TITLE:
  A Non-LTE Model for the H2O 6.3 and 2.7 mu bands in the Middle Atmosphere

AUTHORS:
  Lopez-Puertas, M., G. Zaragoza., B. J. Kerridge and F. W. Taylor

REFERENCE:
  J. Geophys. Res., 100, 9131--9147, 1995 

ABSTRACT:
A non-local thermodynamic equilibrium radiative transfer model is presented 
for the populations of H2O and O2(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 O2(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 in the upper mesosphere and lower thermosphere and excitation from 
the photodissociation of O3 through O2(1) in the lower mesosphere. A
sensitivity study of the H2O and O2(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 H2O pressure-modulated (PM) and 
wideband (WB) channels is presented. The measurements show enhancements in the 
daytime radiances in both channels, ascompared to the nighttime values, above 
about 55 km. The effect is larger in the WB channel. Comparisons with the 
model show that non-LTE excitation of the H2O(010) and (020) levels is 
responsible.