NAMMA Diode Laser Hygrometer(DLH)
Table of Contents
Introduction
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.
Campaign
These 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 Description
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. 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 Convention
Data files are in files of the form:
NAMMA_DLH_yyyymmdd_rx.txt
where,
NAMMA_DLH - Represents the NAMMA project and the DLH instrument
yyyymmdd - Is the year, month and date of the flight
rx - Indicates The revision level
txt - indicates that these are tab delimited text files, using "1001" Gaines-Hipskind format
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:
34 1001 ; Gaines-Hipskind format [http://espoarchive.nasa.gov/archive/docs/formatspec_2_0.pdf]
Diskin, Glenn S.; Podolske, James R.
NASA Langley Research Center; NASA Ames Research Center
Diode Laser Hygrometer (DLH); Water Vapor Mixing Ratio from NASA DC-8
NASA NAMMA MISSION 2006
1 1
2006 08 07 2006 08 29 ; flight-date analysis-date
1
Start_UTC
1
1
-999
H2O(v)_ppmv
1
These data are FINAL
18
PI_CONTACT_INFO: NASA LaRC, MS 483, Hampton, VA 23681; e-mail: glenn.s.diskin@nasa.gov;
PLATFORM: NASA/UND DC-8 Aircraft
LOCATION: DC-8 location data in file NAMMA_ICATS_20060807_1Hz.bin
ASSOCIATED_DATA: NAMMA_ICATS_20060807_1Hz.bin
INSTRUMENT_INFO: Diode Laser Hygrometer - external path in-situ water vapor
DATA_INFO: Water Vapor Volumetric Mixing Ratio in ppmv: p(H2O) / p(ambient) * 10^6
UNCERTAINTY: H2O(v) = 5%
ULOD_FLAG: -7777
ULOD_VALUE: 100000;
LLOD_FLAG: -8888
LLOD_VALUE: 1;
DM_CONTACT_INFO: glenn.s.diskin@nasa.gov
PROJECT_INFO: N/A
STIPULATIONS_ON_USE: PI is interested in knowing intended usage of this dataset
OTHER_COMMENTS: Data in this file are 1 sec; higher bandwidth data available from PI
REVISION: R1
R1: No comments for this revision.
UT_Sec H2O_ppmv
68520.0 12666.56
68521.0 12546.99
68522.0 12637.19
68523.0 12328.16
68524.0 12272.10
68525.0 12200.19
68526.0 12217.56
68527.0 12263.86
68528.0 12556.91
68529.0 12488.31
68530.0 12405.04
Column headers are described as follows:
UT_Sec:
Time in UTC seconds.
H2O_ppmv:
Water Vapor Volumetric Mixing Ratio in ppmv: p(H2O) / p(ambient) * 10^6.
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
Podolske, J. R., 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 Information
The data producer is:
Glenn Diskin
NASA LaRC, MS 483,
Hampton, VA 23681
e-mail: glenn.s.diskin@nasa.gov
To order these data or for further information, please contact:
Global Hydrology Resource Center
User Services
320 Sparkman Drive
Huntsville, AL 35805
Phone: 256-961-7932
E-mail: support-ghrc@earthdata.nasa.gov
Web: http://ghrc.nsstc.nasa.gov/
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