Lidar observations of thermal structure and gravity wave activity in the middle atmosphere

The Rayleigh lidar technique has been applied to study middle atmospheric thermal structure with particular emphasis on the properties and influence of internal gravity waves. Profiles of density and temperature in the upper stratosphere and mesosphere are derived from range-resolved detection of backscattered laser radiation. This technique is capable of resolving the short vertical and temporal scale fluctuations induced by gravity wave motions in the 30 to 60 km altitude region. One hundred and thirty nights of observation were carried out at Toronto mainly over a one year time period between June, 1991, and June, 1992. Observations were also obtained during February and March of 1993 in the high Canadian Arctic at Eureka NWT (80;A persistent temperature inversion with overlying nearly adiabatic lapse was found to be a common feature in the mesosphere above Toronto. This generally occurred at heights below 70 km during winter and above during summer. It is demonstrated that a persistent well-defined turbulent layer will bring about such a temperature structure.;Temperature fluctuations in the upper stratosphere are interpreted as being caused by atmospheric gravity waves. The wave amplitudes were often found to be on the threshold of inducing convective instability while the vertical variation in rms fluctuation and available potential energy density indicated substantial dissipation. The observations are taken to support the main hypothesis of the linear instability theory of gravity wave spectra--that convective instability imposes a limit on wave amplitude. Vertical amplitude growth generally occurred only outside of the saturated spectral range which extended to longer wavelengths with increasing altitude. There was a substantial variability in gravity wave activity on time scales ranging from hours to months. Dramatic changes in the distribution of spectral energy were observed over the course of a few hours. The total resolved available potential energy density in the gravity wave field varied considerably from day to day and seasonally with a maximum in winter and minimum in summer.;A stratospheric sudden warming event was observed at Eureka. The warming occurred first in the upper stratosphere during late February and then at lower levels in early March. This appeared to be associated with a disturbance of the entire middle atmosphere with temperature changes in the mesosphere and lower stratosphere being opposite in sign to those in the upper stratosphere. In comparing averages taken over the warming and non-warming periods separately there appeared to be substantially greater gravity wave dissipation within the upper stratospheric warming.