Progresses and potential applications of thermal infrared kernel-driven model

Abstract:

The thermal radiation directionality (TRD) effect has been widely concerned in the area of thermal infrared remote sensing over 50 years which can lead to the directional brightness temperature (DBT) difference between different viewing directions up to 10 K (Cao et al., 2019a). Many physical models have been proposed to simulate the DBT patterns over different underlying surfaces aimed to achieve the TRD effect correction for the satellite products. In practice, it is advised to handle only TRD models having a limited number of input parameters. The use of thermal infrared kernel-driven models appears a good tradeoff between physical accuracy and operationality.

The DBT in any direction can be calculated after the best estimate of kernel coefficients for a given set of observations with sufficient angular samplings. Four 3-parameter kernel-driven models had been proposed in the thermal infrared domain, two of them are extended directly from the visible and near infrared domains (i.e. RoujeanLagouarde (RL) model (Duffour et al., 2016; Lagouarde and Irvine, 2008) and RossThick-LiSparseR (Ross-Li) model (Hu et al., 2017, 2016; Peng et al., 2011; Ren et al., 2014)) while the other two models were generated from the perspective of thermal infrared radiation (i.e. LiStrahlerFriedl-LiDenseR (LSF-Li) model (Su et al., 2002) and Vinnikov model (Ermida et al., 2017; Vinnikov et al., 2012)). Cao et al. (2019b) compared these four 3-parameter models and found the LSF-Li is the most accurate one. However, the existing kernel-driven models underestimate the hotspot effect, especially for continuous canopies.

In this study, a new general framework of thermal infrared 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 and an isotropic kernel) with the ability to simulate the bowl, dome and bell shapes in the solar principal plane. Four 4-parameter specific models (Vinnikov-RL, LSF-RL, Vinnikov-Chen, and LSF-Chen) within the new framework were further developed to assess their fitting abilities for both continuous and discrete vegetation canopies. To evaluate 4 existing models and 4 new models comprehensively, it was prepared 306 groups of 4SAIL/DART generated multi-angle datasets considering 6 different canopy architectures, 17 component temperatures and 3 solar directions. Results show that the 4 new models behave slightly better than the 4 existing models over discrete canopies whereas they significantly improved the fitting accuracy over continuous canopies. The innovative new general framework with four parameters improve the fitting ability significantly since the addition of one more degree of freedom.

There are three potential applications for the new developed thermal infrared kernel-driven models: (1) to achieve angular correction of land surface temperature products, (2) to estimate the surface upward longwave radiation considering the TRD effect through satellite network, (3) to design the optimize angular configuration for the future simultaneously multi-angle thermal infrared satellite.