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SRL
Point of Contact:
- David Whiteman
- Code 924
- NASA/Goddard Space Flight Center
- Greenbelt, MD 20771
- Telephone: (301)614-6703
- Fax: (301)614-6744
- E-mail: david.whiteman@gsfc.nasa.gov
- SRL Homepage: http://virl.gsfc.nasa.gov/srl/index.htm
Brief Instrument Description of the
Scanning Raman Lidar (SRL)
The NASA/Goddard Space Flight Center Scanning Raman Lidar (SRL) was deployed at the U. S. Navys Atlantic Undersea Test and Evaluation Center (AUTEC) on Andros Island in the Bahamas on July 29, 1998. The SRL will measure atmospheric water vapor and aerosols during both day and night for roughly two months until the completion of the CAMEX-3 hurricane study program in late September. The primary science objectives for the SRL during these two months are 1) to serve as the nucleus of the water vapor calibration/validation facility for CAMEX-3 and 2) to acquire an extended record of high quality water vapor and aerosol data in a sub-tropical location for use by the Tropical Rainfall Measuring Mission (TRMM) community and for initialization of global circulation models.
SRL System Description
A lidar system consists of a laser transmitter, telescope receiver and electronics for data acquisition. The lidar technique consists of firing a pulse of laser light into the atmosphere and recording the return signals due to scattering events in the atmosphere. The distance to the scattering volume element is determined by measuring the time of flight of the laser pulse whereas the intensity of scattering is related to the amount of the quantity being measured. Lidar systems have been built to measure a variety of atmospheric parameters including water vapor, ozone, density and temperature. The SRL employs two different lasers for its measurements of water vapor and aerosols. For optimized nighttime measurements, an XeF excimer laser (351 nm output) is used. For the daytime measurements and to enable an around-the-clock measurement capability, a tripled Nd:YAG laser combined with a dual (narrow and wide) field of view optics design is used.
Light backscattered by molecules and aerosols at the laser wavelength as well as Raman scattered light from water vapor (3654 cm-1), nitrogen (2329 cm-1), and oxygen (1555 cm-1) molecules is collected by a 0.76 m, f/5.2, variable field-of-view (.25 - 2.5 milliradians) Dall-Kirkham telescope mounted horizontally on a 3.7m optical table. The telescope field-of-view is steered with a large (1.2m x .8m) flat mirror which rotates on a horizontal axis and is also mounted on the optical table. All of the SRL instrumentation, including lasers, large aperture telescope and data acquisition electronics, is housed within a single environmentally controlled mobile trailer which also supplies work space for several experimenters.
Two detection channels, operating in the photon counting mode, are employed for each wavelength in order to measure signals throughout the troposphere and lower stratosphere. In normal operation, data are recorded as one-minute profiles corresponding to the accumulation of signals from about 23000 laser shots. The photon counting data have a range resolution of 75 meters.
The scanning capability of the lidar system may be used in two ways. For scanning at any angle in a single scan plane, the optical table may be slid through an opening in the back of the trailer deploying the scan mirror to the outside. This allows a 180 degree horizon to horizon scan capability. By scanning at low elevation angles, the SRL also acquires much higher vertical resolution data near the surface. Using the motorized scan mirror, atmospheric profiles can be acquired at any angle in a single plane perpendicular to the trailer or continuously scanned from horizon to horizon. Alternatively, the scan mirror may be pointed through one of three windowed openings in the trailer, allowing vertical measurements and measurements at 5-10 degrees above the horizon in either direction. We anticipate operating the lidar in this latter congfiguration during CAMEX-3. This allows operations to continue during rain and provides improved thermal stabilty of the instrumentation.
During nighttime operations, one-minute, 75 meter resolution profiles extending to 8-9 km are measured; for daytime operations, ten-minute, 75 meter profiles extending to 3.5-4 km are measured. The lidar water vapor profiles are then used in conjunction with temperature profiles measured by radiosondes as well as derived from coincident downwelling radiances measured by the AERI instrument to compute relative humidity profiles. Precipitable water is derived by integrating the water vapor mixing ratio profile with respect to pressure. The water vapor mixing ratio, which is defined as the mass of water vapor divided by the mass of dry air, is derived from the ratio of Raman water vapor to Raman nitrogen signals. A small correction, which is easily computed using model atmospheric density profiles, is used to account for the difference in atmospheric transmission between the Raman water and nitrogen return signals.
The SRL has also been used extensively to measure aerosol profiles. The aerosol scattering ratio, which is defined as the ratio of the total (aerosol + molecular) scattering to molecular scattering, is computed directly from the lidar data. The aerosol volume backscatter cross section is then computed from the scattering ratio and the molecular backscattering cross section which is derived from the coincident radiosonde pressure and temperature profiles. The SRL is also used to simultaneously and directly measure the aerosol volume extinction cross section using the derivative of the logarithm of the nitrogen Raman return signal. Aerosol optical thickness is then found by integrating the aerosol extinction profiles.
