CAMEX-3 JPL Laser Hygrometer

Table of Contents

Introduction
File Naming convention
Data Format		 References
Contact Information

Introduction

A hygrometer is an instrument which determines the water vapor content of a parcel of air. Direct measurement of water vapor content has been difficult until fairly recently. Historically, typical hygrometers determined the water vapor content of the atmosphere indirectly by measuring the wet bulb temperature (e.g. a psychrometer) or the dewpoint (e.g. using a chilled mirror dewpoint hygrometer).  Determination of water vapor content is a simple matter with either.

Laser hygrometers measure the amount of water vapor directly via absorption of laser light. If the initial strength of the laser and the path length through which it travels is known, measuring the diminution of its intensity will give an indication of the amount of water vapor present.

The JPL Laser Hygrometer uses this open-path principle. A tunable diode laser operating at 1.37mm is mounted as shown in a window blank (an aluminum panel which replaces the passenger window) on the right side (FS 490 right) of the NASA DC-8. The laser and detector are mounted on a circular aluminum disk visible in the upper rectangular 'arm' of the instrument. Exactly 25cm away (on the top side of the triangular shaped blue 'arm' ) is a 0.5 inch diameter mirror. This gives a path length of 50cm- from the laser, down to the mirror and back to the detector. The strut between the 'arms' insures that the path length remains constant in spite of vibrations occurring in the instrument and the length of the arms insures that the instrument is well outside the boundary layer of the aircraft minimizing effects of the aircraft itself upon the measurements. Software in the instrument determines the water vapor content of the atmosphere from a 2 second integrated sample. The laser is normally configured for 1 Hz sampling, but can be adjusted (via software) to a maximum of 8 Hz. Measurement precision is +0.05ppmv (parts per million volume) in the stratosphere.

File Naming Convention

File names appear as: 98ddd_JPLhyg.txt where ddd is the day of the year. As the txt extension would indicate, these data are in ascii form, and appear as below.

Data Format

The data is in ASCII text form, and is comprised of two sections. The first section, the  header, contains information about the date of the data, processing date, and unit designations for each of the data columns. Below is an example of the header data. To the right of several of the rows are key letters in red that reference columns in the following data table (table 2).

Table 1. Header
25   1001
May, Randy
JPL
JPL Laser Hygrometer 
CAMEX-3
1   1
1998 08 06   1998 08 14     [Flight Date     Process Date]
0
Elapsed UT seconds from 0 hours on takeoff date<----------A
1.0e-6 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
99999 99999 99999 99999 99999 99999 99999 99999 99999 
H2O volume mixing ratio<----------------------------------B
Static Pressure (mb), from MMS<---------------------------C
Static Temperature (K) from MMS<--------------------------D
H2O partial pressure (mb)<--------------------------------E
Dew point temperature (C)<--------------------------------F
Saturation v.p. with respect to H2O (mb)<-----------------G
Saturation v.p. with respect to ice (mb)<-----------------H
Relative Humidity (%, relative to H2O sat.)<--------------I
grams H2O per kilogram of air<----------------------------J
 1
JPL Laser Hygrometer. ** Preliminary Data **
1

Note that on the next to last line is the phrase "** Preliminary Data **". These data were processed either during or immediately following the CAMEX-3 experiment. They were NOT reprocessed subsequent to that time as post flight calibrations were consistent with pre-mission values. Conversation with the instrument PI, Dr. Randy May at JPL, confirmed that these data were in final form.

By using the key for each data column from the above table, decoding the data becomes trivial. Mixing ratio, presented as parts per million by volume, are given in column B, which is the most common unit used for stratospheric measurements where this type of instrument was first utilized (for the NASA ER-2 during the 1997 POLARIS mission). That value is shown in column J.
 

Table 2. Data
    A                B             C          D             E              F              G                H            I        J 

   UT(s)       ppmv      P(mb)    T(K)      Ep(mb)       Td(C)    Esh(mb)    Esi(mb)    %RH  g/kg 
 63428.0   157.87  403.60 255.50   0.06372  -49.85   1.53137   1.28993   4.16  0.0987 
 63430.0   157.66  403.00 255.40   0.06354  -49.85   1.51846   1.27783   4.18  0.0985 
 63432.0   156.96  402.40 255.30   0.06316  -49.95   1.50566   1.26584   4.19  0.0981 
 63434.0   155.21  401.80 255.20   0.06236  -50.05   1.49295   1.25395   4.18  0.0970 
 63436.0   153.99  401.10 255.10   0.06177  -50.15   1.48034   1.24217   4.17  0.0962 
 63438.0   153.53  400.60 255.00   0.06151  -50.15   1.46782   1.23048   4.19  0.0960 
 63440.0   153.80  399.90 254.90   0.06150  -50.15   1.45540   1.21889   4.23  0.0961 
 63442.0   154.01  399.30 254.70   0.06150  -50.15   1.43084   1.19602   4.30  0.0963 
 63443.0   153.69  399.00 254.70   0.06132  -50.15   1.43084   1.19602   4.29  0.0961 
 63445.0   153.42  398.40 254.60   0.06112  -50.15   1.41870   1.18474   4.31  0.0959 
 63447.0   153.32  397.70 254.50   0.06097  -50.25   1.40664   1.17355   4.33  0.0958 
 63449.0   153.72  397.00 254.40   0.06103  -50.25   1.39469   1.16245   4.38  0.0961

References

See "Open-Path, Near-IR Tunable Diode Laser Spectrometer for Atmospheric Measurements of H2O", R.D. May, J. Geophys. Res., 103, 19,161-19,172 (1998).

Contact Information

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/