a Sodium Wind/temperature LIDAR and Observed Mesopause Thermal Structure Over Fort Collins, Colorado.
Abstract
A new two-frequency narrowband Na lidar, capable of measuring temperature in the mesopause region has been developed at Colorado State University. The uniqueness of this lidar system lies in its transmitter. Using Doppler -free fluorescence spectrum from a laboratory Na vapor cell, the laser frequency can be tuned and locked accurately as desired and the laser lineshape function can be measured directly. The vertically resolved temperature structure throughout the mesopause region (80-105km) can be measured by actively probing the thermally broadened Na D _2 resonance line with the narrowband Na lidar. Absolute temperature accuracy at the Na layer peak of +/-1.6K with a vertical resolution of 1.5 km and integration time of 4 min were achieved. The Na temperature lidar has been in regular nighttime operation since 1991. By the end of January 1994, 147 nights of quality data evenly distributed over one year period have been collected. Analysis of a total of more than 30,000 photocount files gives rise to high resolution seasonally and nightly averaged temperature and Na density profiles. Based on this extensive data set, the thermal structure of the mesopause region over Fort Collins, CO (40.6 N, 105 W), is discussed. Several features in the midlatitude mesopause, either new or significantly strengthened, are investigated in this study. First, the mesopause month -altitude temperature contour represents thermal structure at a midlatitude. Second, the double temperature minima have been seen each year for three years in spring and fall; this is now believed to be a permanent feature of the mesopause region. The trend of the mean nightly temperature at the lower altitude is a clear signature of a wave-driven diabetic circulation observed at midlatitude. Third, the profiles of annual mean temperature, annual and semi-annual temperature waves are presented. Within experimental errors the mean temperature, annual and semiannual amplitude and phases of temperature wave appear to be an extension of the French Rayleigh lidar result. Fourth, wave analysis has yielded parameters associated with the mesopause climatology, such as sodium abundance, centroid height, rms thickness of the Na layer, mesopause temperature and altitude. Like the polar mesopause, the mesopause altitude follows a bistable pattern. Using a uniquely constructed acoustooptic frequency shifter to give three different frequencies, nu _{rm a} = -652MHz and nu_{rm c} = 188MHz as usual along with nu_{ rm s} = -172MHz at the spectral slope, the Na temperature lidar has been up-graded to permit simultaneous measurements of radial wind and temperature in the mesopause region. With the lidar axis tilted 15^circ from zenith, initial observations were made for three nights in September, 1993, achieving a spatial and temporal resolutions of 1.5 km and 6 min with measurement uncertainty 1.6 K and 3.3 m/s at the peak of the Na layer. Sample individual and nightly mean radial wind and temperature profiles are presented to show the effectiveness of the "up-graded" lidar for investigating thermal and dynamical activities in the mesopause region.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT........58Y
- Keywords:
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- Engineering: Electronics and Electrical; Physics: Atmospheric Science