Mesospheric temperature observations at the USU/CASS Atmospheric Lidar Observatory (ALO)

Abstract

The Center for Atmospheric and Space Sciences (CASS) at Utah State University (USU) operates the ALO for studying the middle atmosphere from the stratosphere to the lower thermosphere. ALO's mid-latitude location (41.74°N, 111.81°W, 1466 m) is very unique in that it is in the middle of an extensive set of rugged mountains, the Rocky Mountains, which are a major orographic source of gravity waves that may give rise to a longitudinal variation in the mesospheric structure. Mesospheric observations between approximately 45 and 90 km have been carried out on many clear nights with the ALO Rayleighscatter lidar since late 1993. They have been carried out, mostly, with a frequency-doubled Nd:YAG laser producing 18 W at 532 nm and a 44-cm zenith-pointing telescope. To obtain better and more complete observations in the future, a considerably bigger steerable telescope, an alexandrite ring laser for resonance scatter, and an expanded data-acquisition system are being developed. The observations in the extensive existing database have been reduced to provide absolute temperature profiles, which provide important information for understanding the physics and chemistry of the middle atmosphere and for examining global change. They have been used to make a mesospheric temperature climatology that has been and is being used to examine secular, annual, seasonal, and tidal variations, to compare with other temperature observations and with modeled temperatures, and to study mesospheric inversion layers. Day-to-day changes in the temperature profiles are also being compared to meteorological parameters to see if mesospheric changes can be related to low-altitude sources. Temporal and spatial fluctuations in the density profiles have also been examined to provide more direct information on gravity wave activity. And, on 24 June 1999 UT, the lidar probed the first known noctilucent cloud to penetrate to this low latitude, approximately 10° equatorward of previously reported sightings and detections.

Connection between the midlatitude mesosphere and sudden stratospheric warmings as measured by Rayleigh‐scatter lidar

“…A RSL operated at the Atmospheric Lidar Observatory on the campus of USU (41.7°N, 111.8°W) from 1993 to 2004 [Herron, 2007;Wickwar et al, 2001]. It employed either a Spectra Physics gaussian coupled resonator (GCR-5) or GCR-6 Nd:YAG laser to transmit 18 W (600 mJ per pulse) or 24 W (800 mJ per pulse), respectively.…”

Observations of a noctilucent cloud above Logan, Utah (41.7°N, 111.8°W) in 1995

“…The single, gated detector is a greensensitive, bialkali photomultiplier tube (Electron Tubes 9954B) in a Peltier-cooled housing. A more extensive description of this system is given by Wickwar et al [2001] and Herron [2004].…”

Visual and lidar observations of noctilucent clouds above Logan, Utah, at 41.7°N

“…The intense low‐altitude returns are blocked by a rotating chopper and by an electronic gate for the cooled photomultiplier tube (Electron Tubes 9954B), which is gated on at 38 km, providing good data from below 45 km to above 90 km. This is essentially the system described by Wickwar et al [1997, 2001]. The elemental sampling gate width is 250 ns or 37.5 m. However, the data reduction integrates over three gates, making the effective gate width 750 ns or 112.5 m. In the analysis a boxcar average can be performed over any multiple of 112.5 m, with the result returned every 112.5 m. With a laser beam divergence of <500 μrad, the horizontal spot size, or resolution, is <41 m at 82‐km altitude.…”