Observation of Tornadoes Using a Compact Polarimetric X-Band Weather Radar

Introduction

On 30 April 2019, an EF-1 tornado occurred in Denton, Texas, the United States. According to Public Information Statement issued by NWS Dallas/Fort Worth (Figure 1), the tornado touched down at midnight shortly before 00:00Z, in Denton and ran for several minutes on the ground. Tree damages at several locations showed that the tornado intensity was EF-1, and its wind speed was on the order of 85-90 mph. The radar XUNT (X-band in University of North Texas), a compact Polarimetric X-band Doppler weather radar, has been operated at the CASA Network in Texas. This radar observed and resolved the tornado and NWS mentioned that it gave them confidence to issue the tornado warning. The present study aims to clarify the characteristics of the tornado vortex and of its parent clouds.

Results of analysis

The NWS statement and the results of the radar XUNT observation are compared in Table 1. The four points (A-D) are defined along the tornado path on the ground in Figure 2. The summary of survey by NWS, are listed at each estimated times in Table 1. The maximum velocity difference (ΔVrad), vortex diameter, convergence and vortex altitude at each point observed by the radar XUNT are also listed in the table.

The temporal changes of ΔVrad and vortex diameter are shown in Figure 3. The temporal change of convergence is also shown in Figure 4. The maximum convergence is observed at 23:55Z before the tornado reaches the point C as shown in Figure 4. At the same time, the maximum value of ΔVrad, about 50 m/s, and the diameter shrinks to 400 m in Figure 3. The maximum velocity is consistent with the NWS statement.

The reflectivity, Doppler velocity and correlation coefficient, rhohv observed by the radar XUNT at 23:56Z are shown in Figure 5. The observation altitude is approximately 120 m. Therefore, the observation information shows not that for a funnel cloud but directly the tornado vortex and/or debris signal due to the tornado. The debris ball can be seen clearly in reflectivity and correlation coefficient of Figure 5. The location of the tornado estimated by the radar observation well matches with the point-C along the tornado path. According to the NEXRAD radar in Fort Worth (KFWS), the echo top is approximately 15 km and is quite deep convection, but neither meso cyclone nor hook echo is observed. This fact implies that the parent cloud of the tornado is non-supercell type.

Conclusions

The results of radar analysis on this tornado were consistent with the NWS statement. Moreover, the radar has resolved the tornado path accurately every minute. This is the reasonable evidence that our radar is a great tool for detecting and tracking tornadoes even for non-supercell tornadoes whose parent clouds don’t have mesocyclones, and then quickening the warning process of NWS.

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