Observatoire français des tornades et orages violents

A strong tornado hit seven cities of northern France in the late evening of Sunday, 3 August 2008, causing severe damage along its 19 km path from Pont-sur-Sambre to Boussois. Three people were killed in the collapse of their house and 18 were injured. More than 1000 houses were damaged and several thousand trees were uprooted or fallen down.

The authors led a damage survey in the hours that followed the disaster, then investigated this case, in order to determine the characteristics of this tornado precisely and to better understand the conditions that led to its formation. Weather radar analysis shows that the convective cell that gave rise to the tornado took on a fairly pronounced S-shaped structure, with a persistent mesocyclone in the central part of the convective system. The synoptic and mesoscale pattern associated with this severe storm was very dynamic, and characterized by a coupling between a low-level jet and a highly divergent jet-stream. The authors have reconstructed a vertical profile for this case study, in order to describe the tornadic environment precisely. The reconstructed profile reveals two main elements, namely an environment having a very modest vertical instability on one hand, and the presence of intense wind shear, notably in the lowest layers of the atmosphere on the other hand. This conclusion is supported by the analysis of many instability and shear parameters.