We have combined ground-based and space-based measurements and modeling of the mesosphere-lower thermosphere
to study the zonal and seasonal variability of the semidiurnal thermal tide. This study uses resonance lidar soundings of
temperatures from 80-100 km at Arecibo, PR (18°N, 67°W) and Maui, HI (20°N, 156°W) and observations from the
SABER instrument aboard TIMED. Findings include general dominance of migrating tides through most of the year,
excepting January, when the lidar-measured Maui phase front is much shallower than that observed by SABER or
Arecibo lidar. At both sites the GSWM-02-predicted phase is later at lower altitudes than observations. GSWM-02 has
difficulty reproducing the observed phase structure for July and August as well, months when observed phases are in
fairly good agreement. Observations also show that the semidiurnal thermal tide phase has a 6-h (or 180-degree) shift
between winter and the other seasons. The winter phase structure appears to set up in late November for Arecibo, and the
structure returns to the non-winter phase between late February and early March. SABER observations show that the
longitudinal phase variation is large in January and small for other seasons. A modal decomposition shows that the (2, 3)
Hough mode is large most of the year but small in January, setting up an asymmetric tidal structure in the solstice
periods. Solstice ground-observed amplitudes are large, while from space the winter amplitudes are small, indicating the
importance of local effects.
A dye ring laser is stabilized to a D2a Doppler-free feature of sodium vapor using a LabVIEW®-based, phase-sensitive servo. Locking precision and stability, at better than ±1 MHz, are suitable for Na lidar applications. This performance was achieved with improved digital filtering and new approaches to the problem. The inverse (type II) Chebyshev discrete filter employed demonstrates superior filtering and computational efficiency plus improved flexibility. New approaches include the determination of optimum modulation frequency, laser-tuning sensitivity, and bandwidth requirements via spectral analyses of the noise spectrum, derivative scan, and modulated spectrum. This practice guides a user in selecting the system operation parameters and negotiating the trade-offs involved when expanding the filter's passband. Allan deviation plots provide a quantitative description of the short- and long-term frequency excursions. A comparison of Allan deviation plots before and after locking shows a substantial improvement in stability throughout time scales from 0.10 to 10 s.
Resonance fluorescence Doppler lidars using Doppler shift and spectral broadening effects are the principal instruments to simultaneously measure wind and temperature in the middle atmosphere. Such lidars demand high accuracy, precision, and stability of the laser optical frequency. Current resonance Doppler lidars suffer various problems in frequency stabilization that limit their locking precision and stability. We have addressed these problems by developing a LabVIEW®-based laser frequency locking system. This new system utilizes wavelength-modulation and phase-sensitive-detection techniques in conjunction with a proportional-integral-derivative feedback servo loop. It achieves better than ±1-MHz locking precision and stability over 1 h. The system also remains locked throughout a series of abrupt disturbance tests. Owing to its high locking precision, immunity to electronic and laser noise, reliability, and flexibility in adapting for various systems, we believe that this new system represents a marked improvement in resonance Doppler lidar technology.
The University of Illinois Fe (iron) Boltzmann temperature lidar was operated at the South Pole (90°S) from November 1999 to October 2001, and then at the Rothera Station (67.5°S, 68.0°W) from December 2002 to March 2005. This lidar transmits two UV wavelengths at 372 and 374 nm, and is able to measure the middle and upper atmosphere temperature, Fe density, polar mesospheric clouds (PMC), and polar stratospheric clouds (PSCs). In this paper, we analyze the PSC data collected in the winters and springs of 2003 and 2004 at Rothera, and compare them with the PSC data collected at the South Pole in the 2000 and 2001. PSCs were observed in the range of 15-28 km during the seasons from May/June to October at both locations. The PSC backscatter ratio, width, and altitude at Rothera are comparable to those at the South Pole. However, Rothera PSCs occur less frequently (~17.7%) and in shorter periods, compared to PSCs at the South Pole (~64.9%). At Rothera, PSC occurrence frequency in 2004 is only half of that in 2003, which is likely due to warmer stratospheric temperatures in 2004 associated with changes of the polar vortex. These are the first ground-based lidar observations of PSC at Rothera, and also the first in West Antarctica.
We made the first lidar observations of polar mesospheric clouds (PMC) at the South Pole and in the southern hemisphere with an Fe Boltzmann temperature lidar in the 1999-2000 and 2000-2001 austral summer seasons. Strong PMC activities were observed at the South Pole and extensive data were collected. Here we summarize the lidar observation results including the interannual, seasonal and diurnal variations of PMC altitude, brightness and occurrence probability. In particular, our data show that PMC at the South Pole are a few kilometers higher than in the northern hemisphere and PMC at the South Pole exhibit seasonal trends in both altitude and brightness. We explore the possible causes through the study of atmospheric thermal structure and upwelling wind by using NCAR TIME-GCM model and then presenting a PMC altitude model. Our initial conclusion is that these hemispheric differences and seasonal trends in PMC altitudes are the combination results of the hemispheric differences in thermal structure and upwelling wind, which are caused by the Earth orbital eccentricity and inter-hemisphere difference in gravity wave forcing.
The University of Illinois Na wind/temperature lidar data collected at the Starfire Optical Range, New Mexico, can be used to present annual variations of the mesospheric mid-night horizontal winds. For each night, the winds profiles from the 11:00pm to 1:00am were smoothed in 1km altitude and then averaged in time to give mid-night winds profiles. All mid-night winds profiles within one month were averaged again to give mid night winds profiles of that month. The annual variations of the mid-night winds in the altitude ranges from 84km to 100km were obtained using a total of 46 nights lidar data distributed over the months except July. Zonal mid-night winds showed strong annual oscillations. Prevailing westerly winds blew over low altitudes in the winter months and over high altitudes in summer months. Meridional mid-night winds also showed annual or semi-annual oscillations. Southward winds dominated below 90km from April to October, and at about 96km during winter months and summer months except January. Correlations were found between the meridional mid-night winds and the mid-night temperatures. When the meridional winds blow from south, temperatures become warm. Results were compared with calculations of the HWM93 and the mid night winds observed with a MF radar at Urbana. Similar wind structures can be found in their results. The major differences are that strong westerly winds above 94km in March and April and strong northward winds at 96km in January were observed with the lidar. The winds values of the lidar and the MF radar data are larger than those of HWM93. Activities of inertial gravity waves and tidals might contaminated estimations of annual winds with the lidar data and contribute partially to their differences.
KEYWORDS: Iron, LIDAR, Atmospheric modeling, Temperature metrology, Clouds, Data modeling, Atmospheric particles, Solar radiation models, Chemical species, Alexandrite lasers
Global climate change resulting from greenhouse gas increase has extended into the middle and upper atmosphere. Modelling studies show that the effects are more significant at high latitudes. Thus, the observations at the poles may provide some of the first conclusive evidence of global changes in the middle and upper atmosphere, and also provide a convenient mean of calibrating general circulation models. However, the middle and upper atmosphere region is difficult to study. To help address the measurement needs, we developed a new robust lidar system using an Fe Boltzmann technique. The new lidar utilizes mesospheric iron as a fluorescence tracer and relies on the temperature dependence of the population difference of two closely spaced Fe transitions. We deployed this new lidar to the North and South Poles from 1999 through 2001 to measure the temperatures and polar mesospheric clouds in the middle and upper atmosphere for both day and nighttime. Important scientific results and issues have emerged from our observations at the poles, especially on the polar mesospheric clouds (PMC) and temperatures. Significant hemispheric differences in PMC altitudes were found during our pole-to-pole measurements. Interpretation of these results has led to a new insight into the solar radiative forcing on the Earth atmosphere and a baseline for calibrating the atmospheric general circulation models. We review the pole-to-pole observations and results in this paper.
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