适用于遥感应用的辐射传输高精度快速计算方法

Rapid Yet Accurate Radiative Transfer Algorithm for Remote Sensing

准确的散射辐射方向特征与辐射传输能量守恒的物理要求是一切算法的有效性准则。在有限计算资源的限制下,寻找最优的有限展开算法,获得能量守恒物理要求下的散射方向特征的准确性,是多年来辐射传输算法的追求目标。在以有限流数离散化辐射传输方程进行数值求解过程中,要保证数值计算中的积分守恒,流数有限要求粒子散射相函数必须截断,而不合适的截断容易导致解的误差,尤其是当大气中存在强前向散射的大粒子时(如云滴、沙尘暴事件过后的沙尘),截断的相函数会出现振荡,从而导致解的虚假振荡特性。散射相函数无限扩展可以消除这种虚假振荡,但容易导致数值积分能量不守恒和解的不稳定。本文从原理上对这种虚假振荡和不稳定现象进行了分析,并给出两种用较少流数达到较高的计算精度的高效计算方法,这些算法对于遥感方法研究、GCM气候模式中辐射强迫的参数化研究以及地气系统的能量收支研究,尤其是目前研究较多的云和沙尘等大粒子气候效应有重要意义。

Energy conservation and accurate angular distribution of radiance are the basic requirements for simulation of radiative transfer processes. Current climate model and operational remote sensing technique demands fast and accurate radiative transfer algorithm. To improve the computational efficiency, the scattering phase function is expanded and truncated as the summation of orthogonal polynomials such as Legendre function. Based on mathematical theory, number of polynomials must agree with streams （ number of grids in angular integration） to satisfy the integration conservation, inappropriate truncation of scattering phase function results in considerable errors, especially for strong forward scattering media such as cloud particles and dust aerosol in the atmosphere. The truncated scattering phase function of dust or cloud, which fluctuate with scattering angle, results in false fluctuation in angular distribution of radiance, the fluctuation could be removed through taking huge number terms in expansion of scattering phase function, but energy conservation is broken, radiation flux is wrong and the results are unstable. Based on mathematical and radiative transfer theory, the phenomenon of false fluctuation and the stability of radiative transfer algorithm are analyzed, several fast and accurate methods are compared. Finally, two fast and accurate algorithms, satisfying the energy conservation and using less streams, are given, which is of significance in remote sensing, parameterization of radiative forcing in global climate models, energy budget estimation of the earth-atmosphere system, in particular, the effect of clouds and dust events.