Lidar 101: Introduction to the JPL lidars
The following paragraphs briefly describe the measurement principles used by the four JPL lidars in operation at the Table Mountain Facility, CA (TMF), and Mauna Loa Observatory, HI (MLO). This page does not intend to review/explain all lidar techniques completely and precisely, but rather to summarize the specific techniques used at JPL. A review, among others, of lidar remote sensing techniques can be found here:
Lidar: Range-Resolved Optical Remote Sensing of the Atmosphere (Springer Series in Optical Sciences (102)) by Claus Weitkamp (Editor)
The general principle of a Lidar (Light Detection And Ranging) system is similar to that of a radar but using light instead of radiowaves. A laser beam is sent up into the atmosphere and is backscattered by the atmospheric molecules and particles. Depending on the wavelength of the emitted light (i.e., its color), the type of scattering molecules or particles, and the type of scattering process, the light collected on the ground can be analyzed to retrieve various atmospheric properties such as ozone, water vapor, or aerosol abundance, as well as temperature or density. The lidar technique has one advantage compared to other techniques such as ground-based or satellite radiometers: the time at which the laser pulse leaves/comes back to the ground is known very precisely. This "active" remote sensing technique therefore yields a very high vertical resolution, on the order of a few meters).
One (perhaps the most simple) technique is commonly called "Rayleigh" lidar. The laser beam emitted into the atmosphere is backscattered at the same wavelength by the air molecules and particles and collected by a telescope. The light is passed through various optical components (filters, splitters, etc.), and hits the surface of one or several "photomultipliers", a very sensitive opto-electronic component that converts the received light into electronic pulses. These pulses are sent to a computer where they are counted and sampled as a function of time or altitude. The signals are corrected for various linear and non-linear effects (extraction of background noise, pile-up/saturation effects, atmospheric extinction, etc.). If the atmosphere is assumed to be free of aerosol particles (i.e., only clean air molecules, typically always true above 30-35 km altitude) the corrected signal is directly proportional to the air number density. By integrating the hydrostatic equation from the topmost point down, we obtain the atmospheric temperature profile. A typical altitude range for good quality temperature profiles is 30-80 km, making Rayleigh lidar the only affordable technique available for temperature measurements at these altitudes. The JPL atmospheric lidar group operates three lidar systems (two at TMF and one at MLO) that include channels dedicated to the measurement of middle atmospheric temperature (10-90 km).
If aerosol particles or clouds are present (usually true below 30 km altitude), additional backscattering by the particles contaminates the "pure Rayleigh" signals described above. Temperature retrieval is no longer possible under these conditions but the presence of additional scattering can actually be used to retrieve aerosol/cloud properties. In this case the lidar is commonly called an "aerosol lidar". However the aerosols/clouds properties cannot be determined very accurately and the use of additional type of scattering (commonly called "Raman", described below) is preferred.
Another common technique is Differential Absorption Lidar (DIAL) which uses the wavelength-dependent absorbing properties of a given atmospheric constituent. In such cases, two laser beams at different wavelengths are emitted into the atmosphere, one being less absorbed than the other by the constituent for which a vertical profile is desired. The difference in the shape/strength of the signals returned at the two wavelengths allows the number density profile of that constituent to be retrieved. There is no need for any instrument calibration with this technique. The JPL atmospheric lidar group operates three DIAL systems for the measurements of tropospheric ozone (TMF) and stratospheric ozone (TMF and MLO).
Another technique commonly called "Raman lidar" (short for "vibrational Raman scattering") uses the inelastic backscattering properties of the atmospheric molecules. When a molecule is hit by light at a given wavelength it backscatters at the same wavelength ("Rayleigh" or "elastic" scattering described above) and also at other wavelengths, each shift in wavelength being a function of the energy transition properties of the molecule. For example, if nitrogen is hit by light at the wavelength of 355 nm (UV light) it will mostly backscatter at the same wavelength but also at 387 nm (and other wavelengths). For nitrogen, the intensity of the Raman-shifted scattering is about 700 times weaker than that of the elastic scattering. Because nitrogen (or oxygen) is well-mixed in the atmosphere up to very high altitudes (>80 km), one can use its Raman-shifting properties (or that of the oxygen) together with that of water vapor to retrieve atmospheric water vapor mixing ratio. After signal corrections similar to that described above for Rayleigh signals, the ratio of the signal received at the wavelength Raman-backscattered by the water vapor to the signal Raman-backscattered by nitrogen (or oxygen) is directly proportional to atmospheric water vapor mixing ratio. The lidar profile is then calibrated using a calibration lamp and/or an external source of water vapor measurement (e.g., radiosonde). The JPL atmospheric lidar group operates one Raman lidar at TMF specifically dedicated to the measurement of tropospheric water vapor from near the ground to the upper troposphere and lower stratosphere.
In specific instrument configurations, information from Raman scattering together with that from Rayleigh scattering can be used to retrieve aerosol/clouds properties, as well as to retrieve atmospheric temperature and/or ozone in the presence of thin aerosol layers. Three of the four lidars of the JPL atmospheric lidar group include Raman channels for the measurement of temperature and ozone below 30 km.
Leblanc, T., R. J. Sica, J. A. E. van Gijsel, S. Godin-Beekmann, A. Haefele, T. Trickl, G. Payen, and F. Gabarrot (2016), Proposed standardized definitions for vertical resolution and uncertainty in the NDACC lidar ozone and temperature algorithms – Part 1: Vertical resolution, Atmos. Meas. Tech., 9(8), 4029-4049. https://doi.org/10.5194/amt-9-4029-2016
Leblanc, T., R. J. Sica, J. A. E. van Gijsel, S. Godin-Beekmann, A. Haefele, T. Trickl, G. Payen, and G. Liberti (2016), Proposed standardized definitions for vertical resolution and uncertainty in the NDACC lidar ozone and temperature algorithms – Part 2: Ozone DIAL uncertainty budget, Atmos. Meas. Tech., 9(8), 4051-4078.
Leblanc, T., R. J. Sica, J. A. E. van Gijsel, A. Haefele, G. Payen, and G. Liberti (2016), Proposed standardized definitions for vertical resolution and uncertainty in the NDACC lidar ozone and temperature algorithms – Part 3: Temperature uncertainty budget, Atmos. Meas. Tech., 9(8), 4079-4101. https://doi.org/10.5194/amt-9-4079-2016