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Keywords:

  • Noctilucent clouds;
  • summer mesosphere;
  • mesospheric temperatures;
  • Rayleigh lidar;
  • global change;
  • diurnal tide

[1] A Rayleigh-scatter lidar has been operated at the Atmospheric Lidar Observatory (ALO) on the Utah State University (USU) campus (41.7°N, 111.8°W) since August 1993. During the morning of 22 June 1995, lidar returns from a noctilucent cloud (NLC) were observed for approximately 1 hr, well away from the twilight periods when NLCs are visible. This detection of an NLC at this latitude shows that the first reported sighting, in 1999 (Wickwar et al., 2002), was not a unique occurrence. This 1995 observation differs from the 1999 one in that temperatures could be deduced. Near the 83-km NLC altitude the temperatures were found to be up to ∼23 K cooler than the 11-year June climatology for ALO. This analysis shows that these cool temperatures arose, not because the whole profile was cooler, but because of a major temperature oscillation or wave with a 22-km vertical wavelength and a ∼0.9 km/hr downward phase speed. This large-amplitude wave has many of the characteristics of the diurnal tide. However, the amplitude would have to be enhanced considerably. These lidar observations were supplemented by OH rotational temperature observations from approximately 87 km. These NLC observations equatorward of 50° have been suggested to be significant harbingers of global change. However, if that were the case, the mechanism is more complicated than a simple overall cooling or an increase in water vapor. Accordingly, we propose enhanced generation of gravity waves that would interact with the diurnal tide to produce a large-amplitude wave, the cold phase of which would give rise to low enough temperatures to produce the NLC. The gravity wave source might be orographic in the Mountain West or convective far to the east or south.