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  • Author or Editor: Dusan Zrnic x
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Alexander Ryzhkov
and
Dusan Zrnić

Abstract

This paper examines some effects of drop size distribution and shape on the rainfall-rate estimates obtained from the specific differential phase. An algorithm that uses exclusively the specific differential phase is presented, and performance of this algorithm is examined by applying it to 15 storm events in Oklahoma that include heavy and light rainfalls. Radar-derived cumulative rainfall is compared to total rain measured by gauges in a dense gauge network in Oklahoma for each of the events.

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Dusan Zrnic
and
Michael Istok

Abstract

Doppler spectra of a tornado were collected with a radar having a large unambiguous velocity range, ±91 m s−1. Thus for the first time a presentation of nonaliased spectra was possible, showing direct measurement of radial velocities. By fitting the tornado model spectrum to data, the radius of maximum winds and tornado center location are deduced. Tornado spectral signature is defined as a double peak, symmetric with respect to the mean wind spectrum. Histograms of maximum measured wind speeds (from spectrum skirts) for two tornadic storms are obtained, and the histograms of velocity difference (between the left and right spectrum skirt) suggest that smaller scale turbulence (<500 m) is principally responsible for spectrum broadness.

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Dušan S. Zrnić

Abstract

A versatile algorithm to generate weatherlike spectra of any desired shape is described, and applications are briefly discussed.

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Glenn R. Smythe
and
Dusan S. Zrnic

Abstract

A technique for tracking patterns of radial velocity and reflectivity data obtained with a single-Doppler radar is described. Application of the technique to two different scans of the same spatial region may lead to the extraction of a field of “wind” vectors with both radial and azimuthal components. This is accomplished by displacing, in range and azimuth, small volumes or “boxes” of data from the earlier scan and then correlating them with boxes of equal dimensions from the later scan. The displacements at which correlation coefficients maximize are assumed to be due to the advection of patterns existing at scales up to the “box” dimensions.

Correlation coefficients of radial velocities are shown, for the clear air cases analyzed, to be higher than those of reflectivity [dB(Z)]. “Winds” retrieved by correlating velocities and reflectivities independently are compared with each other and with winds synthesized from dual-Doppler radar data. Winds from radial velocity correlations agree better with the dual-Doppler winds than do winds from reflectivity correlations. Convective rolls spaced ∼5 km apart are revealed in the planetary boundary layer.

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R. J. Doviak
and
Dusan Zrnić

Abstract

Probert-Jones' radar equation assumes receiver bandwidth large compared to the reciprocal of the transmitted pulse width τ. The advent of coherent radars with precise transmitter frequencies allows consideration of receiver bandwidth “matched” to and sometimes smaller than τ−1 in order to enhance measurement signal-to-noise ratio.

An extension to the radar equation has been made to show explicitly the dependence of echo power on the product of transmitter pulse width and receiver bandwidth. When receiver bandwidth is less than twice τ−1, there is significant loss in echo power. This should be accounted for when estimating reflectivities.

Considerable improvement in Doppler velocity estimation can often be obtained by matching range resolution to the angular one and this has implications of practical importance when moderately sensitive dual-Doppler radars are used to map the mesoscale wind in clear air.

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Dusan S. Zrnic
and
Richard J. Doviak

Abstract

Doppler velocity spectra of a combined Rankine model vortex are computed by assuming a Gaussian antenna pattern, various vortex sizes, pulse volume depths, and reflectivity profiles. Both very narrow and very broad antenna beamwidths may produce bimodal spectra. Most often, the theoretically derived spectra exhibit a rapid power decrease for spectral components near maximum velocity which agrees with an experimental observation previously reported.

In spring 1973, NSSL's 10 cm, high-resolution Doppler radar scanned the vicinity of a large tornado that devastated Union City, Okla. Digital radar samples were recorded and Fourier-analyzed to derive power spectra for sample volumes spaced about the vortex location. Power spectra were examined for white noise type signatures that indicated vortex rotation contained within the radar sample volume. Spectra were simulated using radar and tornado cyclone parameters matched to those existing during the observations to determine spectral features for comparison with those recorded by the pulse-Doppler radar. The reflectivity throughout and around the funnel was uniform and spectra compared well. Although the precise vortex center location could not be deduced its position was consistent with tornado position determined from film documentation. In the gates containing vortex signatures spectral standard deviations were consistently maximal.

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Alexander Ryzhkov
,
Dusan Zrnić
, and
Richard Fulton

Abstract

A radar polarimetric method for areal rainfall estimation is examined. In contrast to the polarimetric algorithm based on specific differential phase K DP, the proposed method does not require rain-rate estimation from K DP inside the area of interest, but it utilizes only values of total differential phase ΦDP on the areal contour. Even if the radar reflectivity and differential phase data inside the area are corrupted by ground clutter, anomalous propagation, biological scatterers, or hail contamination, reliable areal rainfall estimate is still possible, provided that correct ΦDP estimates are available at a relatively small number of range locations in or at the periphery of the contour of this area.

This concept of areal rainfall estimation has been tested on the Little Washita River watershed area in Oklahoma that contains 42 densely located rain gauges. The areal rainfall estimates obtained from the polarimetric data collected with the 10-cm Cimarron radar are in good agreement with the gauge data, with the standard error of about 18%. This accuracy is better than that obtained with the algorithm utilizing areal averaging of pointwise estimates of K DP inside the watershed area.

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Alexander Ryzhkov
,
Dusan Zrnić
, and
David Atlas

Abstract

The following rainfall measurements are compared: 1) the reflectivity factor–rain rate or R(Z) relation, whereby the rain is estimated point by point for mapping or area integration; 2) use of a specific differential phase K DP (between vertical and horizontal polarization) in a relation with rainfall rate for point-by-point mapping and subsequent integration over areas and time; 3) use of a R(K DP) relation together with a relation between K DP and Z to derive a polarimetrically tuned or matched R(Z) relation; and 4) use of empirical relations between the rainfall volume and the time integral of the storm area in which reflectivity is larger than a selected threshold. These methods are tested on five cases—two summer-type convections, one winter convective case, and two events of stratiform rain with embedded convection. Accumulations of rain in a dense gauge network in Oklahoma are used as a standard for comparison with radar measurements. In four of the five cases the rain totals obtained from the R(K DP) relation agree very well with actual gauge accumulations. This is significantly better than the Marshall–Palmer R(Z) relation, which agrees well with gauges for only one event. Matching Z to K DP brought the R(Z) derived rain total to better agreement with gauges in three more cases.

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Petar Bukovčić
,
Alexander Ryzhkov
, and
Dusan Zrnić

Abstract

In a 2018 paper by Bukovčić et al., polarimetric bivariate power-law relations for estimating snowfall rate S and ice water content (IWC), S ( K DP , Z ) = γ K DP α Z β and IWC ( K DP , Z ) = γ 2 K DP α 2 Z β 2 , were developed utilizing 2D video disdrometer snow measurements in Oklahoma. Herein, these disdrometer-based relations are generalized for the range of particle aspect ratios from 0.5 to 0.8 and the width of the canting angle distribution from 0° to 40° and are validated via analytical/theoretical derivations and simulations. In addition, a novel S(K DP, Z dr) polarimetric relation utilizing the ratio between specific differential phase K DP and differential reflectivity Z dr, K DP / ( 1 Z dr 1 ) , is derived. Both K DP and ( 1 Z dr 1 ) are proportionally affected by the ice particles’ aspect ratio and width of the canting angle distribution; therefore, the variables’ ratio tends to be almost invariant to the changes in these parameters. The S(K DP, Z) and S(K DP, Z dr) relations are applied to the polarimetric S-band WSR-88D data obtained from three geographical locations in Virginia, Oklahoma, and Colorado, and their performance is compared with estimations from the standard S(Z) relations and ground snow measurements. The polarimetric estimates of snow accumulations from the three cases exhibit smaller bias in comparison with the S(Z), indicating good potential for more reliable radar snow measurements.

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Petar Bukovčić
,
Dušan Zrnić
, and
Guifu Zhang

Abstract

Observations and analysis of an ice–liquid phase precipitation event, collected with an S-band polarimetric KOUN radar and a two-dimensional video disdrometer (2DVD) in central Oklahoma on 20 January 2007, are presented. Using the disdrometer measurements, precipitation is classified either as ice pellets or rain/freezing rain. The disdrometer observations showed fast-falling and slow-falling particles of similar size. The vast majority (>99%) were fast falling with observed velocities close to those of raindrops with similar sizes. In contrast to the smaller particles (<1 mm in diameter), bigger ice pellets (>1.5 mm) were relatively easy to distinguish because their shapes differ from the raindrops. The ice pellets were challenging to detect by looking at conventional polarimetric radar data because of the localized and patchy nature of the ice phase and their occurrence close to the ground. Previously published findings referred to cases in which ice pellet areas were centered on the radar location and showed a ringlike structure of enhanced differential reflectivity Z DR and reduced copolar correlation coefficient ρ hv and horizontal reflectivity Z H in PPI images. In this study, a new, unconventional way of looking at polarimetric radar data is introduced: slanted vertical profiles (SVPs) at low (0°–1°) radar elevations. From the analysis of the localized and patchy structures using SVPs, the polarimetric refreezing signature, reflected in local enhancement in Z DR and reduction in Z H and ρ hv, became much more evident. Model simulations of sequential drop freezing using Marshall–Palmer DSDs along with the observations suggest that preferential freezing of small drops may be responsible for the refreezing polarimetric signature, as suggested in previous studies.

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