Linear Depolarization Research Articles

Remote sensing of optical characteristics and particle distributions of the upper ocean using shipboard lidar

Passive ocean color remote sensing has revolutionized our ability to quantify the horizontal distribution of phytoplankton across the ocean surface. Lidar technology can provide remotely sensed estimates of the vertical distribution of optical properties and suspended particles in natural waters, significantly improving our ability to model upper ocean biogeochemical processes. In this study, we constructed and deployed a ship-based lidar system to measure laser backscattering and linear depolarization profiles in the coastal Mid-Atlantic ranging from estuarine to oceanic conditions, and across the Gulf of Maine (GoM). The instrument identified layers with different backscattering intensity in stratified waters of the coastal Mid-Atlantic and produced system attenuation coefficients (Ksys) approximating the absorption coefficient (apg) of particulate + dissolved matter. The linear depolarization ratio was strongly related to in situ measurements of the particulate backscattering ratio (bbp/bp). Measurements of Ksys and linear depolarization made across the GoM corresponded well with simultaneous in situ observations performed aboard the M/V Nova Star and by an autonomous glider deployed along the transect. The relationship between Ksys and apg differed between sampling schemes, likely due to differences in the deployment geometries (e.g., height, nadir angle). These results support the proposition that ship-based lidar systems can provide a powerful tool for remotely measuring the vertical distributions of optical properties and geochemical constituents (e.g., particles) in the upper ocean. Continued development of compact lidar systems for deployment on ships, moorings, and autonomous platforms has the potential to greatly improve the quality and scope of a variety of oceanographic investigations.

Scattering Calculations for Asymmetric Raindrops during a Line Convection Event: Comparison with Radar Measurements

AbstractTwo-dimensional video disdrometer (2DVD) data from a line convection rain event are analyzed using the method of moments surface integral equation (MoM-SIE) via drop-by-drop polarimetric scattering calculations at C band that are compared with radar measurements. Drop geometry of asymmetric drop shapes is reconstructed from 2DVD measurements, and the MoM-SIE model is created by meshing the surface of the drop. The differential reflectivity Zdr calculations for an example asymmetric drop are validated against an industry standard code solution at C band, and the azimuthal dependence of results is documented. Using the MoM-SIE analysis on 2DVD drop-by-drop data (also referred to as simply MoM-SIE), the radar variables [Zh, Zdr, Kdp, ρhv] are computed as a function of time (with 1-min resolution) and compared to C-band radar measurements. The importance of shape variability of asymmetric drops is demonstrated by comparing with the traditional (or “bulk”) method, which uses 1-min averaged drop size distributions and equilibrium oblate shapes. This was especially pronounced for ρhv, where the MoM-SIE method showed lowered values (dip) during the passage of the line convection consistent with radar measurements, unlike the bulk method. The MoM-SIE calculations of [Zh, Zdr, Kdp] agree very well with the radar measurements, whereas linear depolarization ratio (LDR) calculations from the drop-by-drop method are found to be larger than the values from the bulk method, which is consistent with the dip in simulated and radar-measured ρhv. Our calculations show the importance of the variance of shapes resulting from asymmetric drops in the calculation of ρhv and LDR.

Calculation of optical properties of light-absorbing carbon with weakly absorbing coating: A model with tunable transition from film-coating to spherical-shell coating

Optical properties of particles consisting of light-absorbing carbon (or soot) and a weakly absorbing coating material are computed at a wavelength of 355 nm and 532 nm. A morphological particle model is used, in which small amounts of coating are applied as a thin film to the surface of the aggregate, while heavily coated aggregates are enclosed in a spherical shell. As the amount of coating material is increased, a gradual transition from film-coating to spherical-shell coating is accounted for. The speed of this transition can be varied by specifying a single parameter. Two different choices of this parameter, corresponding to a slow and a rapid transition from film-coating to spherical-shell coating, respectively, are investigated. For low soot volume fractions the impact of this transition on the linear depolarisation ratio δl is most pronounced. The model that describes a rapid transition to a spherical coating yields results for δl that are more consistent with existing lidar field measurements than the slow-transition model. At 532 nm the relative uncertainty in modelled δl for a rapid transition values due to uncertainties in the aggregate’s geometry and chemical composition are estimated to range from 109 to 243%, depending on the soot volume fraction. At 355 nm the relative uncertainties were estimated to range from 90.9 to 200%.

Coating effect on light scattering by irregularly shaped particles

In this article, we study the impact of water coating on light scattering by irregularly shaped agglomerated debris particles. We compute light-scattering properties of three types of irregular particles, which simulate atmospheric particles before and after coating. We consider two types of the Junge size distributions over sub-micron range of rc. The coating is represented as deposition of same volume of material on original particles. We found that only the degree of linear polarization is sensitive to coating type, especially maximum value of the polarization that is typically acquired at side scattering. Behavior of the degree of linear polarization maximum appears in accordance with the Umov effect. The linear depolarization ratio at backscattering direction δl is insensitive to coating material, but it is dependent from particles size.

A simple biota removal algorithm for 35 GHz cloud radar measurements

Abstract. Cloud radar reflectivity profiles can be an important measurement for the investigation of cloud vertical structure (CVS). However, extracting intended meteorological cloud content from the measurement often demands an effective technique or algorithm that can reduce error and observational uncertainties in the recorded data. In this work, a technique is proposed to identify and separate cloud and non-hydrometeor echoes using the radar Doppler spectral moments profile measurements. The point and volume target-based theoretical radar sensitivity curves are used for removing the receiver noise floor and identified radar echoes are scrutinized according to the signal decorrelation period. Here, it is hypothesized that cloud echoes are observed to be temporally more coherent and homogenous and have a longer correlation period than biota. That can be checked statistically using ∼ 4 s sliding mean and standard deviation value of reflectivity profiles. The above step helps in screen out clouds critically by filtering out the biota. The final important step strives for the retrieval of cloud height. The proposed algorithm potentially identifies cloud height solely through the systematic characterization of Z variability using the local atmospheric vertical structure knowledge besides to the theoretical, statistical and echo tracing tools. Thus, characterization of high-resolution cloud radar reflectivity profile measurements has been done with the theoretical echo sensitivity curves and observed echo statistics for the true cloud height tracking (TEST). TEST showed superior performance in screening out clouds and filtering out isolated insects. TEST constrained with polarimetric measurements was found to be more promising under high-density biota whereas TEST combined with linear depolarization ratio and spectral width perform potentially to filter out biota within the highly turbulent shallow cumulus clouds in the convective boundary layer (CBL). This TEST technique is promisingly simple in realization but powerful in performance due to the flexibility in constraining, identifying and filtering out the biota and screening out the true cloud content, especially the CBL clouds. Therefore, the TEST algorithm is superior for screening out the low-level clouds that are strongly linked to the rainmaking mechanism associated with the Indian Summer Monsoon region's CVS.

Depolarization Ratio Profiles Calibration and Observations of Aerosol and Cloud in the Tibetan Plateau Based on Polarization Raman Lidar

A brief description of the Water vapor, Cloud and Aerosol Lidar (WACAL) system is provided. To calibrate the volume linear depolarization ratio, the concept of “ Δ 90 ° -calibration” is applied in this study. This effective and accurate calibration method is adjusted according to the design of WACAL. Error calculations and analysis of the gain ratio, calibrated volume linear depolarization ratio and particle linear depolarization ratio are provided as well. In this method, the influences of the gain ratio, the rotation angle of the plane of polarization and the polarizing beam splitter are discussed in depth. Two groups of measurements with half wave plate (HWP) at angles of (0 ° , 45 ° ) and (22.5 ° , −22.5 ° ) are operated to calibrate the volume linear depolarization ratio. Then, the particle linear depolarization ratios measured by WACAL and CALIOP (the Cloud-Aerosol Lidar with Orthogonal Polarization) during the simultaneous observations were compared. Good agreements are found. The calibration method was applied in the third Tibetan Plateau Experiment of Atmospheric Sciences (TIPEX III) in 2013 and 2014 in China. Vertical profiles of the particle depolarization ratio of clouds and aerosol in the Tibetan Plateau were measured with WACAL in Litang (30.03° N, 100.28° E, 3949 m above sea level (a.s.l.)) in 2013 and Naqu (31.48° N, 92.06° E, 4508 m a.s.l.) in 2014. Then an analysis on the polarizing properties of the aerosol, clouds and cirrus over the Tibetan Plateau is provided. The particle depolarization ratio of cirrus clouds varies from 0.36 to 0.52, with a mean value of 0.44 ± 0.04. Cirrus clouds occurred between 5.2 and 12 km above ground level (a.g.l.). The cloud thickness ranges from 0.12 to 2.55 km with a mean thickness of 1.22 ± 0.70 km. It is found that the particle depolarization ratio of cirrus clouds become larger as the height increases. However, the increase rate of the particle depolarization ratio becomes smaller as the height increases.

Scanning vertical distributions of typical aerosols along the Yangtze River using elastic lidar

In recent years, China has experienced heavy air pollution, especially haze caused by particulate matter (PM). The compositions, horizontal distributions, transport, and chemical formation mechanisms of PM and its precursors have been widely investigated in China based on near-ground measurements. However, the understanding of the distributions and physical and chemical processes of PM in the vertical direction remains limited. In this study, an elastic lidar was employed to investigate the vertical profiles of aerosols along the Yangtze River during the Yangtze River Campaign of winter 2015. Some typical aerosols were identified and some events were analyzed in three cases. Dust aerosols can be transported from the Gobi Desert to the Yangtze River basin across a long distance at both low and high altitudes in early December. The transport route was perpendicular to the ship track, suggesting that the dust aerosols may have affected a large area. Moreover, during transport, some dust was also affected by the areas below its transport route since some anthropogenic pollutants were mixed with the dust and changed some of its optical properties. Biomass-burning aerosols covering a distant range along the Yangtze River were identified. This result directly shows the impact areas of biomass-burning aerosols in some agricultural fields. Some directly emitted aerosol plumes were observed, and direct effects of such plumes were limited both temporally and spatially. In addition, an aerosol plume with very low linear depolarization ratios, probably formed through secondary processes, was also observed. These results can help us better understand aerosols in large spatial scales in China and can be useful to regional haze studies.

Three-channel single-wavelength lidar depolarization calibration

Abstract. Linear depolarization measurement capabilities were added to the CANDAC Rayleigh–Mie–Raman lidar (CRL) at Eureka, Nunavut, in the Canadian High Arctic in 2010. This upgrade enables measurements of the phases (liquid versus ice) of cold and mixed-phase clouds throughout the year, including during polar night. Depolarization measurements were calibrated according to existing methods using parallel- and perpendicular-polarized profiles as discussed in ). We present a new technique that uses the polarization-independent Rayleigh elastic channel in combination with one of the new polarization-dependent channels, and we show that for a lidar with low signal in one of the polarization-dependent channels this method is superior to the traditional method. The optimal procedure for CRL is to determine the depolarization parameter using the traditional method at low resolution (from parallel and perpendicular signals) and then to use this value to calibrate the high-resolution new measurements (from parallel and polarization-independent Rayleigh elastic signals). Due to its use of two high-signal-rate channels, the new method has lower statistical uncertainty and thus gives depolarization parameter values at higher spatial–temporal resolution by up to a factor of 20 for CRL. This method is easily adaptable to other lidar systems which are considering adding depolarization capability to existing hardware.

Assessing the depolarization capabilities of nonspherical particles in a super-ellipsoidal shape space.

Here we use the state-of-the-art invariant imbedding T-matrix method to theoretically assess the backscattering linear depolarization ratio (LDR) of nonspherical particles in a super-ellipsoidal shape space. Super-ellipsoids have inherent flexibility to model the particle aspect ratio, roundness, and concavity, these being salient characteristics of most atmospheric particles (e.g., sea salt and dust aerosols). The complex refractive index of super-ellipsoids was set up with the real part ranging from 1.1 to 2.0 and the imaginary part from 10-7 to 0.5. To constrain the computational burden, the maximum size parameters for spheroids and super-ellipsoids were set as 100 and 50, respectively. From the LDRs of spheroids, we found that enhanced LDRs (>~60%) are common for optically soft particles. However, as the real part of the refractive index increases (larger than ~1.33), the enhanced LDRs (>~60%) are in high probability observed for nearly-spherical particles, and then disappear as the refractive index exceeds 1.7. To produce the enhanced LDRs, the imaginary part of the refractive index should also be less than ~0.01 such that the backscattered waves from particle-to-air transmission have sizable contributions, as the external reflection of spheroids produces no depolarization. This finding has particular relevance to LiDAR observations of atmospheric particles because the refractive index of most aerosols and hydrometeors at the LiDAR wavelength (e.g., 0.532μm) locates in this region, and aerosols and hydrometeors could have nearly-spherical morphologies. From the LDRs for general super-ellipsoids, we found that the enhanced LDRs (>~60%) exist for nearly-spherical particles with the aspect ratio close to unity, but disappear for super-ellipsoids with an aspect ratio at unity. In addition, the LDRs trend to decrease as the real part of the refractive index increases for convex super-ellipsoids, but show different features for concave super-ellipsoids. Furthermore, super-ellipsoids with different roundness parameters have a distinct dependence on the aspect ratio, which is significantly different from spheroids. The results presented here provide comprehensive references for understanding the LDR change of atmospheric aerosols as the particle shape and refractive index for interpreting LiDAR backscattering signals.

Optical Modeling of Sea Salt Aerosols: The Effects of Nonsphericity and Inhomogeneity

AbstractThe nonsphericity and inhomogeneity of marine aerosols (sea salts) have not been addressed in pertinent radiative transfer calculations and remote sensing studies. This study investigates the optical properties of nonspherical and inhomogeneous sea salts using invariant imbedding T‐matrix simulations. Dry sea salt aerosols are modeled based on superellipsoidal geometries with a prescribed aspect ratio and roundness parameter. Wet sea salt particles are modeled as coated superellipsoids, as spherical particles with a superellipsoidal core, and as homogeneous spheres depending on the level of relative humidity. Aspect ratio and roundness parameters are found to be critical to interpreting the linear depolarization ratios (LDRs) of NaCl crystals from laboratory measurements. The optimal morphology parameters of NaCl necessary to reproduce the measurements are found to be consistent with data gleaned from an electron micrograph. The LDRs of wet sea salts are computed based on inhomogeneous models and compared with the measured data from ground‐based LiDAR. The dependence of the LDR on relative humidity is explicitly considered. The increase in the LDR with relative humidity at the initial phase of deliquescence is attributed to both the size increase and the inhomogeneity effect. For large humidity values, the LDR substantially decreases because the overall particle shape becomes more spherical and the inhomogeneity effect in a particle on the LDR is suppressed for submicron sea salts. However, the effect of inhomogeneity on optical properties is pronounced for coarse‐mode sea salts. These findings have important implications for atmospheric radiative transfer and remote sensing involving sea salt aerosols.

Detection of saharan mineral dust aerosol transport over brazilian northeast through a depolarization lidar

In this study we present results of linear volume depolarization ratio profiles obtained by a depolarization lidar in operation in Natal, Brazil. The DUSTER system has 4 channels, namely: 1064, 532 s/p and 355 nm. This system is calibrated with a half-wave plate using the Δ90° methodology. The data obtained from this system is correlated with AERONET sunphotometer data, and, when available, CALIPSO satellite data. In addition a trajectory model (HYSPLIT) is used to calculate backward trajectories to assess the origin of the dust polluted air parcels. The objective is to create a transport database of Saharan dust.

Aerosol optical properties variability during biomass burning events observed by the eole-aias depolarization lidars over Athens, Greece (2007-2016)

The EOLE multi-wavelength aerosol Ramandepolarization lidar, and the AIAS depolarization lidar, in synergy with a sun photometer (CIMEL), were used, in the period 2007-2016, to provide the vertical profiles of the aerosol optical properties over Athens, Greece. More than 30 biomass burning events (fresh and aged smoke particles) were observed, with smoke layers between 1.5 up to 4-5 km height, while their duration ranged from 1-3 days. Lidar ratio (LR) values ranged from 40-105 sr (at 355 nm) and from 40-100 sr (at 532 nm), while the linear particle depolarization ratio (LPDR) at both 355 and 532 nm, remained <7%. The extinction-related Ångström exponent (AEa) at 355 nm/532 nm) ranged from 0.3 to 2.1. Additionally, a case of a near-range transport of biomass burning aerosols arriving over Athens up to 4 km height, between 27 and 28 June 2016, was studied. For this case, we found LRs of the order of 70±5 sr (355 nm) and 65±15 sr (532 nm) and AEa(355 nm/532 nm) around 1.

Lidar stand-alone retrieval of atmospheric aerosol microphysical properties during SLOPE

Two cases from SLOPE campaign at Granada are analyzed in terms of particle microphysical properties using novel software developed at Potsdam University. Multiwavelength Raman lidar measurements of particle extinction and backscatter coefficients as well as linear particle depolarization ratios are used as input for the software. The result of the retrieval is a 2-dimensional particle volume distribution as a function of radius and aspect ratio, from which the particle microphysical properties are obtained.

Triple-wavelength lidar observations of the linear depolarization ratio of dried marine particles

For aerosol typing with lidar, sea salt particles are usually assumed to be spherical with a consequently low depolarization ratio. Evidence of dried marine particles at the top of the humid marine aerosol layer with a depolarization ratio up to 0.1 has been found at predominately maritime locations on Barbados and in the Southern Atlantic. The depolarization ratio for these probably cubic sea salt particles has been measured at three wavelengths (355, 532 and 1064 nm) simultaneously for the first time and compared to model simulations.

Анализ изменчивости линейного деполяризационного отношения при радиолокационном зондировании неоднородной среды, заполненной гидрометеорами

Анализ изменчивости линейного деполяризационного отношения при радиолокационном зондировании неоднородной среды, заполненной гидрометеорами

Earlinet single calculus chain: new products overview

The Single Calculus Chain (SCC) is an automatic and flexible tool to analyze raw lidar data using EARLINET quality assured retrieval algorithms. It has been already demonstrated the SCC can retrieve reliable aerosol backscatter and extinction coefficient profiles for different lidar systems. In this paper we provide an overview of new SCC products like particle linear depolarization ratio, cloud masking, aerosol layering allowing relevant improvements in the atmospheric aerosol characterization.

Vertical separation of the atmospheric aerosol components by using poliphon retrieval in polarized micro pulse lidar (P-MPL) measurements: case studies of specific climate-relevant aerosol types

POLIPHON (POlarization-LIdar PHOtometer Networking) retrieval consists in the vertical separation of two/three particle components in aerosol mixtures, highlighting their relative contributions in terms of the optical properties and mass concentrations. This method is based on the specific particle linear depolarization ratio given for different types of aerosols, and is applied to the new polarized Micro-Pulse Lidar (P-MPL). Case studies of specific climate-relevant aerosols (dust particles, fire smoke, and pollen aerosols, including a clean case as reference) observed over Barcelona (Spain) are presented in order to evaluate firstly the potential of P-MPLs measurements in combination with POLIPHON for retrieving the vertical separation of those particle components forming aerosol mixtures and their properties.

Polarisation effects in lidar signals: determination, correction, and uncertainty calculation

An analytical framework, based on the Müller-Stokes formalism, is presented, which enables us to determine the impact of polarisation effects of the lidar optics on the lidar signals and on the measured atmospheric linear depolarisation. Furthermore, the equations enable us to investigate the accuracy of the gain ratio calibration of depolarisation channels with different calibration setups, to correct some of the polarisation effects, and to perform uncertainty analyses.

Long-term profiling of mineral dust and pollution aerosol with multiwavelength polarization Raman lidar at the Central Asian site of Dushanbe, Tajikistan: case studies

Abstract. For the first time, continuous vertically resolved aerosol measurements were performed by lidar in Tajikistan, Central Asia. Observations with the multiwavelength polarization Raman lidar PollyXT were conducted during CADEX (Central Asian Dust EXperiment) in Dushanbe, Tajikistan, from March 2015 to August 2016. Co-located with the lidar, a sun photometer was also operated. The goal of CADEX is to provide an unprecedented data set on vertically resolved aerosol optical properties in Central Asia, an area highly affected by climate change but largely missing vertically resolved aerosol measurements. During the 18-month measurement campaign, mineral dust was detected frequently from ground to the cirrus level height. In this study, an overview of the measurement period is given and four typical but different example measurement cases are discussed in detail. Three of them are dust cases and one is a contrasting pollution aerosol case. Vertical profiles of the measured optical properties and the calculated dust and non-dust mass concentrations are presented. Dust source regions were identified by means of backward trajectory analyses. A lofted layer of Middle Eastern dust with an aerosol optical thickness (AOT) of 0.4 and an extinction-related Ångström exponent of 0.41 was measured. In comparison, two near-ground dust cases have Central Asian sources. One is an extreme dust event with an AOT of 1.5 and Ångström exponent of 0.12 and the other one is a most extreme dust event with an AOT of above 4 (measured by sun photometer) and an Ångström exponent of −0.08. The observed lidar ratios (and particle linear depolarization ratios) in the presented dust cases range from 40.3 to 46.9 sr (and 0.18–0.29) at 355 nm and from 35.7 to 42.9 sr (0.31–0.35) at 532 nm wavelength. The particle linear depolarization ratios indicate almost unpolluted dust in the case of a lofted dust layer and pure dust in the near-ground dust cases. The lidar ratio values are lower than typical lidar ratio values for Saharan dust (50–60 sr) and comparable to Middle Eastern or west-Asian dust lidar ratios (35–45 sr). In contrast, the presented case of pollution aerosol of local origin has an Ångström exponent of 2.07 and a lidar ratio (particle linear depolarization ratio) of 55.8 sr (0.03) at 355 nm and 32.8 sr (0.08) at 532 nm wavelength.

Atmospheric Ice Particle Shape Estimates from Polarimetric Radar Measurements and In Situ Observations

AbstractA remote sensing approach to retrieve the degree of nonsphericity of ice hydrometeors using scanning polarimetric Ka-band radar measurements from a U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program cloud radar operated in an alternate transmission–simultaneous reception mode is introduced. Nonsphericity is characterized by aspect ratios representing the ratios of particle minor-to-major dimensions. The approach is based on the use of a circular depolarization ratio (CDR) proxy reconstructed from differential reflectivity ZDR and copolar correlation coefficient ρhυ linear polarization measurements. Essentially combining information contained in ZDR and ρhυ, CDR-based retrievals of aspect ratios are fairly insensitive to hydrometeor orientation if measurements are performed at elevation angles of around 40°–50°. The suggested approach is applied to data collected using the third ARM Mobile Facility (AMF3), deployed to Oliktok Point, Alaska. Aspect ratio retrievals were also performed using ZDR measurements that are more strongly (compared to CDR) influenced by hydrometeor orientation. The results of radar-based retrievals are compared with in situ measurements from the tethered balloon system (TBS)-based video ice particle sampler and the ground-based multiangle snowflake camera. The observed ice hydrometeors were predominantly irregular-shaped ice crystals and aggregates, with aspect ratios varying between approximately 0.3 and 0.8. The retrievals assume that particle bulk density influencing (besides the particle shape) observed polarimetric variables can be deduced from the estimates of particle characteristic size. Uncertainties of CDR-based aspect ratio retrievals are estimated at about 0.1–0.15. Given these uncertainties, radar-based retrievals generally agreed with in situ measurements. The advantages of using the CDR proxy compared to the linear depolarization ratio are discussed.