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M. Sachidananda and D. S. Zrnic

Abstract

In this paper a method is presented for separating overlaid echoes in a Doppler weather radar that uses a staggered pulse repetition time (PRT) transmission scheme to mitigate the effects of range–velocity ambiguities. In the standard staggered PRT technique, the PRT alternates between two values, T 1 and T 2; (T 1 < T 2) and the unambiguous range corresponds to the shorter PRT (T 1). If the weather extends up to the range corresponding to the longer of the two PRTs, some echoes from the short PRT will arrive at the receiver after the transmission of the long PRT. Therefore, echoes from the short and long PRTs that arrive at the same time but originate from range locations spaced cT 1/2 apart will be overlaid. An algorithm is developed to separate the overlaid echoes and estimate the spectral moments of both the overlaid signals. This effectively increases the unambiguous range to cT 2/2, corresponding to the longer PRT. Conditions for which the algorithm could be applied are described, and a strategy on how to use it in a range–velocity ambiguity mitigation scheme is outlined. The method of overlay resolution is tested on simulated time series. These tests illustrate the capability to separate the overlaid echoes and identify characteristics of weather signals for which the algorithm is expected to perform well.

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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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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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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 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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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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M. Sachidananda and D. S. Zrnic

Abstract

In the staggered pulse repetition time (PRT) radar, the phase difference between the autocorrelations at lag T 1 and T 2 is used for velocity estimation. This paper investigates velocity estimates from the autocorrelation at shorter lag, while the longer lag is used to resolve the ambiguity. This velocity estimate is shown to have lower error than the one using the phase difference. This method is preferred in the absence of ground clutter.

In a recent paper on the spectral processing of the staggered PRT sequences, a new algorithm was presented that enables the recovery of spectral moments in presence of ground clutter. Although this algorithm recovers velocity over the extended unambiguous interval corresponding to the difference PRT, there is a small bias error in the velocity and spectrum width estimates due to the loss of some of the signal components in the process of filtering the clutter. An enhancement in the algorithm is suggested that enables removal of this bias by reconstructing the lost spectral components before the spectral moments are estimated. The proposed algorithm completely removes the bias error in the velocity and spectral width most of the time thereby significantly improving these estimates.

The window function and the number of samples used for processing significantly influence the performance of the clutter filter. The window function effect is explored via simulation, and these results are presented.

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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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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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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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