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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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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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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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Hiroshi Uyeda and Dusan S. Zrnić

Abstract

We have developed a procedure that detects and tracks gust fronts automatically. It does not rely on a single method but requires simultaneous operation of two related algorithms. The convergence algorithm measures radial convergence and hence only gusts propagating along radials can be readily detected. The mesocyclone-shear algorithm measures azimuthal shear and is suitable for detecting gusts parallel with radials as well as low-level vortices. Long shear lines that these algorithms detect are classified as gusts whereas symmetric shear features are rejected if their shear and flux or “momentum” are insignificant; otherwise they are classified as low-level vortices. To locate gusts we use second-order polynomials in the range-azimuth plane. It is shown that predicted gust locations from simple linear projections of the least square fitted curves agree very well with actual gust locations.

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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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Valery Melnikov and Dusan S. Zrnić

Abstract

It is shown that the NEXRAD weather radar with enhanced detectability is capable of observing the evolution of convective thermals. The fields of radar differential reflectivity show that the upper parts of the thermals are observable due to Bragg scatter, whereas scattering from insects dominates in the lower parts. The thermal-top rise rate is between 1.5 and 3.7 m s−1 in the analyzed case. Radar observations of thermals also enable estimations of their maximum heights, horizontal sizes, and the turbulent dissipation rate within each thermal. These attributes characterize the intensity of convection.

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M. Sachidananda and Dusan S. Zrnic

Abstract

This paper deals with the recovery of Doppler velocities in the presence of range overlaid echoes. Transmitted pulses are phase shifted to tag the echoes from scatterers, which are separated by the unambiguous range. A new systematic phase code and an algorithm for estimating the mean velocities of overlaid first- and second-trip signals are presented. The return samples are phase corrected to cohere the first- or the second-trip signal, leaving the other signal power spread in a deterministic manner across the Doppler spectrum. An algorithm has been developed to recover the velocity of the weaker signal even if the power ratio of overlaid signals is as large as 40 dB, for spectrum widths of 4 m s−1 or less, and an unambiguous velocity of 32 m s−1. Tests on simulated weather signals indicate that the method, employed in surveillance Doppler radars, can effectively double the unambiguous range without the sacrifice of the unambiguous velocity interval.

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Thomas B. Sanford and Dus̆an S. Zrnić
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Keith A. Brewster and Dusan S. Zrnić

Abstract

Doppler radars offer unique data from which it is possible to estimate the turbulent eddy dissipation rates, ε. If the inertial subrange extends to lengths longer than the radar resolution volume size, ε can be obtained from the Doppler spectrum width. Spatial spectra of mean Doppler velocities can also yield ε estimates but only if a significant portion of the analysis length is contained within the inertial subrange. We compare dissipation rate estimates obtained with the two independent measurement techniques. At close range and vertical incidence, agreement between the two independent estimates of ε is within 10%. Furthermore, the slope of the spatial energy densities is very close to −5/3 predicted by Kolmogorov. The energy input is mainly from buoyancy-driven updrafts and the transition wavelength (about 3 km) between the input scale and the inertial subrange is consistent with the updraft-downdraft circulation cell, which is about 10 km. For a more distant storm at a range of 60 km, the filtering of mean velocities by the resolution volume precludes precise estimation of ε from spatial spectra of mean velocities.

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John W. Conway and Dušan S. Zrnić

Abstract

The origin and importance to embryo and hail production of a region of drops advected above the freezing level in the updraft of a severe Colorado hailstorm is examined using radar polarization measurements in conjunction with dual-Doppler and trajectory analysis. These drops, which give a distinct radar signature termed the “differential reflectivity column,” originate from 1) melted hydrometeors that fall from the back-sheared anvil, through the embryo curtain, and are recirculated into the storm updraft, and 2) in situ drop growth within the updraft. Some of the drops refreeze and likely produce frozen-drop hailstone embryos.

Numerous hailstone trajectories are found to cross either through, or over, the drop column where the hailstones undergo a significant growth phase. Two separate hailstone fallout regions are identified. Some hailstones in the northern fallout region show anticyclonic trajectories and in situ updraft and column growth. Others grow while crossing the top of the vault. Hailstones in the southern region exhibit growth while passing cyclonically through the column or over the vault.

A new method to determine hydrometeor fall speeds from radar polarization measurements for use with dual-Doppler analysis is introduced.

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