热红外地表方向性辐射温度与半球辐射温度关系研究

地表温度是陆面过程的一个重要影响因素,利用地表温度的遥感反演算法只能获取卫星传感器观测角度条件下的地表温度（即某个方向上的辐射温度）,但地球表面普遍存在非同温像元,反演得到的像元地表辐射温度具有方向性特征。本文利用热红外辐射传输模型4 SAIL（Scattering by Arbitrarily Inclined Leaves）,以及方向性热辐射参数化模型,针对非同温均匀冠层,考虑冠层结构、太阳位置和观测角等因素的影响,模拟得到方向性辐射温度数据,与半球辐射温度数据比较,得到估算半球辐射温度的最佳观测角度。此外,开展热红外地面观测试验,对热红外地表辐射温度的角度效应,以及利用模拟数据得到的半球辐射温度最佳观测角度进行了验证。结果表明,当太阳高度角较低时,均匀草地的地表辐射温度,会随着观测天顶角的增大而增加,受观测方位角的影响较小,当观测天顶角为75°时,倾斜观测与垂直观测得到的辐射温度差值达到2.7 K,说明热辐射存在明显的方向性特征。同时,将热红外地表方向性辐射温度与同步观测的半球辐射温度进行对比分析,当叶面积指数小于1.0时,半球辐射温度的最佳替代角度为51°,与模拟结果相符。

Study of the Relationship Between Thermal Infrared Directional and Hemispherical Radiative Temperatures

Land surface temperature (LST) is one of the key parameters in land surface processes, and hemispherical radiative surface temperature is also an important input parameter for land surface models. However, notably the LST derived from satellite scale data is affected by the variation of viewing zenith angles. Therefore, it is necessary to develop methods to estimate the hemispherical radiative temperature from the satellite-derived LST before inputting the hemispherical radiative temperature into land surface models. This paper firstly analyzes the relationship between the directional and hemispherical radiative temperatures simulated by a thermal-infrared radiative transfer model and a parameterized model for flat and homogeneous canopy, considering leaf inclination distribution function (LIDF), leaf area index (LAI), solar zenith angle and viewing azimuth angle. Then, a best directional radiative temperature is proposed to substitute the hemispherical radiative temperature. In addition, this paper also briefly describes the angular effect of the radiative surface temperature via field experimental study. The experiments were conducted over a homogenous and flat grassy lawn using two KT-15.85D infrared radiometers mounted on a multi-angle observation device. The grass radiative surface temperature under different viewing angles was measured by one radiometer through rotating the arm of the multi-angle observation device. The radiative surface temperature at nadir was measured by the other radiometer on a fixed arm. The results reveal that the radiative surface temperature increases with the increase of viewing zenith angle and it depends slightly on the variation of viewing azimuth angle. Comparing the off-nadir radiative temperatures to those measured at nadir, it exhibits a maximum difference of 2.7 K when the viewing zenith angle is at 75°, which implies that the angular effect of infrared radiation does exist. Comparing and analyzing the directional and hemispherical radiative temperatures, we found that the directional radiative temperature measured at a viewing zenith angle of 51° can be the best substitute for the hemispherical radiative temperature when the LAI is below 1.0.