The basic principle of the lidar is a pulsed laser beam that is sent into the atmosphere, where it is scattered and absorbed by atmospheric molecules and aerosols. The backscattered light is collected with the receiving telescopes and filtered spectrally, converted to electrical signal and digitized for further analysis. The atmospheric response to the laser pumping at a single laser line consists of several spectral components. These are mainly represented by the elastic scattered light, the pure rotational Raman spectra (PRRS) and the rotational-vibrational Raman spectra of atmospheric molecules. Isolating individual spectral features of the atmospheric response, we apply the Raman lidar technique to characterize the scattering properties of atmospheric aerosols and to measure the air temperature and humidity.
The different products that can be obtained from the lidar are particle backscatter, particle extinction, depolarization ratio, lidar ratio, water vapor mixing ratio, aerosol optical depth, and temperature. The combination of particle backscatter measured at three wavelengths, particle extinction measured at two wavelengths, and particle depolarization ratio will also give information on the aerosol types.
• Particle backscatter is derived directly as the ratio of atmospheric lidar returns in the elastic and the pure rotational Raman channels. Thanks to the relatively small difference in wavelength of backscattered light for these two channels the corresponding differential atmospheric extinction is negligible. An example of July 15th, 2010, measured at Deebles Point can be found here.
• Particle extinction is calculated directly from the atmospheric attenuation of the pure rotational Raman signal. A narrow field-of-view of the receiving telescopes in combination with a narrow spectral bandwidth of the pure rotational Raman lidar channels allows the daytime extinction measurement even in equatorial regions. [Example July 15th, 2010, Deebles Point]
• Depolarization ratio: the linear depolarization ratio, measured at 532nm, gives information about the spherical shape of particles. By implementing an individual receiving telescope for the depolarization channel allows minimizing the number of optical elements in the light path. This allows achieving reasonably high polarization purity in the cross-parallel channel, sufficient to detect variation of particle depolarization at a level of the molecular depolarization [Example July 15th, 2010, Deebles Point].
• Lidar ratio: is the ratio of measured particle extinction versus backscatter coefficient (with extinction and backscatter calculated both on the same coarse grid, required for achieving sufficient accuracy of extinction measurements) [Example July 15th, 2010, Deebles Point].
• Water vapor mixing ratio is derived as a ratio of backscatter signals in vibrational Raman branch of water vapor and nitrogen molecules. Due to extremely strong scattered sunlight background at the location of observational site at daytime, we operate the water vapor channel only during night.
• Air temperature: pure rotational Raman lidar technique is implemented for air temperature profiling. Near range and far range detection channels are equipped both with two temperature channels operating in UV and in visible spectral range. This allows higher accuracy of combined air temperature profile. An example of derived temperature profiles during ALOMAR (Norway) are here.
New Potential for the Raman Lidar
The upgraded Raman lidar system at the Southern Great Plains ARM site in Oklahoma. Operational since 1995, this fully autonomous system is nearly identical to the systems deployed at the Darwin, Australia, ARM site in December 2010 and at the Oliktok Point, Alaska, ARM site in September 2014.
The installation of new detection electronics, and the subsequent reduction in the amount of neutral density attenuation in some of the detector channels, opened up new Raman lidar research areas. Before the 2004 upgrade, the maximum temporal and spatial resolutions for the water vapor mixing ratio and aerosol scattering ratio (and hence backscatter coefficient) were 1 min and 75 m; after the upgrade, the resolutions were improved to 10 sec and 7.5 m.
Some new exciting research areas have also resulted from the higher temporal resolution, especially when observations from other instruments are included in the analysis
• deriving water vapor flux observations using coincident Raman and Doppler lidar measurements and
• characterizing entrainment in cumulus clouds using Raman lidar, an atmospheric emitted radiance interferometer, a cloud radar, a microwave radiometer, and surface measurements.
The ARM program’s primary goal for the Raman lidar was to provide routine measurements of water vapor through the boundary layer across the diurnal cycle. The system’s unique and powerful measurements have been used in an extremely wide range of research—a much larger range of research than was originally anticipated. Value-added Raman lidar data products include time-resolved profiles of
• The Raman lidar system installed at the Darwin, Australia, ARM site in December 2010.
water vapor mixing ratio,
• relative humidity,
• aerosol scattering ratio,
• aerosol volume backscatter coefficient,
• aerosol extinction coefficient,
• aerosol optical depth,
• linear depolarization ratio,
• cloud mask and cloud base height, and