An analysis of the influence of hydrometeors on the linear depolarization ratio and differential reflectivity of the radar signals

Abstract. A better quantification of aerosol properties is required for improving the modelling of aerosol effects on weather and climate. This task is methodologically demanding due to the diversity of the microphysical properties of aerosols and the complex relation between their microphysical and optical properties. Advanced lidar systems provide spatially and temporally resolved information on the aerosol optical properties that is sufficient for the retrieval of important aerosol microphysical properties. Recently, the mass concentration of transported volcanic ash, which is relevant for the flight safety of aeroplanes, was retrieved from measurements of such lidar systems in southern Germany. The relative uncertainty of the retrieved mass concentration was on the order of ±50%. The present study investigates improvements of the retrieval accuracy when the capability of measuring the linear depolarization ratio at 1064 nm is added to the lidar setup. The lidar setups under investigation are based on those of MULIS and POLIS of the Ludwig-Maximilians-Universität in Munich (Germany) which measure the linear depolarization ratio at 355 and 532 nm with high accuracy. The improvements are determined by comparing uncertainties from retrievals applied to simulated measurements of this lidar setup with uncertainties obtained when the depolarization at 1064 nm is added to this setup. The simulated measurements are based on real lidar measurements of transported Eyjafjallajökull volcano ash. It is found that additional 1064 nm depolarization measurements significantly reduce the uncertainty of the retrieved mass concentration and effective particle size. This significant improvement in accuracy is the result of the increased sensitivity of the lidar setup to larger particles. The size dependence of the depolarization does not vary strongly with refractive index, thus we expect similar benefits for the retrieval in case of measurements of other volcanic ash compositions and also for transported desert dust. For the retrieval of the single scattering albedo, which is relevant to the radiative transfer in aerosol layers, no significant improvements were found.

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Quantitative data analysis of ESAR data

Advances in Radio Science ◽

10.5194/ars-11-291-2013 ◽

2013 ◽

Vol 11 ◽

pp. 291-295

Author(s):

N. Phruksahiran ◽

M. Chandra

Keyword(s):

Ground Surface ◽

Quantitative Information ◽

Polarization Property ◽

Data Sets ◽

Program Code ◽

Quantitative Data Analysis ◽

Linear Depolarization Ratio ◽

German Aerospace ◽

Differential Reflectivity

Abstract. A synthetic aperture radar (SAR) data processing uses the backscattered electromagnetic wave to map radar reflectivity of the ground surface. The polarization property in radar remote sensing was used successfully in many applications, especially in target decomposition. This paper presents a case study to the experiments which are performed on ESAR L-Band full polarized data sets from German Aerospace Center (DLR) to demonstrate the potential of coherent target decomposition and the possibility of using the weather radar measurement parameter, such as the differential reflectivity and the linear depolarization ratio to obtain the quantitative information of the ground surface. The raw data of ESAR has been processed by the SAR simulator developed using MATLAB program code with Range-Doppler algorithm.

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Benefit of depolarization ratio at λ = 1064 nm for the retrieval of the aerosol microphysics from lidar measurements

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-7-5095-2014 ◽

2014 ◽

Vol 7 (5) ◽

pp. 5095-5115

Author(s):

J. Gasteiger ◽

V. Freudenthaler

Keyword(s):

Optical Properties ◽

Mass Concentration ◽

Single Scattering ◽

Sufficient Information ◽

Depolarization Ratio ◽

Aerosol Composition ◽

Microphysical Properties ◽

Lidar Measurements ◽

Increased Sensitivity ◽

Linear Depolarization Ratio

Abstract. A better quantification of aerosol microphysical and optical properties is required to improve the modelling of aerosol effects on weather and climate. This task is methodologically demanding due to the huge diversity of aerosol composition and of their shape and size distribution, and due to the complexity of the relation between the microphysical and optical properties. Lidar remote sensing is a valuable tool to gain spatially and temporally resolved information on aerosol properties. Advanced lidar systems provide sufficient information on the aerosol optical properties for the retrieval of important aerosol microphysical properties. Recently, the mass concentration of transported volcanic ash, which is relevant for the flight safety of airplanes, was retrieved from measurements of such lidar systems in Southern Germany. The relative uncertainty of the retrieved mass concentration was on the order of ±50%. The present study investigates improvements of the retrieval accuracy when the capability of measuring the linear depolarization ratio at 1064 nm is added to the lidar setup. The lidar setups under investigation are based on the setup of MULIS and POLIS of the LMU in Munich which measure the linear depolarization ratio at 355 nm and 532 nm with high accuracy. By comparing results of retrievals applied to simulated lidar measurements with and without the depolarization at 1064 nm it is found that the availability of 1064 nm depolarization measurements reduces the uncertainty of the retrieved mass concentration and effective particle size by a factor of about 2–3. This significant improvement in accuracy is the result of the increased sensitivity of the lidar setup to larger particles. However, the retrieval of the single scattering albedo, which is relevant for the radiative transfer in aerosol layers, does hardly benefit from the availability of 1064 nm depolarization measurements.

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Microphysical and Polarimetric Radar Modeling of Hydrometeor Refreezing

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0314.1 ◽

2021 ◽

Author(s):

Dana M. Tobin ◽

Matthew R. Kumjian

Keyword(s):

Polarimetric Radar ◽

Horizontal Polarization ◽

Radar Signature ◽

Liquid Portion ◽

Linear Depolarization Ratio ◽

Bin Microphysics ◽

Ice Shell ◽

Differential Reflectivity ◽

The Impact ◽

Polar Correlation

AbstractA unique polarimetric radar signature indicative of hydrometeor refreezing during ice pellet events has been documented in several recent studies, yet the underlying microphysical causes remain unknown. The signature is characterized by enhancements in differential reflectivity (ZDR), specific differential phase (KDP), and linear depolarization ratio (LDR), and a reduction in co-polar correlation coefficient (ρhv) within a layer of decreasing radar reflectivity factor at horizontal polarization (ZH). In previous studies, the leading hypothesis for the observed radar signature is the preferential refreezing of small drops. Here, a simplified, one-dimensional, explicit bin microphysics model is developed to simulate the refreezing of fully melted hydrometeors, and coupled with a polarimetric radar forward operator to quantify the impact of preferential refreezing on simulated radar signatures. The modeling results demonstrate that preferential refreezing is insufficient by itself to produce the observed signatures. In contrast, simulations considering an ice shell growing asymmetrically around a freezing particle (i.e., emulating a thicker ice shell on the bottom of a falling particle) produce realistic ZDR enhancements, and also closely replicate observed features in ZH, KDP, LDR, and ρhv. Simulations that assume no increase in particle wobbling with freezing produce an even greater ZDR enhancement, but this comes at the expense of reducing the LDR enhancement. It is suggested that the polarimetric refreezing signature is instead strongly related to both the distribution of the unfrozen liquid portion within a freezing particle, and the orientation of this liquid with respect to the horizontal.

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

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-17-0196.1 ◽

2018 ◽

Vol 35 (6) ◽

pp. 1169-1180 ◽

Cited By ~ 3

Author(s):

Sanja B. Manić ◽

Merhala Thurai ◽

V. N. Bringi ◽

Branislav M. Notaroš

Keyword(s):

Rain Event ◽

Size Distributions ◽

Surface Integral ◽

Azimuthal Dependence ◽

Industry Standard ◽

Standard Code ◽

Radar Measurements ◽

Linear Depolarization Ratio ◽

Drop Size Distributions ◽

Differential Reflectivity

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.

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Comparisons of Electromagnetic Scattering Properties of Real Hailstones and Spheroids

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-17-0344.1 ◽

2019 ◽

Vol 58 (1) ◽

pp. 93-112 ◽

Cited By ~ 6

Author(s):

Zhiyuan Jiang ◽

Matthew R. Kumjian ◽

Robert S. Schrom ◽

Ian Giammanco ◽

Tanya Brown-Giammanco ◽

...

Keyword(s):

Electromagnetic Scattering ◽

3D Structure ◽

Laser Scanner ◽

The United States ◽

Dipole Approximation ◽

Horizontal Polarization ◽

Differential Phase ◽

Irregular Shapes ◽

Linear Depolarization Ratio ◽

Differential Reflectivity

AbstractSevere (>2.5 cm) hail causes >$5 billion in damage annually in the United States. However, radar sizing of hail remains challenging. Typically, spheroids are used to represent hailstones in radar forward operators and to inform radar hail-sizing algorithms. However, natural hailstones can have irregular shapes and lobes; these details significantly influence the hailstone’s scattering properties. The high-resolution 3D structure of real hailstones was obtained using a laser scanner for hail collected during the 2016–17 Insurance Institute for Business and Home Safety (IBHS) Hail Field Study. Plaster casts of several record hailstones (e.g., Vivian, South Dakota, 2010) were also scanned. The S-band scattering properties of these hailstones were calculated with the discrete dipole approximation (DDA). For comparison, scattering properties of spheroidal approximations of each hailstone (with identical maximum and minimum dimensions and mass) were calculated with the T matrix. The polarimetric radar variables have errors when using spheroids, even for small hail. Spheroids generally have smaller variations in the polarimetric variables than the real hailstones. This increased variability is one reason why the correlation coefficient tends to be lower in observations than in forward-simulated cases using spheroids. Backscatter differential phase δ also is found to have large variance, particularly for large hailstones. Irregular hailstones with a thin liquid layer produce enhanced and more variable values for reflectivity factor at horizontal polarization ZHH, differential reflectivity ZDR, specific differential phase KDP, linear depolarization ratio (LDR), and δ compared with dry hailstones; is also significantly reduced.

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Particle settling and convective mixing in the Saharan Air Layer as seen from an integrated model, lidar, and in-situ perspective

10.5194/acp-2016-480 ◽

2016 ◽

Author(s):

Josef Gasteiger ◽

Silke Groß ◽

Bernadett Weinzierl ◽

Daniel Sauer ◽

Volker Freudenthaler

Keyword(s):

Dust Particles ◽

Depolarization Ratio ◽

Western Atlantic ◽

Particle Counter ◽

Convective Mixing ◽

Saharan Air Layer ◽

Lidar Measurements ◽

Linear Depolarization Ratio ◽

Aerosol Properties

Abstract. Long-range transport of aerosol in the Saharan Air Layer (SAL) across the Atlantic plays an important role for weather, climate, and ocean fertilization. However, processes occurring within the SAL and their effects on aerosol properties are still unclear. In this work we study particle settling and convective mixing within the SAL based on measured and modeled vertical aerosol profiles in the upper 1 km of the transported SAL. We use ground-based POLIS lidar measurements and airborne particle counter measurements over the Western Atlantic, as well as space-based CALIOP lidar measurements from Africa to the Western Atlantic. In our model we take account of the optical properties and the Stokes gravitational settling of irregularly-shaped Saharan dust particles. We test two hypotheses about the occurrence of convective mixing within the SAL over the Atlantic to explain the aerosol properties observed by the lidars and the particle counter. Our first hypothesis (H1) assumes that no mixing occurs in the SAL leading to an altitude separation of super-micron dust particles as a result of settling. The second hypothesis (H2) assumes that convective mixing occurs in the SAL during the day allowing large super-micron dust particles to stay airborne longer than without convective mixing. In general, a decrease of the particle linear depolarization ratio towards the SAL top is found in the measured lidar data but the decrease is much weaker than modeled in case of H1. The in-situ data on particle number concentrations show a presence of large particles near the SAL top that is inconsistent with H1. Furthermore, the analysis of the CALIOP measurements reveals that the average vertical profile of the linear depolarization ratio of the aerosols in the upper 1 km of the SAL does not change along its transport path over the Atlantic. These findings indicate H2 to be much more likely than H1, giving evidence that convective mixing occurs within the SAL over the Atlantic with significant consequences for the evolution of the size distribution of the super-micron dust particles during transport.

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The potential of elastic and polarization lidars to retrieve extinction profiles

Atmospheric Measurement Techniques ◽

10.5194/amt-13-893-2020 ◽

2020 ◽

Vol 13 (2) ◽

pp. 893-905 ◽

Cited By ~ 5

Author(s):

Elina Giannakaki ◽

Panos Kokkalis ◽

Eleni Marinou ◽

Nikolaos S. Bartsotas ◽

Vassilis Amiridis ◽

...

Keyword(s):

Aerosol Optical Depth ◽

Optical Depth ◽

Dust Particles ◽

Depolarization Ratio ◽

Lidar System ◽

Lidar Measurements ◽

Lidar Ratio ◽

Linear Depolarization Ratio ◽

Aerosol Types ◽

532 Nm

Abstract. A new method, called ElEx (elastic extinction), is proposed for the estimation of extinction coefficient lidar profiles using only the information provided by the elastic and polarization channels of a lidar system. The method is applicable to lidar measurements both during daytime and nighttime under well-defined aerosol mixtures. ElEx uses the particle backscatter profiles at 532 nm and the vertically resolved particle linear depolarization ratio measurements at the same wavelength. The particle linear depolarization ratio and the lidar ratio values of pure aerosol types are also taken from literature. The total extinction profile is then estimated and compared well with Raman retrievals. In this study, ElEx was applied in an aerosol mixture of marine and dust particles at Finokalia station during the CHARADMExp campaign. Any difference between ElEx and Raman extinction profiles indicates that the nondust component could be probably attributed to polluted marine or polluted continental aerosols. Comparison with sun photometer aerosol optical depth observations is performed as well during daytime. Differences in the total aerosol optical depth are varying between 1.2 % and 72 %, and these differences are attributed to the limited ability of the lidar to correctly represent the aerosol optical properties in the near range due to the overlap problem.

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