Open Path Diode Laser Hygrometer (DLH) Instrument for Tropospheric and Stratospheric Water Vapor Studies


Research Staff: James R. Podolske

The Diode Laser Hygrometer (DLH), developed by NASA’s Langley and Ames Research Centers, has flown on the NASA DC-8 during several field missions including SUCCESS, VOTE, TOTE, SONEX, PEM-Tropics A and B, SOLVE, AFWEX, and the TRACE-P campaign of 2001. The optical layout of this sensor consists of the compact laser transceiver mounted to a DC-8 window port and a sheet of retro-reflecting "road sign" material applied to the DC-8 engine enclosure that completes the optical path. This sensor approach has a number of advantages including compactness, simple installation, fast response time (50 msec), no wall or inlet effects, and a wide dynamic measurement range (several orders of magnitude).

Using differential absorption detection techniques similar to those described in the literature, gas-phase water (H2O(v)) is sensed along a 28.5 meter external path. For dry conditions (generally altitudes greater than 6 km) the diode laser wavelength is locked onto a strong, isolated line at 7139.1 cm-1 while for altitudes typically less than 6 km the laser wavelength is locked onto a weaker line at 7133.9 cm-1. By normalizing the laser differential absorption signal with the laser power signal, the H2O(v) measurement is unaffected by clouds, haze, plumes, etc. thereby enabling high spatial resolution measurements in and around clouds. The H2O(v) mixing ratio is computed by an algorithm from the differential absorption magnitude, ambient pressure and temperature, and coefficients derived from laboratory calibration of the sensor.

During the AFWEX campaign in December 2000, DLH was flown with a suite of in situ and remote sensing water instruments to assess the level of agreement between the various techniques. Analysis of data is currently underway.


Collaborators: Glenn Diskin and Glen Sachse, Langley Research Center

Point of Contact: James Podolske, 650/604-4853,