May 23 - July 29
Observations of a Supercell and Weak Tornado Made With a Rapid-Scan, Polarimetric Mobile Radar
Alex Lyakhov, David Bodine, and Robert Palmer
What is already known:
- There is little data of weak, brief, supercell tornadoes from rapid-scan, polarimetric radars.
- Little is known about how rapidly evolving processes—such as tornadogenesis— correlates with polarimetric signatures.
- May be possible to correlate distinct polarimetric and velocity signatures with tornado intensities.
What this study adds:
- This case study showed that the polarimetric inflow and differential reflectivity arc signatures weakened rapidly around tornado dissipation.
- Azimuthal shear decreased within the low-level mesocyclone following tornado dissipation, but did not immediately decrease within the midlevel mesocyclone.
Abstract:
A rapid-scan, X-band, polarimetric, mobile Doppler radar is used to collect horizontal reflectivity, differential reflectivity, cross correlation coefficient and radial velocity data of a supercell that produced two EF-0 tornadoes in Osage County, OK on 18 June 2011. Volume scans of the first tornado, which lasted a few minutes, were acquired every 30 s. Analysis of data reveals several common polarimetric radar signatures associated with supercells including the low-level inflow, low-level hail and differential reflectivity arc signatures. The low-level inflow and differential reflectivity arc signatures both decreased in prominence around the time of tornado dissipation. No tornadic debris signature was noted, likely owing to the fact that the tornado was too weak to loft heavy debris, suggesting it is difficult for polarimetric radars to detect weak tornadoes. A Three Body Scatter Spike was also evident in the data, suggesting the presence of large hail aloft. Doppler velocity data reveal that mid-level mesocyclone intensification is not a pre-requisite for tornadogenesis. A trend of increasing azimuthal shear with time up to tornado dissipation is observed in the lowest two elevation scans, as well as within the low and midlevel mesocyclones. Azimuthal shear decreased after tornado dissipation in the lowest two scans and the low-level mesocyclone, but not with the midlevel mesocyclone. Furthermore, an anticyclonic circulation accompanied the cyclonic mesocyclone. A hook signature in the reflectivity field was observed with the mesoanticyclone, which later morphed into a linear feature.