[Written by for the Accent on Applications section of Photonics News by Shane Mayor]
Fine-scale remote probing of the atmospheric boundary layer (the lowest 1-3 km above the surface) is important for understanding thunderstorm generation, air pollution meteorology, and exchange of gases and energy between the surface and the free atmosphere. During the summer of 1996, scientists from the National Center for Atmospheric Research (NCAR) Atmospheric Technology Division (ATD) and the National Oceanic and Atmospheric Administration (NOAA) Environmental Technology Laboratory (ETL) deployed three lidars, operating at wavelengths ranging from the UV to the middle infrared, at a University of Illinois field site near Champaign to observe high resolution structure of aerosol, winds, and ozone as the boundary layer evolved from early morning to late evening. The data gathered during the one month experiment will be used 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 NCAR's scanning aerosol backscatter lidar (SABL) operating at 532 and 1064 nm wavelength, NOAA-ETLs ground-based UV differential absorption lidar (DIAL), and a high-resolution Doppler lidar (HRDL) operating at 2-micron wavelength. Approximately 100 hours of simultaneous vertically pointing DIAL and HRDL data were collected which for ozone flux research. Over 200 hours of HRDL (pointing vertical and scanning) and SABL data (vertical only) were collected during the 4 week experiment.
SABL uses a Nd:YAG laser with a doubling crystal to transmit 15 ns pulses of green (50 mJ per pulse) and infrared light (75 mJ per pulse) at up to 60 Hz. Backscatter from atmospheric aerosols is collected by a 14" Cassegrain telescope, detected by a PMT (green) and APD (IR), and digitized by two 40 MHz digitizers. Data is written to 8-mm magnetic tapes. SABL's compact and rugged design enables airborne, shipboard, or ground-based deployment.
SABL's first deployment was aboard the NCAR C-130 research aircraft in November and December of 1995 during the Aerosol Characterization Experiment (ACE). LIFT was the first ground-based deployment for the instrument. SABL provides vertical profiles of aerosol backscatter with very high temporal and spatial resolution (up to 3.75 m) which can be used to indicate the altitude of the top of the PBL. It is also well suited for measuring the distance to the edge of an optically thick cloud or profiling through thin cirrus clouds.
The DIAL, developed by Yanzeng Zhao of NOAA-ETL, provides vertical profiles of ozone concentration. The system is Nd:YAG based and operates at 266 nm (quadrupled 1064 nm), 289 nm (Raman shifted and mixed 532 nm), and 355 nm (tripled 1064 nm). The system uses a three beam transmitter and single receiver with each transmit beam overlaping 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. As a participant of an ongoing study by California Air Resource Bureau (CARB) the ozone DIAL was previously deployed in Claremont, CA and Victorville, CA to study tropospheric ozone pollution in the Los Angeles area.
The HRDL system, developed by Christian Grund of NOAA-ETL, uses an injection-seeded Tm:Lu,YAG laser operating at 2.0218-m m. The pulse laser is acousto-optically Q-switched and double end-pumped by fiber-coupled 785 nm CW laser diodes. At a 200 Hz laser repetition rate,HRDL transmits 1 mJ 200-300 ns FWHM pulses. A room temperature InGaAs PIN diode is used for heterodyne detection. The 0.35 m diameter off-axis Mersenne receiver telescope also serves to expand the transmitted laser pulses. Two 12-bit digitizers operating at 50 MHz provide samples of the signal to four Arial Hydra cards, each with four TMS320C40 DSP's,to accomplish real time signal processing, display, and recording. 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 cm/s velocity resolution.
All three lidars were housed in portable trailers and parked on property owned by the University of Illinois during LIFT. The rural east-central Illinois site was selected because of the extremely flat terrain which simplifies PBL processes, good aerosol scattering, and the nearby permanent installation of radar wind profilers operated by the NOAA Aeronomy Laboratory. By combining measurements from the different lidars and radars, other information such as fluxes of heat and ozone will be obtained. Performance of the lidars will be compared with radar profilers for boundary layer research.
LIFT was the third very successful lidar field project accomplished by the joint optical remote sensing program between NOAA-ETL and NCAR-ATD which is less than 2 years old. For further information contact Dr. Micheal Hardesty or Shane Mayor, National Center for Atmospheric Research, P.O. Box 3000, Boulder CO 80307-3000, 303-497-2002.