Abstract Radiometric measurements in the Thermal Infrared (TIR) domain exhibit an angular variation over most surface types, known as the Thermal Radiation Directionality (TRD) phenomenon. A primary objective of the ongoing development of TRD physical models is to perform a correction of the angular effects to obtain comparable land surface temperature products. In practice, it is advised to handle only the models having a limited number of input parameters for the purpose of operational applications. The use of semi-empirical kernel-driven models (KDMs) appears to be a good tradeoff between physical accuracy and computational efficiency as it was already demonstrated through a broad usage in the optical domain. It remains that the existing state-of-the-art 3-parameter TIR KDMs (RossThick-LiSparseR, LiStrahlerFriedl-LiDenseR, Vinnikov, and RoujeanLagouarde) underestimate the hotspot phenomenon, especially for continuous canopies marked by a narrow peak. In this study, a new general framework of TIR kernel-driven modeling is proposed to overcome such issue. It is a linear combination of three kernels (including a base shape kernel, a hotspot kernel with adjustable width and an isotropic kernel) with the ability to simulate the bowl, dome and bell shapes in the solar principal plane. Four specific 4-parameter models (Vinnikov-RoujeanLagouarde, LiStrahlerFriedl-RoujeanLagouarde, Vinnikov-Chen, and LiStrahlerFriedl-Chen, named “base shape kernel - hotspot kernel”) within the new framework were studied to assess their abilities to mimic the patterns of the directional brightness temperature for both continuous and discrete vegetation canopies. These four 4-parameter KDMs and four 3-parameter KDMs were comprehensively evaluated with 306 groups of simulated multi-angle datasets generated by a modernized analytical 4-stream radiative transfer model based on the Scattering by Arbitrarily Inclined Leaves (4SAIL), and a Discrete Anisotropic Radiative Transfer (DART) model considering different solar zenith angles (SZA), canopy architectures and component temperatures, and 2 groups of airborne measured multi-angle datasets over continuous maize and discrete pine forest. Results show that the four 4-parameter KDMs behave better than the four existing 3-parameter KDMs over continuous canopies (e.g. R2 increases from 0.661~0.970 to 0.940~0.997 and RMSE decreases from 0.17~0.71 to 0.07~0.16 when SZA = 30°) and discrete canopies (e.g. R2 increases from 0.791~0.989 to 0.976~0.996 and RMSE decreases from 0.10~0.84 to 0.08~0.21 when SZA = 30°). The new general framework with four parameters (three kernel coefficients and an adjustable hotspot width) improves the fitting ability significantly, compared to the four existing three-parameter KDMs, given the addition of one more degree of freedom. Results show that the coefficients of the base shape kernel, hotspot kernel and isotropic kernel are related to the temperature difference between leaf and background, temperature difference between sunlit component and shaded component, and the nadir brightness temperature, respectively. However, the estimated hotspot width depends on vegetation structure. The new kernel-driven modeling framework has the potential to be a tool for angular correction of multi-angle satellite observations and angular optimization of future multi-angle TIR sensors.

中文翻译：

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热红外领域内核驱动建模的通用框架

摘要 热红外 (TIR) 域中的辐射测量在大多数表面类型上表现出角度变化，称为热辐射方向性 (TRD) 现象。正在进行的 TRD 物理模型开发的主要目标是对角效应进行校正，以获得可比较的地表温度产品。在实践中，为了操作应用的目的，建议仅处理具有有限数量输入参数的模型。半经验内核驱动模型 (KDM) 的使用似乎是物理精度和计算效率之间的良好折衷，因为它已经通过光学领域的广泛使用得到了证明。现有最先进的 3 参数 TIR KDM（RossThick-LiSparseR、LiStrahlerFriedl-LiDenseR、Vinnikov、和 RoujeanLagouarde）低估了热点现象，特别是对于以窄峰为标志的连续檐篷。在这项研究中，提出了一个新的 TIR 内核驱动建模的通用框架来克服这个问题。它是三个内核（包括基本形状内核、宽度可调的热点内核和各向同性内核）的线性组合，能够模拟太阳主平面中的碗形、圆顶形和钟形。研究了新框架内的四个特定 4 参数模型（Vinnikov-RoujeanLagouarde、LiStrahlerFriedl-RoujeanLagouarde、Vinnikov-Chen 和 LiStrahlerFriedl-Chen，命名为“基本形状内核 - 热点内核”），以评估它们模仿连续和离散植被冠层的定向亮温。这四个 4 参数 KDM 和四个 3 参数 KDM 综合评估了 306 组模拟多角度数据集，这些数据集由基于任意倾斜叶散射 (4SAIL) 的现代化分析 4 流辐射传输模型和离散的考虑不同太阳天顶角 (SZA)、冠层结构和组件温度的各向异性辐射传输 (DART) 模型，以及连续玉米和离散松林上的 2 组机载测量多角度数据集。结果表明，四个 4 参数 KDM 在连续冠层上的表现优于现有的四个 3 参数 KDM（例如，当 SZA = 30°时，R2 从 0.661~0.970 增加到 0.940~0.997，RMSE 从 0.17~0.71 减少到 0.07~0.16） ) 和离散檐篷（例如 R2 从 0.791~0.989 增加到 0.976~0。当 SZA = 30°时，996 和 RMSE 从 0.10~0.84 下降到 0.08~0.21）。与现有的四个三参数 KDM 相比，具有四个参数（三个内核系数和一个可调热点宽度）的新通用框架显着提高了拟合能力，并增加了一个自由度。结果表明，基形核、热点核和各向同性核的系数分别与叶背温差、日照分量与阴影分量温差、天底亮温有关。然而，估计的热点宽度取决于植被结构。