Planck weighted transmittance and correction of solar reflection for broadband infrared satellite channels

Three approximations methods to calculate the level-to-space transmittance, used in fast radiative transfer model, are studied for five infrared broadband channels. Two of them are Planck-weighted transmittances, in which one involves the effective layer temperature of a layer (PW1), and the other involves temperatures at the interface between layers (PW2). The third one is not including temperature at all (ORD). It is found that under all circumstances method PW1 is better than method PW2 compared to the line-by-line (LBL) calculation. Planck-weighted methods are more sensitive to atmospheric surface temperatures compared to ORD method. When surface temperature increase, the bias from PW methods most likely increases, while ORD method shows more consistent results. However, for those channels with very broad spectral intervals and larger variations of Planck function, PW methods show better results. The sensitivities of these methods to the profile vertical structure show that PW1 is much better than PW2 and ORD to closely following the LBL results. Based on these simulations and comparisons, when the band correction is larger (greater than 1), PW1 method should be used to take account of the Planck radiance changing with the transmittance within the band spectral. When considering the solar contribution in daytime, correction of the solar reflection has been made for near infrared (NIR) broadband channels (~3.7 µm) when using PW1 transmittance. The solar transmittance is predicted by using explanatory variables as PW1 transmittance, secant of zenith angle, and surface temperature. With the correction, the error reduces to more reasonable level. The solar reflection correction and PW transmittance are significant to accurately simulate the NIR broadband channel radiances from the sensors on the future national operational environmental satellite systems, in particular the Joint Polar Satellite System (JPSS) and the Geostationary Operational Environmental Satellite R-Series (GOES-R).