NONLOCAL THERMODYNAMIC-EQUILIBRIUM MODEL FOR H2O 6.3 AND 2.7-MU-M BANDS IN THE MIDDLE ATMOSPHERE

A non-local thermodynamic equilibrium radiative transfer model is presented for the populations of H2O and O-2(1) vibrational levels in the middle atmosphere. Radiative transfer in the H2O bands is treated using the Curtis matrix method, and an exhaustive review of the collisional processes and their rate constants affecting the populations of these levels has been carried out. The near resonant vibrational-vibrational coupling between H2O(010) and O-2(1) is crucial for establishing their respective populations in the mesosphere. The population of H2O(010) starts departing from LTE significantly above about 65 km at night, this precise altitude being dependent on the temperature structure. At daytime, non-LTE begins at approximately the same height but is significantly enhanced with respect to nighttime. The principal additional daytime excitation processes are absorption of solar radiation by H2O at 2.7 and 6.3 mu m in the upper mesosphere and lower thermosphere and excitation from the photodissociation of O-3 through O-2(1) in the lower mesosphere. A sensitivity study of the H2O and O-2(1) vibrational temperatures to the atmospheric and model parameters has been carried out. A preliminary analysis of ISAMS/UARS measurements in the 6.9-mu m H2O pressure-modulated (PM) and wideband (WB) channels is presented. The measurements show enhancements in the daytime radiances in both channels, as compared to the nighttime values, above about 55 km. The effect is larger in the WE channel. Comparisons with the model show that non-LTE excitation of the H2O(010) and (020) levels is responsible.