Analysis of the impact of atmospheric horizontal inhomogeneity on long-wave atmospheric transmission and sky background radiation at low elevation angles

To understand the impact of atmospheric horizontal inhomogeneity on infrared radiation from the sky background, this paper innovatively employs a target ray-tracing method to construct an inhomogeneous key atmospheric parameter model distributed along inclined line-of-sight paths. Using the Combined Atmospheric Radiative Transfer software (CART), we simulate and calculate the long-wave infrared atmospheric transmittance and atmospheric background radiation under different observation paths in three regions: Yushu in northwest China, Hefei in central China, and Maoming along the southeast coast. The results indicate that for low observation elevation angles, the horizontal inhomogeneity of atmospheric temperature and humidity significantly affects the radiation transfer process. Specifically, in Hefei, the relative deviation in atmospheric transmittance between different azimuths (southward and northward) reaches up to 17% in summer when the observation elevation angle ranges from 4° to 6°, while it decreases to 1% to 3% when the angle ranges from 8° to 10°. For atmospheric downward radiation, the relative deviation between different azimuths (southward and northward) is 16% in winter when the observation elevation angle ranges from 1° to 4°, and it decreases to 1% to 3% when the angle ranges from 6° to 10°. Additionally, the study reveals that the atmospheric transmittance in all three regions generally exhibits a trend of being higher in the south and lower in the north across different observation azimuths, whereas the atmospheric downward radiation shows the opposite pattern, being higher in the north and lower in the south. The aforementioned findings demonstrate that the radiation transfer characteristics of the actual atmosphere in the long-wave band exhibit horizontal inhomogeneity, which holds significant implications for optoelectronic engineering fields such as target detection and identification at low elevation angles.