An Earlier Lidar Observation of a Noctilucent Cloud above Logan, Utah (41.7º N, 111.8º W)

Salt Lake Community College

A Rayleigh-scatter lidar has been operated at the Atmospheric Lidar Observatory (ALO) on the Utah State University (USU) campus (41.7º N, 111.8º W) for the last 11 years. During the morning of 22 June 1995 a noctilucent cloud (NLC) was observed with the lidar, for approximately one hour, well away from the twilight periods when NLCs are visible. This sighting of an NLC at this latitude shows that the first 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. The hourly profiles are at least 20 K cooler than the 11-year June climatology for ALO near the NLC altitude. However, the cool temperatures arose because of a major temperature oscillation or wave, not because the whole profile was cooler. These lidar observations were supplemented by OH rotational observations from approximately 87 km, which also showed unusually cold temperatures on this night. While these NLC observations equatorward of 50° may be significant harbingers of global change, the mechanism is more complicated than a simple overall cooling or increase in water vapor.