Wednesday, 6 October 2004
Our previous conceptual studies of WSR-88D measurements of simulated tornadoes resulted in an improved but limited understanding of the effects of radar range and angular sampling and enhanced resolution on Doppler velocity signatures of tornadoes. The drawback of the simulation, however, was the assumption of uniform reflectivity distribution within the tornado vortex. Until recently, how the horizontal profile of reflectivity across the vortex varied with height and stage of tornado evolution was not well understood. High-resolution DOW (Doppler on Wheels) observations of tornadoes reveal concentric/spiral bands of reflectivity surrounding a low-reflectivity eye of a tornado vortex. Aloft, a reflectivity minimum occurs inside the tornado core with a high-reflectivity annulus being slightly wider than the tornado's core radius at which maximum tangential velocity occurs. The annulus tapers towards the ground. The minimum in reflectivity within the tornado core implies the presence of centrifuging of radar scatterers by the vortex. Differences between uniform and nonuniform reflectivity distribution across the tornado core can have an impact on the Doppler velocity measurements. When centrifugal accelerations are large, the motion of radar targets may differ significantly from the tornado's swirling motion. Thus, centrifugal effects indirectly contribute to underestimation of the tornado's maximum wind speeds. In addition to centrifuging action, strong horizontal gradients of reflectivity distribution within the tornado core can affect the weighting of Doppler samples. A tornado model was used to simulate the characteristics of a tornado. The model used important processes that included centrifugal effects on precipitation particles by the swirling flow. In the steady, axisymmetric tornado vortex, the particles moved at different speeds than the rotating air of the tornado. As a simulated WSR-88D scanned across the tornado vortex, the radar sampled the swirling precipitation particles in order to investigate the centrifugal effects of the particles on the Doppler velocity values. The results of the simulation will be presented at the Conference.
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