VALIDATION OF MEASUREMENTS OF WATER-VAPOR FROM THE HALOGEN OCCULTATION EXPERIMENT (HALOE)

The Halogen Occultation Experiment (HALOE) experiment is a solar occul tation limb sounder which operates between 2.45 and 10.0 mu m to measu re the composition of the mesosphere, stratosphere, and upper troposph ere. It flies onboard the Upper Atmosphere Research Satellite (UARS) w hich was launched in September 1991. Measurements are made of the tran smittance of the atmosphere in a number of spectral channels as the Su n rises or sets behind the limb of the atmosphere. One of the channels , at 6.60 mu m, is a broadband filter channel tuned to detect absorpti on in the nu(2) band of water vapor. This paper describes efforts to v alidate the absolute and relative uncertainties (accuracy and precisio n) of the measurements from this channel. The HALOE data have been com pared with independent measurements, using a variety of observational techniques, from balloons, from the ground, and from other space missi ons, and with the results of a two-dimensional model. The results show that HALOE is providing global measurements throughout the stratosphe re and mesosphere with an accuracy within +/-10% over most of this hei ght range, and to within +/-30% at the boundaries, and to a precision in the lower stratosphere of a few percent. The H2O data are combined with HALOE measurements of CH4 in order to test the data in terms of c onservation of total hydrogen, with most encouraging results. The obse rved systematic behavior and internal consistency of the HALOE data, c oupled with these estimates of their accuracy, indicate that the data may be used for quantitative tests of our understanding of the physica l and chemical processes which control the concentration of H2O in the middle atmosphere.