Microwave spectroscopy of H₂O in the stratosphere and mesosphere

Abstract

Ground-based microwave spectroscopy is a powerful technique for remote sensing of minor constituents, such as H₂O, in the middle atmosphere that might otherwise be difficult to measure quantitatively. The 6₁₆–5₂₃ H₂O transition at 22.2 GHz is particularly well suited for this application. It is optically thin and at a low frequency which allows straightforward inversion of spectra to recover altitude profiles up to ∼80 km altitude. Because of considerable radioastronomical interest in this transition, which is non-thermally excited in the interstellar medium, many suitable receiver systems are available^1–3. Monitoring of the H₂O altitude profile on short and long time scales is practical and has been going on from Haystack Observatory for several years^2,4,5. The principal limitation of this experiment is not receiver sensitivity—a few hours of integration are sufficient to produce an H₂O atmospheric emission spectrum suitable for inversion—but, rather, baseline curvature in the receiver–spectrometer system. Baseline effects have limited the effective bandwidth of the Haystack experiments to 6.67 MHz making the effective lower altitude limit of profile inversion ∼50 km. They are also a dominant source of error in recovered profiles in the 50–60 km range. We present here the first results of observations with a new receiver system developed specifically to observe atmospheric H₂O which allowed a spectrometer bandwidth of 40 MHz to be used and, thus, allowed the H₂O profile from ∼35 to 80 km to be recovered. The resulting profile, although only a ‘snapshot’ of a probably time-dependent phenomenon, is consistent with previous measurements in the mesosphere and stratosphere.