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(a) The NWRT phased-array antenna (photo, courtesy of A. Zahrai, NSSL), and the configuration of receiving apertures R 1 , R 2 for SA weather radar interferometry for (b) azimuth SAI and (c) elevation SAI.
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The theory of measuring crossbeam wind, shear, and turbulence within the radar's resolution volume V6 is described. Spaced-antenna weather radar interferometry is formulated for such measurements using phased-array weather radar. The formulation for a spaced-antenna interferometer (SAI) includes shear of the mean wind, allows turbulence to be aniso...
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... NWRT is the first phased-array weather radar operating in the 10-cm-wavelength band (the same as WSR-88Ds). The NWRT has been adapted from a monopulse antenna used for target detection and track- ing. The antenna from an AN/SPY-1A radar (Brookner 1988) and a transmitter from a WSR-88D weather ra- dar are used for the NWRT (Fig. 1a). The NWRT transmits with all of the array elements uniformly ex- cited and receives signals with tapered weighting. The antenna has three ports (i.e., a sum, azimuth difference, and elevation difference). Although the antenna's dif- ference channels were disabled when it was transferred to the National Severe Storms Laboratory in ...
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... a capability that does not exist with mechanically steered beams. The NWRT phased-array antenna allows SAI wind mea- surements without any change to the antenna hard- ware. The sum and azimuth difference signals can be used to form an azimuth SAI with two virtual receivers, one for each of the left and right halves of the antenna, as shown in Fig. 1b. Similarly, an elevation SAI can be constructed (Fig. 1c) from the sum and the elevation difference signals. A PAR in which one has access to many more elements would allow the receiving anten- nas to be overlapped so that better performance for wind measurements can be achieved in low signal-to- noise environments ( Zhang et al. ...
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... beams. The NWRT phased-array antenna allows SAI wind mea- surements without any change to the antenna hard- ware. The sum and azimuth difference signals can be used to form an azimuth SAI with two virtual receivers, one for each of the left and right halves of the antenna, as shown in Fig. 1b. Similarly, an elevation SAI can be constructed (Fig. 1c) from the sum and the elevation difference signals. A PAR in which one has access to many more elements would allow the receiving anten- nas to be overlapped so that better performance for wind measurements can be achieved in low signal-to- noise environments ( Zhang et al. ...
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... antenna patterns for the monopulse sum and difference channels are shown in Zhang et al. (2005). Here, the beam cross sections for the full and half- receiving apertures are sketched in Fig. 2. Figure 2a shows the full-aperture beam as a circle, and the beam of the azimuth half aperture is shown as an ellipse, corresponding to the azimuth SAI (Fig. 1b). The re- duced azimuth resolution is due to the reduced antenna size in the azimuthal direction. Rotating the antenna patterns in Fig. 2a by 90° leads to Fig. 2b, which shows the beam cross sections for the zenith SAI (Fig. 1c). ...
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... beam as a circle, and the beam of the azimuth half aperture is shown as an ellipse, corresponding to the azimuth SAI (Fig. 1b). The re- duced azimuth resolution is due to the reduced antenna size in the azimuthal direction. Rotating the antenna patterns in Fig. 2a by 90° leads to Fig. 2b, which shows the beam cross sections for the zenith SAI (Fig. 1c). ...
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... winds from the cross-correlation function using a cross-correlation ratio (CCR) method ( Zhang et al. 2003), or the full- correlation analysis (FCA) method (Briggs 1984). For example, the apparent azimuth baseline wind compo- nent [i.e., ay (0)] can be calculated from the cross- correlation function for signals from SAI's azimuth re- ceivers (Fig. 1b) for which z 12 0 in (13). The loga- rithm of the cross-correlation magnitudes at equal positive and negative lags leads ...
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Citations
... More recent work has considered the application of the technique to UHF boundary layer radar (Cohn et al. 2001), and to weather radar (e.g., Zhang and Doviak 2007;Venkatesh and Frasier 2013), and has revisited the theoretical formulation of the analysis. Venkatesh and Frasier (2013) provide a good brief contemporary review of SA analysis techniques. ...
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... Interest in applying this method to prole precipitation is relatively recent. To date, iterature on spaced-antenna weather radars has either been theoretical (Zhang and Doviak (2007), Zhang and Doviak (2008)) or based on numerical simulations ). Although Hardwick et al. (2005) have documented spaced-antenna observations earlier, empirical studies evaluating the precision of SA retrievals on precipitation echoes have not been reported. ...
Spaced-antenna retrievals are a potential means of higher spatial-resolution wind-eld retrievals than currently available single radar algorithms in the weather radar community. Relevant literature to date has either been theoretical (Zhang and Doviak (2007)) or based on simulations ((Venkatesh and Frasier (2013)). On a dataset with smoothly varying wind-proles, spaced-antenna (SA) measurements are compared to Doppler beam swinging (DBS) and Velocity Azimuth Display (VAD)-like variational retrievals of baseline wind. Qualitative features of the G-SZl, FSA, DBS and VAD retrievals are shown to be consistent.
... The noise has a different effect on the CCF. The H and V channel noise sequences are independent so they do not bias the CCF estimation at lag 0. Since it is widely accepted that the weather signal ACF/CCF can be well modeled by a Gaussian function [1], [2], [20], [21], [25]- [27] and the ACF/CCF at other lags is not biased by the second-trip contamination, the Gaussian model can be used to fit the ACF/CCF for first-trip weather. Consequently, the estimation of radar moments such as power (S h,v ), spectrum width (σ), correlation coefficient (ρ hv ), and differential phase (Φ DP ) can be effectively improved. ...
This study presents a new identification and mitigation scheme of second trip contamination for pulsed Doppler polarimetric weather radars with the ability of random phase coding. This scheme can be easily implemented in a magnetron radar without any hardware changes. For relatively weak contamination, identification and mitigation are based on a multilag processing method, which uses multiple lags of both the auto- and cross-correlation functions to estimate radar moments. For relatively strong contamination, instantaneous phase variations of horizontal and vertical polarization channels are combined into a simple fuzzy-logic scheme to complete the identification. Data from the C-band OU-PRIME radar are used to demonstrate the effectiveness of the proposed scheme for identification and mitigation of second-trip echoes.
... Concomitantly with its primary task, namely sky scanning, the phased array can use some of its elements to perform other secondary tasks. For instance, two or three elements inside the array can be selected to estimate the wind velocity, not only radially, but also horizontally and vertically [3], [4]. This method is particularly useful when a high-resolution spatial and temporal estimation is needed, as in the case of a hurricane or tornado. ...
... The application of the spaced-antenna method, if proven practical for weather radars, is a potential means of single radar based high angular resolution wind-vector retrieval (Zhang and Doviak (2007), Pazmany et al. (2004)). However, to arrive at designs for spaced-antenna implementation and accurately interpret observations using a spacedantenna weather radar, numerical simulations are essential. ...
... Spaced-Antenna concepts have been reviewed in several works (Briggs et al. 1950;Briggs 1984;Larsen and Röttger 1989;Doviak et al. 1996;Holloway et al. 1997;Zhang and Doviak 2007), and can be explained through the cross-correlation function of back-scattered electric fields sampled by two mono-static antenna systems A 1 and A 2 separated by a baseline ∆x. Effectively, spaced-antenna wind-estimation is based on a model for the correlation function expectation (or equivalently spectra) given by ...
... Therefore, with this implementation, the effective SA baseline is half the physical distance between the centers of the "left" and "right" receive apertures. This is similar to the scheme employed by the MAPR UHF radar (Cohn et al. , 2001 and to that described using the NWRT S-band radar (Zhang and Doviak 2007). The primary strength of the phased-array is in the ability to synthesize various SA baselines and effective aperture sizes through the programmable amplitudeweighting on transmit and receive. ...
In Engineering, complete control of the simulation environment is used to efficiently arrive at designs for subsequent implmentation and minimize trial and error. Here, a monte-carlo based phased-array weather radar simulator is developed. The results obtained are validated with theoretical predictions for the auto-and cross-spectra and the correlation function expectation. Future work includes using this simulator as a tool to better interpret observations made using the spaced-antenna system.
... The noise has a different effect on the CCF. The H and V channel noise sequences are independent so they do not bias the CCF estimation at lag 0. Since it is widely accepted that the weather signal ACF/CCF can be well modeled by a Gaussian function [1], [2], [20], [21], [25]- [27] and the ACF/CCF at other lags is not biased by the second-trip contamination, the Gaussian model can be used to fit the ACF/CCF for first-trip weather. Consequently, the estimation of radar moments such as power (S h,v ), spectrum width (σ), correlation coefficient (ρ hv ), and differential phase (Φ DP ) can be effectively improved. ...
This paper presents a new algorithm to identify and mitigate the second trip contamination for the polarmetric weather radar employing the random phase coding. The algorithm is based on an auto/cross-correlation function (ACF/CCF) multi-lag processing method and the optimal use of instantaneous phase from polarimetric channels. It is specially designed for polarimetric weather signals with a random phase. Therefore, it is convenient to be implemented in a magnetron weather radar. The data processing of C-band polarimetric weather radar OU-PRIME demonstrates the effectiveness of identification and mitigation of the second trip echo.
... Its pulse-to-pulse beam-steering capability allows accurate measurements in a shorter dwell time, yielding faster data updates than those obtained with a mechanically rotating dish (Heinselman et al. 2007; Yu et al. 2007; ZrnicétZrnicét al. 2007). The NWRT uses an antenna from the AN/SPY1-A monopulse radar of the Aegis system (Sensi 1988), which allows the study of spaced antenna interferometry (SAI) for crossbeam wind measurement (Zhang and Doviak 2007) and subvolume inhomogeneity/ object detection (Zhang and Doviak 2008). The NWRT has a unique capability to scan the beam, mechanically and/or electronically, allowing rapid sequentia weather measurements (i.e., less than 23 s between observations) of the same reflectivity field with several squinted beams, each having a different sidelobe pattern and beamwidth. ...
The phased-array radar (PAR) of the National Weather Radar Testbed (NWRT) has a unique hybrid (mechanical and electrical) azimuth scan capability, allowing weather observations with different antenna patterns. Observations show the standard deviation of the sample mean power of weather echoes received through the main lobe of a set of squinted beams is less than the clutter received via sidelobes. This then allows use of a multipattern technique to cancel sidelobe echoes from moving scatterers, echoes that cannot be filtered with a ground-clutter canceler. Although the multipattern technique was developed to cancel clutter received through sidelobes, results show clutter from objects moving within the beam can also be canceled.
... Here, we will investigate the possible causes for this overestimation, as well as a possible solution to overcome this problem. When the vertical motions in the convective boundary layer are active (Flowers et al., 1994; Zhang and Doviak, 2007) the horizontal wind estimates are biased due to the vertical velocity variability (shear) (Scipí on et al., 2009b). This bias is one of the causes of the overestimation of the horizontal velocity variances. ...
Values of turbulence kinetic energy (TKE) and TKE dis-sipation rate can be obtained from radar estimates of velocity and Doppler spectral width. However, these es-timates usually do not have sufficient temporal resolu-tion and lead to filtered results that do not allow straight-forward theoretical interpretation. A procedure is pro-posed for evaluation of velocity distribution properties from radar measurements using flow fields generated by means of numerical large eddy simulation (LES). The TKE estimates are obtained from LES turbulent velocity fields through averaging in space and, alter-natively/complementary, by averaging in time. These estimates are compared with measurements retrieved from a virtual radar, which is embedded within the LES. The optimal averaging time to obtain steady consistent statistics is investigated. Wind shears are known to cause a bias in the TKE estimates from radar measure-ments. This effect is also studied by analyzing the virtual radar data in conjunction with turbulence statistics from LES. The values of turbulence dissipation rate are esti-mated from LES data through a parameterized expres-sion that relates the dissipation rate to sub-grid TKE and turbulence length scale. Estimates of dissipation rate from the virtual radar are obtained from the turbulence contribution to the spectral width after all other contri-butions are taken into account. The dissipation rate estimates from the virtual radars, vertical and oblique beams, and from LES are compared. Findings from this study will ultimately feed into turbu-lence parameter retrieval algorithms developed for ac-tual radar systems.
... SA is another technique [Briggs and ad D. H. Shinn, 1950] that has been used to obtain wind profiles and turbulence in the boundary layer [Cohn et al., 2001]. If the wind is uniform, SA can measure the cross-beam wind, as well as the radial wind component within the radar resolution volume [Doviak and Zrni´c, 2006; Zhang and Doviak, 2007]. The Full Correlation Analysis (FCA) method was introduced by Briggs [1984] as a technique which uses both the auto-and cross-correlation functions to estimate wind. ...
... where˜u = u o + s u z o and˜v = v o + s v z o are the DBSmeasured zonal and meridional wind biased by the horizontal shear of the vertical wind in the x and y direction [Zhang and Doviak, 2007]. The solution to the set of equations (multiple beams) shows that the horizontal shear effect caused by the vertical velocity can not be separated from the horizontal wind estimates. ...
... Finally, s u , s v , and s w are the three components of the vertical velocity shear. The effects of the vertical wind w o and vertical shear s w are typically small and will be omitted in the future [Holloway et al., 1997; Zhang and Doviak, 2007]. ...
The two techniques most often used to retrieve the three-dimensional wind (zonal, meridional, and vertical) from wind profiling radars are Doppler Beam Swinging (DBS) and Spaced Antenna (SA). These well-known techniques are based on the assumption of homogene-ity across the region defined by the radar beam direc-tions. However, this assumption is not always valid due to the presence of spatial inhomogeneities in the wind field and shear. The SA method incorporates only a ver-tical beam for transmission. The backscattered signal is then received using spatially separated antennas. As a result, the SA technique relies less heavily on spatial homogeneity compared to DBS. However, SA could be more sensitive to vertical velocity variations, especially in active convective boundary layer cases. In DBS, the time needed to complete a typical DBS scan can also make temporal variability a concern, which is often on the order of a few minutes. The present study employs a combination of a virtual radar and Large-Eddy Simulations (LES) in order to evaluate different wind-profiling radar methods of esti-mating wind in the presence of horizontal shear of ver-tical velocity. Measurements from DBS are compared with those from SA. For this study, the DBS method was configured for five beam directions (each with a beam width of 9 degrees) in the four cardinal directions with a zenith angle of 15.5 degrees and a vertical beam. Dwell time for each beam was set to 30 s, producing a re-visit time of 2.5 min. The SA method was designed with three receivers: one located at the center of the LES sub-domain, the others 1 m to the zonal and meridional directions, respectively. The SA dwell time is set to 60 s. Results from both DBS and SA were compared to the "true" fields obtained directly from the LES.
... It serves to test the operational utility of phased array radars (PAR) in monitoring the weather [1] and analyzing the evolution of severe storms to provide early warning. Examples of ongoing research at the testbed include beam-multiplexing [2], simultaneous aircraft tracking and weather surveillance [3], spacedantenna interferometry (SAI) to measure crossbeam wind and to detect/locate sub-volume inhomogeneity [4], [5], monopulse processing [6], and the extraction of atmospheric refractivity from the scattering of ground targets [7]. Preliminary results of storm evolution [8] and microburst [9] comparisons between the PAR, WSR-88D, and Terminal Doppler Weather Radar are promising. ...
The national weather radar testbed (NWRT) has a 10-cm phased array radar that is used primarily for monitoring the weather. It is attractive compared to the current parabolic dish weather surveillance radar-88 Doppler (WSR-88D) because of its capability to electronically steer. When combined with the recently developed beam multiplexing technique that uses a small number of contiguous samples and clever spatial sampling, this radar can obtain very rapid update scans and is extremely advantageous in monitoring severe weather. However, the small number of contiguous samples makes filtering of the clutter signal problematic in the temporal/spectral domain and can limit the performance of this radar in clutter dominated conditions. By exploiting the spatial correlation of the auxiliary channel signals, the effect of clutter contamination can be reduced to overcome this problem. In this work, two spatial filtering techniques that use low-gain auxiliary receive channels are presented, and the effect of clutter mitigation is investigated using numerical simulations for a tornadic thunderstorm dominated by clutter return. Parameters under investigation include signal-to-noise ratio, clutter-to-signal ratio, number of time series samples, varying clutter spectral widths, and maximum weight constraints.
