Impact of non-LTE processes on middle atmospheric water vapor retrievals from simulated measurements of 6.8 μm Earth limb emission

We present an efficient and accurate algorithm for the retrieval of water vapor (H2O) volume mixing ratio (vmr) in the middle atmosphere from satellite measurements of 6.8 μm Earth limb emission, which rigorously includes non-local thermodynamic equilibrium (non-LTE) processes. A number of retrieval studies have been conducted with this algorithm leading to three major conclusions about the retrieval of H2O in the middle atmosphere: (1) satellite measurements of 6.3 μm limb emission must be made to high altitudes (85-90 km), where the few optically thick H2O absorption lines become optically thin, in order to prevent large errors in retrieved stratospheric and mesospheric H2O vmr; (2) the impact of non-LTE on nighttime H2O retrievals is greater than previously thought; (3) the enhancements in the daytime H2O channel radiances of the NIMBUS 7 LIMS experiment may be entirely due to non-LTE emission from the H2O 6.3 μm bands.