Multiple-scattering influence on extinction- and backscatter-coefficient measurements with Raman and high-spectral-resolution lidars
TL;DR: A formalism describing the influence of multiple scattering on cloud measurements with Raman and high-spectral-resolution lidars is presented and it is found that, for typical measurement geometries of ground-based lidars, as many as five scattering orders contribute significantly to the backscattered light.
Abstract: A formalism describing the influence of multiple scattering on cloud measurements with Raman and high-spectral-resolution lidars is presented. Model calculations including both particulate and molecular scattering processes are performed to describe the general effects of multiple scattering on both particulate and molecular lidar backscatter signals. It is found that, for typical measurement geometries of ground-based lidars, as many as five scattering orders contribute significantly to the backscattered light. The relative intensity of multiple-scattered light is generally larger in signals backscattered from molecules than in signals backscattered from particles. The multiple-scattering formalism is applied to measurements of water and ice clouds taken with a Raman lidar. Multiple-scattering errors of measured extinction coefficients are typically of the order of 50% at the bases of both water and ice clouds and decrease with increasing penetration depth to below 20%. In contrast, the multiple-scattering errors of backscatter coefficients are negligible in ice clouds and below 20% in water clouds.
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TL;DR: An intercomparison of the algorithms used to retrieve aerosol extinction and backscatter starting from Raman lidar signals has been performed by 11 groups of lidar scientists involved in the European Aerosol Research Lidar Network and demonstrates that the data-handling procedures used by all the lidar groups provide satisfactory results.
Abstract: An intercomparison of the algorithms used to retrieve aerosol extinction and backscatter starting from Raman lidar signals has been performed by 11 groups of lidar scientists involved in the European Aerosol Research Lidar Network (EARLINET). This intercomparison is part of an extended quality assurance program performed on aerosol lidars in the EARLINET. Lidar instruments and aerosol backscatter algorithms were tested separately. The Raman lidar algorithms were tested by use of synthetic lidar data, simulated at 355, 532, 386, and 607 nm, with realistic experimental and atmospheric conditions taken into account. The intercomparison demonstrates that the data-handling procedures used by all the lidar groups provide satisfactory results. Extinction profiles show mean deviations from the correct solution within 10% in the planetary boundary layer (PBL), and backscatter profiles, retrieved by use of algorithms based on the combined Raman elastic-backscatter lidar technique, show mean deviations from solutions within 20% up to 2 km. The intercomparison was also carried out for the lidar ratio and produced profiles that show a mean deviation from the solution within 20% in the PBL. The mean value of this parameter was also calculated within a lofted aerosol layer at higher altitudes that is representative of typical layers related to special events such as Saharan dust outbreaks, forest fires, and volcanic eruptions. Here deviations were within 15%.
238 citations
DOI•
30 Sep 2003220 citations
Cites methods from "Multiple-scattering influence on ex..."
...The main goal of the Raman algorithm intercomparison experiment is to test the correctness and accuracy of the algorithms used by each group within the EARLINET network for the retrieval of the aerosol extinction profile starting from nitrogen Raman lidar signals (Ansmann et al., 1990; Bösenberg, 1998; Godin et al., 1999; Measures, 1984; Bösenberg and Theopold, 1988; Wandinger, 1998; Whiteman, 1999)....
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TL;DR: In this paper, three ground-based Raman lidars and an airborne high-spectral-resolution lidar were operated during SAMUM 2006 in southern Morocco to measure height profiles of the volume extinction coefficient, the extinction-to-backscatter ratio and the depolarization ratio of dust particles in the Saharan dust layer at several wavelengths.
Abstract: Three ground-based Raman lidars and an airborne high-spectral-resolution lidar (HSRL) were operated during SAMUM 2006 in southern Morocco to measure height profiles of the volume extinction coefficient, the extinction-to-backscatter ratio and the depolarization ratio of dust particles in the Saharan dust layer at several wavelengths. Aerosol Robotic Network (AERONET) Sun photometer observations and radiosoundings of meteorological parameters complemented the ground-based activities at the SAMUM station of Ouarzazate. Four case studies are presented. Two case studies deal with the comparison of observations of the three ground-based lidars during a heavy dust outbreak and of the ground-based lidars with the airborne lidar. Two further cases show profile observations during satellite overpasses on 19 May and 4 June 2006. The height resolved statistical analysis reveals that the dust layer top typically reaches 4–6 km height above sea level (a.s.l.), sometimes even 7 km a.s.l.. Usually, a vertically inhomogeneous dust plume with internal dust layers was observed in the morning before the evolution of the boundary layer started. The Saharan dust layer was well mixed in the early evening. The 500 nm dust optical depth ranged from 0.2–0.8 at the field site south of the High Atlas mountains, Angstr¨ om exponents derived from photometer and lidar data were between 0–0.4. The volume extinction coefficients (355, 532 nm) varied from 30–300 Mm −1 with a mean value of 100 Mm −1 in the lowest 4 km a.s.l.. On average, extinction-to-backscatter ratios of 53–55 sr (±7–13 sr) were obtained at 355, 532 and 1064 nm.
219 citations
Cites background from "Multiple-scattering influence on ex..."
...…CALIPSO results are not corrected for multiple-scattering effects, which can cause a significant underestimation of the particle optical depth and the related column lidar ratio (Young et al., 2006) in any case of weak, moderate or strong extinction of light by the dust particles (Wandinger, 1998)....
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TL;DR: The results indicate that, for the range of temperatures encountered in the troposphere, the magnitude of the temperature-dependent effect can reach 10% or more for narrowband Raman water-vapor measurements.
Abstract: The essential information required for the analysis of Raman lidar water vapor and aerosol data acquired by use of a single laser wavelength is compiled here and in a companion paper [Appl. Opt. 42, 2593 (2003)]. Various details concerning the evaluation of the lidar equations when Raman scattering is measured are covered. These details include the influence of the temperature dependence of both pure rotational and vibrational-rotational Raman scattering on the lidar profile. The full temperature dependence of the Rayleigh-Mie and Raman lidar equations are evaluated by use of a new form of the lidar equation where all the temperature dependence is carried in a single term. The results indicate that, for the range of temperatures encountered in the troposphere, the magnitude of the temperature-dependent effect can reach 10% or more for narrowband Raman water-vapor measurements. Also, the calculation of atmospheric transmission, including the effects of depolarization, is examined carefully. Various formulations of Rayleigh cross-section determination commonly used in the lidar field are compared and reveal differences of as much as 5% among the formulations. The influence of multiple scattering on the measurement of aerosol extinction with the Raman lidar technique is considered, as are several photon pulse pileup-correction techniques.
168 citations
TL;DR: In this article, the formation of the ice phase in tropical altocumulus has been studied with multiwavelength aerosol-cloud Raman lidar, wind Doppler lidar and radiosonde, providing information on geometrical and optical properties, cloud phase, cloud top temperature, updraft and downdraft velocity, and fall speed of ice crystals.
Abstract: [1] The formation of the ice phase in tropical altocumulus has been studied with multiwavelength aerosol-cloud Raman lidar, wind Doppler lidar, and radiosonde, providing information on geometrical and optical properties, cloud phase, cloud top temperature, updraft and downdraft velocity, and fall speed of ice crystals. The observations were conducted at Praia (15°N, 23.5°W), Cape Verde, in the tropical North Atlantic in the framework of the Saharan Mineral Dust Experiment (SAMUM) project in January and February 2008. More than 200 different altocumulus layers were analyzed. The coldest liquid cloud had a temperature of −36°C and appeared at a height of 9800 m. Tropical altocumulus is found to be geometrically (262 ± 137 m) and optically thin (0.69 ± 0.61), mostly short-lived, and horizontally small with extents of less than 50 km in 80% of the cases. A clear relationship between the occurrence of the ice phase in altocumulus and cloud top temperature is observed, even more clear after the removal of effects of cloud seeding, which is found to be an important process of ice production in lower layers of multilayer altocumulus systems. Because almost all altocumulus layers (99%) showed a liquid cloud top (region in which ice nucleation begins), we conclude that deposition and condensation ice nucleation are unimportant processes during the initial phase of altocumulus glaciation. A pronounced impact of aerosols such as mineral particles known to be favorable ice nuclei is not found in this region with strong dust-smoke outbreaks from Africa. The different phases of an almost complete life cycle of an altocumulus were monitored over 5 hours. The observed processes of droplet and ice formation are discussed based on height-resolved depolarization-ratio (cloud phase) and vertical-velocity time series.
160 citations
Cites background from "Multiple-scattering influence on ex..."
...This behavior indicates a decreasing impact of multiple scattering on the solutions of the extinction coefficient with increasing laser-light penetration into the cloud [Wandinger, 1998]....
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...…backscatter coefficients are computed from signal ratios (ratio of the measured elastic backscatter signal to the respective nitrogen Raman signal) so that the multiple scattering effect widely cancels out [Wandinger, 1998] because both profiles are affected in almost the same way by this effect....
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...The strength of the multiple scattering effect depends on the distance of the cloud layer from the lidar, the size distribution and number concentration of the scatterers, and the receiver field of view (RFOV) of the lidar [Wandinger, 1998]....
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References
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TL;DR: Examples covering the measured range of extinction-to-backscatter ratios (lidar ratios) in ice clouds are presented and simple backscatter lidars can provide reliable information about the cloud optical depth and the mean cloud lidar ratio.
Abstract: Height profiles of the extinction and the backscatter coefficients in cirrus clouds are determined independently from elastic- and inelastic- (Raman) backscatter signals. An extended error analysis is given. Examples covering the measured range of extinction-to-backscatter ratios (lidar ratios) in ice clouds are presented. Lidar ratios between 5 and 15 sr are usually found. A strong variation between 2 and 20 sr can be observed within one cloud profile. Particle extinction coefficients determined from inelastic-backscatter signals and from elastic-backscatter signals by using the Klett method are compared. The Klett solution of the extinction profile can be highly erroneous if the lidar ratio varies along the measuring range. On the other hand, simple backscatter lidars can provide reliable information about the cloud optical depth and the mean cloud lidar ratio.
668 citations
TL;DR: A method is presented that permits the determination of atmospheric aerosol extinction profiles from measured Raman lidar signals, no critical input parameters are needed, which could cause large uncertainties of the solution.
Abstract: A method is presented that permits the determination of atmospheric aerosol extinction profiles from measured Raman lidar signals. No critical input parameters are needed, which could cause large uncertainties of the solution, as is the case in the Klett method for the inversion of elastic lidar returns.
496 citations
TL;DR: In this paper, a combined Raman elastic-backscatter lidar has been developed, where a XeCl excimer laser is used as the radiation source and inelastic Raman backscatter signals are spectrally separated from the elastic signal with a filter or grating polychromator.
Abstract: A combined Raman elastic-backscatter lidar has been developed. A XeCl excimer laser is used as the radiation source. Inelastic Raman backscatter signals are spectrally separated from the elastic signal with a filter or grating polychromator. Raman channels can be chosen to register signals from CO2, O2, N2, and H2O. Algorithms for the calculation of the water-vapor mixing ratio from the Raman signals and the particle extinction and backscatter coefficients from both elastic and inelastic backscatter signals are given. Nighttime measurements of the vertical humidity distribution up to the tropopause and of particle extinction, backscatter, and lidar ratio profiles in the boundary layer, in high-altitude water and ice clouds, and in the stratospheric aerosol layer are presented. Daytime boundary-layer measurements of moisture and particle extinction are made possible by the improved daylight suppression of the grating polychromator. Test measurements of the CO2 mixing ratio indicate the problems for the Raman lidar technique in monitoring other trace gases than water vapor.
415 citations
TL;DR: A high spectral resolution lidar technique to measure optical scattering properties of atmospheric aerosols is described, and Aerosol optical properties, such as the backscatter ratio, optical depth, extinctionCross section, scattering cross section, and theBackscatter phase function are derived.
Abstract: A high spectral resolution lidar technique to measure optical scattering properties of atmospheric aerosols is described. Light backscattered by the atmosphere from a narrowband optically pumped oscillator-amplifier dye laser is separated into its Doppler broadened molecular and elastically scattered aerosol components by a two-channel Fabry-Perot polyetalon interferometer. Aerosol optical properties, such as the backscatter ratio, optical depth, extinction cross section, scattering cross section, and the backscatter phase function, are derived from the two-channel measurements.
270 citations
TL;DR: A high-spectral-resolution lidar that uses an iodine absorption filter and a tunable, narrow-bandwidth Nd:YAG laser is demonstrated and provides better performance than the Fabry–Perot etalon that it replaces.
Abstract: A high-spectral-resolution lidar that uses an iodine absorption filter and a tunable, narrow-bandwidth Nd:YAG laser is demonstrated. Measurements of aerosol scattering cross section and optical depth are presented. The iodine absorption filter provides better performance than the Fabry–Perot etalon that it replaces.
187 citations