Fast forward radiative transfer modeling of 4.3 μm nonlocal thermodynamic equilibrium effects for infrared temperature sounders

Retrieval algorithms for downlooking infrared sounders typically avoid using channels from the 4.3 μm CO₂ region that probe the mid- and upper-atmosphere due to very high altitude Non Local Thermodynamic Equilibrium (NLTE) emission, which can add as much as 10 K to the measured daytime brightness temperatures (BT). In this paper we report a fast radiative transfer model for a nadir sounding instrument (AIRS) that includes the effects of NLTE, allowing the retrieval algorithm to use many short wave CO₂ channels for upper-air soundings. Model biases, and standard deviations are very similar for both day and night. This work allows an infrared sounder to probe the upper atmosphere much more completely using only short wave 4.3-4.5 μm channels.