Abstract

The impact of beam broadening on the quality of radar polarimetric data in the presence of nonuniform beam filling (NBF) is examined both theoretically and experimentally. Cross-beam gradients of radar reflectivity Z, differential reflectivity Z _DR, and differential phase Φ_DP within the radar resolution volume may produce significant biases of Z _DR, Φ_DP, and the cross-correlation coefficient ρ _hv. These biases increase with range as a result of progressive broadening of the radar beam. They are also larger at shorter radar wavelengths and wider antenna beams.

Simple analytical formulas are suggested for estimating the NBF-induced biases from the measured vertical and horizontal gradients of Z, Z _DR, and Φ_DP. Analysis of polarimetric data collected by the KOUN Weather Surveillance Radar-1988 Doppler (WSR-88D) demonstrates that frequently observed perturbations of the radial Φ_DP profiles and radially oriented “valleys” of ρ _hv depression can be qualitatively and quantitatively explained using the suggested NBF model.

Abstract

A simple model of the radar scattering by atmospheric particles is used to interpret all elements of the covariance scattering matrix. The components of the covariance scattering matrix and corresponding polarimetric variables are expressed via a limited number of integral parameters that characterize distributions of sizes, shapes, and orientations of meteorological scatterers.

The co–cross-polar correlation coefficients ρ _xh and ρ _xυ measured in the horizontal–vertical linear polarization basis are the major focus of this study. It is shown that the magnitudes of both coefficients are almost entirely determined by orientation of particles and do not depend on particle sizes and shapes. The phases of these coefficients can be used to detect the presence of melting hail or wet snow in the radar resolution volume.

A model of the mean canting angle of raindrops varying along a propagation path is developed to examine effects of propagation on the depolarization variables such as ρ _xh , ρ _xυ , and linear depolarization ratio. Analysis shows that depolarization variables are very sensitive to the mean canting angle averaged over a long propagation path.

Abstract

Simultaneous transmission and reception of horizontally and vertically polarized waves is a preferable choice technique for dual-polarization weather radar. One of the consequences of such a choice is possible cross-coupling between orthogonally polarized waves. Cross-coupling depends on depolarizing properties of propagation media, and it is usually negligible in rain because the net mean canting angle of raindrops is close to zero.

Snow crystals at the tops of thunderstorm clouds are often canted in the presence of strong electric fields and produce noticeable cross-coupling between radar signals at horizontal and vertical polarizations if both signals are transmitted and received simultaneously. As a result, peculiar-looking radial signatures of differential reflectivity Z _DR and differential phase Φ_DP are commonly observed in the crystal regions of thunderstorms.

The paper presents examples of strong depolarization in oriented crystals from the data collected by the polarimetric prototype of the Weather Surveillance Radar-1988 Doppler (WSR-88D) and a theoretical model that explains the results of measurements. It is shown that the sign and magnitude of the Z _DR and Φ_DP signatures strongly depend on the orientation of crystals and a system differential phase on transmission.

Abstract

In the presence of partial beam blockage (PBB), weather radar measurements can experience significant bias that directly compromises the accuracy of the hydrologic applications. Techniques for the calibration of the radar reflectivity factor Z and differential reflectivity Z _DR, measured with dual-polarization weather radars in the presence of partial beam obstruction, are examined in this paper.

The proposed Z _DR calibration technique utilizes radar measurements of Z _DR in light rain and dry aggregated snow at unblocked and blocked elevations. This calibration technique was tested for the National Severe Storms Laboratory’s (NSSL’s) Cimarron radar that suffers from PBB, and a polarimetric prototype of the Weather Surveillance Radar-1988 Doppler (WSR-88D) that does not experience PBB. Results indicate that the Z _DR bias that is associated with PBB can be calibrated with an accuracy of 0.2–0.3 dB, provided that the dataset is sufficiently large.

Calibration of Z in the presence of PBB is based on the idea of self-consistency among Z, Z _DR, and the specific differential phase K _DP in rain. The self-consistency calibration of Z from the Cimarron radar is performed following an area–time integral method. Integration is partitioned into small azimuthal sectors to assess the azimuthal modulation of the Z bias. The suggested technique is validated by direct comparisons of reflectivity factors that are measured by the Cimarron radar and the unobstructed operational WSR-88D radar. It is shown that the azimuthal modulation of Z that is caused by PBB is well captured, and the accuracy of the Z calibration is within 2–3 dB.

Abstract

Chaff contaminates estimates of precipitation amounts; hence, it is important to remove (or censor) its presence from the fields of radar reflectivity. It is demonstrated that efficient and direct identification of chaff is possible with a polarimetric radar. Specifically considered are the horizontal and vertical polarization basis and covariances of corresponding returned signals. Pertinent polarimetric variables are the copolar correlation coefficient, differential reflectivity, and the linear depolarization ratio. Two models are used to compute the expected values of these variables. In one, chaff is approximated with a Hertzian dipole and, in the other, with a thin wire antenna. In these models chaff is assumed to have a uniform distribution of flutter angles (angle between the horizontal plane and chaff axis). The two models produce nearly equivalent results. Also shown are polarimetric signatures of chaff observed in the presence of precipitation. Inferences about chaff's orientation are made from comparisons between measured and observed differential reflectivity and the cross-correlation coefficient.

Abstract

Rainfall estimation from specific differential phases in meteorological situations with significant anomalous propagation (AP) is discussed. It is shown that the correlation coefficient between horizontally and vertically polarized backscatter signals and local variability of the total differential phase can be good identifiers of ground clutter–contaminated data. Further, it is suggested how to estimate rainfall in regions of ground clutter caused by AP.

Abstract

The intrinsic uncertainty of radar-based retrievals in snow originates from a large diversity of snow growth habits, densities, and particle size distributions, all of which can make interpreting radar measurements of snow very challenging. The application of polarimetric radar for snow measurements can mitigate some of these issues. In this study, a novel polarimetric method for quantification of the extinction coefficient and visibility in snow, based on the joint use of radar reflectivity at horizontal polarization Z and specific differential phase K _DP, is introduced. A large 2D-video-disdrometer snow dataset from central Oklahoma is used to derive a polarimetric bivariate power-law relation for the extinction coefficient, ${\sigma }_{\mathit{e}}\left(K_{\mathrm{DP}},Z\right)=\gamma K_{\mathrm{DP}}^{\alpha }{Z}^{\beta }$ . The relation is derived for particle aspect ratios ranging from 0.5 to 0.8 and the width of the canting angle distribution ranging from 0° to 40°, values typical of aggregated snow, and validated via theoretical and analytical derivations/simulations. The multiplier of the relation is sensitive to variations in particles’ densities, the width of the canting angle distribution, and particles’ aspect ratios, whereas the relation’s exponents are practically invariant to changes in the latter two parameters. This novel approach is applied to polarimetric S-band WSR-88D data and verified against previous studies and in situ measurements of the extinction coefficient for four snow events in the eastern United States. The polarimetric radar estimates of the extinction coefficient exhibit smaller biases in comparison to previous studies concerning the ground measurements. The results indicate that there is good potential for reliable radar estimates of visibility from polarimetric weather radars, a parameter inversely proportional to the extinction coefficient.

Abstract

Specific differential propagation phase (K _DP) is examined for estimating convective rainfall in Colorado and Kansas. Estimates are made at S band with K _DP alone and in combination with radar reflectivity (Z _H). Results are compared to gauge observations by computing bias factors, defined as the sum of gauge-measured rainfalls divided by the sum of radar estimates at gauges reporting rainfall, and the correlation coefficient between the gauge and radar-estimated amounts. Rainfall accumulations computed from positive-only values of K _DP (provided Z _H ≥ 25 dBZ) yield bias factors that vary from 0.76 to 2.42 for 3 storms in Colorado and from 0.78 to 1.46 for 10 storms in Kansas. Correlation coefficients between gauge-observed and radar-estimated rainfalls are 0.76 to 0.95. When negative K _DP’s are included as negative rainfall rates, bias factors range from 0.81 to 3.00 in Colorado and from 0.84 to 2.31 in Kansas. In most storms, the correlation between gauge and radar rainfalls decreases slightly.

In an experiment with the K _DP/Z _H combination, rainfall rates are computed from K _DP when K _DP is ≥0.4° km⁻¹ and from Z _H for K _DP < 0.4° km⁻¹ and Z _H ≥ 25 dBZ. Neglect of the negative K _DP’s and substitution of the always positive Z _H rainfall rates result in a tendency to overestimate rainfall. Bias factors are 0.63–1.46 for Colorado storms and 0.68–0.97 for Kansas storms, and correlation coefficients between gauge and radar amounts are 0.80–0.95. In yet another test with the K _DP/Z _H pair, rainfall estimates are computed from K _DP when Z _H ≥ 40 dBZ and from Z _H when 25 ⩽ Z _H < 40 dBZ. For this experiment, bias factors range from 0.90 to 1.91 in Colorado and from 0.88 to 1.46 in Kansas. Correlation coefficients are 0.80–0.96.

Since bias factors and correlation coefficients between estimated rainfalls and gauge observations for K _DP are similar to those for radar reflectivity, there was no obvious benefit with K _DP rainfalls for a well-calibrated radar. Large underestimates with K _DP in two storms were attributed to rainfalls dominated by small drops. In one storm, the problem was aggravated by widespread negative K _DP’s thought related to vertical gradients of precipitation. An advantage of K _DP-derived rainfall estimates confirmed here is an insensitivity to anomalous propagation.

Abstract

Characteristics of the magnitude and phase of correlation coefficients between horizontally and vertically polarized returns from ground clutter echoes are quantified by analyzing histograms obtained with an 11-cm wavelength weather surveillance radar in Norman, Oklahoma. The radar receives simultaneously horizontal and vertical (SHV) electric fields and can transmit either horizontal fields or both vertical and horizontal fields. The differences between correlations obtained in this SHV mode and correlations measured in alternate H, V mode are reviewed; a histogram of differential phase obtained in Florida using alternate H, V mode is also presented. Data indicate that the backscatter differential phase of clutter has a broad histogram that completely overlaps the narrow histogram of precipitation echoes. This is important as it implies that a potent discriminator for separating clutter from meteorological echoes is the texture of the differential phase. Values of the copolar cross-correlation coefficient from clutter overlap completely those from precipitation, and effective discrimination is possible only if averages in range are taken. It is demonstrated that the total differential phase (system and backscatter) depends on the polarimetric measurement technique and the type of scatterers. In special circumstances, such as calibrating or monitoring the radar, clutter signal can be beneficial. Specifically, system differential phase can be estimated from histograms of ground clutter, receiver differential phase can be estimated from precipitation returns, and from these two, the differential phase of transmitted waves is easily computed.

Abstract

To obtain accurate radar quantitative precipitation estimation (QPE) for extreme rainfall events such as land-falling typhoon systems in complex terrain, a new method was developed for C-band polarimetric radars. The new methodology includes a correction method based on vertical profiles of the specific differential propagation phase (VPSDP) for low-level blockage and an optimal relation between rainfall rate ( ) and the specific differential phase ( ). In the VPSDP-based correction approach, a screening process is applied to fields, where missing or unreliable data from lower tilts caused by severe beam blockage are replaced with data from upper and unblocked tilts. The data from upper tilts are adjusted to account for variations in the vertical profile of . The corrected field is then used for rain-rate estimations. To acquire an accurate QPE result, a new relation for C-band polarimetric radars was derived through simulations using drop size distribution (DSD) and drop shape relation (DSR) observations from typhoon systems in Taiwan. The VPSDP-based correction method with the new relation was evaluated using the typhoon cases of Morakot (2009) and Fanapi (2010).