Near-Horizontal Radiative Transfer Characteristics and Applications to Remote Sensing

Abstract

In this paper, near-horizontal sky radiance characteristics are studied through a numerical simulation and experiment. From the radiative transfer calculation results, it is found that as the total atmospheric columnar optical depth τ_T ≥ 5, the ratio of the downward radiative flux to the downward sky radiance at 2° elevation angle is about 8. 16, almost independent of the optical depth and the solar zenith and azimuth angle. This interesting characteristic is verified by the experiment. A joint lidar-radiometer-balloon system is used to measure sky radiance, aerosol or cloud optical depth, ana the extinction coefficient distribution in the boundary layer and so on. As far as the mean result of 82 groups of data for fogy and cloudy days is concerned, the ratios mentioned above for wavelengths 4500, 5500, 6500, 7500, and 8500 Åare, respectively, 8.206, 8.152, 8.193, 7.91, and 8.01. Furthermore, an approximate expression of the ratio suitable for any optical depth is established, and the near-horizontal radiative transfer characteristic is used to determine the aerosol or cloud optical depth and simplify the slant visibility formula.