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Valery Melnikov

ABSTRACT

The impacts of the differential phase of incident radar waves (ψ i) on measured differential reflectivity (Z DR), differential phase, and correlation coefficient from ice cloud particles are presented for radars employing simultaneous transmission and reception of orthogonally polarized waves (SHV radar design). The maximal values of Z DR and the differential phase upon scattering (δ) from ice particles are obtained as functions of ψ i. It is shown that SHV δ from ice particles can exceed a dozen degrees whereas the intrinsic δ is of a few hundredths of a degree. In melting layers, the δ values from particles obeying the Rayleigh scattering law can be several degrees depending on ψ i so that, to explain such δ values, an assumption of resonance scattering is not necessary. The phase δ affects the estimation of specific differential phase (K DP) in icy media and, therefore, the phase δ should be measured. The radar differential phase upon transmission ψ t is a part of ψ i and, therefore, affects the δ values. A radar capability to alter ψ i by varying ψ t could deliver additional information about scattering media.

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Valery Melnikov

Abstract

The mean axis ratio (length/width) and the degree of orientation of cloud ice particles are retrieved from radar differential reflectivity (Z DR) and the copolar correlation coefficient (ρ hv) measured with the S-band WSR-88D radar. Hardware differential phases and amplifications in the polarimetric channels affect measured Z DR and ρ hv and are taken into consideration in the retrieval procedure. The retrieval is performed for particles in shapes of hexagonal prisms, which are closer to shapes of real cloud particles than frequently used spheroids. The median retrieved axis ratio for prisms is larger than that for spheroids. The statistical 1σ retrieval errors caused by fluctuations of radar returns are about 40% in areas of signal-to-noise ratios stronger than 10 dB. The values of the degree of orientation lie in an interval from 2° to 23°, which points to significant perturbations in the orientations of ice particles most likely caused by the wind field.

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Valery M. Melnikov

Abstract

Estimators of the linear depolarization ratio (LDR) and cross-polarization correlation coefficients (ρ xh) free from noise biases are devised. The estimators are based on the 1-lag correlation functions. The 1-lag estimators can be implemented with radar with simultaneous reception of copolar and cross-polar returns. Absence of noise biases makes the 1-lag estimators useful in eliminating variations of the system gain and in observations of heavy precipitation with enhanced thermal radiation. The 1-lag estimators allow for measurements at lower signal-to-noise ratios than the conventional algorithms.

The statistical biases and standard deviations of 1-lag estimates are obtained via the perturbation analysis. It is found that both the 1-lag and conventional estimates of ρ xh experience strong statistical biases at ρ xh less than 0.3 (i.e., at low canting angles of oblate hydrometeors), and a procedure to correct for this bias is proposed.

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Valery Melnikov and Jerry M. Straka

Abstract

A novel method of retrieving the mean axis ratio (width/length) and standard deviation of orientation angles (σθ, which is called herein the intensity of fluttering) of ice cloud particles from polarimetric radar data is described. The method is based on measurements of differential reflectivity Z DR and the copolar correlation coefficient in cloud areas with Z DR > 4 dB. In three analyzed cases, the values of the retrieved axis ratio were in an interval from 0.15 to 0.4 and σθ found in an interval from 2° to 20°. The latter values indicate that the particles experienced light to moderate fluttering. Ambiguities in the retrievals because of uncertainties in the bulk ice density of the particles and possible presence of columnar crystals are considered. The retrieval method is applicable for centimeter-wavelength radars; the analyzed data were collected with the dual-polarization S-band Weather Surveillance Radar-1988 Doppler (WSR-88D).

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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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Valery M. Melnikov and Dusan S. Zrnić

Abstract

Herein are proposed novel estimators of differential reflectivity Z DR and correlation coefficient ρ hv between horizontally and vertically polarized echoes. The estimators use autocorrelations and cross correlations of the returned signals to avoid bias by omnipresent but varying white noise. These estimators are considered for implementation on the future polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D) network. On the current network the reflectivity factor is measured at signal-to-noise ratios (SNRs) as low as 2 dB and the same threshold is expected to hold for the polarimetric variables. At such low SNR and all the way up to SNR = 15 dB, the conventional estimators of differential reflectivity and the copolar correlation coefficient are prone to errors due to uncertainties in noise levels caused by instability of radar devices, thermal radiations of precipitation and the ground, and wideband radiation of electrically active clouds. Noise variations at SNR less than 15 dB can bias the estimates beyond apparatus accuracy. For brevity the authors refer to the estimators of Z DR and ρ hv free from noise bias as the “1-lag estimators” because these are derived from 1-lag correlations. The estimators are quite robust and the only weak assumption for validity is that spectral widths of signals from vertically and horizontally polarized returns are equal. This assumption is verified on radar data. Radar observations demonstrate the validity of these estimator and lower sensitivity to interference signals than the conventional algorithms.

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Valery M. Melnikov and Richard J. Doviak

Abstract

Weather radar observations of stratiform precipitation often reveal regions having very large measured Doppler spectrum widths, exceeding 7, and sometimes 10, m s−1. These widths are larger than those typically found in thunderstorms; widths larger than 4 m s−1 are associated with moderate or severe turbulence in thunderstorms. In this work, stratiform precipitation has been found to have layers of widths larger than 4 m s−1 in more than 80% of cases studied, wherein the shear of the wind on scales that are large compared to the dimensions of the radar resolution volume is the dominant contributor to spectrum width. Analyzed data show that if width ≤7 m s−1, and if the layers are not wavy or patchy, these layers have weak turbulence. On the other hand, regions having widths >4 m s−1 in patches or in wavelike structures are likely to have moderate to severe turbulence with the potential to be a hazard to safe flight. To separate the contributions to spectrum width from wind shear and turbulence and to evaluate the errors in turbulence estimates, data have been collected with elevation increments much less than a beamwidth. Despite beamwidth limitations, the small elevation increments reveal impressive structures in the fields. For example, the “cat’s eye” structure associated with Kelvin–Helmholtz waves is clearly exhibited in the fields of spectrum width observed at low-elevation angles, but not in the reflectivity or velocity fields. Reflectivity fields in stratiform precipitation are featureless compared to spectrum width fields.

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Valery M. Melnikov and Richard J. Doviak

Abstract

A new Doppler spectrum width estimator using the absolute power differences (APDs) at lag one is presented, and its performance is evaluated using simulated signals as well as those recorded from the National Severe Storms Laboratory's Research and Development WSR-88D. The APD estimate bias, its standard error of estimation, and the frequency of complex widths are compared with that obtained with other single lag estimators [e.g., the pulse-pair logarithm (PPL) estimator]. For narrow spectra and signal-to-noise ratios more than 15 dB, the APD estimator has lower bias, lower standard deviation, and a lesser number of complex width estimates than the PPL estimator. Spectrum width fields, observed when this estimate technique is applied to the logarithms of echo power from an X-band radar, reveal meteorologically significant features that are not often seen in weather radar displays in regions of relatively small spectrum widths.

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Valery M. Melnikov and Dušan S. Zrnić

Abstract

Pulse-to-pulse switching of polarizations (alternate transmission mode) is considered for polarimetric phased array radar (PAR). It is argued that the performance of the radar in terms of data quality should match or exceed the achieved standards of the Weather Surveillance Radar-1988 Doppler (WSR-88D). It turns out that the most stringent demand on the radar concerns the surveillance scan at the lowest elevations wherein the polarimetric variables are free of overlaid echoes, while ground clutter is significantly reduced. The scan uses a long pulse repetition time that has repercussion on the standard errors of the polarimetric variables and hence the choice of polarimetric mode. Herein the dwell time of this scan serves as a benchmark for comparisons of the accuracy of estimates. Because weather PAR should provide useful information at low signal-to-noise ratios (SNR) as low as those measured by the WSR-88D, the statistics of polarimetric variables, known at high SNR, is extended to low SNRs. It follows that the alternate mode would not match the performance of the simultaneous mode in the surveillance scans on the WSR-88D. Quasi-simultaneous transmission and reception of horizontally polarized and vertically polarized waves is discussed as a cost-effective alternative.

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Valery M. Melnikov and Dusan S. Zrnić

Abstract

The authors demonstrate that there are maximum measurable (saturation) spectrum widths for standard autocovariance techniques, the 0,1-lag autocovariance estimator and the 1,2-lag estimator. The maximal mean measurable spectrum widths from the two estimators depend on the number of samples and are substantially lower than the Nyquist velocity. Furthermore the maximal mean spectrum width of the 1,2-lag algorithm is approximately 2 times smaller than the maximum mean width of the 0,1-lag estimator. Simulated signals, solar noise, and weather signals are processed to verify theoretical predictions.

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