Simplified Characterization of Earth-limb Radiance Calculation for Longwave Infrared Bands

Atmospheric path radiance is generally calculated via solving radiative transfer equation, in which the Curtis-Godson (CG) approximation is adopted to discretize the inhomogeneous atmosphere into tens to hundreds of homogeneous sublayers for calculation accuracy. However, as the density of the atmosphere decrease with altitude, the sub-layers at lower altitude contribute major radiance. Theoretically, a couple of sub-layers can achieve similar calculation results with significant improvement of efficiency. In this paper, a simplified characterization model of Earth-limb radiance calculation is developed for longwave infrared (LWIR) bands. To this end, the atmospheric radiative transfer ARTM model developed in our previous work, namely the BHU-ATM, is used to calculate extinction coefficients for each sublayer and simulate total atmospheric path radiance. A simplified model consisting of three sub-layers is built to calculate total atmospheric emission. For a set of given atmospheric radiance in several bands, the major parameters in the simplified model are equivalent spectral absorption coefficients and equivalent temperatures, which are inverted by means of Sequential Quadratic Programming (SQP). The developed model is used to simulate Earth-limb radiance in other bands belong to LWIR spectrum, which is compared with the results of the BHU-ATM to assess precision. Additionally, different sets of inversion bands are selected to analyze the optimal bands for equivalent characterization of Earth-limb emission.