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Dušan S. Zrnić

This is a version of a speech presented at the 27th Conference on Radar Meteorology. Relative advantages of polarimetry are contrasted with the advantages accrued by the introduction of radar into meteorology and by the addition of Doppler measurements. A description of present interests as perceived by the author follows, and possible future trends are suggested.

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Dúsan S. Zrnić

Abstract

The concept of the polarimetric scattering matrix applicable to hydrometeors is reviewed to indicate the total number of measurands that is possible from a radar system with two orthogonal linear polarizations. It is shown how to obtain this complete set of polarimetric measurands together with Doppler spectral moments through a single receiver by proper choice of polarization in a transmit-receive sequence pair.

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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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Robert Rabin and Dusan Zrnic

Abstract

The VAD technique is applied to unevenly spaced data obtained with two nearby Doppler weather radars in the optically clear atmosphere. Assuming that the power of higher order harmonies can be neglected, a least-squares fit method obtains the zeroth and first harmonies. The VAD results are compared with a detailed dual Doppler-radar analysis of a nearby area. The divergence found by both methods is explained in terms of the synoptic weather situation. The value of single Doppler weather radar is demonstrated in determining subsynoptic vertical winds in clear air. Inherent errors are briefly discussed.

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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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Svetlana Bachmann and Dusan Zrnić

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Echoes in clear air from two types of biological scatterers mixed within the resolution volumes over a large region, observed with S-band dual polarization radar, are presented. This case occurred in the evening of 7 September 2004, at the beginning of the fall migrating season of song birds (passerines). Polarimetric spectral analyses are used for distinguishing birds and insects in multimodal spectra. Spatial continuity of spectral peaks shows clear separation of insect (wind) speeds from bird speeds. Spectral densities of polarimetric variables exhibit vastly different values at speeds corresponding to insects than from those of birds, allowing the separation of the two scatterer types. Therefore, the statistics of the intrinsic polarimetric variables computed from spectral densities are unbiased and closer to the ensemble statistics of the echo type than the ones obtained with standard processing techniques. A novel display of the spectral densities versus azimuth, termed spectral velocity–azimuth display (SVAD), is constructed for easier viewing and interpretation of the velocity field. Analyses of the SVADs reveal the mean velocities and the directions of the two types of scatterers.

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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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Dusan Zrnić and David Schvartzman

Abstract

We review cubic phase codes for mitigating ambiguities in range and velocity before introducing two specific codes. The two have periodicities of 5 and 7 samples for both the transmitted and the modulation code sequences. The short periods are suitable for generating codes of arbitrary length starting with about 15. We abbreviate the two codes with L5 and L7 and describe generation of the codes starting with kernels (i.e., minimum length sequences that repeat to generate the codes of desired lengths). The L5 modulation code produces 5 spectral replicas of the coded signal and the L7 produces 7. We apply the L7 code to a sinusoid and reveal spectra of the modulated signals from several ambiguous range intervals. Through simulation, we show application to weatherlike signals and construct examples whereby two weather signals and ground clutter are overlaid. Using theory, we define the operating region of the codes in the signal parameter space. The region covers a wide range of overlaid returned powers and spectrum widths; it is obtained from simulations involving the L codes and the SZ(8/64) code. The technique is effective in distinguishing the returns from two trip regions separated by no more than L − 2 ambiguous range intervals and reconstructing the corresponding spectral moments. The L5 and L7 codes protect from trip returns up to the fifth and seventh, making them suitable for short-wavelength (3 and 5 cm) radars as their PRTs must be relatively short to accommodate the expected spread of velocities in storms.

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Alexander Ryzhkov and Dusan Zrnic

Abstract

Effects of beamfilling nonuniformities on the differential phase and rainfall estimation that use specific differential phase are examined. Two physical models are considered: an isolated rain cell with azimuthal dimensions comparable to the beamwidth and a sharp rain boundary approximating a squall line. The model results are compared to observations. An extension of the analysis to include observations of the melting layer is made. From this analysis emerges an interpretation of the radial profile of differential phase that varies from previous explanations. The current interpretation might be favored due to its simplicity and reliance on accepted physical properties of the melting layer.

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M. Sachidananda and Dusan S. Zrnic

Abstract

A procedure to filter the ground clutter from a dual-polarized, staggered pulse repetition time (PRT) sequence and recover the complex spectral coefficients of the weather signal is presented. While magnitude spectra are sufficient for estimation of the spectral moments from staggered PRT sequences, computation of differential phase in dual-polarized radars requires recovery of the complex spectra. Herein a method is given to recover the complex spectral coefficients after the ground clutter is filtered. Under the condition of “narrow” spectra, it is possible to recover the differential phase, ΦDP, and the copolar correlation coefficient, ρ hv, accurately, in addition to the differential reflectivity, Z DR. The technique is tested on simulated time series and on actual radar data. The efficacy of the method is demonstrated on plan position indicator (PPI) plots of polarimetric variables.

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