针对遥感应用中的三维云层辐射传输计算问题,提出一种水平方向邻域多次散射近似模型。通过分析水平方向相邻云体单元间的辐射通量密度变化机制,建立相邻云体水平辐射交换方程。引入太阳方向补偿函数,修正太阳入射角对辐射传输的影响。利用I3RC Phase Ⅱ淡积云场(Cu)数据、基于实测修正的多尺度叠加分形算法得到的层积云场(Sc)数据和高积云场(Ac)数据进行实验。实验结果表明,与独立像元近似(IPA)模型和严格单次散射结合爱丁顿多次散射(SSEddMS)模型相比,在0~60°太阳天顶角范围内,所提模型的上行方向辐射源函数(USF)平均相对误差优于13%;在低太阳天顶角条件下,计算精度提升超过15%;在不同观测条件下,像元级辐射计算精度优于5%。所提模型能够适用于具有不同光学厚度与水平非均匀性的三维云层,在稳定性、适用性及计算精度方面均具有显著优势。

An approximation model of multiple-scattering among horizontally adjacent fields is proposed as a means of improving the three-dimensional (3D) radiative transfer calculation for clouds in remote-sensing applications. Horizontal radiative-exchange equations are established after analyzing the mechanism of radiation flux density variation among horizontally adjacent cloud units. By introducing the solar-compensation function, the influence of the solar incidence angle upon radiative transfer is corrected. The experiment is conducted using I3RC Phase Ⅱ cumulus (Cu), stratocumulus (Sc) and altocumulus (Ac) data generated by the multi-scale superposition fractal algorithm based on actual observation and correction. The experimental results show that compared with the independent pixel approximation (IPA) and combined strict single-scattering and Eddington multiple-scattering (SSEddMS) models, the mean relative error of the upwelling source function (USF) calculated using the proposed model is better than 13% when the solar zenith angle is in the range of 0°-60°. The accuracy of the proposed model is improved by even more than 15% under low solar zenith angles. The accuracy of the pixel-level-radiance calculation of the proposed model falls within 5% under different observational conditions. Furthermore, it can be applied to 3D clouds with different optical thicknesses and horizontal non-uniformities. This has obvious advantages for stability, applicability, and accuracy.