HALOE observations of H 2 O and CH 4 are compared with in situ techniques aboard the ER‐2 aircraft during the northern winter of 1991/92, in particular for the dates 911208, 920108, 920217, 920222 and 920320 when the spatial coincidences are close, within ±1° latitude and ±12° longitude. The temporal coincidence is within 4 hours. The version 9 retrievals of HALOE profiles are used, since they contain a self‐consistent aerosol correction. The results reveal the limitations of comparing high resolution in situ aircraft data with a remote sounding limb scanner. Of the 5 comparison dates, 3 had HALOE/ER‐2 coincidences which occurred near the edge of the Arctic vortex (911208, 920108, 920217); the vertical variability in the HALOE results and the horizontal variability in the ER‐2 observations on these days show that the vortex edge is not a region where exact agreement can be expected except by chance. On a 4th comparison date, there was substantial overlap away from the vortex edge, although for some species the aircraft data show considerable variability near the coincidence point. On the 5th comparison date, the ER‐2 had no overlap in altitude with the lowest HALOE observations; however, a short linear interpolation over ∼1 km altitude results in smooth composite profiles. Generally speaking the agreement between HALOE and the ER‐2 at overlap altitudes is about 12% in the case of water vapor, which shows low horizontal and vertical variability. The difference was shown to contain atmospheric variability in one case by using the HALOE viewpath superimposed on a potential vorticity map to to examine a pair of HALOE observations straddling the coincidence point in longitude. The agreement for methane, with limited data having altitude overlap, is better than 6%.
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Tuck et al. (1993) studied this question.