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 pres ented for the populations of H2O and O-2(1) vibrational levels in the middle atmosphere. Radiative transfer in the H2O bands is treated usin g the Curtis matrix method, and an exhaustive review of the collisiona l processes and their rate constants affecting the populations of thes e levels has been carried out. The near resonant vibrational-vibration al coupling between H2O(010) and O-2(1) is crucial for establishing th eir respective populations in the mesosphere. The population of H2O(01 0) starts departing from LTE significantly above about 65 km at night, this precise altitude being dependent on the temperature structure. A t daytime, non-LTE begins at approximately the same height but is sign ificantly 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 an d excitation from the photodissociation of O-3 through O-2(1) in the l ower 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 c hannels, as compared to the nighttime values, above about 55 km. The e ffect is larger in the WE channel. Comparisons with the model show tha t non-LTE excitation of the H2O(010) and (020) levels is responsible.