CAMEX-4 JPL Laser Hygrometer

Table of Contents

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 then determined using tables for formulii from either of these values.

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 principle in an open-path instrument; that is, the laser path is in the free stream passing by the aircraft.

A tunable diode laser operating at 1.37mm is mounted in a window blank (an aluminum panel which replaces the passenger window) on the port side of the NASA DC-8. The laser and detector are mounted on a circular aluminum disk in the upper rectangular 'arm' of the instrument. Exactly 25cm away is a 0.5 inch diameter mirror from which the laser beam is reflected. This gives a path length of 50cm- from the laser, down to the mirror and back to the detector. 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.

Each mission (no more than one per day) for which there were data collected has a file of the form:

c4djlh_2001.267_010418.dat
where c4jlh represents a CAMEX-4 dataset for the JPL Laser Hygrometer, 2001.267 is the yyyy.ddd and 010418 is the mission number.

The data is in Gains Hipskind format (see link for tutorial). This ascii based file is comprised of two sections. The first section is the header and 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).

28   1001                                                                       
Herman, Robert L.                                                               
JPL                                                                             
JPL Laser Hygrometer                                                            
CAMEX-4                                                                         
1   1                                                                           
s001 09 24   2002 06 20                                                         
0                                                                               
Elapsed UT seconds from 0 hours on takeoff date                                 
10                                                                              
1.0e-6 1.0 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 99999                     
H2O volume mixing ratio                                                         
Static Pressure (mb), from MMS                                                  
Static Temperature (K) from MMS                                                 
H2O partial pressure (mb)                                                       
Dew point temperature (C)                                                       
Saturation v.p. with respect to H2O (mb)                                        
Saturation v.p. with respect to ice (mb)                                        
Relative Humidity (%, relative to ice)                                          
Relative Humidity (%, relative to liquid water)                                 
Grams H2O per kilogram of air                                                   
 3                                                                              
Water vapor measured by the JPL Laser Hygrometer                                
utilizes static temperature and pressure measured                               
by MMS.

Table 2 - The second part of the datafile are the actual data measurements themselves. An example is shown here.

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).

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/