Atmospheric lidar is a class of instruments that uses laser light to study atmospheric properties from the ground up to the top of the atmosphere. Such instruments have been used to study, among other, atmospheric gases, aerosols, clouds, and temperature.
History
The basic concepts to study the atmosphere using light were developed before World War II. In 1930, E.H. Synge proposed to study the density of the upper atmosphere using a searchlight beam . In the following years, searchlight beams were used to study cloud altitude using both scanning and pulsed light.R. Bureau: La Météorologie 3, 292 (1946) Advanced techniques to study cloud properties using scattered light with different wavelengths were also proposed. With the first experiments, light scattering patterns were observed in the troposphere that were not compatible with a pure molecular atmosphere. This incompatibility was attributed to suspended haze particles. Similar techniques were also developed in the U.S.S.R. The searchlight beam technique continued to improve after the end of the War, with more precise instruments and new atmospheric parameters, like temperature At the same time, pulsed light was used to construct a rangefinder to measure the distance of objects, but remained only an experimental design.
In 1960, T. Maiman demonstrated the first functional laser at Hughes Research Laboratories. The demonstration was a pivotal moment for lidar development. Soon afterwards, engineers at Hughes Aircraft Company developed a laser rangefinder using ruby laser light. The new device, named colidar (coherent light detection and ranging), gained widespread publicity . In 1962, L. Smullin and G. Fiocco used a ruby laser to detect echoes from the Moon. During their experiments they observed light scattered in the upper atmosphere that they attributed to dust particles. Soon, several research groups constructed similar devices to observe the atmosphere. By 1969, "over 20 lasers were in use by meteorologists in the United States on at least a semi-routine basis" for various applications including aerosol measurements, sub-visible cirrus and noctilucent clouds observations, and visibility measurement
Design
A simplified representation of a lidar set-up is demonstrated in Figure 1. The transmission unit consists of a laser source, followed by a series of mirrors, and a beam expander which sends the collimated light beam vertically up to the open atmosphere. Part of the transmitted radiation is scattered by atmospheric components (i.e., gases, molecules, aerosols, clouds) backward to the lidar, where it is collected by a telescope. The backscattered light is driven to an optical analyzer where the optical signal is first spectrally separated, amplified and transformed to an electrical signal. Finally, the signal is digitized and stored in a computer unit.
Applications
Clouds Lidars have been proven useful for classification of cloud types (i.e., cumuli versus cirrus). Cloud boundaries can be retrieved from a ground-based lidar operating at a visible and/or near-infrared band. Cloud-base height can be identified by the time difference between the transmittance of the laser pulse to the sky and the detection of the backscattered light by the telescope. The laser beam is always attenuated when it penetrates through the clouds. However, when a powerful laser (e.g., Nd:YAG laser with high energy per pulse) is used, cloud tops can be retrieved too. Another physical parameter that can be retrieved is the cloud phase. By using a linear polarized laser beam, a linear particle depolarization ratio (δ) can be defined as the ratio of measured perpendicular backscatter intensity over parallel backscatter intensity with respect to the transmitter polarization axis:
When this parameter is zero (the backscattered signal is linearly polarized), the cloud contains liquid spherical droplets. However, when the cloud contains ice crystals, backscattered light arrives at the receiver unit with a cross-polarized component, and δ has a higher value (0 < δ < 1). Liquid droplets tend to behave as symmetrical scattering elements, while ice crystals are asymmetrical.Scotland, R. M., K. Sassen, and R. Stone, "Observations by lidar of linear depolarizations of hydrometeors," J. Appl. Meteorol., 10, 1011--1017, 1971
The use of the polarization ratio generally includes an implicit assumption that the particles in the volume are randomly oriented. The polarization properties of oriented particles cannot be properly represented by the depolarization ratio. Ice crystals are known to horizontally orient when they are large enough that drag forces overcome the randomizing effects of Brownian motion. Rain is also generally oriented, where drag forces flatten the drops along the fall direction. In such cases, the measured depolarization ratio may depend on the particular polarization state used by the lidar system. Some polarization lidar systems can measure the entire backscatter phase matrix, thereby avoiding the ambiguity of the depolarization ratio when oriented particles are present.
Aerosol particles One of the biggest uncertainties for climate change is the importance of aerosol direct and indirect effects. The uncertainties were stressed in the 4th Assessment Report by the Intergovernmental Panel for Climate Change (IPCC). The large diversity of aerosol optical properties, including their sources and the meteorological processes they are subjected to, requires vertically resolved measurements, which can only be performed with routine lidar observations. Networks of aerosol lidars such as the European Aerosol Research Lidar Network (EARLINET) The EARLINET publishing group 2000-2010; M. Adam, Alados-Arbolas, L., Althausen, D., Amiridis, V., Amodeo, A., Ansmann, A., Apituley, A., Arshinov, Y., Balis, D., Belegante, L., Bobrovnikov, S., Boselli, A., Bravo-Aranda, J. A., Bsenberg, J., Carstea, E., Chaikovsky, A., Comern, A., D'Amico, G., Daou, D., Dreischuh, T., Engelmann, R., Finger, F., Freudenthaler, V., Garcia-Vizcaino, D., Garca, A. J. F., Gei, A., Giannakaki, E., Giehl, H., Giunta, A., de Graaf, M., Granados-Muoz, M. J., Grein, M., Grigorov, I., Gro, S., Gruening, C., Guerrero-Rascado, J. L., Haeffelin, M., Hayek, T., Iarlori, M., Kanitz, T., Kokkalis, P., Linn, H., Madonna, F., Mamouri, R.-E., Matthias, V., Mattis, I., Menndez, F. M., Mitev, V., Mona, L., Morille, Y., Muoz, C., Mller, A., Mller, D., Navas-Guzmn, F., Nemuc, A., Nicolae, D., Pandolfi, M., Papayannis, A., Pappalardo, G., Pelon, J., Perrone, M. R., Pietruczuk, A., Pisani, G., Potma, C., Preiler, J., Pujadas, M., Putaud, J., Radu, C., Ravetta, F., Reigert, A., Rizi, V., Rocadenbosch, F., Rodrguez, A., Sauvage, L., Schmidt, J., Schnell, F., Schwarz, A., Seifert, P., Serikov, I., Sicard, M., Silva, A. M., Simeonov, V., Siomos, N., Sirch, T., Spinelli, N., Stoyanov, D., Talianu, C., Tesche, M., De Tomasi, F., Trickl, T., Vaughan, G., Volten, H., Wagner, F., Wandinger, U., Wang, X., Wiegner, M., Wilson, K. M. a., 2014. Earlinet all observations (2000-2010) were established to investigate aerosol properties, along with transport and modification phenomena, in a coherent way on a regional to continental scale. As of 2015, EARLINET consists of 27 lidar stations hosting more than 44000 profiles. Elastic-backscatter lidars (EBL) have been used extensively to investigate clouds and aerosol layers since the 1960s.Fiocco, G., Grams, G., 1964 Observation of an aerosol layer at 20 km by an optical radar. Journal of the Atmospheric Sciences 21, 323 EBLs detect the total backscattered signal (particle and molecular contributions). Profiles of the extinction coefficient have to be estimated using the molecular signal and the assumption of a conditionally "constant" (roughly speaking) aerosol extinction to backscatter ratio, called the lidar ratio. The main equation involved, known as the lidar equation is:
where P(r) is the power of the backscattered radiation received by the lidar telescope in distance r, E is transmitted laser-pulse energy, L is the lidar constant summarizing its optical and detection characteristics, O(r) is the overlap function,Weitkamp, C., 2005. Lidar: Range-Resolved Optical Remote Sensing of the Atmosphere. Springer Series in Optical Sciences. Springer. and and are the aerosol/molecular backscatter- and extinction coefficient respectively. Molecular backscatter and extinction can be derived by meteorological data, therefore the only unknowns in the lidar equation are and . However the lidar ratio, as an intensive aerosol property, strongly depends on the size, morphology and chemical composition of the particles and is highly variable with respect to height, which often risks the extinction profile credibility. The process for calculating backscatter- and extinction coefficient profiles from EBL returns is widely known as the Klett method Klett J. D., 1981. Stable analytical inversion solution for processing lidar returns, Applied Optics 20, 211. and was originally formalised by Hitschfeld and Bordan in 1954.Hitschfeld, W., Bordan J., 1954. Errors Inherent in the Radar Measurement of Rainfall at Attenuating Wavelengths, Journal of Meteorology 11, 58. The aforementioned defect on estimating extinction profiles is overcome by Raman (inelastic) backscatter lidar and high spectral resolution lidar (HSRL). Raman lidar works by additionally measuring the inelastic backscatter by nitrogen and/or oxygen molecules.Ansmann, A., Riebesell, M., Weitkamp, C., 1990. Measurement of atmospheric aerosol extinction profiles with a Raman lidar. Optics Letters 15, 746. HSRL uses a processing approach but obtains the additional measure of molecular only backscatter at the transmitted wavelength by blocking the spectrally narrow aerosol returns and passing the spectrally broad molecular returns.S. T. Shipley, D. H. Tracy, E. W. Eloranta, J. T. Trauger, J. T. Sroga, F. L. Roesler, and J. A. Weinman, 1983. High spectral resolution lidar to measure optical scattering properties of atmospheric aerosols. 1: Theory and instrumentation. Appl. Opt. 22, 3716-3724E. W. Eloranta, Chapter 5: High Spectral Resolution Lidar in Lidar: Range-Resolved Optical Remote Sensing of the Atmosphere, C. Weitkamp, ed. (Springer, 2005) These techniques provide a direct calculation of the extinction coefficient, eliminating the need for a lidar ratio assumption since any additional terms involved (e.g. the molecular extinction coefficient) are handled by meteorological (e.g. radiosoundings) and standard-atmosphere data. After some mathematical manipulations of the lidar equation the extinction-related equation reads:
where the subscripts "inc" and "sca" refer to the incident laser light and the shifted backscattered light respectively (in HSRL these terms are the same thus further simplifying the equation, but the distinction is needed in the case of Raman lidar), N is the nitrogen / oxygen molecule number density and is the Ångström exponent. A drawback of this method is the presence of a derivative in the resulting extinction coefficient formula () which results in potential numerical instability, introducing an active field of research.
Inversion of aerosol microphysical properties