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Lidar Atmospheric Sensing Experiment |
Vaporized water is an invisible gas that is present everywhere in the atmosphere. The distribution of atmospheric water vapor is of fundamental importance to weather and climate, atmospheric radiation studies, global hydrological cycle and atmospheric chemistry. The LASE program was initiated as an effort to produce an autonomous system for measuring water vapor levels from airborne and spaceborne platforms using LIDAR technology.
The ability of a Differential Absorption Lidar (DIAL) system to measure vertical profiles of water vapor in the lower atmosphere has been demonstrated both in ground-based and airborne experiments. In these experiments, tunable lasers are used that require real-time experimenter control to locate and lock onto the atmospheric water vapor absorption line for the DIAL measurements. The Lidar Atmospheric Sensing Experiment (LASE) is the first step in a long-range effort to develop and demonstrate autonomous DIAL systems from airborne and spaceborne platforms. The LASE instrument was developed to measure water vapor, aerosol, and cloud profiles from a high altitude extended range U-2 (ER-2) aircraft. This article presents the science of the LASE program, describes the LASE system design, and discusses the measurement capability of the system.
The measurement of tropospheric water vapor profiles and column content with the LASE system can be used in various atmospheric investigations, including studies of air mass modification, latent heat flux, the water vapor component of the hydrological cycle, and atmospheric transport using water vapor as a tracer of atmospheric motions. The simultaneous measurement of aerosol and cloud distributions can provide important information on atmospheric structure and transport, and many meteorological parameters can also be inferred from these data. In addition, the impact of subvisible and visible aerosol/cloud layers on passive satellite measurements and radiation budgets can be assessed. The atmospheric science investigations that can be conducted with LASE are greatly enhanced because measurements of water vapor profiles and column content are made simultaneously with aerosol and cloud distributions.
The LASE system parameters are given here. The transmitter consists of a Ti:sapphire laser pumped by a double pulsed Nd:YAG laser. The frequency of the Ti:sapphire laser is controlled by injection seeding using a diode laser that is frequency locked to a water vapor line in the 815-nm region. The "on" and "off" wavelengths are separated by less than 70 pm. The laser pulses are sequentially transmitted with about 400 microseconds separation. This permits the use of the same avalanche photodiodes (APD) for detecting the lidar returns. The use of low and high light level APD's provides linear response to atmospheric and cloud/ground returns, respectively. Lidar returns at 5 Hz are digitized and recorded, and when possible, the data are telemetered to the LASE ground station for real-time processing and experiment control. Operation with strong and weak absorption regions of a preselected water vapor line can be made during the mission to optimize the measurement of water vapor in different altitude regions.
The LASE system has proven to be a reliable, accurate, and sensitive water vapor profiler with the ability to measure water vapor mixing ratios over a large dynamic range (0.01 g/kg to 20 g/kg). Aerosol backscatter ratios can be measured from ground to 20 km with a vertical resolution of 30 m and a horizontal resolution of 40 m.
LASE is currently being reconfigured to fly on NASA DC-8 and P-3 aircraft, in addition to the ER-2. Proposals are underway for the instrument to participate in a number of major field programs managed by NASA's Office of Mission to Planet Earth. Information gathered by this instrument will become an integral part of many other atmospheric studies. As the first step towards the long range goal of developing a spaceborne LIDAR instrument, LASE is already doing important work for atmospheric research.
The LASE GIF image files for the following campaigns are currently available to the public on the Lidar Applications Group WWW page and LASE digital profile data are available via the EOS at Langley DACC.
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NASA Langley Science Directorate Homepage. |
