Quantitative Evaluation of Polarimetric Estimates from Scanning Weather Radars using a Vertically Pointing Micro Rain Radar

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-20-0062.1 ◽

2020 ◽

Author(s):

Ricardo Reinoso-Rondinel ◽

Marc Schleiss

Keyword(s):

Drop Size ◽

Limited Attention ◽

Size Distributions ◽

Weather Radars ◽

X Band ◽

Microphysical Processes ◽

Drop Size Distributions ◽

Differential Reflectivity ◽

The Impact ◽

Rain Radar

AbstractConventionally, micro rain radars (MRRs) have been used as a tool to calibrate reflectivity from weather radars, estimate the relation between rainfall rate and reflectivity, and study microphysical processes in precipitation. However, limited attention has been given to the reliability of the retrieved drop size distributions DSDs from MRRs. This study sheds more light on this aspect by examining the sensitivity of retrieved DSDs to the assumptions made to map Doppler spectra into size distributions, and investigates the capability of an MRR to assess polarimetric observations from operational weather radars. For that, an MRR was installed near the Cabauw observatory in the Netherlands, between the IDRA X-band radar and the Herwijnen operational C-band radar. The measurements of the MRR from November 2018 to February 2019 were used to retrieve DSDs and simulate horizontal reflectivity Ze, differential reflectivity ZDR, and specific differential phase KDP in rain. Attention is given to the impact of aliased spectra and right-hand side truncation on the simulation of polarimetric variables. From a quantitative assessment, the correlations of Ze and ZDR between the MRR and Herwijnen radar were 0.93 and 0.70, respectively, while those between the MRR and IDRA were 0.91 and 0.69. However, Ze and ZDR from the Herwijnen radar showed slight biases of 1.07 and 0.25 dB. For IDRA, the corresponding biases were 2.67 and -0.93 dB. Our results show that MRR measurements are advantageous to inspect the calibration of scanning radars and validate polarimetric estimates in rain, provided that the DSDs are correctly retrieved and controlled for quality assurance.

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Drop Shapes, Model Comparisons, and Calculations of Polarimetric Radar Parameters in Rain

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech2051.1 ◽

2007 ◽

Vol 24 (6) ◽

pp. 1019-1032 ◽

Cited By ~ 114

Author(s):

M. Thurai ◽

G. J. Huang ◽

V. N. Bringi ◽

W. L. Randeu ◽

M. Schönhuber

Keyword(s):

Drop Size ◽

Resonance Region ◽

Oblate Spheroid ◽

Size Distributions ◽

Spherical Drop ◽

X Band ◽

Median Volume ◽

Drop Size Distributions ◽

The Impact ◽

Good Agreement

Drop shapes derived from a previously conducted artificial rain experiment using a two-dimensional video disdrometer (2DVD) are presented. The experiment involved drops falling over a distance of 80 m to achieve their terminal velocities as well as steady-state oscillations. The previous study analyzed the measured axis ratios (i.e., ratio of maximum vertical to maximum horizontal chord) as a function of equivolumetric spherical drop diameter (Deq) for over 115 000 drops ranging from 1.5 to 9 mm. In this paper, the actual contoured shapes of the drops are reported, taking into account the finite quantization limits of the instrument. The shapes were derived from the fast line-scanning cameras of the 2DVD. The drops were categorized into Deq intervals of 0.25-mm width and the smoothed contours for each drop category were superimposed on each other to obtain their most probable shapes and their variations due to drop oscillations. The most probable shapes show deviation from oblate spheroids for Deq > 4 mm, the larger drops having a more flattened base, in good agreement with the equilibrium (nonoblate) shape model of Beard and Chuang. Deviations were noted from the Beard and Chuang model shapes for diameters larger than 6 mm. However, the 2DVD measurements of the most probable contour shapes are the first to validate the Beard and Chuang model shapes for large drops, and further to demonstrate the differences from the equivalent oblate shapes. The purpose of this paper is to document the differences in radar polarization parameters and the range of error incurred when using the equivalent oblate shapes versus the most probable contoured shapes measured with the 2DVD especially for drop size distributions (DSDs) with large median volume diameters (>2 mm). The measured contours for Deq > 1.5 mm were fitted to a modified conical equation, and scattering calculations were performed to derive the complex scattering amplitudes for forward and backscatter for H and V polarizations primarily at 5.34 GHz (C band) but also at 3 GHz (S band) and 9 GHz (X band). Calculations were also made to derive the relevant dual-polarization radar parameters for measured as well as model-based drop size distributions. When comparing calculations using the contoured shapes against the equivalent oblate spheroid shapes, good agreement was obtained for cases with median volume diameter (D0) less than around 2 mm. Small systematic differences in the differential reflectivity (Zdr) values of up to 0.3 dB were seen for the larger D0 values when using the oblate shapes, which can be primarily attributed to the shape differences in the resonance region, which occurs in the 5.5–7-mm-diameter range at C band. Lesser systematic differences were present in the resonance region at X band (3–4 mm). At S band, the impact of shape differences in the polarimetric parameters were relatively minor for D0 up to 2.5 mm. Unusual DSDs with very large D0 values (>3 mm) (e.g., as can occur along the leading edge of severe convective storms or aloft due localized “big drop” zones) can accentuate the Zdr difference between the contoured shape and the oblate spheroid equivalent, especially at C band. For attenuation-correction schemes based on differential propagation phase, it appears that the equivalent oblate shape approximation is sufficient using a fit to the axis ratios from the 80-m fall experiment given in this paper. For high accuracy in developing algorithms for predicting D0 from Zdr, it is recommended that the fit to the most probable contoured shapes as given in this paper be used especially at C band.

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Comparison of dual-polarization radar-derived and disdrometer drop-size distributions with S- and X- band radars

10.32469/10355/63388 ◽

2017 ◽

Author(s):

◽

Jordan A. Wendt

Keyword(s):

Drop Size ◽

Radar Data ◽

Precipitation Event ◽

Drop Diameter ◽

Size Distributions ◽

Dual Polarization ◽

X Band ◽

The Difference ◽

Drop Size Distributions ◽

Differential Reflectivity

There have been many studies on the evaluations of drop-size distributions and the parameters that affect these distributions, however, few, if any, have directly compared the relationship between the radar-derived parameters and those parameters that are disdrometer-derived. This study focuses on many different features of thunderstorms that changes the structure of the drop-size distribution (DSD) including: Horizontal reflectivity (ZH), differential reflectivity (ZDR), median drop diameter (D0), the shape parameter of the gamma-distributed DSD ([mu]), and the slope parameter of the gamma-distributed DSD (lambda). This work compares data collected by two disdrometers (OTT PARSIVEL and the Campbell Scientific Present Weather Sensor 100) against DSD parameters derived from dual-polarization radar observations. Using the Warning Decision Support System-Integrated Information (WDSS-II), radar data was merged at 1-km resolution to account for the movement of the precipitation systems before comparing to the 10-minute disdrometer data intervals. It was found that to accurately estimate DSDs from the perspective of using a weather radar, a larger precipitation event is needed. At the beginning and end of a precipitation event the difference between the radar retrieved values of D0, [mu], and [lambda] and those sampled by the disdrometer were much greater than during the middle of the event. Throughout the majority of the cases, the radar-derived reflectivity values were consistently lower than those collected by the disdrometers.

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Quasi-Vertical Profiles—A New Way to Look at Polarimetric Radar Data

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-15-0020.1 ◽

2016 ◽

Vol 33 (3) ◽

pp. 551-562 ◽

Cited By ~ 58

Author(s):

Alexander Ryzhkov ◽

Pengfei Zhang ◽

Heather Reeves ◽

Matthew Kumjian ◽

Timo Tschallener ◽

...

Keyword(s):

Continuous Monitoring ◽

Radar Data ◽

Vertical Profiles ◽

Remote Sensors ◽

Weather Radars ◽

X Band ◽

Potential Benefits ◽

High Vertical Resolution ◽

Microphysical Processes ◽

Differential Reflectivity

AbstractA novel methodology is introduced for processing and presenting polarimetric data collected by weather surveillance radars. It involves azimuthal averaging of radar reflectivity Z, differential reflectivity ZDR, cross-correlation coefficient ρhv, and differential phase ΦDP at high antenna elevation, and presenting resulting quasi-vertical profiles (QVPs) in a height-versus-time format. Multiple examples of QVPs retrieved from the data collected by S-, C-, and X-band dual-polarization radars at elevations ranging from 6.4° to 28° illustrate advantages of the QVP technique. The benefits include an ability to examine the temporal evolution of microphysical processes governing precipitation production and to compare polarimetric data obtained from the scanning surveillance weather radars with observations made by vertically looking remote sensors, such as wind profilers, lidars, radiometers, cloud radars, and radars operating on spaceborne and airborne platforms. Continuous monitoring of the melting layer and the layer of dendritic growth with high vertical resolution, and the possible opportunity to discriminate between the processes of snow aggregation and riming, constitute other potential benefits of the suggested methodology.

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Shipborne Polarimetric Weather Radar: Impact of Ship Movement on Polarimetric Variables at C Band

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-13-00242.1 ◽

2014 ◽

Vol 31 (7) ◽

pp. 1557-1563 ◽

Cited By ~ 5

Author(s):

M. Thurai ◽

P. T. May ◽

A. Protat

Keyword(s):

Drop Size ◽

Size Distributions ◽

Correction Factors ◽

Distribution Parameters ◽

Radar Measurements ◽

Nonzero Mean ◽

Polarimetric Weather Radar ◽

Drop Size Distributions ◽

Differential Reflectivity ◽

Ship Movement

Abstract The effect of ship motion on shipborne polarimetric radar measurements is considered at C band. Calculations are carried out by (i) varying the “effective” mean canting angle and (ii) separately examining the elevation dependence. Scattering from a single oblate hydrometeor is considered at first. Equations are derived (i) to convert the measured differential reflectivity for nonzero mean canting angles to those for zero mean canting angle and (ii) to do the corresponding corrections for nonzero elevation angles. Scattering calculations are also performed using the T-matrix method with measured drop size distributions as input. Dependence on mean volume diameter is examined as well as variations of the four main polarimetric parameters. The results show that as long as the ship movement is limited to a roll of less than about 10°–15°, the effects are tolerable. Furthermore, the results from the scattering simulations have been used to provide equations for correction factors that can be applied to compensate for the “apparent” nonzero canting angles and nonzero elevation angles, so that drop size distribution parameters and rainfall rates can be estimated without any bias.

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Testing the Drop-Size Distribution-Based Separation of Stratiform and Convective Rain Using Radar and Disdrometer Data from a Mid-Latitude Coastal Region

Atmosphere ◽

10.3390/atmos12030392 ◽

2021 ◽

Vol 12 (3) ◽

pp. 392

Author(s):

Merhala Thurai ◽

Viswanathan Bringi ◽

David Wolff ◽

David Marks ◽

Charanjit Pabla

Keyword(s):

Drop Size ◽

Coastal Region ◽

Size Distributions ◽

Weighted Mean ◽

Convective Rain ◽

Microphysical Processes ◽

Drop Size Distributions ◽

Using Data ◽

Improved Technique ◽

Different Characteristics

Stratiform and convective rain are associated with different microphysical processes and generally produce drop-size distributions (DSDs) with different characteristics. Previous studies using data from (a) a tropical coastal location, (b) a mid-latitude continental location with semi-arid climate, and (c) a sub-tropical continental location, found that the two rain types could be separated in the NW–Dm space, where Dm is the mass-weighted mean diameter and NW is the normalized intercept parameter. In this paper, we investigate the same separation technique using data and observations from a mid-latitude coastal region. Three-minute DSDs from disdrometer measurements are used for the NW- versus Dm-based classification and are compared with simultaneous observations from an S-band polarimetric radar 38 km away from the disdrometer site. Specifically, RHI (range-height indicator) scans over the disdrometer were used for confirmation. Results show that there was no need to modify the separation criteria from previous studies. Three-minute DSDs from the same location were used as input to scattering calculations to derive retrieval equations for NW and Dm for the S-band radar using an improved technique and applied to the RHI scans to identify convective and stratiform rain regions. Two events are shown as illustrative examples.

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Testing the Drop-Size Distribution Based Separation of Stratiform and Convective Rain Using Radar and Disdrometer Data from a Midlatitude Coastal Region

Environmental Sciences Proceedings ◽

10.3390/ecas2020-08125 ◽

2020 ◽

Vol 4 (1) ◽

pp. 13

Author(s):

Merhala Thurai ◽

Viswanathan Bringi ◽

David Wolff ◽

David Marks ◽

Charanjit Pabla

Keyword(s):

Drop Size ◽

Coastal Region ◽

Size Distributions ◽

Weighted Mean ◽

Convective Rain ◽

Mean Diameter ◽

Microphysical Processes ◽

Drop Size Distributions ◽

Using Data ◽

Different Characteristics

Stratiform and convective rain are associated with different microphysical processes and generally produce drop-size distributions (DSDs) with different characteristics. A previous study, using data from a tropical coastal location found that the two rain types could be separated in the NW–Dm space, where Dm is the mass-weighted mean diameter and NW is the normalized intercept parameter. The separation method has also been tested using data and observations from a midlatitude continental location with semiarid climate, and a subtropical continental location. In this paper, we investigate the same separation technique using data and observations from a midlatitude coastal region. Three-minute DSDs from disdrometer measurements were used for the NW versus Dm based classification and were compared with simultaneous observations from an S-band polarimetric radar 38 km away from the disdrometer site. Specifically, range-height indicator (RHI) scans over the disdrometer were used for confirmation. The results showed that there was no need to modify the separation criteria from previous studies. Scattering calculations using the three-minute DSDs were used to derive retrieval equations for Nw and Dm for the S-band radar and applied to the RHI scans to identify convective and stratiform rain regions. Two events are shown as illustrative examples.

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Observed Bulk Hook Echo Drop Size Distribution Evolution in Supercell Tornadogenesis and Tornadogenesis Failure

Monthly Weather Review ◽

10.1175/mwr-d-20-0353.1 ◽

2021 ◽

Author(s):

Kristofer S. Tuftedal ◽

Michael M. French ◽

Darrel M. Kingfield ◽

Jeffrey C. Snyder

Keyword(s):

Drop Size ◽

Thermodynamic Characteristics ◽

Radar Data ◽

Number Concentration ◽

Size Distributions ◽

Polarimetric Radar ◽

Dual Polarization ◽

Near Surface ◽

Microphysical Processes ◽

Drop Size Distributions

AbstractThe time preceding supercell tornadogenesis and tornadogenesis “failure” has been studied extensively to identify differing attributes related to tornado production or lack thereof. Studies from the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) found that air in the rear-flank downdraft (RFD) regions of non- and weakly tornadic supercells had different near-surface thermodynamic characteristics than that in strongly tornadic supercells. Subsequently, it was proposed that microphysical processes are likely to have an impact on the resulting thermodynamics of the near-surface RFD region. One way to view proxies to microphysical features, namely drop size distributions (DSDs), is through use of polarimetric radar data. Studies from the second VORTEX used data from dual-polarization radars to provide evidence of different DSDs in the hook echoes of tornadic and non-tornadic supercells. However, radar-based studies during these projects were limited to a small number of cases preventing result generalizations. This study compiles 68 tornadic and 62 non-tornadic supercells using Weather Surveillance Radar–1988 Doppler (WSR-88D) data to analyze changes in polarimetric radar variables leading up to, and at, tornadogenesis and tornadogenesis failure. Case types generally did not show notable hook echo differences in variables between sets, but did show spatial hook echo quadrant DSD differences. Consistent with past studies, differential radar reflectivity factor (ZDR) generally decreased leading up to tornadogenesis and tornadogenesis failure; in both sets, estimated total number concentration increased during the same times. Relationships between DSDs and the near-storm environment, and implications of results for nowcasting tornadogenesis, also are discussed.

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