process between photons and canopy-soil system. Three problems would be focused on, including the influence of diffuse skylight, the anisotropic of soil reflectance and the leaf clumping effect of the noncontinuous vegetation. Finally, the model would be validated and improved through Monte Carlo simulation and field experiments.

反照率是能量平衡中的重要参数，植被反照率更与气候变化和生态环境变化密切相关。目前，遥感反照率产品反演中最广泛采用的核驱动模型，但该模型依赖于多角度观测数据表达地表植被复杂的二向反射特性，导致反照率产品的时空分辨率受到限制。解决这一问题需要打破通过半球积分计算反照率的思维，直接从能量守恒定律出发，建立反照率的机理化模型。对植被覆盖地表，光谱不变量理论为描述光子在冠层中的辐射传输过程提供了简明的参量化方法，将微观尺度叶子对光子的反射特性和冠层尺度多次散射特性直接关联起来。本项目期望以光谱不变量理论为基础，通过计算植被和土壤吸收辐射比例，建立植被反照率机理模型。在已有研究的基础上，模型将考虑太阳直射光和天空散射光的影响、土壤反射特性的各向异性、辐射在植被内及土壤和植被间的多次反弹、非连续植被叶片的聚集效应等关键因素。项目实施结果将促进植被遥感建模与生态系统过程研究。

对项目的背景、主要研究内容、重要结果、关键数据及其科学意义等做简单概述.反照率是地表辐射能量平衡的重要参数，植被反照率更与气候变化和生态环境变化密切相关。目前，遥感反照率产品反演中最广泛采用的核驱动模型，但该模型依赖于多角度观测数据表达地表植被复杂的二向反射特性，导致反照率产品的时空分辨率受到限制。本项目期望通过建立植被反照率物理机理模型，解决植被反照率计算所受半经验模型的限制。基本思路是跳出通过半球积分计算反照率的思维，直接从能量守恒定律出发，建立植被冠层参数和反照率之间的机理化模型关系。.对植被覆盖地表，光谱不变量理论为描述光子在冠层中的辐射传输过程提供了简明的参量化方法，将微观尺度叶子对光子的反射特性和冠层尺度多次散射特性直接关联起来。本文以光谱不变量理论为基础，通过计算植被和土壤吸收辐射比例，建立ECSI（the Energy Conservation and Spectral Invariants）植被反照率机理模型。在已有研究的基础上，模型考虑太阳直射光和天空散射光的影响、土壤反射特性的各向异性、辐射在植被内及土壤和植被间的多次反弹、非连续植被叶片的聚集效应等关键因素。.将植被反照率模型应用到非连续植被冠层时，关键“接口”是方向性聚集指数输入值。通过本项目的研究，提出了解析表达和计算机模拟两种方法，以孔隙率为入口计算行播作物和森林冠层的聚集指数，并在两种方法相互印证下分析了聚集指数的角度变化特征，使反照率模型的可行性大大提高。

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