三维植被辐射传输模型的研制及其在陆面模式中的应用

The process of solar radiative transfer (shortwave) at the land surface is important to energy, water and carbon balance, especially for vegetated areas. Therefore, the canopy radiative transfer process plays a fundamental role in the land surface modeling. There are two major methods currently used in the land surface models. However, the method using look-up tables can hardly describe the dynamics of radiation transfer processes, and the most commonly used two-stream model has its problems on the assumption considering the Plant Functional Types (PFTs) within a grid to be independent of each other, and their leaves to be horizontally homogeneous, which is unrealistic in most cases. In addition, the 3D schemes developed in vegetation/remote sensing area, due to many reasons, can hardly be translated into simple analytical solutions suitable for climate models. Climate models run their computations on a global mesh with as high resolution as is feasible. In addition, with the advent of global quantitative derivation of terrestrial properties by remote sensing in terms of types, resolution and accuracy, a more detailed description of radiative transfer processes on the land surface is both possible and necessary. The objective of this project is trying to develop an advanced 3D canopy radiative transfer model suitable for land surface modeling, couple it into CLM/CoLM model with comprehensive validation and evaluation. The new developed 3D model will serve as a better scheme to simulate the process of solar radiative transfer at the land surface in terms of capability and accuracy, which can provide a better feedback to the atmospheric models and increase the capability of climate/earth system modeling.

植被辐射传输方案是陆面模式中最为关键的方案之一，对于地表能量平衡、水循环及碳循环有着非常重要的影响。现有陆面模式的植被辐射传输方案主要分为两种类型：一是查找表法，但缺乏动态性；二是最为通用的二流近似模型，假设模式网格内的各种功能型植被之间相互独立，同一类型植被之间无缝覆盖，叶子水平均一分布，并未考虑植被三维（3D）结构的影响。由于各方面原因，已有的3D植被辐射传输模型难以在陆面模式中应用。本项目旨在发展一个适用于陆面模式的3D植被辐射传输模型，将其耦合进通用陆面模式中，并进行验证与评估，使之能更准确地计算地表反照率及太阳辐射在植被和土壤之间的分配，提高陆面模式模拟的准确度，为大气模式提供更好的反馈。

次网格中的植被结构是陆面模式的基础框架。目前陆面模式对次网格植被结构的表征可分为两种：一种是以地表覆盖类型（LCT, land cover type）划分，另一种是以植被功能型（PFT, plant functional type）划分。两种方式都属于一维模型，并都存在一定的问题。本项目从自然界植被一般形态出发，考虑植被的三维结构，即树冠形状、树冠水平分布以及垂直分层。在此结构上，改进了我们之前发展的三维植被辐射传输模型，对模型进行优化，使之更好的描述树冠形状，考虑叶倾角分布，并改进了传统的二流近似模型。通过理想实验与蒙特卡洛模拟结果比较，新发展的三维模型相比于传统的一维模型具有更好的模拟精度。在与陆面模式耦合方面，采用最新的MODIS地表植被覆盖度资料MOD44B V051对全球陆面格点进行统计、分类，生成陆面模式地表植被结构输入资料。以此为基础，将新发展的三维模型与最新的CESM-CLM4.5耦合。通过离线模拟实验，并与观测资料比较，结果表明新发展的模型对地表反照率和植被光合作用的模拟精度都有明显提高。本项目发展的模型是目前全球陆面模式唯一可用的考虑植被三维结构的辐射传输模型，能更真实地描述实际情况下地表植被结构及其辐射传输过程，提高对反照率、光合作用模拟的准确性。目前该项目成果已参与国家发改委资助的大科学装置——地球系统模拟器的陆面分模块编制。2017年被列入国家重大研发计划《全球高分辨率陆面过程模式研发与应用》课题一中的专题之一。