Showing papers on "Depolarization ratio published in 2020"

Smoke of extreme Australian bushfires observed in the stratosphere over Punta Arenas, Chile, in January 2020: optical thickness, lidar ratios, and depolarization ratios at 355 and 532 nm

Abstract: . We present particle optical properties of stratospheric smoke layers observed with multiwavelength polarization Raman lidar over Punta Arenas (53.2 ∘ S, 70.9 ∘ W), Chile, at the southernmost tip of South America in January 2020. The smoke originated from the record-breaking bushfires in Australia. The stratospheric aerosol optical thickness reached values up to 0.85 at 532 nm in mid-January 2020. The main goal of this rapid communication letter is to provide first stratospheric measurements of smoke extinction-to-backscatter ratios (lidar ratios) and particle linear depolarization ratios at 355 and 532 nm wavelengths. These aerosol parameters are important input parameters in the analysis of spaceborne CALIPSO and Aeolus lidar observations of the Australian smoke spreading over large parts of the Southern Hemisphere in January and February 2020 up to heights of around 30 km. Lidar and depolarization ratios, simultaneously measured at 355 and 532 nm, are of key importance regarding the homogenization of the overall Aeolus (355 nm wavelength) and CALIPSO (532 nm wavelength) lidar data sets documenting the spread of the smoke and the decay of the stratospheric perturbation, which will be observable over the entire year of 2020. We found typical values and spectral dependencies of the lidar ratio and linear depolarization ratio for aged stratospheric smoke. At 355 nm, the lidar ratio and depolarization ratio ranged from 53 to 97 sr (mean 71 sr) and 0.2 to 0.26 (mean 0.23), respectively. At 532 nm, the lidar ratios were higher (75–112 sr, mean 97 sr) and the depolarization ratios were lower with values of 0.14–0.22 (mean 0.18). The determined depolarization ratios for aged Australian smoke are in very good agreement with respective ones for aged Canadian smoke, observed with lidar in stratospheric smoke layers over central Europe in the summer of 2017. The much higher 532 nm lidar ratios, however, indicate stronger absorption by the Australian smoke particles.

Optical properties of Central Asian aerosol relevant for spaceborne lidar applications and aerosol typing at 355 and 532 nm

Abstract: . For the first time, a dense data set of particle extinction-to-backscatter ratios (lidar ratios), linear depolarization ratios, and backscatter- and extinction-related Angstrom exponents for a Central Asian site are presented. The observations were performed with a continuously running multiwavelength polarization Raman lidar at Dushanbe, Tajikistan, during an 18-month campaign (March 2015 to August 2016). The presented seasonally resolved observations fill an important gap in the database of aerosol optical properties used in aerosol typing efforts with spaceborne lidars and ground-based lidar networks. Lidar ratios and depolarization ratios are also basic input parameters in spaceborne lidar data analyses and in efforts to harmonize long-term observations with different space lidar systems operated at either 355 or 532 nm. As a general result, the found optical properties reflect the large range of occurring aerosol mixtures consisting of long-range-transported dust (from the Middle East and the Sahara), regional desert, soil, and salt dust, and anthropogenic pollution. The full range from highly polluted to pure dust situations could be observed. Typical dust depolarization ratios of 0.23–0.29 (355 nm) and 0.30–0.35 (532 nm) were observed. In contrast, comparably low lidar ratios were found. Dust lidar ratios at 532 nm accumulated around 35–40 sr and were even lower for regional background dust conditions (20–30 sr). Detailed correlation studies (e.g., lidar ratio vs. depolarization ratios, Angstrom exponent vs. lidar ratio and vs. depolarization ratio) are presented to illuminate the complex relationships between the observed optical properties and to identify the contributions of anthropogenic haze, dust, and background aerosol to the overall aerosol mixtures found within the 18-month campaign. The observation of 532 nm lidar ratios ( sr) and depolarization ratios (around 15 %–20 %) in layers with very low particle extinction coefficient ( sr) suggests that direct emission and emission of resuspended salt dust (initially originated from numerous desiccating lakes and the Aralkum desert) have a sensitive impact on the aerosol background optical properties over Dushanbe.

Variability in lidar-derived particle properties over West Africa due to changes in absorption: towards an understanding

Abstract: . Measurements performed in western Africa (Senegal) during the SHADOW field campaign are analyzed to show that spectral dependence of the imaginary part of the complex refractive index (CRI) of dust can be revealed by lidar-measured particle parameters. Observations in April 2015 provide good opportunity for such study, because, due to high optical depth of the dust, exceeding 0.5, the extinction coefficient could be derived from lidar measurements with high accuracy and the contribution of other aerosol types, such as biomass burning, was negligible. For instance, in the second half of April 2015, AERONET observations demonstrated a temporal decrease in the imaginary part of the CRI at 440 nm from approximately 0.0045 to 0.0025. This decrease is in line with a change in the relationship between the lidar ratios (the extinction-to-backscattering ratio) at 355 and 532 nm ( S355 and S532 ). For instance in the first half of April, S355∕S532 is as high as 1.5 and the backscatter Angstrom exponent, Aβ , is as low as −0.75 , while after 15 April S 355 / S 532 = 1.0 and Aβ is close to zero. The aerosol depolarization ratio δ532 for the whole of April exceeded 30 % in the height range considered, implying that no other aerosol, except dust, occurred. The performed modeling confirmed that the observed S355∕S532 and Aβ values match the spectrally dependent imaginary part of the refractive index as can be expected for mineral dust containing iron oxides. The second phase of the SHADOW campaign was focused on evaluation of the lidar ratio of smoke and estimates of its dependence on relative humidity (RH). For five studied smoke episodes the lidar ratio increases from 44±5 to 66±7 sr at 532 nm and from 62±6 to 80±8 sr at 355 nm, when RH varied from 25 % to 85 %. Performed numerical simulations demonstrate that observed ratio S355∕S532 , exceeding 1.0 in the smoke plumes, can indicate an increase in the imaginary part of the smoke particles in the ultraviolet (UV) range.

The dual-field-of-view polarization lidar technique: A new concept in monitoring aerosol effects in liquid-water clouds – Case studies

Abstract: . In a companion article (Jimenez et al., 2020), we introduced a new lidar method to derive microphysical properties of liquid-water clouds (cloud extinction coefficient, droplet effective radius, liquid-water content, cloud droplet number concentration Nd) at a height of 50–100 m above cloud base together with aerosol information (aerosol extinction coefficients, cloud condensation nucleus concentration NCCN) below the cloud layer so that detailed studies of the influence of given aerosol conditions on the evolution of liquid-water cloud layers with high temporal resolution solely based on lidar observations became possible now. The novel cloud retrieval technique makes use of lidar observations of the volume linear depolarization ratio at two different receiver field-of-views (FOVs). In this part 2, the new dual-FOV polarization lidar technique is applied to cloud measurements in pristine marine conditions at Punta Arenas in southern Chile. A multiwavelength polarization Raman lidar, upgraded by integrating a second polarization-sensitive channel to permit depolarization ratio observations at two FOVs, was used for these measurements at the southernmost tip of South America. Two case studies are presented to demonstrate the potential of the new lidar technique. Successful aerosol-cloud-interaction (ACI) studies based on measurements with the upgraded aerosol-cloud lidar in combination with a Doppler lidar of the vertical wind component could be carried with one minute temporal resolution at these pristine conditions. In a stratocumulus layer at the top of the convective boundary layer, we found values of Nd and NCCN (for 0.2 % water supersaturation) ranging from 15–100 cm−3 and 75–200 cm−3, respectively, and associated activation ratios (Nd / NCCN) of 0.25–0.5 during updraft periods. The studies of the aerosol impact on cloud properties yielded ACI values close to 1. The impact of aerosol water-uptake on the ACI studies was analyzed with the result that the highest ACI values were obtained when considering aerosol proxies (light extinction coefficient or NCCN) measured at heights about 500 m below cloud base (and thus for dry aerosol conditions).

Optical characterization of pure pollen types using a multi-wavelength Raman polarization lidar

Abstract: . We present a novel algorithm for characterizing the optical properties of pure pollen particles, based on the depolarization ratio values obtained in lidar measurements. The algorithm was first tested and validated through a simulator and then applied to the lidar observations during a 4-month pollen campaign from May to August 2016 at the European Aerosol Research Lidar Network (EARLINET) station in Kuopio (62 ∘ 44 ′ N, 27 ∘ 33 ′ E), in Eastern Finland. With a Burkard sampler, 20 types of pollen were observed and identified from concurrent measurements, with birch (Betula), pine (Pinus), spruce (Picea), and nettle (Urtica) pollen being the most abundant, contributing more than 90 % of the total pollen load, regarding number concentrations. Mean values of lidar-derived optical properties in the pollen layer were retrieved for four intense pollination periods (IPPs). Lidar ratios at both 355 and 532 nm ranged from 55 to 70 sr for all pollen types, without significant wavelength dependence. An enhanced depolarization ratio was found when there were pollen grains in the atmosphere, and an even higher depolarization ratio (with mean values of 0.25 or 0.14) was observed with the presence of the more non-spherical spruce or pine pollen. Under the assumption that the backscatter-related Angstrom exponent between 355 and 532 nm should be zero for pure pollen, the depolarization ratio of pure pollen particles at 532 nm was assessed, resulting in 0.24±0.01 and 0.36±0.01 for birch and pine pollen, respectively. Pollen optical properties at 1064 and 355 nm were also estimated. The backscatter-related Angstrom exponent between 532 and 1064 nm was assessed to be ∼0.8 ( ∼0.5 ) for pure birch (pine) pollen; thus the longer wavelength would be a better choice to trace pollen in the air. Pollen depolarization ratios of 0.17 and 0.30 at 355 nm were found for birch and pine pollen, respectively. The depolarization values show a wavelength dependence for pollen. This can be the key parameter for pollen detection and characterization.

Falling Mixed-Phase Ice Virga and their Liquid Parent Cloud Layers as Observed by Ground-Based Lidars

Abstract: Falling mixed-phase virga from a thin supercooled liquid layer cloud base were observed on 20 occasions at altitudes of 2.3–9.4 km with ground-based lidars at Wuhan (30.5 °N, 114.4 °E), China. Polarization lidar profile (3.75-m) analysis reveals some ubiquitous features of both falling mixed-phase virga and their liquid parent cloud layers. Each liquid parent cloud had a well-defined base height where the backscatter ratio R was ~7.0 and the R profile had a clear inflection point. At an altitude of ~34 m above the base height, the depolarization ratio reached its minimum value (~0.04), indicating a liquid-only level therein. The thin parent cloud layers tended to form on the top of a broad preexisting aerosol/liquid water layer. The falling virga below the base height showed firstly a significant depolarization ratio increase, suggesting that most supercooled liquid drops in the virga were rapidly frozen into ice crystals (via contact freezing). After reaching a local maximum value of the depolarization ratio, both the values of the backscatter ratio and depolarization ratio for the virga exhibited an overall decrease with decreasing height, indicating sublimated ice crystals. The diameters of the ice crystals in the virga were estimated based on an ice particle sublimation model along with the lidar and radiosonde observations. It was found that the ice crystal particles in these virga cases tended to have smaller mean diameters and narrower size distributions with increasing altitude. The mean diameter value is 350 ± 111 µm at altitudes of 4–8.5 km.

Wavelength dependence of ice cloud backscatter properties for space-borne polarization lidar applications

Abstract: We investigated the use of backscatter properties of atmospheric ice particles for space-borne lidar applications. We estimated the average backscattering coefficient (β), backscatter color ratio (χ), and depolarization ratio (δ) for ice particles with a wide range of effective radii for five randomly oriented three-dimensional (3D) and three quasi-horizontally oriented two-dimensional (2D) types of ice particle using physical optics and geometrical integral equation methods. This is the first study to estimate the lidar backscattering properties of quasi-horizontally oriented non-pristine ice crystals. We found that the χ-δ relationship was useful for discriminating particle types using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data. The lidar ratio (S)-δ relationship, which is determined using space-borne high-spectral-resolution lidar products such as EarthCARE ATLID or future space-borne lidar missions, may also produce robust classification of ice particle types because it is complementary to the χ-δ relationship.

Using polarized Raman spectroscopy to study the stress gradient in mineral systems with anomalous birefringence

Abstract: Polarized Raman spectroscopy was applied to garnet hosts which exhibit anomalous birefringence around inclusions of zircon and quartz to elucidate the spatial distribution of the anisotropic strain fields in the vicinity of the host-inclusion boundary. We show that there is a direct relationship between the stress-induced birefringence and the Raman scattering generated by the fully symmetric phonon modes (the A1g modes in cubic crystals). Our experimental results coupled with selected finite element models show that the ratio between the measured Raman peak intensity collected in cross and parallel polarized scattering geometries of totally symmetric modes represents a useful tool to constrain the radial stress profile in the host around the inclusions. Further, we demonstrate how group-theoretical considerations and tensor analysis of the morphic effect (external-field-induced change of the symmetry) on the phonons and the optical properties of the host can help to derive useful information on the symmetry of the stress field. Finally, we show experimentally that, under the same amount of applied stress, this approach is more sensitive than the commonly used approach of measuring differences in phonon frequencies and provides better opportunities to map the spatial variations of strain. This approach is an alternative technique to study structural phenomena associated with anomalous birefringence in host crystals surrounding stressed inclusions and could be applied to other systems in which similar optical effects are observed.

Spatio-temporal discrimination of molecular, aerosol and cloud scattering and polarization using a combination of a Raman lidar, Doppler cloud radar and microwave radiometer

Abstract: The combined data from the ESA Mobile Raman Polarization and Water Vapor Lidar (EMORAL), the LATMOS Bistatic Doppler Cloud Radar System for Atmospheric Studies (BASTA), and the INOE Microwave Radiometer (HATPRO-G2) have been used to explore the synergy for the spatio-temporal discrimination of polarization and molecular, aerosol and cloud scattering. The threshold-based methodology is proposed to perform an aerosol-cloud typing using the three instruments. It is demonstrated for 24 hours of observations on 10 June 2019 in Rzecin, Poland. A new scheme for target classification, developed collaboratively by the FUW and the OUC, can help determine molecules, aerosol (spherical, non-spherical, fine, coarse), cloud phase (liquid, ice, supercooled droplets) and precipitation (drizzle, rain). For molecular, aerosol, and cloud discrimination, the thresholds are set on the backward scattering ratio, the linear particle depolarization ratio and the backscatter colour ratio, all calculated from lidar signals. For the cloud phase and precipitation categorization, the thresholds are set on the reflectivity and the Doppler velocity derived from cloud radar signals. For boundary layer particles, precipitation, and supercooled droplets separation, the thresholds are set on the profiles of temperature and relative humidity obtained by the microwave radiometer. The algorithm is able to perform separation even under complicated meteorological situation, as in the presented case study.

Lidar Observed Optical Properties Of Tropical Cirrus Clouds Over Gadanki Region

Abstract: The presence of cirrus cloud has its impact on the earth’s radiation budget. In order to study the effect of cirrus clouds in the tropical regions, it is essential to understand and characterize their optical properties. The optical properties of high altitude cirrus clouds are obtained using the polarization diversity ground based Mie lidar instrument at a tropical latitude station in the Indian subcontinent. Lidar measurements are taken for one year (2013) at National Atmospheric Research Laboratory (NARL), located at Gadanki (13.5° North, 79.2° East; 375m AMSL), India and are used for the present investigation. Altitude variations of optical depth and depolarization ratio are discussed. In the altitude range of 10–17 km, the range of the optical depth and depolarization ratio of cirrus cloud was found to be 0.01–0.4 and 0.1–0.4 respectively. The interdependence of optical depth as a function of depolarization ratio is analyzed and a positive correlation is observed (0.3950). From the measured optical depth values, it is categorized that 8%, 77% and 14% of the cirrus clouds are sub-visual, thin and thick clouds. The monthly and seasonal variations of optical properties of cirrus clouds were analyzed. Summary of cirrus cloud layer statistics and the statistical variation (seasonal) of the optical properties of cirrus clouds is presented for the period of study.

Compact All-Fiber Polarization Coherent Lidar Based on a Polarization Modulator

Abstract: A compact all-fiber polarization coherent Lidar is demonstrated to simultaneously measure the velocity and depolarization ratio. In the Lidar system, a dual-polarization light beam is generated in a polarization modulator (PolM). The light beam consists of a linearly polarized optical carrier and an optical sideband with an orthogonal polarization state. If the light is depolarized, the polarization directions of the optical carrier and the sideband are rotated. Both of them can be detected along the polarization direction of the original optical carrier. In the receiver, the backscattered light along the polarization direction of the original optical carrier is selected and beats with a frequency-shifted optical local oscillator (LO) signal at a photodetector. A beat signal consisting of two single-frequency components will be generated. The velocity is extracted from the frequencies of the two single-frequency components, while the depolarization ratio can be extracted from the intensity ratio of the two single-frequency components. An experiment is carried out, in which a 159-MHz signal is applied to the PolM, and an 80-MHz signal is used to produce the frequency-shifted optical LO. Two single-frequency signals at 80 and 79 MHz are generated at the receiver and used to simultaneously measure the velocity and depolarization of a target in real time.

Depolarization Ratios of Methane Raman Bands as a Function of Pressure.

Abstract: In this work, we measured the intensities of Q-branches of the ν1, ν2 and ν3 bands in the polarized and depolarized methane Raman spectra in the pressure range of 1-60 atm. It was established that the pressure dependence of depolarization ratios of the ν2 and ν3 bands are negligible. In turn, the depolarization ratio of the ν1 band increases with increasing pressure and reaches approximately 0.0045 at 60 atm. These data are more precise than previously published ones because ν1 band intensities were determined taking into account the contribution of overlapping lines of ν3 band. The presented data will be useful in calculating the methane polarizabilities at high pressure, as well as in calculating methane Raman spectra for measuring the natural gas composition using Raman spectroscopy.

Using the Physical Optics Approximation for Estimating the Light Scattering Properties of Large Dust Particles for Lidar Applications

Abstract: The physical optics approximation was used to solve the light scattering problem of large dust particles with irregular shape. The comparison between the physical optics approximation and the DDA method was made for particles with volume-equivalent size parameters of 44.21. A full light scattering matrix (Mueller matrix) was calculated for particles with volume-equivalent size parameters from 44 up to 220 for two wavelengths: 0.532 and 1.064 μm. The dependences of the lidar and depolarization ratio, and extinction coefficient on the number of spatial orientations of the particle were obtained. It was shown that the elements of the light backscattering matrix obey power laws.

Retrieving microphysics of cirrus clouds from lidar-radar and depolarization ratios

Abstract: Simultaneous measurement of lidar and radar signals returned from the same cirrus clouds is a prospective method for retrieving the cloud microphysics, i.e. size and shape of the ice crystals constituting the cloud. In this study, the ratio of the backscattered signals of lidar and radar called the lidar-radar ratio has been calculated for the first time for typical shapes of ice crystals and wide distribution of the crystals over their sizes. It is shown that it is the lidar-radar ratio that is most sensitive to crystal sizes while the lidar depolarization ratio is most sensitive to crystal shapes.

Optical path system for measuring forward and backward scattering and depolarization ratio of single particulate matter

Abstract: The invention discloses an optical path system for measuring the forward and backward scattering and depolarization ratio of a single particulate matter. The optical path system comprises an incidentlight collimation and depolarization module, a laser calibration detection module, a single particle scattering module, a backward polarized light detection module, a forward scattered light measurement module and a light splitting lens, the incident light collimation and depolarization module, the light splitting lens, the single particle scattering module and the forward scattered light measurement module are sequentially installed, and the laser calibration detection module and the backward polarized light detection module are oppositely arranged on the opposite faces of the light splittinglens. The optical path system is simple in structure and high in stability, by establishing the reasonable combination of a collimating lens and a condensing lens, the optical path system is reasonably designed, and the optical path system is suitable for being widely applied to the pollutant monitoring of the monitoring stations, so that the detection level of the atmospheric pollutant physicalproperties is greatly improved.

Polarization Raman Lidar for Atmospheric Monitoring in the Vipava Valley

Abstract: We report on the design, construction and performance of a polarization Raman lidar, built for atmospheric monitoring in the Vipava valley in SW Slovenia, a regional air pollution hot-spot where aerosols are expected to originate from a number of different sources. Its key features are automatized remote operation capability and indoor deployment, which provide high duty cycle in all weather conditions. System optimization and performance studies include the calibration of the depolarization ratio, merging of near-range (analog) and far-range (photon-counting) data, determination of overlap functions and validation of the retrieved observables with radiosonde data.

Feasibility Studies of the Three-Wavelength Mie-Scattering Polarization Scheimpflug Lidar Technique

Abstract: A three-wavelength Mie-scattering polarization Scheimpflug lidar system, utilizing 808-nm, 520-nm and 405-nm multimode laser diodes as light sources and two CMOS sensors as detectors, is developed for the studies of the aerosol extinction coefficient, depolarization ratio and the Angstrom exponent. Atmospheric monitoring has been carried out on a near horizontal path from 23:00 January 14th to 06:00 January 15th , 2019 at Dalian, which is a coast city in Northern China. By studying the depolarization, aerosol extinction coefficient and Angstrom exponent, it has been found out that a strong north wind blew away local spherical haze particles and brought external non-spherical large-size particles. The measurement results indicated a promising future of employing the present three-wavelength polarization Scheimpflug lidar system in the applications of atmospheric remote sensing.

The polarization characteristics of cirrus cloud using lidar and radar in Hefei

Abstract: Lidar and radar are two useful tools for cirus clouds, which can provide the possibility to retrieve the vertical profiles of both the number density of particles and their microphysical characteristics, especially, when they operate together simultaneously. In this study, a multi-wavelength Lidar and a 35 Ghz Radar with polarization channels are employed to measure the properties of cirrus clouds in Hefei. The backscattering coefficient at 0.532 μm from lidar, and the effective reflectivity factor from radar are inversed for use. Furthermore, the quantities responsible for microphysics can be extracted and explained as the dimensionless values, such as the depolarization ratio and color ratio. Then these optical properties for cirrus cloud during campaigns are analyzed and compared with calculating results.

Characterisation of Biomass Burning Aerosols in the Southern Hemispheric Midlatitudes by Multiwavelength Raman Lidar

Abstract: Vertically resolved multiwavelength aerosol Raman lidar observations were conducted in the pristine environment of the Southern-hemisphere midlatitudes at Punta Arenas, Chile (53.1346°S, 70.8834°W). In contrast to the usually prevailing clean and pristine conditions at this site, two pronounced lofted aerosol layers were observed up to 4.2 and 4.4 km height on 4 and 5 February 2019, respectively. The layers mainly consisted of biomass burning aerosols originating from the region of Central Chile, where wildfires were also observed. Based on spectrally resolved backscatter and extinction coefficients, lidar ratios and depolarization ratio a detailed characterization of the aerosol optical properties is presented.

Studies on Aerosol Optical Properties at High Altitude Station in Western Himalayas Using Raman Lidar

Abstract: The aerosol optical properties have been investigated using the Raman lidar system for the month of November 2018 at the western Himalayan station of Palampur. Before deriving the optical properties, the lidar data has been applied with initial pre-processing such as Dead time correction, atmospheric noise correction, temporal and spatial averaging, range correction, gluing etc. The optical properties such as backscatter coefficient, extinction coefficient and linear depolarization ratio have been derived by using the inversion algorithm proposed by Fernald. The results show that the backscatter coefficient was found in the range from 9.00E-9 m−1 sr−1 to 4.97E-6 m−1 sr−1 and the extinction coefficient was found in the range from 3.16E-7m-1 to 1.74E-4m-1 . The Linear depolarization ratio was in the range from 0.0179 to 0.621 with lower values at near heights suggesting the dominance of spherical particles at the lower heights. We have also observed a cloud layer at a height of 9.5 km to 12.1 km with high depolarization ratio during the observation period on 22/11/2018.

Comparing Halo Doppler lidar depolarization ratio with PollyXT

Abstract: Depolarization ratio is highly valuable in lidar-based aerosol classification and can be used to quantify the contributions of different aerosol types to elevated layers [1]. Typically, aerosol particle depolarization ratio is determined at relatively short wavelengths of 355 nm and/or 532 nm, though some multi-wavelength case studies including 1064 nm have shown strong spectral dependency [2,3]. Here, we demonstrate that Halo Photonics Stream Line Doppler lidars can be used to retrieve aerosol particle depolarization ratio at 1.5 μm wavelength.

How to determine the shape of nuclear molecules with polarized gamma-rays

Abstract: A method has been recently proposed to establish the geometry of the alpha-cluster arrangement in $^{12}$C making use of polarized gamma-rays. The ratio of intensities of scattered radiation at 90 degrees along and perpendicular to the initial direction of the electric field vector, called depolarization ratio, is a key quantity that allows to underpin the nature of totally symmetric modes of vibrations. This allows to connect with the underlying point-group structure and therefore to the geometric shape of the nuclear molecule. This method is reviewed for $^{12}$C and extended to other configurations, such as three unequal clusters and four identical clusters (e.g. $^{16}$O).

Backscatter radar-lidar ratios for sizing ice crystals of cirrus clouds

Abstract: Cirrus clouds consisting mainly of ice crystals are important components of the atmosphere which essentially modulate the radiative budget of the Earth. Until now, the microphysical properties (i.e., size and shape) of the ice crystals, as well as their number density are poorly known because of their great variability in time and space and difficulties of field measurements. At present, cirrus clouds are widely studied by various ground-based, airborne and spaceborne instruments. Among such instruments, lidars and radars are promising devices providing active remote sensing of the clouds. In the report, we present the results of the calculations the radar-lidar ratio explicitly at a reasonable model for the size and shapes of the cirrus ice crystals using the physical-optics approximation. We show that it is the radar-lidar ratio that is mainly informative for retrieving crystal sizes. Also, we calculate the depolarization ratios for both lidar and radar. We obtain that the lidar depolarization ratio is effective for estimating crystal shapes in cirrus clouds. Such data would be useful for interpreting any data obtained simultaneously by radars and lidars.

Monitoring Ice Crystals Clouds: Investigation of the Lidar Depolarization Ratios

Abstract: A cirrus cloud dataset from a groundbased lidar in a sub-arctic station is analyzed in terms of the particle depolarization ratio. The depolarization values -δ- showed a broad distribution, ranging between 0.25 and 0.7, with a mean value of 0.38 ± 0.07. This variability is examined in correlation with temperature dependencies and the cirrus optical depth. Depolarization values greater than 0.45 are observed from temperatures between -60oC and -40oC, where horizontally oriented planar ice crystals are to be expected. Two case studies are also, discussed and analyzed in terms of depolarization height dependence.