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  • Author or Editor: A. Ryzhkov x
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J. Picca
and
A. Ryzhkov

Abstract

A comparative analysis of a supercell hailstorm using simultaneous observations with S-band and C-band polarimetric radars supported by abundant ground-truth reports is presented in this study. The storm occurred on 16 May 2010 and produced a swath of extremely damaging hail across a large portion of the Oklahoma City, Oklahoma, metro area. Hail sizes over 10 cm in diameter and hail drifts upward of 1.5 m in height were reported. Both S-band (KOUN) and C-band [University of Oklahoma Polarimetric Radar for Innovations in Meteorology and Engineering (OU-PRIME)] polarimetric radars in Norman, Oklahoma, sampled the storm at ranges less than 60 km, so that high-resolution dual-wavelength polarimetric data were obtained. At C band, this analysis mostly presents raw Z and Z DR (due to problems with differential phase resulting from an incorrect censoring threshold in the examined case) while taking into account the possibility of attenuation in the interpretation of these data. Among the issues investigated in the study are the relation of hail size measured at the surface to the polarimetric signatures at both wavelengths, the difference between polarimetric signatures at the two wavelengths of hail aloft and near the surface (where melting hail is mixed with rain), and the three-body scatter spike (TBSS) signature associated with large hail.

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A. Ryzhkov
,
M. Pinsky
,
A. Pokrovsky
, and
A. Khain

Abstract

The radar observation operator for computation of polarimetric radar variables from the output of numerical cloud models is described in its most generic form. This operator is combined with the Hebrew University of Jerusalem cloud model with spectral microphysics. The model contains 7 classes of hydrometeors and each class is represented by size distribution functions in 43 size bins. The performance of the cloud model and radar observation operator has been evaluated for the case of a hailstorm in Oklahoma on 2 February 2009. It is shown that the retrieved fields of polarimetric radar variables at C and S microwave bands are generally consistent with results of observations. The relationship between microphysical and polarimetric signatures is illustrated.

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A. Ryzhkov
and
D. S. Zrnic

Abstract

On 9 June 1993, a squall line passed over the National Severe Storms Laboratory Cimarron radar and polarimetric data of this event were recorded. The line produced heavy rain and at one time was oriented north-south, extending over the radar site. At that time intense rainfall occurred over the radar. Polarimetric radar data from this event are examined to explore the utility of polarization radar techniques for rainfall monitoring and to evaluate the rain accumulation algorithm of the National Weather Services WSR-88D radar. The Twin Lakes WSR-88D radar observed the same squall line but from a different viewing angle. An unexpectedly large attenuation was experienced by the 10-cm-wavelength radiation, leading to large errors in conventional rain estimation techniques. An independent assessment of the rain measurements is made using rain accumulation in a dense network of surface rain gauges.

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A. V. Ryzhkov
and
D. S. Zrnić

Abstract

In this paper, the fields of three radar polarimetric variables-differential reflectivity Z DR, specific differential phase K DP, and correlation coefficient between horizontally (H) and vertically (V) polarized echoes ρ hv -along with radar reflectivity Zh , are examined within two Oklahoma mesoscale convective systems (MCSs). The analysis of the whole set of polarimetric variables reveals at least three types of hydrometeor populations in the precipitation within thew MCSs. It seems to be possible to discriminate between pure liquid raindrops, drops with ice cores inside them, and mixed-phase precipitation containing rain and hail using joint analysis of all the polarimetric measurands available. Hail-bearing zones are characterized by significant reduction of Z DR and ρ hv , as well as large values of Zh . Specific differential phase K DP is usually high in these zones, and sometimes a pronounced differential phase shift upon scattering is evident.

Experimental data show that the differential phase ΦDP and its derivative K DP are reliable indicators of liquid water in heavy precipitation. A negative bias of Z DR due to differential attenuation in precipitation could be significant in this type of storm. The validity of the correction scheme for Z DR estimates based on the ΦDP evaluation proposed in earlier theoretical papers was examined. It was found that differential attenuation was underestimated at least twofold in the previous theoretical predictions.

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P. Zhang
,
D. Zrnić
, and
A. Ryzhkov

Abstract

A new method for mitigation of partial beam blockage that uses the consistency between reflectivity factor Z and specific differential phase K DP and their radial integrals in rain is presented. The immunity of differential phase ΦDP to partial beam blockage is utilized to estimate the bias of reflectivity factor caused by beam blockage. The algorithm is tested on dual-polarization radar data collected by the NCAR S-band polarimetric Dopper radar system (S-Pol) during the Southwest Monsoon Experiment/Terrain-Influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX) in June 2008 in Taiwan. Corrected reflectivity factors in the blocked sectors are compared with corresponding values deduced from a digital elevation model (DEM) to show the advantage of the suggested method in areas where obstacles such as high-rise buildings cause additional blockage that is not accounted for by DEM. The accuracy and robustness of the method is quantitatively evaluated using a series of radar volume scans obtained in three rainfall events.

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A. V. Ryzhkov
and
D. S. Zrnic

Abstract

Polarimetric signatures of snow precipitation for six Oklahoma snowstorms are examined. The available data consist of specific differential phase K DP, differential reflectivity Z DR, cross-correlation coefficient ρ hv, and radar reflectivity factor Z. These data were obtained with the 10-cm-wavelength Cimarron polarimetric weather radar. The data suggest that in pure snow the average values of K DP and Z DR do not follow a systematic trend with change of the radar reflectivity factor if Z < 35 dBZ; this is not the case in rain. Precipitation is qualified as snow if the average Z DR is less than 0.2 dB for Z < 35 dBZ. The presence of a bright band with a pronounced ρ hv minimum and Z DR maximum is a good discernible feature for discriminating between snow and rain. Thus, a localized deep minimum of the cross-correlation coefficient delineates the transition region between snow and rain in the horizontal direction if sufficiently large snowflakes are generated in the transition area. Otherwise, a sharp change of Z DR can be used to localize the position of the snow–rain line.

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D. S. Zrnić
and
A. Ryzhkov

Abstract

The following advantages of rain measurements using specific differential phase are examined: 1) immunity to beam blockage, 2) immunity to ground clutter canceling, and 3) case to isolate effects of anomalous propagation. We quantify immunity to beam blockage via examples of measurements that corroborate theoretical expectations. Comparisons of rain accumulations between radar and rain gauges are included. We also contrast beam-filling effects on rain estimates from reflectivity and specific differential phase.

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A. V. Ryzhkov
and
D. S. Zrnić

Abstract

Several polarimetric radar estimators of rain rate R and rainwater content M are examined. The accuracy of the estimators is analyzed using a gamma drop size distribution (DSD) simulation and a radar wavelength of 11 cm. The estimators that use combined measurements of specific differential phase K DP and differential reflectivity ZDR are superior to the estimators of R and M obtained from reflectivity factors Z h and Z v at orthogonal polarizations or K DP only. The standard deviation of the R(K DP, ZDR) estimate is 2–3 times less than for the best of the R(Z h , Z v ) estimators. The statistical accuracy of the M(K DP, ZDR) estimator is at least 1.5–2 times better than for the M(Z h , Z v ) estimator. Cumulative rainfalls obtained with the estimators are compared with accumulations recorded with 42 rain gauges in the Little Washita River basin. Biases, errors, and reasons for the superior performance of the R(K DP, ZDR) estimator are explained in terms of the microphysical processes in the squall line that contributed to the rainfall.

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A. V. Ryzhkov
,
D. S. Zrnic
, and
B. A. Gordon

Abstract

Two variants of a polarimetric method to determine ice water content are presented. One uses specific differential phase and differential reflectivity and the other uses specific differential phase; both quantities are for a 10-cm wavelength. Theoretical considerations indicate that these polarimetric methods are suited for pristine ice crystals. Ice water content of lightly to moderately aggregated crystals might also be estimated. Verification of the proposed method is made using in situ data collected by the T-28 instrumented aircraft. Comparison with two estimators that use the reflectivity factor suggests that the polarimetric methods are better and can quantify correctly ice water content in the range above 0.1 g m−3.

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R. J. Doviak
,
V. Bringi
,
A. Ryzhkov
,
A. Zahrai
, and
D. Zrnić

Abstract

This paper reports on the steps taken by the National Severe Storms Laboratory (NSSL) to 1) develop open system hardware to facilitate upgrades to the WSR-88D (NEXRAD) radar and 2) improve identification of the type of precipitation and its quantitative measure. An engineering evaluation is made to determine if the WSR-88D antenna assembly with minimum modification could be used in a polarimetric mode. The polarimetric characteristics and radiation patterns of a research WSR-88D are briefly discussed. Considerations for the choice of polarimetric basis and design options are described. A polarimetric scheme employing simultaneous transmission of horizontally (H) and vertically (V) polarized waves is suggested for the WSR-88D, which eliminates an expensive, high-power switch. A theoretical evaluation is performed to determine the effects that feed alignment, drop canting, and backscatter depolarization have on the measurements of polarimetric parameters made with simultaneous transmission and reception of H and V signals. Experiments with the Colorado State University–Universities of Chicago and Illinois radar are performed to compare polarimetric variables obtained with alternate and simultaneous transmissions of H, V waves. Both simultaneous reception in two receivers and alternate reception in one receiver have been used.

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