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Three Lidars Team Up to Probe the Atmosphere

Photonics Spectra
Dec 1996
Ruth A. Mendonsa

CHAMPAIGN, Ill. -- In a joint optical remote-sensing program, scientists from the National Center for Atmospheric Research (NCAR) Atmospheric Technology Div. and the National Oceanic and Atmospheric Administration (NOAA) Environmental Technology Laboratory, both in Boulder, Colo., collaborated on a month-long project to probe the atmospheric boundary layer. Fine-scale remote probing of this layer, the lowest above the Earth's surface, is used to learn more about thunderstorm generation, air pollution meteorology and exchange of gases and energy between the surface and the free atmosphere.

In the project, dubbed LIFT (lidars in flat terrain), the researchers deployed three lidars at a University of Illinois field site to observe the high-resolution structures of aerosol, winds and ozone as the boundary layer evolved from early morning to late evening. All three lidars were housed in portable trailers on property owned by the University of Illinois. The site was selected because of its extremely flat terrain, which simplifies the planetary boundary layer processes, provides good aerosol scattering and because the site offers access to the nearby radar wind profilers operated by the NOAA Aeronomy Laboratory. Researchers will use the data gathered during the experiment to evaluate the combination of different types of lidar measurements for quantifying boundary layer height, turbulence, fluxes and entrainment.

Included in the suite of lidars was NACP's scanning aerosol backscatter lidar (SABL) operating at 532 and 1064 nm, which provides vertical profiles of aerosol backscatter with very high temporal and spatial resolution. SABL uses an Nd:YAG laser with a doubling crystal to transmit 15-ns pulses of green and infrared light at up to 60 Hz. LIFT was the first ground-based deployment of this instrument.

The laboratory's ground-based UV differential absorption lidar (DIAL), which provides vertical profiles of ozone concentration, was the second ground-based lidar. It is Nd:YAG-based and operates at 266, 289 and 355 nm. DIAL uses a three-beam transmitter and single receiver with each transmit beam overlapping the receiver field of view at a different altitude in order to measure ozone concentrations from ground level to 3 km with a range resolution of 15 m.

Last in the trio is a high-resolution Doppler lidar (HRDL) operating at 2 µm. This system uses an injection-seeded Tm:Lu,YAG laser operating at 2.0218 µm. The pulse laser is acousto-optically Q-switched and double end pumped by fiber-coupled 785-nm CW laser diodes. While HRDL measures radial air motion along the laser beam, the full-hemispheric beam-scanning capability can be used in a variety of ways to obtain horizontal and vertical wind components and turbulence statistics with 30-m range resolution and centimeter-per-second velocity resolution.

By combining the measurements from the different lidars, other information such as fluxes of heat and ozone will be obtained. LIFT was the third successful lidar field project accomplished by the two organizations.


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