Linear Depolarization Ratio Values Research Articles

Scattering and absorption properties of spheroidal soot-sulfate aerosols

We analyze the results of numerically exact modeling of scattering and absorption characteristics of randomly oriented polydisperse spheroidal soot particles covered by spheroidal nonabsorbing sulfate shell. Using numerically exact T-matrix method, at wavelengths 0.355 μm, 0.532 μm and 1.064 μm, we perform computations of the absorption cross section (Cabs), absorption Ångström exponent (AAE), backscattering linear depolarization ratio (LDR) and scattering matrix elements for spheroids with core volume-equal-sphere effective radius (further in the text, the term "effective radius" is used) Rc,eff = 0.1, 0.15 μm, soot volume fraction .f = 7% and 15%. Mostly, we consider the case of oblate spheroids with the axis ratio ε ranging from 1.1 to 1.8. Our size-averaged results demonstrate, in particular, that: (i) the absorption cross section depends on the degree of nonsphericity very weakly, but substantially increases with increasing aerosol size and decreases with increasing wavelength; (ii) the absorption Ångström exponent also shows a weak dependence on the spheroid axis ratio, but it is quite sensitive to the choice of the wavelength pair, particle size and amount of soot/sulfate; (iii) the backscattering linear depolarization ratio is very sensitive to particle nonsphericity, size, amount of sulfate coating and wavelength, and concentric soot-sulfate spheroidal particles could cause a wide range of the LDR values that significantly decrease with increasing wavelength. We also conclude that at backscattering angles, the elements of the normalized scattering matrix exhibit specific changes which depend on particle shape, size, and amount of coating material.

Aspects of melting layer and fall streaks in stratiform cloud system over the Western Ghats, India from Ka-band polarimetric radar observations

During the Indian summer monsoon, high temporal and spatial resolution observations from Ka-band dual-polarization radar deployed in the Western Ghats (WG), India are utilized to investigate the vertical structure of the stratiform precipitating system. First, the radar reflectivity (Z) measurements are corrected for rain attenuation, based on the relationship between specific attenuation (A) and Z. The disdrometer dataset is used to calculate the expected value of A at Ka-band for a given Z using microwave scattering simulation. The radar data are also corrected for wet radome transmission loss, gaseous, and melting layer (ML) attenuation. Within the ML, a change in Z and other radar polarimetric variables suggest hydrometeors phase change. In addition to Z, polarimetric measurements such as differential reflectivity (ZDR), linear depolarization ratio (LDR), and copolar correlation coefficient between horizontal and vertical channel (ρHV) are used to determine the ML height. Ka-band polarimetric radar signatures near the ML are characterized by a low ρHV (∼0.93), high Z (∼25–30 dBZ), ZDR (∼3 dB), and LDR (∼ −15 dB) values. In the stratiform precipitation region, well-defined fall streaks are observed above ML, which descends through 0 °C isotherm into the rain region. When the fall streaks are present above the ML, Z increases below the ML, indicating a seeder-feeder mechanism. Because there is little observational evidence concerning the vertical distribution of stratiform clouds over the Indian subcontinent, the current study may help to understand stratiform clouds and the ice processes that occur within them.

Optical properties of morphologically complex black carbon aerosols: Effects of coatings

Optical properties are computed for fractal-like aggregate black carbon (BC) aerosols coated with different substances. Two models are used for these aerosols: (i) the coated aggregate model (model I), where coating is added to the voids and surroundings of BC fractal-like aggregates; and (ii) the closed-cell aggregate model (model II), where coating is added concentrically to each monomer of BC fractal-like aggregates. Our results favor choosing the coated aggregate model (model I) to simulate scattering and absorption by coated BC aerosols because this model is morphologically more realistic, and because this model yields mass absorption cross section (MAC) and backscattering linear depolarization ratio (LDR) values that are consistent with field measurements. Moreover the corresponding computed degree of linear polarization (DoLP) and LDR values are very sensitive to changes in the coating volume fraction (fvol) and the coating refractive index (m). With the same absorbing BC core, the MAC value increases steadily with increasing fvol and m values. For example, using a density of 1.8 g/cm3 and BC refractive index 1.95 + i0.79, the calculated MAC values for uncoated BC aggregates range from 6.2 to 6.8 m2/g at 0.55 µm. When coating material is applied to the BC aggregates, the calculated MAC values for model I particles increase to between 9.8 and 13.2 m2/g (depending on m) when fvol = 87.5%. The backscattering LDR values also tend to increase with the increasing m values for the shapes and sizes considered in our study. For model I particles, the backscattering LDR values span a wide range of 4.2–27.8% at a wavelength of 0.35 µm at fvol = 87.5% when m increases from 1.33 to 1.55. Our results are relevant to analyses of polarimetric and lidar observations of smoke particles, especially when these particles undergo hygroscopic growth.

Hydrometeor Distribution and Linear Depolarization Ratio in Thunderstorms

The distribution of hydrometeors in thunderstorms is still under investigation as well as the process of electrification in thunderclouds leading to lightning discharges. One indicator of cloud electrification might be high values of the Linear Depolarization Ratio (LDR) at higher vertical levels. This study focuses on LDR values derived from vertically pointing cloud radars and the distribution of five hydrometeor species during 38 days with thunderstorms which occurred in 2018 and 2019 in Central Europe, close to our radar site. The study shows improved algorithms for de-aliasing, the derivation of vertical air velocity and the classification of hydrometeors in clouds using radar data. The comparison of vertical profiles with observed lightning discharges in the vicinity of the radar site (≤1 km) suggested that cloud radar data can indirectly identify “lightning” areas by high LDR values observed at higher gates due to the alignment of ice crystals, likely because of an intensified electric field in thunderclouds. Simultaneously, the results indicated that at higher gates, there is a mixture of several hydrometeor species, which suggests a well-known electrification process by collisions of hydrometeors.

Spectrally dependent linear depolarization and lidar ratios for nonspherical smoke aerosols

We use the numerically exact T-matrix method to model light scattering and absorption by aged smoke aerosols at lidar wavelengths ranging from 355 to 1064 nm assuming the aerosols to be smooth spheroids or Chebyshev particles. We show that the unique spectral dependence of the linear depolarization ratio (LDR) and extinction-to-backscatter ratio (or lidar ratio, LR) measured recently for stratospheric Canadian wildfire smoke can be reproduced by a range of model morphologies, a range of spectrally dependent particle refractive indices, and a range of particle sizes. For these particles, the imaginary part of the refractive index is always less than (or close to) 0.035, and the corresponding real part always falls in the range [1.35, 1.65]. The measured spectral LDRs and LRs could be produced by nearly-spherical oblate spheroids or Chebyshev particles whose shapes resemble those of oblate spheroids. Their volume-equivalent effective radii should be large enough (reff = 0.3 µm or greater) to produce the observed enhanced LDRs. Our study demonstrates the usefulness of triple-wavelength LDR measurements as providing additional size information for a more definitive characterization of the particle morphology and composition. Non-zero LDR values indicate the presence of nonspherical aerosols and are highly sensitive to particle shapes and sizes. On the other hand, the LR is a strong function of absorption and is very responsive to changes in the particle refractive index.

The Potential Use of the Linear Depolarization Ratio to Distinguish between Convective and Stratiform Rainfall to Improve Radar Rain-Rate Estimates

AbstractA major source of errors in radar-derived quantitative precipitation estimates is the inhomogeneous nature of the vertical reflectivity profile (VPR). Operational radars generally scan in azimuth at constant elevation (PPI mode) and provide limited VPR information, so predetermined VPR shapes with limited degrees of freedom are needed to correct for the VPR in real time. Typical stratiform VPRs have a sharp peak below the 0° isotherm, known as the “bright band,” caused by the presence of large melting snowflakes, but this feature is not present in convective cores where the melting ice is in the form of graupel or compact ice. Inappropriate correction assuming a brightband VPR can lead to underestimation of rain rates, with particular impacts in intense convective storms. This paper proposes the use of high values of linear depolarization ratio (LDR) measurements to confirm the presence of large melting snowflakes and lower values for melting graupel or high-density ice as a prerequisite to selecting a suitable profile shape for VPR correction. Using a climatologically representative dataset of short-range, high-resolution C-band vertical profiles, the peak value of the LDR in the melting layer is shown to have robust skill in identifying VPRs without bright band, with the “best” performance at a threshold of −20 dB. Further work is proposed to apply this result to improving corrections for VPR at longer range, where the limited effect of beam broadening on LDR peaks could provide advantages over other available methods.

Effects of Antenna Patterns on Cloud Radar Polarimetric Measurements

AbstractThis paper presents an experimental analysis of the antenna system effects on polarimetric measurements conducted with cloud radars operating in the linear depolarization ratio (LDR) mode. Amplitude and phase of the copolar and cross-polar antenna patterns are presented and utilized. The patterns of two antennas of different quality were measured at the Hungriger Wolf airport near Hohenlockstedt, Germany, during the period from 28 January to 1 February 2014. For the measurements a test transmitter mounted on a tower and the scanning 35-GHz (Ka band) cloud radar MIRA-35, manufactured by METEK GmbH and operated in the receiving mode, were used. The integrated cross-polarization ratios (ICPR) are calculated for both antennas and compared with those measured in light rain. Correction algorithms for observed LDR and the co-cross-channel correlation coefficient ρ are presented. These algorithms are aimed at removing/mitigating polarization cross-coupling effects that depend on the quality of radar hardware. Thus, corrected LDR and ρ are primarily influenced by scatterer properties. The corrections are based on the decomposition of the coherency matrix of the received signals into fully polarized and nonpolarized components. The correction brings LDR values and the co-cross-channel correlation coefficients from two radars with different antenna systems to a close agreement, thus effectively removing hardware-dependent biases. Uncertainties of the correction are estimated as 3 dB for LDR in the range from −30 to −10 dB. In clouds, the correction of the co-cross-channel correlation coefficient ρ results in near-zero values for both vertically pointed radars.

Linear Depolarization Ratios of Columnar Ice Crystals in a Deep Precipitating System over the Arctic Observed by Zenith-Pointing Ka-Band Doppler Radar

AbstractThis study demonstrates that linear depolarization ratio (LDR) values obtained from zenith-pointing Ka-band radar Doppler velocity spectra are sufficient for detecting columnar ice crystals. During a deep precipitating system over the Arctic on 7 December 2013, the radar recorded LDR values up to −15 dB at temperatures corresponding to the columnar ice crystal growth regime. These LDR values were also consistent with scattering calculations for columnar ice crystals. Enhancements in LDR were suppressed within precipitation fallstreaks because the enhanced LDR values of columnar ice crystals were masked by the returns from the particles within the fallstreaks. However, Doppler velocity spectra of LDR within the fallstreak distinguished populations of slower-falling particles with high LDR (>−15 dB) and faster-falling particles with much lower LDR, suggesting that columnar ice crystals with high LDR coexisted with larger isometric particles that produced low LDR while dominating the total copolar reflectivity, thereby decreasing LDR. The measurements suggest that the columnar ice crystals originated in liquid-cloud layers through secondary ice production.

Aerosol characteristics over Delhi national capital region: a satellite view

Multi-sensor aerosol data sets are analysed to examine the aerosol characteristics over the Delhi national capital region. Both the Multiple-angle Imaging Spectroradiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) capture the seasonal cycle of aerosol optical depth (AOD) as observed by ground-based measurements. However, AOD from MISR shows a low bias relative to AOD from MODIS, which increases linearly at high AOD conditions. A large difference (by >25 W m–2 per unit AOD) in the top-of-atmosphere direct radiative forcing efficiency derived from MODIS and MISR-retrieved AOD is observed during the winter and pre-monsoon seasons relative to the other seasons. The ubiquitous presence of dust (as indicated by non-spherical particle fraction to AOD and linear depolarization ratio values) is observed throughout the year. The aerosol layer is mostly confined to within 2 km of surface in the winter and post-monsoon seasons, while it expands beyond 6 km in the pre-monsoon and monsoon seasons. Columnar AOD is found to be highly sensitive to aerosol vertical distribution. The applicability of multi-sensor data sets and climatic implications are discussed.

A study on the use of radar and lidar for characterizing ultragiant aerosol

AbstractFrom 19 April to 19 May 2010, volcanic aerosol layers originating from the Eyjafjallajökull volcano were observed at the Institute of Methodologies for Environmental Analysis of the National Research Council of Italy Atmospheric Observatory, named CIAO (40.60°N, 15.72°E, 760 m above sea level), in Southern Italy with a multiwavelength Raman lidar. During this period, ultragiant aerosols were also observed at CIAO using a colocated 8.45 mm wavelength Doppler radar. The Ka‐band radar signatures observed in four separate days (19 April and 7, 10, and 13 May) are consistent with the observation of nonspherical ultragiant aerosols characterized by values of linear depolarization ratio (LDR) higher than –4 dB. Air mass back trajectory analysis suggests a volcanic origin of the ultragiant aerosols observed by the radar. The observed values of the radar reflectivity (Ze) are consistent with a particle effective radius (r) larger than 50–75 µm. Scattering simulations based on the T‐matrix approach show that the high LDR values can be explained if the observed particles have an absolute aspect ratio larger than 3.0 and consist of an internal aerosol core and external ice shell, with a variable radius ratio ranging between 0.2 and 0.7 depending on the shape and aspect ratio. Comparisons between daytime vertical profiles of aerosol backscatter coefficient (β) as measured by lidar and radar LDR reveal a decrease of β where ultragiant particles are observed. Scattering simulations based on Mie theory show how the lidar capability in typing ultragiant aerosols could be limited by low number concentrations or by the presence of an external ice shell covering the aerosol particles. Preferential vertical alignment of the particles is discussed as another possible reason for the decrease of β.

Micro pulse lidar observations of mineral dust layer in the lower troposphere over the southwest coast of Peninsular India during the Asian summer monsoon season

A large aerosol plume with optical depth exceeding 0.7 engulfs most parts of the Arabian Sea during the Asian summer monsoon season. Based on Micro Pulse Lidar observations during the June–September period of 2008 and 2009, the present study depicts, for the first time, the existence of an elevated dust layer occurring very frequently in the altitude band of 1–3.5 km over the west coast of peninsular India with relatively large values of linear depolarization ratio ( δ L ). Large values of δ L indicate the dominance of significantly non-spherical aerosols. The aerosol optical depth of this layer (0.2) is comparable to that of the entire atmospheric column during dust-free days. Back-trajectory analysis clearly shows the advection of airmass from the arid regions of Arabia and the west Arabian Sea, through the altitude region centered around 3 km. This is in contrast to the airmass below 1 km originating from the pristine Indian Ocean region which contains relatively spherical aerosols of marine origin with δ L generally <0.05.

Polarimetric Signatures from Ice Crystals Observed at 95 GHz in Winter Clouds. Part I: Dependence on Crystal Form

Abstract Based on observations made with an airborne 95-GHz polarimetric cloud radar and in situ microphysical probes, the dependence of ZDR and linear depolarization ratio (LDR) values on ice crystal type and radar beam orientation was examined. Distinct ranges of ZDR and LDR values at various radar beam orientations were identified for simple planar and columnar crystals and for melting particles. The results also show that, based on ZDR and LDR values for different beam orientations, dendritic crystals can be distinguished from simpler hexagonal and branched crystals. Polarimetric signatures are almost exclusively associated with unrimed or slightly rimed crystals, therefore the presence of such signatures can help to identify cloud regions where such crystals dominate. The data generally agrees with previously reported results, though some differences are also noted. The observed ZDR and LDR values for simple crystal types are in reasonable agreement with theoretical predictions.

Millimeter wave scattering from spatial and planar bullet rosettes

The electromagnetic scattering characteristics of several bullet-rosette ice crystal forms are computationally evaluated at 35-, 94-, and 220-GHz frequencies and compared with those of stellar crystals, hexagonal plates, and columns. One of the bullet rosettes is a planar crystal with four branches, the other two are spatial rosettes with six and eight branches. Two orientation models are used, one represents highly oriented crystals for which side and vertical incidence directions are considered, and the other represents: randomly oriented crystals (the incidence direction does not affect this case). It is observed that the linear depolarization ratio (LDR), as well as the copolarized correlation coefficient (/spl rho//sub h/spl nu//), can be used to differentiate columns from planar (including plates and stellar crystals) and spatial crystals based on their values at vertical incidence or their trends as a function of the elevation angle. For the random orientation case, LDR and /spl rho//sub h/spl nu// can differentiate columns from spatial crystals (except for sizes larger than 1.2 mm at 220 GHz) but not from planar crystals. Furthermore, the elevation angle dependence of LDR and Z/sub DR/ (differential reflectivity) has the potential for differentiating columnar, planar, and spatial crystals for sizes from a few tenths of a millimeter to 2 mm at 220 GHz, and from about 1 to 2 mm at 94 GHz. At 35 GHz, spatial crystals smaller than 2-mm resemble spherical particles in terms of their Z/sub DR/ and LDR signatures. The results for high-density (0.9 g cm/sup -3/) and low-density (representing hollow crystals) crystal models show significant differences in the values of LDR, Z/sub DR/, /spl rho//sub h/spl nu//, and the backscattering cross sections.

CSU-CHILL Polarimetric Radar Measurements from a Severe Hail Storm in Eastern Colorado

Abstract Polarimetric radar measurements made by the recently upgraded CSU-CHILL radar system in a severe hailstorm are analyzed permitting for the first time the combined use of Zh, ZDR, linear depolarization ratio (LDR), KDP, and ρhυ to infer hydrometeor types. A chase van equipped for manual collection of hail, and instrumented with a rain gauge, intercepted the storm core for 50 min. The period of golfball-sized hail is easily distinguished by high LDR (greater than or equal to −18 dB), negative ZDR (less than or equal to −0.5 dB), and low ρhυ (less than or equal to 0.93) values near the surface. Rainfall accumulation over the entire event (about 40 mm) estimated using KDP is in excellent agreement with the rain gauge measurement. Limited dual-Doppler synthesis using the CSU-CHILL and Denver WSR-88D radars permit estimates of the horizontal convergence at altitudes less than 3 km above ground level (AGL) at 1747 and 1812 mountain daylight time (MDT). Locations of peak horizontal convergence at these t...

Depolarization of radar signals due to multiple scattering in rain

The linear depolarization ratio (LDR) of radar returns from rain is studied. Pulse intensities of copolarized and cross polarized components are calculated by using the second-order solution of the time-dependent radiative transfer equation for a finite rain layer composed of spherical raindrops. Theoretical results show significant differences in LDR values between X and Ka bands for light to moderate rainfall rates and a rapid increase in the LDR reaching a level of about -8.5 dB at radar ranges near the rear edge of the rain layer. These characteristics of the LDR are in good agreement with observations from an air-borne dual-frequency, dual-polarized radar and suggest that a part of the depolarized radar power is caused by second-order multiple scattering effects.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>