We describe a new two‐frequency lidar for measuring Na temperature profiles that uses a stabilized cw single‐mode dye laser oscillator (rms frequency jitter < 1 MHz) followed by a pulsed‐dye power amplifier (140 MHz FWHM linewidth) which is pumped by an injection‐locked Nd: YAG laser. The laser oscillator is tuned to the two operating frequencies by observing the Doppler‐free structure of the Na D2 fluorescence spectrum in a vapor cell. The lidar technique and our initial observations of the temperature profile between 82 and 102 km at Ft. Collins, CO (40.6°N,105°W) are described. Absolute temperature accuracies at the Na layer peak of better than ±3 K with a vertical resolution of 1 km and an integration period of approximately 5 min were achieved.

The principle and practice of narrow-band light detection and ranging (lidar) for temperature measurements are discussed, with emphasis on a new two-frequency technique for measuring mesospheric Na temperature and density profiles. The uniqueness of this narrow-band lidar lies in the transmitter whose line-shape function can be measured directly. The frequency of the laser output can be monitored simultaneously during data acquisition with Doppler-free fluorescence spectroscopy by using a laboratory Na cell. These measurement techniques along with the procedures for data analysis are described in detail. At present the absolute temperature accuracy at the Na layer peak is +/-3 K (+/-4 K) with a vertical resolution of 1 km and an integration period of 5 min (2.5 min). Potential applications and furtherimprovements in this lidar technique are also discussed.

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Fourteen nights of high‐resolution Na temperature lidar data were collected at the Urbana Atmospheric Observatory (40°N, 88°W) over a 6‐month period from January through June 1991. The temperature structure in the 80‐ to 105‐km region exhibited strong variability due to the influence of gravity waves, tides, and sporadic Na layers (Nas). Extreme temperatures of 140 K and 255 K at 90 km were observed. A cooling rate of 8 K/month was exhibited from winter to summer at 85–90 km, and in late May and early June a well‐defined cold mesopause formed near 85 km with temperatures typically between 150 and 165 K. This mesopause structure produced an associated enhancement in the Brunt‐Vaisala frequency near 90 km. The average Brunt‐Vaisala period decreased from 5.2 min during the winter months to 4.3 min during spring and early summer. Na density and temperature perturbations are 180° out of phase on the top side of the Na layer and in phase on the bottom side as predicted by Gardner and Shelton (1985). The rms relative temperature and horizontal wind perturbations averaged 4.7% and 19.8 m/s, respectively.