Optical remote-sensing systems have contributed enormously to our understanding of glo- bal weather patterns, ozone depletion, pollution transport, and even global warming. Satellite-based imagers and light detection and ranging (lidar) profilers have been used for decades to map the vertical and global structure of clouds, trace gases, pollutants, and atmospheric parameters such as winds and temperature. Lidar systems in particular have benefited from recent advances in optical technologies and now can profile the atmosphere from the surface to the edge of space. In 1994 the space shuttle Discovery carried the first spaceborne atmospheric lidar profiler into orbit. Several instruments are currently in various stages of development to study the global distribution of atmospheric aerosols and clouds, as well as important constituents such as water vapor, ozone, and carbon dioxide (CO2).
The upper atmosphere is inaccessible to in-situ probing from aircraft, balloons, and satellites. Only rocket probes and remote-sensing instruments can study the composition and structure of this region; thus, lidar technologies, the optical counterpart to radar, are especially important for monitoring the upper atmosphere from the middle stratosphere (from about 30 km altitude) to the lower thermosphere (to about 110 km altitude). This region has been receiving increased attention because of the growing interest in global climate change.
Modeling studies and observations have shown that global change resulting from trace gas variations is not confined to the lower atmosphere but also extends into the middle and upper atmospheres.1 Greenhouse gases such as CO2 and methane (CH4) warm the lower atmosphere by absorbing infrared radiation emitted from the Earth's surface. These gases also are efficient radiators, cooling the upper atmosphere by sending heat into space. Models predict that doubling CO2 concentrations will cool the stratopause (about 50 km) by 10 to 12 K and the mesopause region (from 80 to 100 km) by 6 to 12 K.2probing sodium
Lidar systems typically integrate high-energy pulsed lasers, large optical telescopes, and range-gated photon-counting detectors to derive atmospheric profiles. Two primary types of lidar systems are being used to probe the upper atmosphere. Rayleigh/aerosol systems use optical scatter from air molecules and ice particles to infer temperature, wind, and aerosol profiles up to about 90 km altitude. Resonance fluorescence lidars use resonant backscattering from atomic metal layers in the mesopause region and lower thermosphere (80 to 110 km). The metal layers are formed by meteoric ablation, and lidar systems have been developed to probe mesospheric sodium (Na), iron (Fe), potassium (K), calcium (Ca), calcium ions (Ca+), and lithium (Li). Because of its relatively high abundance and large resonant backscatter cross section, Na has been widely studied. Na-based systems provide the highest resolution and most accurate wind and temperature measurements available. Rugged K and Fe systems have been developed recently to make upper-atmosphere temperature measurements at remote sites and from research ships and aircraft, however.
A Na lidar profile obtained at the Starfire Optical Range (Kirtland AFB; Albuquerque, NM) illustrates the molecular scattering between 30 and 80 km and the resonant scattering from Na between 80 and 105 km (see figure 1). To determine temperature using the backscattered molecular signal, we integrate the lidar-measured atmospheric density profile downward using the hydrostatic equation, ideal gas law, and an assumed upper-level temperature point usually chosen from a model. By inserting multiple narrowband Fabry-Perot filters in the detector optical path, we also can determine the Doppler shift of the backscattered molecular signal and hence the radial wind velocity.
Figure 1. The photon-count profile obtained by the University of Illinois Na Wind/Temperature lidar at the Starfire Optical Range shows molecular scattering and resonant Na scattering. The molecular scattered signal is proportional to atmospheric density.
To derive temperature and wind data from the backscattered Na signal, we tune the laser over the Na D2 spectral line, which is near 589 nm. The spectral bandwidth is related to temperature, and the spectral center line is related to Doppler shift associated with the radial winds.3 Temperature, radial wind velocity, and Na density can be determined by measuring the backscattered signal at as few as three different frequencies within the Na spectrum.
Although simple in concept, Na wind/temperature lidars employ sensitive ring dye lasers, pulsed dye amplifiers, and complex frequency-locking techniques to achieve the required tens of kilohertz frequency accuracy necessary for wind and temperature observations. Because measurement accuracy and useful resolution are related to signal strength, the most accurate measurements are obtained by using large telescopes such as the 3.5-m facility operated by the Air Force Research Laboratory at the Starfire Optical Range. This system has even been used to probe the winds and temperatures within several persistent meteor ablation trails left in the wake of bright fireballs.4 Although Na lidars are now capable of producing exquisite measurements of upper-atmospheric winds and temperatures, their use is restricted to ground-based sites with environmentally controlled labs to accommodate the sensitive and complex laser systems. polar monitoring
The polar regions are more sensitive to global climate change effects than other areas. Profiles of atmospheric parameters and constituents at the geographic poles can provide a convenient means of validating and calibrating global circulation models. Key parameters such as temperature profiles have been measured at the poles only in the troposphere and lower stratosphere to altitudes below 30 km, however, using balloon-borne sensors. Clearly, polar upper-atmosphere data are vital for testing the atmospheric circulation models that underlie our understanding of global climate change, but collecting data from this remote and inaccessible region with sophisticated remote-sensing instruments is challenging.
To help address this crucial measurement need, the University of Illinois (Urbana, IL) lidar group, in collaboration with the Aerospace Corp. (El Segundo, CA) and the National Center for Atmospheric Research (NCAR; Boulder, CO), recently developed a robust Fe Boltzmann lidar system for measuring temperature profiles in the upper atmosphere. Deployed by research aircraft or operated at remote sites, the system includes two 0.4-m-diameter telescopes and two injection-seeded pulsed alexandrite lasers that are frequency doubled to probe the Fe resonance lines at 372 and 374 nm. The nominal average output power of each laser is about 3 W at these wavelengths. The lidar can make observations during the day and at night. Temperatures are measured at altitudes from 30 to 80 km by summing the 372 and 374 nm molecular scattered signals and using the Rayleigh technique. Above 80 km, temperatures are measured by ratioing the two Fe signals and using the Fe Boltzmann technique.5
Figure 2. Xinzhao Chu of the University of Illinois adjusts one of the alexandrite lasers in the Fe Boltzmann lidar during a flight aboard the NSF/NCAR Electra aircraft. (University of Illinois).
In June and July 1999, we flew the lidar system over the North Pole aboard the National Science Foundation/NCAR Electra aircraft during the Arctic Mesopause Temperature Study (see figure 2). Six months later the instrument was moved to the Amundsen-Scott South Pole Station to measure the atmosphere temperature structure throughout the year. During these campaigns the instrument made the first measurements of upper-atmosphere temperatures, Fe densities, and polar mesospheric clouds over both poles during mid-summer.6 By combining the lidar measurements with balloon data, we now can determine the temperature profile from the surface to the edge of space (see figure 3). The system will remain at the South Pole through 2002 to establish the baseline temperature structure that can be used to assess future changes associated with climate change and to compare them to model predictions.
Figure 3. The composite temperature profile measured over the geographic South Pole uses data from an Fe/Rayleigh lidar and balloon.
Unlike the lower atmosphere, the upper atmosphere is colder during summer than in winter. Polar mesospheric clouds form at altitudes near 85 km over each of the polar caps during mid-summer, when temperatures fall below 125° C. These clouds are the highest on Earth. Their visual counterparts are the summertime noctilucent clouds, which typically are seen at latitudes greater than 50° during the night, when the surface is in darkness but the upper atmosphere is still sunlit. The brightness and geographic extent of noctilucent clouds have been increasing during the past four decades. Scientists believe these changes may be related to increasing levels of atmospheric CO2 and CH4, which in the upper atmosphere lead to cooler temperatures and increased levels of water vapor.
In the 1999/2000 southern and northern hemisphere summer seasons, we used the Fe Boltzmann lidar to measure the volume backscatter coefficient profiles of polar mesospheric cloud and noctilucent cloud layers over the North and South Poles and the Gulf of Alaska. Surprisingly, the altitudes of the polar mesospheric clouds over the South Pole were consistently 2 to 3 km higher than those over the North Pole. This feature may be related to hemispheric differences in the upper-atmosphere temperature structure.
Lidar technologies are making crucial contributions to studies of the chemistry and dynamics of the upper atmosphere. It is now possible to profile winds and temperatures throughout the atmosphere to altitudes as high as 110 km. These observations are leading to fundamental advances in our knowledge of the upper atmosphere and to the impact of global climate change. During the 40 years since the invention of the laser, lidar technologies have firmly established themselves as one of the key tools for probing the Earth's atmosphere. oe REFERENCES
1. R. Roble, et al., Geophys. Res. Lett. 16, 1441-1444, 1989.
2. R. Portmann et al., Geophys. Res. Lett. 22, 1733-1736, 1995.
3. R. Bills, et al., Opt. Eng., 30 , pp. 13-21, January 1991.
4. Chu, X., et al., Geophys. Res. Lett. 27, 1807-1810, 2000.
5. J. Gelbwachs, Appl. Opt. 33, 7151-7156, 1994.
6. C. Gardner, et al., Geophys. Res. Lett., in press, 2001.
Chester Gardner is VP for economic development & corporate relations and acting VP for academic affairs, University of Illinois, Urbana, IL.