Vortex-SE 2016 UMass X-Pol Radar Data Stephen Frasier, William Heberling, Joe Waldinger University of Massachusetts Microwave Remote Sensing Laboratory 151 Holdsworth Way Amherst, MA 01003 (413) 545-0779 frasier@umass.edu jwaldinger@umass.edu Robin Tanamachi, Dan Dawson Department of Earth, Atmospheric, and Planetary Sciences (EAPS) Purdue University 550 Stadium Mall Drive West Lafayette, IN 47907 (765) 496-2866 rtanamachi@purdue.edu dandawson@purdue.edu Data Contact: frasier@umass.edu 1.0 Data Set Overview Polarimetric and Doppler Radar measurements from various deployment locations during 2016 VORTEX-SE IOPs (dates UTC). IOP 1: 3/14 from Courtland, AL (Courtland Airport) IOP 2: 3/24 from Tanner, AL (I-65 overpass) IOP 3: 3/31 from Florence, AL IOP 4: 4/27 from Tanner, AL (I-65 overpass) IOP 5: 4/29 from Tanner, AL (I-65 overpass) IOP 6: 4/30 from Courtland, AL (Courtland Airport) 2.0 Instrument Description The UMass X-Pol mobile radar is a dual-polarized X-band Doppler radar. It employs a magnetron transmitter and coherent-on-receive detection for Doppler measurements. H- and V-polarizations are transmitted and received simultaneously (STSR). Basic instrument parameters are as follows: Center Frequency 9.41 GHz Bandwidth 1 MHz Pulse Rates 2.4/1.6/0.51 kHz (triple PRT) Transmit Power 5 kW (per polarization) Antenna Gain 41 dB Beam width 1.2 deg Range resolution 150 m Range reporting 60 m Max/min velocity +/-38.2 m/s 3.0 Data Collection & Processing UMass X-Pol transmits a triple PRT waveform consisting of 1-microsecond pulses separated by pulse periods T_ij: T_01: 0.416 ms (2.4 kHz) T_12: 0.625 ms (1.6 kHz) T_20: 1.958 ms (511 Hz). The first two pulse intervals are used for dual-PRT velocity estimation, while the final pulse interval is used to maintain the magnetron duty cycle at 0.1%. It provides a long unambiguous range of 294 km. The time-average pulse rate is 1 kHz. Raw I and Q time series are recorded, and data are processed in blocks of 33 pulses (11 of each pulse interval). Echo Powers in each polarization (, ) and their cross- correlation () are accumulated over all pulses in each block prior to cohering for velocity estimation. From these, reflectivity factor and polarimetric products are computed. Individual pulse echoes are subsequently cohered using a phase sample from the transmitter leakage recorded by the receiver. Only the H channel is used for velocity estimation. Velocities and spectral width are estimated using pulse-pair methods described in Doviak & Zrnic. Reflectivity factor is calculated from measured SNR and from nominal system parameters. Differential refectivity is calibrated by assuming = 0 dB for low reflectivity echoes from rain. Attenuation and differential attenuation are estimated from differential phase as described in remarks below. 4.0 Data Format Data are provided in DoRaDE sweep format. Tools exist to convert these to the newer CF-Radial format (e.g. RadxConvert). Eleven data products are provided: ZH: Reflectivity Factor (dBZ) w/o attenuation correction VE: Unfolded Velocity (m/s) derived from 1.6/2.4 kHz. Folds at +/-38.2 VF: Folded Velocity (m/s) derived from 2.4 kHz alone. Folds at +/- 19.1 VW: Spectrum Width (m/s) ZD: Differential Reflectivity (dB) w/o differential attenuation correction RH: H-V Correlation Coefficient w/o SNR correction PH: H-V Differential Phase (deg) including system differential phase SH: H-Pol Raw Power (long range). See Remark 1 SV: V-Pol Raw Power (long range). See Remark 1 AH: Path-integrated Attenuation (dB). See Remark 2 AD: Path-integrated Differential Attenuation (dB). See Remark 2. 5.0 Data Remarks Remark 1: Most fields are subject to the possible presence of multiple-trip echoes. The nearest range ambiguity is at 62.5 km. The fields SH and SV are obtained from the last pulse in each triplet and have no multiple trip echoes. These fields are raw signal power in dB(arbitrary units). Signal-to-noise ratio is easily computed from these by converting to linear units (e.g. SH --> 10^(SH/10)) and applying the following equations: SNR_H = SH/NH - 1. SNR_V = SV/NV - 1. where NH and NV are the average background noise level. These may be used as a data quality filter and/or to mask spurious second trip echoes. Remark 2: The fields AH and AD are estimated from PH using a simple- minded procedure. Data are thresholded on RH > 0.8 and ZH > 5. Then, radial by radial, thresholded PH are smoothed with a 15-point (1 km) median filter, the initial phase (from the 8th filtered point) is subtracted, and the radial is again filtered. Finally, a 3x3 median filter is applied to the image to mitigate any radial-to-radial striation. From the filtered PH, the attenuation fields are obtained as follows: AH (dB) = 0.28*PH AD (dB) = 0.04*PH Remark 3: In IOPs 1 and 2, and part of 3, the local oscillator was not properly tuned such that the magnetron frequency could drift partially out of the receiver band. This resulted in a degradation of sensitivity with time and noisier velocity estimates. Corrections for sensitivity loss have been implemented based on reduction of transmitter leakage level from nominal. In some instances, a significant discrepancy exists between VF (folded) and VE (unfolded) velocity estimates. In such cases, it is recommended to rely on VF rather than VE. Remark 4: In IOPs 4-6 a receiver gain reduction of approximately 10 dB was observed in the V-pol receiver channel. V-pol sensitivity appears unaffected, just a reduction in signal level. A correction for ZD has been applied, and other products are unaffected. 6.0 References Junyent, F., 2003. "Design, development and initial field deployment of an X band polarimetric, Doppler radar," M.S.E.E. thesis, Dept. of Electrical and Computer Engineering, University of Massachusetts, Amherst, 121 pp. Snyder, J.C., H. B. Bluestein, V. Venkatesh, S. J. Frasier, 2013. "Observations of Polarimetric Signatures in Supercells by an X-Band Mobile Doppler Radar," Mon. Wea. Rev., 141, 3-29.