Richard Ferrare, Edward Browell, Syed Ismail, Susan Kooi, Vince Brackett, Marian Clayton, Anthony Notari, Carolyn Butler, John Barrick, Glenn Diskin, Barry Lesht, Frank Schmidlin, Dave Turner, David Whiteman, Larry Miloshevich
Measurements of water vapor profiles over the Southern Great Plains acquired by two different lidars are presented. NASA's airborne DIAL Lidar Atmospheric Sensing Experiment (LASE) system measured water vapor, aerosol, and cloud profiles during the ARM/FIRE Water Vapor Experiment (AFWEX) in November-December 2000 and during the International H2O Project (IHOP) in May-June 2002. LASE measurements acquired during AFWEX are used to characterize upper troposphere water vapor measured by ground-based Raman lidars, radiosondes, and in situ aircraft sensors. LASE measurements acquired during IHOP are being used to better understand the influence water vapor variability on the initiation of deep convection and to improve the quantification and prediction of precipitation associated with these storms. The automated Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) Raman Lidar (CARL) has been routinely measuring profiles of water vapor mixing ratio, relative humidity, aerosol extinction, aerosol backscattering, and aerosol and cloud depolarization during both daytime and nighttime operations. Aerosol and water vapor profiles acquired since March 1998 are used to investigate the seasonal variability of the vertical distributions of water vapor and aerosols.
KEYWORDS: Sensors, Semiconductor lasers, Absorption, Calibration, Modulation, Temperature metrology, Amplifiers, Stratosphere, Signal detection, Data modeling
An instrument has been developed for the measurement of water vapor in the troposphere and lower stratosphere. This instrument, dubbed the NASA Langley/Ames Diode Laser Hygrometer (DLH), has been flown on 10 missions aboard NASA's DC-8 aircraft. The DLH utilizes an open-path, double-pass configuration, where the path is defined on one end by a laser transceiver mounted on the interior of a modified window panel, and on the other by a panel of retroreflecting material mounted on the DC-8's outboard engine nacelle. The DLH operates on one of two spectral absorption lines in the 1.4 μm spectral region, in a wavelength-modulated (WM) mode, with the laser locked to the center of the absorption line encountered in a reference cell. The spectral line used is determined by the local conditions - a weak line is used at low altitudes and a stronger one at high altitudes - and is changed at various times during a flight by the operator. Signal detection is accomplished by demodulating the return signal at twice the driving frequency (2F detection). The returned laser power (DC) is also measured. The DLH is calibrated in the laboratory at various combinations of pressure and water vapor density. From the calibration data and a multiparameter spectral model, a set of coefficients is developed, and these coefficients are used to convert the measured 2F/DC ratio, along with local temperature and pressure (which are measured by separate instruments aboard the aircraft), to water vapor mixing ratio.
We report the application of diode laser spectroscopy to the non-intrusive measurement of water vapor and helium content within the enclosures of the Charters of Freedom that were sealed by NBS approximately 50 years ago. The instrument was adapted from the Diode Laser Hygrometer, developed through the NASA Global Tropospheric Experiment program to make open-path water vapor measurements outside the NASA DC-8 research aircraft. Operating in the 1.4-micron wavelength region, laser wavelength scans of water vapor absorption lines enabled the determination of the moisture level and the amount of helium relative to air within the Charters enclousres (an indicator of enclosure leaks). These measurements revealed that at least five of the seven Charters of Freedom enclosures had not leaked since they were sealed approximately 50 years ago. The measurements also revealed that water vapor within these enclosures ranged from 29 to 55% greater than anticipated.
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