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  • View in gallery

    RaXPol radar deployed for field operations on 11 Jun 2011 near Thomas, OK (photo courtesy of J. Snyder).

  • View in gallery

    RaXPol simplified radar-system block diagram.

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    Illustration of the pulse sequence. In standard rapid-scan mode, the radar-transmitted frequency is hopped (shifted) by at least the pulse bandwidth after each pulse pair to ensure independent sampling during the brief 4.8-ms averaging interval. To avoid second-trip echo contamination, V- and H-channel power (PV and PH, respectively) and H–V correlation coefficient (ρhv) are estimated from the first sample in each pair.

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    Enhanced Fujita scale 5 (EF-5) tornado southwest of El Reno, OK, at ~2058/59 UTC 24 May 2011, as viewed to the south from RaXPol while collecting data (photo courtesy of H. Bluestein).

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    Example of variables in a dataset collected at close range in an EF-5 tornado at 2001:34 UTC 24 May 2011 in central Oklahoma in standard rapid-scan mode. Elevation angle was 1°; range rings are marked every km. (top, left to right) Equivalent radar reflectivity factor Ze (dBZ), copolar (H–V) correlation coefficient ρhv, and differential reflectivity Zdr (dB). (middle, left to right) Folded (unedited) Doppler velocity Vf (m s−1); unfolded Doppler velocity V (m s−1); and spectrum width (m s−1); cyclonic vortex signature (red–green couplet) is seen in the unfolded velocities, along with low-Zdr and low-ρhv debris signatures. Relatively high spectrum widths (SW) are seen inside the debris signature and near the cyclonic vortex signature. (bottom)Variables color coded with larger legends. Partial beam blockage is very evident in the Ze field in the east-northeastern sector. Nyquist interval for the unedited, folded Doppler velocities is ±38.54 m s−1.

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    Example of a (top left) weak-echo hole aloft in a tornado (16° elevation angle, ~1.4 km ARL) on 24 May 2011 southwest of El Reno, OK, with equivalent radar reflectivity factor (dBZ). (top right) Unfolded Doppler velocity (m s−1); cyclonic tornado vortex signature circled. (middle) low-ρhv debris signature that is much wider than the weak-echo hole and encompasses the tornado vortex signature. Range rings are shown every 1 km. (bottom) Variables color coded with larger legends.

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    Sequence of unfolded Doppler velocity field at 1° elevation angle over a 20-s period of a developing tornado in central Oklahoma, every ~2 s, from 2058:52 to 2059:12 UTC 24 May 2011. Range rings shown every kilometer. A larger (than in each panel) Doppler velocity (m s−1) color scale shown at the bottom.

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    (top left) Midlevel BWER and (top right) curved Zdr bands, (bottom left) Arc of ρhv, and differential phase φdp (°), at 2047:08 UTC 24 May 2011, at 18° elevation angle. Range rings shown every 5 km. Arrows in the top-left panel point to regions of less attenuation and to the BWER; arrow in the top-right panel points to an enhanced Zdr arc–curved band; arrow in the bottom-left panel points to the BWER. Melting layer is evident in the relatively low ρhv ring, low φdp partial ring, and high Zdr partial ring, near the 7.5–10-km range (2.3–3 km ARL). (These rings and partial rings are not to be confused with those found around an updraft in a convective storm above the freezing level). Partial beam blockage is evident in the ρhv and φdp fields to the southwest.

  • View in gallery

    As in Fig. 7, but for H–V–copolar cross-correlation coefficient ρhv. The D in the top left and bottom right panel marks trackable debris; other two arrows indicate stationary, apparent ground targets. Low values of ρhv coincide with the debris ball seen in Z (cf. Fig. 5) and arcs of debris just to the right and below the ellipse of low ρhv.

  • View in gallery

    Relationship between the area of ρhv < 0.9 (enclosed by black outline in selected panels), thought to envelop debris, and other radar variables: (top left) equivalent radar reflectivity factor Ze (dBZ); (top right) ρhv; (middle left) Ze (dBZ); (middle right) unfolded, unedited Doppler velocity (m s−1); (bottom left) Zdr (dB); and (bottom right) SW (m s−1).

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    Strobe-mode pulse pattern. In this mode, the pulse pattern of Fig. 3 is condensed into two longer, stepped frequency pulses to eliminate the beam smearing associated with the 5-ms averaging interval. Pulse width is denoted by PW.

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    Stepped frequency subpulse segments of the strobe mode pulse pattern. Each strobe pulse is amplitude tapered using the Tukey window function with a taper coefficient of α = 0.8 (Tukey 1967; Harris 1978). Subpulse lengths were also increased to 1.1 μs to maintain a 1-μs half-power subpulse width after the amplitude tapering. Horizontally and vertically polarized pulse segments are denoted by PH and PV, respectively. Horizontally and vertically polarized pulse-pair segments are denoted by PPH and PPV, respectively.

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    Stepped frequency subpulse segments of the strobe mode pulses have 3-MHz subpulse-to-subpulse frequency steps. With 11-subpulse segments, the total transmitted pulse bandwidth is 34 MHz.

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    Strobe-mode received signal power (dBm), including the transmitted pulse (solid), one of the isolated subpulse segments (dashed), and the corresponding zero range gate (vertical dots) as a function of range gate count.

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    (left) Rapid-scan strobe-mode equivalent radar reflectivity factor and (right) Doppler velocity images obtained from a convective storm on 12 Jul 2011 in Oklahoma. Outer range ring is at 10 km.

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    Comparison of (left) strobe and (right) conventional rapid-scan mode radar reflectivity of a high-gradient region of the convective storm shown in Fig. 15. Improved angular resolution of data in (left), collected about a minute later than data in (right), is evident in the sharper reflectivity gradient (indicated by the arrows).

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A Mobile Rapid-Scanning X-band Polarimetric (RaXPol) Doppler Radar System

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  • 1 ProSensing Inc., Amherst, Massachusetts
  • | 2 School of Meteorology, University of Oklahoma, Norman, Oklahoma
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Abstract

A novel, rapid-scanning, X-band (3-cm wavelength), polarimetric (RaXPol), mobile radar was developed for severe-weather research. The radar employs a 2.4-m-diameter dual-polarized parabolic dish antenna on a high-speed pedestal capable of rotating the antenna at 180° s−1. The radar can complete a 10-elevation-step volume scan in about 20 s, while maintaining a 180-record-per-second data rate. The transmitter employs a 20-kW peak-power traveling wave tube amplifier that can generate pulse compression and frequency-hopping waveforms. Frequency hopping permits the acquisition of many more independent samples possible than without frequency hopping, making it possible to scan much more rapidly than conventional radars. Standard data products include vertically and horizontally polarized equivalent radar reflectivity factor, Doppler velocity mean and standard deviation, copolar cross-correlation coefficient, and differential phase. This paper describes the radar system and illustrates the capabilities of the radar through selected analyses of data collected in the U.S. central plains during the 2011 spring tornado season. Also noted are opportunities for experimenting with different signal-processing techniques to reduce beam smearing, increase sensitivity, and improve range resolution.

Corresponding author address: Andrew L. Pazmany, ProSensing Inc., 107 Sunderland Rd., Amherst, MA 01002. E-mail: pazmany@prosensing.com

Abstract

A novel, rapid-scanning, X-band (3-cm wavelength), polarimetric (RaXPol), mobile radar was developed for severe-weather research. The radar employs a 2.4-m-diameter dual-polarized parabolic dish antenna on a high-speed pedestal capable of rotating the antenna at 180° s−1. The radar can complete a 10-elevation-step volume scan in about 20 s, while maintaining a 180-record-per-second data rate. The transmitter employs a 20-kW peak-power traveling wave tube amplifier that can generate pulse compression and frequency-hopping waveforms. Frequency hopping permits the acquisition of many more independent samples possible than without frequency hopping, making it possible to scan much more rapidly than conventional radars. Standard data products include vertically and horizontally polarized equivalent radar reflectivity factor, Doppler velocity mean and standard deviation, copolar cross-correlation coefficient, and differential phase. This paper describes the radar system and illustrates the capabilities of the radar through selected analyses of data collected in the U.S. central plains during the 2011 spring tornado season. Also noted are opportunities for experimenting with different signal-processing techniques to reduce beam smearing, increase sensitivity, and improve range resolution.

Corresponding author address: Andrew L. Pazmany, ProSensing Inc., 107 Sunderland Rd., Amherst, MA 01002. E-mail: pazmany@prosensing.com

1. Introduction

The need for rapidly scanning1 weather radars for observing fast-changing weather phenomena such as convective storms, microbursts, small-scale features in hurricanes, and the process of convective development has been well established (Keeler and Frush 1983; Bluestein et al. 2001; Heinselman et al. 2012). Finescale precipitation features in severe convective storms, for example, can evolve on time scales of only tens of seconds or less, while conventional and research weather radars typically take 15–60 s to complete a single plan position indicator (PPI) scan. Volume scans made up of a number of PPIs at different elevation angles usually require several minutes or more to complete. Very fast-scanning military radars with electronically scanned antenna arrays have been operating for decades, but the migration of this technology to weather radars has been slow and difficult, primarily because of cost and the more demanding nature of observing meteorological targets than point targets. Fast-scanning meteorological radars have been previously developed but do not have polarimetric capabilities. Wurman and Randall (2001) developed a frequency-scanned X-band (3-cm wavelength) mobile radar, with a 2.4-m-diameter slotted waveguide array antenna, having a scan rate of 36° s−1 specifically for the study of tornadic storms. An X-band, phase/frequency electronically scanned U.S. Army air-defense radar with a 1.5 m × 1.8 m antenna scanning at 180° s−1 was recently converted for weather observations (Bluestein et al. 2010; French et al. 2013). Also, a U.S. Navy Spy-1 S-band phased array radar was recently acquired by the National Severe Storms Laboratory in Norman, Oklahoma, in order to develop adaptive beam-forming techniques for weather observations (Zrnic, D. S. et al. 2007; Weadon et al. 2009). The testing of a mobile, X-band, rapid imaging radar (Isom et al. 2009), the atmospheric imaging radar (AIR), is in progress.

Polarimetric Doppler radars have been used to study microphysical processes and to identify tornado-debris signatures (TDS) using fixed-site S-band (10-cm wavelength) and C-band (5-cm wavelength) radars (e.g., Ryzhkov et al. 2005; Kumjian and Ryzhkov 2008); but these radars are not rapid-scan radars. Mobile, polarimetric Doppler radars operating at X band have been used for the same purposes (e.g., Bluestein et al. 2007a; Tanamachi et al. 2012; Snyder et al. 2010, 2013; Wurman et al. 2012), but, again, these radars are not rapid-scan radars.

Electronic-scanning (via phase or frequency shifts) dual-polarization radars, on the other hand, are still in the early research phase (Zhang et al. 2011; Salazar et al. 2010; Orzel et al. 2011; Palumbo et al. 2012) and have not yet been deployed in fielded systems. However, dual-polarized prime-focus parabolic dish antennas with high gain, low sidelobes, and very good cross-polarization isolation are readily available commercially. Such a conventional polarimetric dish antenna was used to complete volume scans over a limited area (80° sector with 0.5° azimuthal spacing at 12 elevation angles) in about 70 s in a Weather Survellance Radar-1988 Doppler (WSR-88D) radar system in an experimental mode (Kumjian et al. 2010). In this paper we describe a much faster mechanically scanned, polarimetric, mobile X-band radar system that can complete a 360° azimuth scan in less than 2 s and a 10-elevation-step volume scan in about 20 s while maintaining a 1° resolution in azimuth. It is not as rapidly scanning as existing rapid-scan nonpolarimetric Doppler radars such as the MWR-05XP (Bluestein et al. 2010) or the Rapid Doppler on Wheels (DOW) (Wurman and Randall 2001), but it is much more rapidly scanning than existing polarimetric Doppler radars (e.g., Bluestein et al. 2007a; Wurman et al. 2012). A radar that scans 3 times as fast as other radars, for example, can cover 3 times the number of elevation angles in a volume with the same update time.

The keys to this mechanically rapid-scanning radar are a high-speed pedestal that can rotate a 2.4-m-diameter antenna, perhaps close to the largest that can be safely operated from a mobile platform, at the fastest practical measurement speed, and frequency hopping. The selection of the operating frequency is a compromise between limiting attenuation in precipitation and achieving high angular resolution. To match or exceed the 1° beamwidth of the national network of WSR-88D weather radars, the operating frequency has to be at least 9 GHz (X band) with a 2.4-m antenna. With conventional pulsed weather radars, the antenna pedestal scan speed has to be limited to maintain a sample volume angular resolution similar to the antenna half-power beamwidth. A good compromise of only 50% degradation in angular resolution is a scan speed that moved the antenna beam one beamwidth during the integration time [i.e., unity normalized rotation rate in Doviak and Zrnic (1992, p. 196, Fig. 7.25)]. This maximum scan speed (Vped) can be approximated as
e1
where N is the number of radar pulses in the averaging interval, θ3dB is the half-power beamwidth, and 2rmax/c is the pulse repetition interval (PRI) determined by the maximum unambiguous range (rmax) and the speed of light (c). This maximum practical scan rate turns out to be about 187° s−1, with a 1° beamwidth (θ3dB) antenna, a measurement range of about 40 km and a minimum of 10 pulse pairs used to estimate equivalent radar reflectivity factor (Ze), and radial velocity (VR).
The resulting short dwell time of ~5 ms is not adequate to provide enough independent samples to estimate accurately the mean radar parameters to a useful precision. To ensure that “independent” samples are no more than 10% correlated, the sampling interval (Ts) has to be limited so [Doviak and Zrnic 1992, p. 128, Eqs. (6.11a) and (6.11b)]
e2
where σV is the velocity spectrum width and λ is the radar wavelength. So, for a velocity spectrum width of 1 m s−1, the decorrelation time is about 4 ms at X band (λ = 3 cm), yielding only one or two independent samples during a 5-ms data acquisition time. The solution to this problem is the well-established frequency-diversity technique described in Hildebrand and Moore (1990), Girardin-Gondeau et al. (1991), Doviak and Zrnic (1992, p. 180), and Sadowy et al. (1997). In rapid-scanning mode, the radar transmits a sequence of pulse pairs, each pair shifted in frequency by at least the pulse bandwidth to ensure independence. A drawback of this technique is that it does not permit ground-clutter filtering, but it has, as will be noted later, the benefit of reducing the second-trip echo contamination in the measured data.

Using this technique of combining a fast mechanical scanned polarimetric parabolic dish antenna with a frequency-agile radar system, the University of Oklahoma teamed with ProSensing Inc. to develop a polarimetric rapid-scanning mobile weather radar. The instrument was developed in 2010 and began participating in field experiments in 2011. The purpose of this paper is to document the radar system and to demonstrate its capabilities by presenting examples of high spatial and temporal resolution data collected from severe storms and tornadoes during the spring 2011 tornado season in the U.S. central plains. Although the signal-processing techniques employed (especially frequency hopping) and the individual hardware components (especially the rapidly rotating pedestal) used in the radar system are not novel, the way in which they are combined is.

2. System description

RaXPol is a high-power rapid-scanning X-band polarimetric Doppler radar system, mounted on a Ford F-550 crew-cab chassis truck (Fig. 1). The truck is equipped with a three-point leveling system, a 7-kW diesel generator, and a differential global positioning system (GPS) for monitoring position and heading. Key components of the radar system include a high-speed elevation over azimuth pedestal; a low-sidelobe 2.4-m-diameter dual-linear polarized parabolic dish antenna with a conical dish-dome; and a 20-kW peak, 200-W (1% duty) average-power traveling wave tube amplifier (TWTA).

Fig. 1.
Fig. 1.

RaXPol radar deployed for field operations on 11 Jun 2011 near Thomas, OK (photo courtesy of J. Snyder).

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

A simplified block diagram of the frequency-agile RaXPol radar transceiver section is shown in Fig. 2 and the system parameters are summarized in Table 1. A 10-MHz crystal oscillator, located in the local oscillator (LO) signal generator section, serves as the reference to all the oscillators and timing circuits of the radar. Transmission is initiated by the transmit pulse generator circuit, which can produce a user-defined radio frequency (RF) pulse with arbitrary frequency modulation and amplitude taper. The only limitations on the transmitted pulse are the 40-MHz bandwidth of the transceiver and the 40-ms maximum pulse length of the TWTA. The transmission (TX) up-converter section mixes the transmit pulse to the 9.73-GHz center transmit frequency and can hop the frequency from pulse to pulse to ensure independent radar parameter samples. The transmit pulse is amplified by the TWTA to 20-kW peak power and then split by a magic hybrid T splitter to send equal power to the vertically (V) and horizontally (H) polarized ports of the 2.4-m-diameter parabolic dish antenna. Each receiver channel contains a passive two-stage (gas and solid state) limiter, a 1-dB-noise-figure–low-noise amplifier, a bandpass filter, and a single-sideband down converter. The transmitted frequency hops are compensated by the receiver LO signal to keep the receiver output intermediate frequency (IF) centered at 90 MHz. The digital receiver samples the 90-MHz V and H received signals at a rate of 120 MHz and then uses a 30-MHz digital LO signal and a programmable low-pass filter to obtain the complex envelope (I and Q) samples. The complex series of V and H samples are decimated to a user-selectable range gate spacing from 7.5 to 75 m, before transfer to the server computer for processing.

Fig. 2.
Fig. 2.

RaXPol simplified radar-system block diagram.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

Table 1.

RaXPol key system parameters.

Table 1.

A multicore computer performs all the remaining signal processing: pulse compression, clutter filtering (applied only to nonrapid-scan data), calculation of various moments, averaging, and assimilation with auxiliary data (pedestal, GPS, inclinometers, component alarms and temperatures, etc.). The data acquisition system can transmit large blocks of processed data via a network connection to client computers for real-time display and can record the raw I/Q time series from the digital receiver or averaged data products. These available data products include V- and H-channel-measured power (for estimating Ze and Zdr), complex autocorrelation at one or two lags (Doppler velocity mean and standard deviation) and zero-lag V–H (copolar) cross-correlation coefficient (ρhv) and differential phase (φdp) from which specific differential phase (Kdp) may be computed. The data system can also compute, display, and record the power spectrum of the V and H received signals and V–H cross spectra.

3. Rapid-scan operation

a. Standard mode

In standard rapid-scan mode, the radar transmits uniformly spaced pulse pairs, or groups of staggered PRI pulse triplets, while shifting the frequency of each group by at least the pulse bandwidth to speed the convergence of the averaged radar parameters to the mean, as shown in Fig. 3. At the 180° s−1 scan speed, the antenna moves one beamwidth in 5.6 ms. The data acquisition system is configured to collect a record in 4.8 ms by averaging data from 12 pulse pairs, using a constant PRI of 200 μs. To avoid second-trip echo contamination, V- and H-channel power (PV and PH) and correlation coefficient (ρhv) are estimated from the first sample in each pair. Thus, frequency hopping also reduces second-trip echo contamination of data collected after the first pulse in each group.

Fig. 3.
Fig. 3.

Illustration of the pulse sequence. In standard rapid-scan mode, the radar-transmitted frequency is hopped (shifted) by at least the pulse bandwidth after each pulse pair to ensure independent sampling during the brief 4.8-ms averaging interval. To avoid second-trip echo contamination, V- and H-channel power (PV and PH, respectively) and H–V correlation coefficient (ρhv) are estimated from the first sample in each pair.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

In rapid-scan mode, the pedestal azimuth speed is approximately 180° s−1. In PPI scan mode, the antenna beam elevation is fixed; but in 2011in volume-scan mode, changed after every azimuth sweep. The elevation transition steps were programmed to occur toward the front of the truck, so the continuous and constant elevation-scan sections are toward the rear, where there is a clear line of sight to the horizon. In 2012, the scanning program was changed, so that the user now chooses the location of the elevation change. According to the old strategy, much of the data acquired at the lowest elevation angle occurs while the radar beam is in transition from the highest elevation angle to the lowest, and then overshoots the lowest elevation angle and has to recover upward.

b. Results from data collection in standard mode

Data were collected in rapid-scan mode in a large and powerful tornado (Fig. 4) during a major tornado outbreak on 24 May 2011. The data were of high quality and show typical patterns of equivalent radar reflectivity factor, Doppler velocity, differential reflectivity, H–V (copolar) correlation coefficient, standard deviation of Doppler velocity, and differential phase. For example, when the tornado was very intense, at 1° elevation angle [~70 m above radar level (ARL)] spiral bands of reflectivity and a “debris ball,” a region of relatively high reflectivity associated with debris rather than precipitation, are evident (Fig. 5). At higher elevation angles, a weak-echo hole (e.g., Fujita 1981; Wakimoto and Martner 1992; Wurman et al. 1996; Bluestein et al. 2003, 2004, 2007b; Wakimoto et al. 2011; Palmer et al. 2011; Tanamachi et al. 2012) is evident (Fig. 6). Weak-echo holes in tornadoes are thought to occur when debris is accelerated radially outward in response to the centrifugal force (Dowell et al. 2005). It is thought that the weak-echo hole is not found, however, right near the ground, owing to radially inward accelerations in the inertial region of the boundary layer (e.g., Wilson and Rotunno 1986; Bluestein et al. 2007b). A cyclonic Doppler-velocity signature is coincident with the debris ball, maximum (outbound) Doppler velocities of at least 116 m s−1 are found, and there is evidence that there are a few places of outbound velocities up to 125 m s−1, though the spectrum width there is as high as 5–10 m s−1 (Fig. 5). These are the second highest wind speeds ever documented in a tornado by Doppler radar; on 3 May 1999, Wurman et al. (2007) documented wind speeds of 135 m s−1 in a tornado. The so-called debris ball, a debris signature associated with randomly oriented airborne debris (e.g., Ryzhkov et al. 2005; Kumjian and Ryzhkov 2008; Palmer et al. 2011), is characterized by a region of relatively low (e.g., <0.6) ρhv (Fig. 5), moderate-to-high Ze, low Zdr (Fig. 5), and a significant rotational couplet in VR. The ρhv field, in particular, is extremely effective in discriminating nonmeteorological scatterers (e.g., debris) from meteorological scatterers.

Fig. 4.
Fig. 4.

Enhanced Fujita scale 5 (EF-5) tornado southwest of El Reno, OK, at ~2058/59 UTC 24 May 2011, as viewed to the south from RaXPol while collecting data (photo courtesy of H. Bluestein).

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

Fig. 5.
Fig. 5.

Example of variables in a dataset collected at close range in an EF-5 tornado at 2001:34 UTC 24 May 2011 in central Oklahoma in standard rapid-scan mode. Elevation angle was 1°; range rings are marked every km. (top, left to right) Equivalent radar reflectivity factor Ze (dBZ), copolar (H–V) correlation coefficient ρhv, and differential reflectivity Zdr (dB). (middle, left to right) Folded (unedited) Doppler velocity Vf (m s−1); unfolded Doppler velocity V (m s−1); and spectrum width (m s−1); cyclonic vortex signature (red–green couplet) is seen in the unfolded velocities, along with low-Zdr and low-ρhv debris signatures. Relatively high spectrum widths (SW) are seen inside the debris signature and near the cyclonic vortex signature. (bottom)Variables color coded with larger legends. Partial beam blockage is very evident in the Ze field in the east-northeastern sector. Nyquist interval for the unedited, folded Doppler velocities is ±38.54 m s−1.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

Fig. 6.
Fig. 6.

Example of a (top left) weak-echo hole aloft in a tornado (16° elevation angle, ~1.4 km ARL) on 24 May 2011 southwest of El Reno, OK, with equivalent radar reflectivity factor (dBZ). (top right) Unfolded Doppler velocity (m s−1); cyclonic tornado vortex signature circled. (middle) low-ρhv debris signature that is much wider than the weak-echo hole and encompasses the tornado vortex signature. Range rings are shown every 1 km. (bottom) Variables color coded with larger legends.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

The ability of RaXPol to document very rapid evolution in a tornado wind field is demonstrated in Fig. 7, in which the Doppler velocity field is shown every ~2 s over a 26-s period, at ~70 m ARL. The receding Doppler velocities (positive, brown increasing to red) increase from 50 to 60 m s−1 to over 80 m s−1 over this short time interval, and there appears to be good continuity in Doppler velocity in the sequence of images.

Fig. 7.
Fig. 7.

Sequence of unfolded Doppler velocity field at 1° elevation angle over a 20-s period of a developing tornado in central Oklahoma, every ~2 s, from 2058:52 to 2059:12 UTC 24 May 2011. Range rings shown every kilometer. A larger (than in each panel) Doppler velocity (m s−1) color scale shown at the bottom.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

The ability of RaXPol to document storm-scale features often seen in supercells at midlevels (e.g., 4–7 km ARL), with high spatial resolution, is seen in Fig. 8, in which a bounded weak-echo region (BWER) and Zdr bands near the right forward flank are evident. It is important to demonstrate that quality data can be obtained at midlevels because polarimetric quantities can yield information about cloud microphysical processes associated with a supercell's main updraft, and these processes can provide insight into cloud dynamics (Kumjian and Ryzhkov 2008). Smoothly spatially varying features in ρhv and φdp are also seen. In particular, evidence of the melting layer is found at the 7.5–10-km range as a partial ring of reduced values of ρhv (e.g., Brandes and Ikeda 2004; Giangrande et al. 2008) and φdp, and increased values of Zdr (only near the bottom of the melting layer) (Giangrande et al. 2008) near 2.3–3 km ARL. The φdp field otherwise varies relatively smoothly and generally increases monotonically with range. The prominence of the melting layer signature on φdp is greater at X band than it is at S band (Loney et al. 2002; Zrnić and Ryzhkov 1999), typically on account of greater backscatter differential phase present at X band. Measurements of φdp tend to be relatively noisy around the melting layer in part because of appreciable backscatter differential phase (Zrnić et al. 1993). In the regions where there is significant attenuation, it increases relatively rapidly with range [i.e., specific differential phase Kdp (not shown) is relatively large]. For comparison, the freezing level, marking the top of the melting layer, was ~4 km AGL earlier in the day (based on a sounding, not shown, at Norman, Oklahoma, on 1200 UTC 24 May 2011).

Fig. 8.
Fig. 8.

(top left) Midlevel BWER and (top right) curved Zdr bands, (bottom left) Arc of ρhv, and differential phase φdp (°), at 2047:08 UTC 24 May 2011, at 18° elevation angle. Range rings shown every 5 km. Arrows in the top-left panel point to regions of less attenuation and to the BWER; arrow in the top-right panel points to an enhanced Zdr arc–curved band; arrow in the bottom-left panel points to the BWER. Melting layer is evident in the relatively low ρhv ring, low φdp partial ring, and high Zdr partial ring, near the 7.5–10-km range (2.3–3 km ARL). (These rings and partial rings are not to be confused with those found around an updraft in a convective storm above the freezing level). Partial beam blockage is evident in the ρhv and φdp fields to the southwest.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

Finally, for the first time we are able to document the evolution of polarimetric variables in a tornado on time scale as short as 2 s. For the purposes of illustration, Fig. 9 depicts the evolution of ρhv at low elevation angle in a violent tornado at ~2-s intervals. The temporal continuity of the low-ρhv debris signature is excellent; in particular, the appendage marked “D” can be seen progressing northeastward with the parent storm and back toward the west around the tornado. Also seen are two comma-shaped low-ρhv signatures. These features cannot be easily traced in radar reflectivity or other polarimetric variables (Fig. 10).

Fig. 9.
Fig. 9.

As in Fig. 7, but for H–V–copolar cross-correlation coefficient ρhv. The D in the top left and bottom right panel marks trackable debris; other two arrows indicate stationary, apparent ground targets. Low values of ρhv coincide with the debris ball seen in Z (cf. Fig. 5) and arcs of debris just to the right and below the ellipse of low ρhv.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

Fig. 10.
Fig. 10.

Relationship between the area of ρhv < 0.9 (enclosed by black outline in selected panels), thought to envelop debris, and other radar variables: (top left) equivalent radar reflectivity factor Ze (dBZ); (top right) ρhv; (middle left) Ze (dBZ); (middle right) unfolded, unedited Doppler velocity (m s−1); (bottom left) Zdr (dB); and (bottom right) SW (m s−1).

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

c. Strobe mode

Temporal averaging, with mechanically scanned weather radars, smears the data in the scanning direction, consequently degrading the angular resolution of the radar images. This smearing can be especially significant at high scan rates unless the averaging time can be kept very short. RaXPol averages data from only 12 pulse pairs in rapid-scan mode, when using a 5-kHz pulse repetition frequency (PRF; 30-km unambiguous range), yet this processing still degrades angular resolution by almost an additional half beamwidth, to about 1.5°; the effective beamwidth is less than the sum of the actual beamwidth and the rotation rate multiplied by the integration time because the convolution between the antenna pattern and the uniform averaging interval weighs the scatterers in the middle of the averaging interval more than at the edges (Doviak and Zrnić 1992, p. 196, Fig. 7.25). Although the amount of smearing with RaXPol in rapid-scanning configuration is moderate, the loss of angular resolution becomes increasingly severe with narrower beam antennas, a higher number of samples, or even faster scan rates. Beam smearing can be essentially eliminated by use of a stepped frequency modulation (FM) strobe technique, which reduces the averaging period to the time required for a single pulse pair and thus preserves the 1° antenna beamwidth angular resolution. This technique was previously implemented for weather observation with the Electra Doppler Radar (ELDORA) (Hildebrand et al. 1996), also for the purpose of reducing beam smearing during rapid scanning.

The “strobe” technique is illustrated in Fig. 11. The idea is to combine all the pulse pairs of the averaging interval of a standard pulse pattern shown in Fig. 3 to a single strobe pulse pair (Fig. 11), such that the first strobe pulse contains all the first pulses of the standard pulse pairs and the second strobe pulse contains all the second pulses. Each strobe subpulse segment is amplitude tapered, and the subpulse frequency shift is increased to improve the isolation between the subpulses and their corresponding backscattered signal, as shown in Figs. 12 and 13. The radar receiver bandwidth and data-acquisition sampling rate also has to be increased to be able to capture all the pulse segments simultaneously. The received signal from the various subpulse segments are separated using a bank of digital filters, each tuned to a specific subpulse center frequency. The isolated subpulse returns are then processed just like standard measurements. Figure 14 illustrates the received strobe pulse signal and one of the subpulse components obtained after digital filtering.

Fig. 11.
Fig. 11.

Strobe-mode pulse pattern. In this mode, the pulse pattern of Fig. 3 is condensed into two longer, stepped frequency pulses to eliminate the beam smearing associated with the 5-ms averaging interval. Pulse width is denoted by PW.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

Fig. 12.
Fig. 12.

Stepped frequency subpulse segments of the strobe mode pulse pattern. Each strobe pulse is amplitude tapered using the Tukey window function with a taper coefficient of α = 0.8 (Tukey 1967; Harris 1978). Subpulse lengths were also increased to 1.1 μs to maintain a 1-μs half-power subpulse width after the amplitude tapering. Horizontally and vertically polarized pulse segments are denoted by PH and PV, respectively. Horizontally and vertically polarized pulse-pair segments are denoted by PPH and PPV, respectively.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

Fig. 13.
Fig. 13.

Stepped frequency subpulse segments of the strobe mode pulses have 3-MHz subpulse-to-subpulse frequency steps. With 11-subpulse segments, the total transmitted pulse bandwidth is 34 MHz.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

Fig. 14.
Fig. 14.

Strobe-mode received signal power (dBm), including the transmitted pulse (solid), one of the isolated subpulse segments (dashed), and the corresponding zero range gate (vertical dots) as a function of range gate count.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

A few drawbacks of the strobe technique are range sidelobes due to finite isolation between the subpulses, illustrated by the subpulse signal (dashed line) near the transmitted pulse in Fig. 14, and the increased minimum range due to the longer strobe pulses. With a 1.1-μs subpulse length and 11 pulse segments, this minimum range is approximately /2 = 1.8 km, where tau is the strobe pulse length. Furthermore, the radar transmitter, receiver, and data system must have much wider bandwidth to be able to receive all the frequency-spaced subpulse segments simultaneously. Finally, because of the high sample rate and additional processing required to filter the subpulse segments, real-time signal processing or display has not yet been implemented in strobe mode; instead, raw I/Q data have been recorded and postprocessed.

The strobe technique was tested with RaXPol on a small thunderstorm in Oklahoma on 12 July 2011. The antenna was rotating at close to 180° s−1, and a 1-μs, 11-subpulse strobe pattern was used to obtain eleven independent samples for the estimation of each reflectivity and Doppler velocity data point. The subpulse segments were spaced by 3 MHz, so a minimum of 34 MHz bandwidth was required for the measurement. The data system recorded raw I/Q samples at a 40-MHz rate and the data were postprocessed. An example set of radar images obtained from this dataset is shown in Fig. 15, and a comparison of strobe and beam-smeared conventional rapid-scan mode in a rain shaft with a high reflectivity gradient is shown in Fig. 16. Further documentation of the utility of strobe mode in severe convective storms is in progress and the results are forthcoming.

Fig. 15.
Fig. 15.

(left) Rapid-scan strobe-mode equivalent radar reflectivity factor and (right) Doppler velocity images obtained from a convective storm on 12 Jul 2011 in Oklahoma. Outer range ring is at 10 km.

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

Fig. 16.
Fig. 16.

Comparison of (left) strobe and (right) conventional rapid-scan mode radar reflectivity of a high-gradient region of the convective storm shown in Fig. 15. Improved angular resolution of data in (left), collected about a minute later than data in (right), is evident in the sharper reflectivity gradient (indicated by the arrows).

Citation: Journal of Atmospheric and Oceanic Technology 30, 7; 10.1175/JTECH-D-12-00166.1

4. Conclusions

Using the combination of a high-speed antenna pedestal and a frequency-agile radar system, RaXPol is capable of completing a 360° PPI scan in 2 s. Furthermore, RaXPol is a polarimetric radar that can collect Zdr, ρhv, and φdp in addition to the standard reflectivity and Doppler velocity radar parameters in rapid-scan mode. Since the delivery to the University of Oklahoma in April 2011, RaXPol has been used not only to document three tornadoes during a tornado outbreak in central Oklahoma on 24 May, but also to document a supercell in southwestern Oklahoma that produced the largest hail ever recorded in the state (in addition to a funnel cloud) on 23 May; a damaging microburst and hailstorm in central Oklahoma on 14 June when collocated with the S-band polarimetric Doppler radar KOUN; bats emerging from their cave in Texas in late June; the landfall of Hurricane Irene in North Carolina on 27 August; a tornadic supercell in southwestern Oklahoma on 18 March 2012; and other subsequent events, such as the complete life cycles of three close-range tornadoes in Kansas and Oklahoma on 25 and 29 May 2012, respectively. The radar might also be useful for studies of precipitation formation in convective storms and lightning studies, which might be benefited by high temporal resolution mapping of polarimetric variables. Detailed analyses of all the tornado data from the case shown for illustrative purposes in this paper are in progress, and the scientific findings will be reported in future papers.

RaXPol's programmable transmit waveform synthesizer and raw data recording mode are useful for testing new weather radar waveforms and signal-processing techniques. This capability was employed to begin to investigate a novel strobe mode that eliminates beam smearing by transmitting pairs of stepped FM pulses and processing the received signal using a bank of digital receivers. Pulse-compression techniques are currently also being tested to increase the range resolution of the radar so that a longer pulse can be transmitted to improve sensitivity; if the transmitted pulse is chirped and compressed in the receiver, then the original fine-range resolution can be recovered (Kurdzo et al. 2012). Thus, pulse compression may be useful when there is strong attenuation and when probing the clear-air boundary layer. The outcome of these studies is beyond the scope of this paper and will also be reported on in future papers. Results are encouraging for the future development of higher-resolution rapid-scanning radars operating at higher frequencies scanning at even higher speeds.

Acknowledgments

This work was supported by the National Science Foundation MRI Grant ATM-0821231 and Grant ATM-0934307 to the University of Oklahoma. We also thank John Meier and Bob Palmer (OU ARRC) for maintaining the radar and providing space for its storage.

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1

The definition of “rapid scanning” or “rapid scan” depends on the user and/or the phenomenon scanned. In this paper, we mean scanning at a rate of hundreds of degrees per second rather than tens of degrees per second.

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