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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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D. S. Zrnić
and
N. Balakrishnan

Abstract

The reflectivity factor (Z), rainfall rate (R) relationship for weather radars that probe precipitation at low elevation angles is sensitive to polarization. It is shown how to transform a relation that is valid with one polarization (vertical, horizontal or circular) to relations that are applicable to the other two polarizations. We present errors that occur if the transformations are not applied, and an example from literature in which two seemingly different Z, R relations are equivalent, tied by the polarization transformation.

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M. Sachidananda
and
D. S. Zrnić

Abstract

Processing of simultaneous measurements of differential polarization parameters (differential reflectivity, Z DR, and differential phase shift, ϕDP) and Doppler spectral moments is discussed. It is shown how Z DR and ϕDP modulate the Doppler signal and what effects they have on the autocovariance at lag 1 if a sequence of alternately polarized fields (linear horizontal, H, and vertical, V) is transmitted. A scheme to overcome these effects is proposed and demonstrated on recorded time series data from a radar with polarization diversity.

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D. S. Zrnić
and
V. M. Melnikov

Abstract

A measurement procedure to determine transmitted differential phase between horizontally and vertically polarized radiation of a dual-polarization radar is presented. It is applicable to radars that transmit and receive simultaneously horizontally and vertically (SHV) polarized waves. The method relies solely on weather data with no instrument intrusions whatsoever. It takes data at vertical incidence while the antenna rotates in azimuth. That way, a large number of samples is collected to reduce statistical errors in estimates. The theory indicates that the transmitted differential phase appears prominently in the backscatter signals off the melting layer. That and relations between various elements of the backscattering matrix are used to derive a set of nonlinear equations whereby the differential phase on transmission is one of the unknowns. Steps for solving these equations are presented as well as a demonstration of the results on radar data. A simplified algorithm that bypasses the coupled nonlinear equations is exposed. Conditions under which the simplification can be applied are presented. These restrict the range of the transmitted differential phase for which the simplified procedure may be applied.

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M. Sachidananda
and
D. S. Zrnić

Abstract

This paper presents an analysis of the accuracy of rain rate estimates from data observed with a radar that has alternating horizontal and vertical polarization. Theoretical accuracies of rain rates from the reflectivity, the differential reflectivity and the differential propagation phase shift are considered via-a-vis the drop size distribution (DSD) variability, using a computer simulation procedure.

First measurements of the differential propagation phase shift have been provided by the National Severe Storms Laboratory's dual-polarized radar, in addition to the reflectivity and the differential reflectivity. An examination of the radar data has revealed factors that could affect the rain rate estimates to a greater extent than the often contended DSD variability in the case of differential reflectivity method. Errors caused by sidelobe contamination significantly affect the differential phase shift data, so that a large spatial scale averaging is required to obtain reasonably accurate rain rate estimates, thus limiting the spatial resolution possible.

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D. S. Zrnić
and
R. J. Doviak

Abstract

A perturbation method is applied to spheroidal oscillating drops to determine the effect of fundamental axisymmetric oscillations on the power spectra and polarimetric measurements. It is shown that vibration magnitudes that produce less than 10% change in equilibrium axial ratios are insufficient to produce measurable changes in the differential reflectivity or differential phase constant, yet they can lead to a detectable increase in sidebands of the power spectra. Larger oscillations do produce measurable effects on the polarimetric variables.

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N. Balakrishnan
and
D. S. Zrnić

Abstract

Precipitation comprising rain and hail is studied. Specifically, techniques to identify and quantify such precipitation in terms of rain and hail fall rates using dual polarized radar data, are presented. Included for consideration are Z H, the reflectivity factor for horizontal polarization, Z DR, the differential reflectivity, and K DP, the differential propagation constant. A variety of simple models of mixed-phase precipitation are first examined. Electromagnetic scattering computations are performed to simulate and study the behavior of Z H, Z DR, and K DP. It is shown that it is possible to distinguish the mixed-phase precipitation from either rain or hail by using Z H, K DP pair and also to infer the thermodynamic phase and orientation from Z H, Z DR pair. On the basis of physical principles, it is shown that K DP senses primarily liquid water in the form of raindrops even when these are mixed with hailstones. The self-consistency Of Z H, Z DR, and K DP is then exploited to estimate both the rain and hail fall rates. The ability of the methods to estimate rain and hail fall rates is demonstrated with actual radar data from two Oklahoma storms.

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N. Balakrishnan
and
D. S. Zrnic

Abstract

We examine the utility of the correlation coefficient between linear orthogonally polarized echoes for determining precipitation type and gauging hail size. Models and measurements from pure rain coincide in predicting very high correlations (0.98); similar results are obtained with pure hail. Several mechanisms could cause the lowering of correlation but the behavior of the examined data is definitely attributed to a mixture of hydrometeor types. This decrease is an indicator of hail size; it is shown theoretically that in at least two other realistic situations the correlation would decrease with hail size. For the examined case a model of hail shape and orientation during fall is able to reproduce the essential features of polarimetric measurements. It suggests, together with our data and data from other investigators, that substantial negative differential reflectivity (about −1 dB) in a region of high reflectivity factor values is caused by hailstones larger than about 2 cm in diameter.

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A. Zahrai
and
D. S. Zrnić

Abstract

The NSSL Doppler radar has been upgraded to include polarimetric measurements and remote operations. We discuss details of thew upgrades and specifics concerning real-time computations of polarimetric variables. Tests and calibrations have proven invaluable for initial quality control of the system, and those that have proven most useful are presented here. Finally, examples of data fields in the polarimetric and single polarization modes are compared to illustrate the specifics and scientific usefulness of polarimetric measurements.

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