Angular characteristics of the Pathfinder AVHRR land thermal data and radiative transfer modeling

In order to understand the angular characteristics observed from the Pathfinder AVHRR land thermal data, a simplified narrow-band radiative transfer model is developed based on a series of approximations for the vertical structures of the atmospheric conditions, which also take into account the directional variation of the upwelling surface radiation. The LOWTRAN-7 code is then applied to validate the present model. Different water vapor content, air temperature and surface temperature are used to simulate the AVHRR ch4/ch5 radiance at the top of the atmosphere as the function of the viewing zenith angle. A directional snow emissivity model and measured emissivity of the agricultural soil are inputed into this model to evaluate the effects of the surface anisotropy on the AVHRR ch4/ch5 radiance. Upwelling radiance usually decreases as the viewing zenith angle increases in thermal regions. However, the angular pattern may reverse when the air temperature is larger than surface temperature