Range correction for radar-derived azimuthal shear: applications to a tornado detection algorithm

The current Tornado Detection Algorithm (TDA) used with the Weather Surveillance Radar- 1988 Doppler network utilizes an input velocity field that is often noisy and subject to de-aliasing errors. The current TDA also relies on azimuthal shear calculations, which are affected by noisy velocity data and can degrade significantly with range. Because of these and other data accuracy issues, the current TDA is prone to producing false detections and inaccurate circulation tracks.

Coincident with the advent of new radar-derived products and ongoing research involving new weather radar systems (e.g., Phased Array Radar), the National Severe Storms Laboratory is developing an improved TDA. A primary component of this algorithm will be the local, linear least squares derivatives (LLSD) azimuthal shear field. The LLSD method uses rotational derivatives of the velocity field and is less affected by noisy velocity data in comparison to the more traditional “peak-to-peak” azimuthal shear calculations.

Initial detections will be made on a field of maximum low-level LLSD shear and diagnosed for potentially tornadic characteristics. Although LLSD shear is less range-dependent than peak-to-peak shear, some range dependency is unavoidable. A preliminary study of 31 tornadoes indicated that the threshold LLSD shear value needed to detect tornadoes was moderately dependent on range from the radar. A regression analysis was completed to determine the relationship between range and shear values such that range-corrected shear values could be estimated.

Predictors in the regression equation include circulation diameter and calculated LLSD shear. The circulation diameter was estimated by calculating the distance between minimum and maximum velocity values at a constant range. This value is assumed to represent the diameter of a mesocyclone-scale circulation, with the understanding that small-scale circulations will not be resolvable at far ranges. The resulting regression equation was applied to range-degraded shear values from tornadic circulations in the initial test set. Range-corrected shear values were compared to actual tornado intensities, as determined by damage surveys, to assess their validity.