地球表面热红外辐射的方向效应

Anisotropy of the Thermal Infrared Radiation leaving the Earth's Surface

利用装有红外辐射计的测角系统研究了自然地表面的红外辐射的方向特征,检测了几种典型自然地表面的地面温度的角度关系.地面亮度温度的测量值随观测天顶角、方位角和太阳位置而改变.草地上的测量反映了植被覆盖典型的方向特征;在粗糙土壤地面上,当太阳辐射方向与观测方向十分接近时,观测到了“热斑效应”;在人造土坡上,测量到较大的亮度温度的角度变化;而在平静水面的观测中,当水面未处于辐射热平衡状态时,在观测天顶角小于60°时,没有观测到发射辐射的角度效应.这些结果表明:地面红外辐射方向特征引起的亮度温度的差异与地形及太阳辐射在地表面上的反射方向效应有关.这些方向变化的特性可导致卫星遥感地面温度反演的偏差.

The paper concentrates on investigating anisotropy of the thermal infrared radiation for the natural ground surfaces. The angular dependency of this parameter is examined by using a goniometer system with an infrared radiometer for several different natural surfaces. As expected, the measured brightness temperatures change with view zenith angle, view azimuth angle, and the Sun position. The measurements on the grass surface present a typical anisotropy of the vegetative cover on the ground. Particularly, the ＂hot spot effect＂ has been observed on the rough soil surface when the view direction is very close to the sunlit direction. Larger temperature difference was found over the artificial soil slope. When the water surface is not in thermal stabilization and the view zenith angle is less than 60＊ the angular dependency of water emissivity does not appear in the field measurements. Obviously, the measured differences of brightness temperature produced by anisotropy of thermal radiation are correlated with terrain and the anisotropy of reflection of solar radiation for typical land surfaces. One should be aware of all these variability, in particular when analyzing IR data provided by satellite sensors. Indeed, these may lead to large biases on estimating the surface temperature.