Tornado spectral signature observed by WSR-88D

A key component of current tornado detection algorithms is a search for the presence of strong localized azimuthal shear of the radial wind. However, because the radar sample volume increases with distance from the radar, the shear signature deteriorates (becomes smoothed and difficult to identify) as the range of the tornado increases. In order to mitigate this problem, a method has been proposed to extract additional information about tornado signatures from Level I Doppler spectral data. These data have the potential of revealing flow dynamics within the radar sample volume.

In this work, we present results of a tornado detection method applied to both simulated spectral data and real spectral data. Preliminary results with simulated spectral data obtained from a Rankine combined vortex reveal significant tornado spectral signatures, which deviate from conventional Gaussian-shaped weather Doppler spectra. For our real data tests, we used one of the few weather radars have the capability of collecting time series data (Level I data), the WSR-88D (KOUN) research radar at the National Severe Storm Laboratory (NSSL) in Norman, Oklahoma. Spectra from tornado outbreaks in Central Oklahoma on May 8 and 9, 2003 are presented. Flattened and non-Gaussian spectral feature are apparent in both cases. Results suggest that a valid tornado spectrum can be used to facilitate tornado detection.