J. R. Yu

High-spectral-resolution fluorescence light detection and ranging for mesospheric sodium temperature measurements

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.

Observed thermal structure of a midlatitude mesopause

The Colorado State Na temperature lidar has been in regular nighttime operation since 1991. By the end of January 1993, 94 nights of quality data with more than 4 hrs each night have been collected. Analysis of a total of 22,999 photocount files gives rise to high resolution seasonally and nightly averaged temperature and Na density profiles, from which the thermal structure of a midlatitude mesopause emerges. Contrary to the reference atmosphere of CIRA 1986, two prevailing temperature minima are seen at altitudes 86.3 ± 2.5 km and 99.0 ± 2.9 km with the lower minimum having the dominating effect. The trend of the mean nightly temperature of the lower minimum is observed to vary monotonically between a low of 172 K at summer solstice and a high of 212 K at nearly one month following winter solstice. As in the polar regions, this is a clear signature of a wave-driven diabatic circulation observed at a midlatitude. The temperature of the upper minimum fluctuates around 189 ± 8 K year-round. Like the polar mesopause, the mesopause altitude follows a bistable pattern, although the seasonal variation of the mesopause temperature is much more complicated. Based on the seasonal variations of these two temperature minima, the observed patterns of mesopause altitude and temperature can be explained. Although the mechanisms responsible for the formation of double temperature minima are not clear at present, the lower temperature minimum with a robust 40 K annual temperature change plays the role of the mesopause commonly known.

Simultaneous three‐frequency Na lidar measurements of radial wind and temperature in the mesopause region

Using a uniquely constructed acoustooptic frequency shifter to provide three different excitation frequencies, {nu}{sub a} = 652 MHz and {nu}{sub c} = 188 MHz along with {nu}{sub s} = 172 MHz at the spectral slope, the Colorado State Na temperature lidar has been up-graded to permit simultaneous measurements of radial wind and temperature in the mesopause region. Initial observations were made at Ft. Collins, CO (40.6{degrees}N, 105{degrees}W) for three nights in September, 1993 (a total of 20.2 h and 1,204 profiles). With a power-aperture product of 0.06 W-m{sup 2}, the lidar measured RMS uncertainties with spatial and temporal resolutions of 1.5 km and 6 min are assessed to be 1.6 K and 3.1 m/s at the peak of the Na layer. Observational results for the nights of September 3, 4 and 21 (UT) that contain new thermal and dynamical information of the mesopause are presented to show the potential of simultaneous wind/temperature measurements. 13 refs., 4 figs., 1 tab.

Vertical structure of the midlatitude temperature from stratosphere to mesopause (30–105 km)

Data sets from Rayleigh lidar (1979–93) at the Observatory of Haute-Provence (44°N, 6°E) and at Biscarrosse (44°N, 1°W) of southern France, and from narrowband Na lidar (1991–4) at Fort Collins, Colorado (41°N, 105°W) are used to deduce the vertical temperature structure of the midlatitude middle atmosphere from 30–105 km. Nightly averaged temperatures measured across the Atlantic at comparable latitudes between 81 and 90 km showed seasonal variations tracking one another. Harmonic analyses of the measured profiles at these sites have been compared to the reference atmosphere, CIRA 1986. Considerable differences are noted in the annual mean temperatures in the mesopause region. Two temperature minima at 86 and 99 km are observed, suggesting heating and cooling mechanisms not previously incorporated into the reference atmosphere. Using the new 3D TIME-GCM of Roble and Ridley which includes both dynamical and chemical processes, we account for this anomalous behavior by model simulation for the first time. Except the existence of strong semi-annual oscillations, the observed annual and semi-annual temperature variations are compatible with CIRA 1986. Observed annual temperature and solar flux forcing are out-of-phase between 65 and 99 km, reflecting a dynamics dominated atmosphere at these altitudes.

Na temperature lidar measurements of gravity wave perturbations of wind, density and temperature in the mesopause region

High resolution temperature profiles of the mesopause region above Fort Collins, CO (40.6°N,105°W) were measured with a Na lidar on the nights of March 2–3 and April 15–16, 1990, during the ALOHA-90 campaign. This paper reports the initial scientific analysis of these data which were used to compute (1) the altitude profiles of relative atmospheric temperature perturbations, (2) the mean Brunt-Vaisala frequency in the mesopause region, and (3) the vertical shear variance of horizontal winds. On March 2–3 and April 15–16, the rms temperature perturbations were 5.7% and 7.1%, the average Brunt-Vaisala periods were 4.9 min and 4.6 min, and the wind shear variances were 878 (ms−1/km)² and 967 (ms−1/km)², respectively.

Doppler-free saturation fluorescence spectroscopy of Na atoms for atmospheric application

The well-known theory of absorption and fluorescence is briefly reviewed in a systematic manner for the Na D transitions. The resulting formalism is applied to simulation of Doppler-free saturation fluorescence spectra. With only one adjusting parameter, the nonradiative rate chosen to represent the time a thermal atom takes to move across the laser beams, the simulated Doppler-free spectra match the measured ones well for both D(1) and D(2) transitions over one decade of excitation intensities. Relative to the weighted center of the six D(2) hyperfine transition lines, the frequencies of the dominant Doppler-free features have been determined from a simulated spectrum to within ±0.1 MHz to be -651.4, 187.8, and 1068.0 MHz, respectively, for D(2a), crossover, and D(2b) resonances. These features may be used as accurate frequency references for atmospheric spectroscopy. They are essential for the operation of the newly developed narrow-band Na fluorescence lidar for wind and temperature measurements in the mesopause region.

Seasonal variations of the thermal structure of the mesopause region at Urbana, IL (40° N, 88° W) and Ft. Collins, CO (41° N, 105° W)

Beginning in 1991, Na lidars were used to make routine measurements of the temperature structure between 80 and 105 km altitude during 65 nights at Urbana, IL and 116 nights at Ft. Collins, CO. More than 13,000 temperature profiles representing more than 1100 h of observations were used to characterize the seasonal temperature variations in the upper mesosphere and lower thermosphere. The seasonal behavior is quite similar at the two sites. The mesopause altitude is near 100 km in winter during Nov–Feb and near 86 km in summer during May–Jul. The mesopause temperature varies from 190 K in mid-winter to 175 K in mid-summer. The transitions between winter and summer mesopause structures occur rapidly in Mar–Apr and Aug–Sep. The temperature differences between the two sites are less than ±15 K and appear to be the result of atmospheric planetary, tidal, and gravity wave perturbations.

Simultaneous lidar and airglow temperature measurements in the mesopause region

A scientific flight over Fort Collins CO (40.6°N, 105°W) was conducted on the night of April 15-16, 1990, during the ALOHA-90 campaign. In this Colorado mission, a direct comparison between measured mesospheric O 2 and OH rotational temperatures with the airglow instruments on board the Electra and simultaneously measured Na temperatures by a ground-based narrowband Na lidar has been made. This first comparison resulted in general agreement in measured temperatures between different instruments

Lidar-observed temperature structures and gravity-wave perturbations of the mesopause region in the springs of 1990–1992 over Fort Collins, CO

The Na temperature lidar data taken during the nine nights in springs 1990 and 1991 at Fort Collins, CO, have been re-analyzed by a refined procedure, which takes into account the effects of (i) the more accurately determined lidar operational frequencies, (ii) the proper inclusion of extinction in the Na layer and (iii) additional temporal and spatial smoothing. Depending on altitudes, the new lidar frequencies and the effect of Na extinction combined to lower the calculated temperature values by 2–8 K. Although the occasionally noted large rapid change (in 15 min) in the centroid temperature is reduced from ∼40 K to ∼20 K due to the added smoothing first employed by Bills and Gardner, the general conclusion presented in the recently published paper [1] remains valid. In this paper, we also present the analysis of new data (a total of 17 nights) taken during spring 1992 which further substantiates the fact that the averaged mesopause in spring rises from a lower altitude before midnight to a higher altitude after midnight. In addition, parameters depicting atmospheric gravity-wave perturbations have been derived from the measured temperature profiles. Using the data collected in 26 spring nights, we have determined the averaged relative density (temperature) perturbation, Brunt Vaisala period and Richardson number to be, respectively, 5.1±1.1%, 5.1±0.3 min, and 1.8±1.0.

Initial spring temperature profiles of the mesopause region over Fort Collins, CO, measured by the Colorado State Na temperature lidar

After its first measurement in late August, 1989, the new Na temperature lidar has been in operation during springs 1990 and 1991 at Fort Collins, CO. A total of nine nights (over 2600 profiles) of mesopause temperature measurements, each for a period longer than 4 hours, have been taken. We present these high quality initial profiles which demonstrate the effectiveness of the new two-frequency narrowband lidar technique for mesopause temperature measurements. The average temperature profiles suggest that the spring mesopause temperatures range from 168 K to 205 K. The mesopause heights are around 88 km before the midnight and around 99 km after the midnight. The nightly averaged temperature profiles in the mesopause region display considerable variability. The richness in new geophysical information obtainable with a Na temperature lidar is made evident by noting strong perturbations in a short time scale (15 min) in the data taken on March 11, 1990 and March 18, 1991.