Radar analysis of a tornado over hilly terrain on 23 July 1996

Abstract

A Doppler radar case study of an F1–2 tornado in the Upper Rhine valley in Germany is presented. The Ziegelhausen tornado touched down on the hilly terrain near Heidelberg in the evening of 23 July 1996. A mini-supercell was the parent storm of the short-lived funnel. During its intensification the Cb-cell behaved like an ordinary right-moving supercell, but along with transition to its tornadic phase a hook echo developed at the storm's right rear flank, indicative of anticyclonic rotation. In addition a mesocyclonic in-cloud vortex signature was found higher up and and proved to be more persistent than the small anticyclonic circulation at the hook echo tip. So both, parent storm and tornado vortex were most likely cyclonic, with the damage swath to the left of the mesocyclone's ground track. The false anticyclonic hook echo appeared to be caused by small-scale orographic forcing. Terrain structure near the tornado site may have induced a low-level anticyclonic lee-vortex in the wake of an isolated mountain. Vertical vorticity advection and orogenic horizontal deformation of the airflow could then serve to consistently explain both the false hook and its sense of rotation. Climatologically, the Ziegelhausen tornado was representative for most German tornado outbreaks. Neither was the parent thunderstorm nowcast as potentially tornadic nor was any warning issued to the public.