SCANNING RAMAN LIDAR
|The SRL makes use of Raman scattering in the atmosphere to measure various atmospheric properties. This permits the study and improved understanding of mesoscale dynamics, warm and cold clouds and aerosols. The main scientific quantities that are measured by the SRL are:
- Water vapor mixing ratio, which is the mass of water vapor divided by the mass of dry air in a given volume of the atmosphere.
- Aerosol scattering ratio, which is the total scattering due to aerosols and molecules divided by the scattering just from molecules.
- Aerosol extinction which is the attenuation of the laser beam due just to aerosol particles (as opposed to molecules).
- Aerosol depolarization ratio, which is the amount of the polarized laser beam that is scattered perpendicular to the beams original polarization – this measurement helps to distinguish irregularly shaped particles from spherical ones.
- Liquid water mixing ratio, which is the mass of liquid water divided by the mass of dry air in a given volume.
Picture of the Scanning Raman
Lidar deployed at Andros Island, Bahamas during CAMEX-3.
The SRL uses a tripled Nd:YAG laser combined with two telescopes using different fields of view to measure high altitude and low altitude signals. Light backscattered by molecules and aerosols at the laser wavelength as well as Raman scattered light from water vapor (3657 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. This telescope is typically operated using 0.25 milliradian field of view and acquires the high altitude signals. A smaller 0.25 m telescope is mounted inside of the larger telescope and operates at approximately 1 milliradian field of view. This smaller telescope makes the depolarization measurements and is used for the low altitude Raman measurements. 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.
The SRL uses up to 12 data acquisition channels that simultaneously sample the signal in both analog to digital and photon counting modes. This permits a single instrumental configuration to be used for both daytime and nighttime measurements. The coupling of narrow field of view, narrow spectral band filters and these combined AD/PC electronics has permitted the SRL to greatly improve the daytime water vapor mixing ratio measurement capability of water vapor Raman lidar systems. An example of this daytime measurement capability is shown below where the random error has also been characterized. The results of this analysis indicate that the SRL is able to measure water vapor mixing ratio in the boundary layer with less than 10% random error with ~2 minute temporal resolution and spatial resolution varying between 60 – 200 meters.
All of the SRL instrumentation, including laser, large aperture telescope and data acquisition electronics, is housed within a single environmentally controlled mobile trailer which also supplies work space for several experimenters.
Water vapor mixing ratio measured by the SRL during the dryline event.
Temporal resolution is 3 minutes, vertical smoothing varied
between 90 meters at 0.5 km to 330 meters above 4 km.
The calibration of the data was determined with respect to
SuomiNet GPS mounted on the SRL trailer.
Please click on image for full view.
Diagram of the NASA GSFC Scanning Raman Lidar.