|
|
NAMMA Diode Laser Hygrometer(DLH) Table of Contents
The Diode Laser Hygrometer (DLH), an instrument developed for the measurement of water vapor in the troposphere and lower stratosphere by NASA’s Langley and Ames Research Centers, has flown on the NASA DC-8 aircraft during several field campaigns. The DLH is a near-infrared spectrometer operating near 1.4 µ, and was developed for in situ measurements of atmospheric water vapor (H 2O(v)) from aircraft platforms. It is based upon near-infrared tunable diode technology. This spectrometer provides true in situ monitoring of water vapor concentrations with precision levels exceeding those of existing Lyman α and frost point hygrometers. CampaignThese data files were generated during support of the NASA African Monsoon Multidisciplinary Analyses (NAMMA) campaign, a field research investigation sponsored by the Science Mission Directorate of the National Aeronautics and Space Administration (NASA). This mission was based in the Cape Verde Islands, 350 miles off the coast of Senegal in west Africa. Commencing in August 2006, NASA scientists employed surface observation networks and aircraft to characterize the evolution and structure of African Easterly Waves (AEWs) and Mesoscale Convective Systems over continental western Africa, and their associated impacts on regional water and energy budgets. For more information about the NAMMA Campaign, go to the NAMMA web site: http://namma.nsstc.nasa.gov/ Instrument DescriptionThe 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. A laser power normalization scheme enables the sensor to accurately measure water vapor even when flying through clouds. An algorithm calculates H 2O(v) concentration based on the differential absorption signal magnitude, ambient pressure and temperature, and spectroscopic parameters that are measured in the laboratory. File Naming ConventionData files are in files of the form:
where, Data Format The DLH data files are 1 second ASCII data and follow the "1001" Gaines-Hipskind format as described in the document entitled - "ASCII File Format Specification for Data Exchange" found at: http://espoarchive.nasa.gov/archive/docs/formatspec_2_0.pdf. Additional information about the data can be found in the file header. An example of the contents of a DLH data file is shown in below:
Column headers are described as follows: UT_Sec:
H2O_ppmv: References G. S. Diskin, J. R. Podolske, G. W. Sachse, and T. A. Slate, "Open-Path Airborne Tunable Diode Laser Hygrometer", Soc. of Photoopt. Instrum. Eng., Bellingham, Wash., (2002). J. R. Podolske, G. W. Sachse, and G. S. Diskin, "Calibration and data retrieval algorithms for the NASA Langley/Ames Diode Laser Hygrometer for the NASA Transport and Chemical Evolution Over the Pacific (TRACE-P) mission", J. Geophys. Res., 108 (D20), 8792-8798 (2003). Vay, S. A., B. E. Anderson, G. W. Sachse, J. E. Collins, Jr., J. R. Podolske, C. H. Twohy, B. Gandrud, K. R. Chan, S. L. Baughcum, and H. A. Wallio, DC-8-based observations of aircraft CO, CH4, N2O, and H2O(g) emission indices during SUCCESS, Geophys. Res. Lett., in press, 1998a Glen W. Sachse; Karen B. Bartlett; James Podolski; Nikita S. Pougatchev; Aircraft Measurements and Analysis of CO, CH4, N2O, CO2, and H2O(v) in Support of TRACE P , G. W. Sachse, and G. S. Diskin (2003), Calibration and data retrieval algorithms for the NASA Langley/Ames Diode Laser Hygrometer for the NASA Transport and Chemical Evolution Over the Pacific (TRACE-P) mission, J. Geophys. Res., 108(D20), 8792, doi:10.1029/2002JD003156. Contact InformationThe data producer is:
To order these data or for further information, please contact:
|