Initiation of a severe thunderstorm over the Mediterranean Sea

The Mediterranean basin is regularly affected by severe weather associated with deep convection. Although convective systems are usually linked to coastal orography, some severe thunderstorms develop and mature over the sea. A recent example is the severe thunderstorm that affected the island of Mallorca in the afternoon of 4 October 2007. The storm formed early in the morning offshore of Murcia, and steadily became organized into a squall line. Arriving in Palma city, this squall line produced severe gusts, heavy rain and several tornadoes. The initiation and evolution of convection in these kinds of maritime events depend on both synoptic and mesoscale features. Representing such interactions is a challenge for numerical weather prediction. The aim of this study is to determine the prominent factors involved in the initiation and evolution of the damaging squall line, by means of high resolution numerical experiments. We also focus on squall line mesovortices to explain the potential for tornado development and the role of Mallorcan orography on their evolution.Simulations performed with the mesoscale model Méso-NH allowed relevant mechanisms for initiation and development of the strong squall line to be identified. The squall line initiates in an area with conditional instability, characterized by a cut-off and a southerly jet aloft and by moist, warm air at low-levels along a front. In addition, the area of low-level convergence offshore of Murcia, associated with the front and enhanced by a low downstream of the Atlas range, was shown to be crucial during the early stage of the convective system. The dry layer in the mid-troposphere and the strong sheared environment provide elements for understanding the development of such a damaging squall line. Moreover, a very high resolution experiment (600 m mesh) gave a very realistic representation of the squall line, including mesovortices ahead of the gust front, which confirmed the potential of this strong convective system for the genesis of small-scale vortices that may precede tornado development. A sensitivity experiment pointed out the prominent role of Mallorcan orography in straightening mesovortices approaching Palma city, and provided interesting elements for the understanding of the localization of tornadoes that occurred on 4 October 2007.

Research Highlights
► The 4 October 2007 severe convective event over the Mediterranean Sea.
► Simulation of a squall line with a fine scale model : Méso-NH.
► Simulations show the potential of the severe squall line for tornadogenesis.
► An original organization of mesovortices in the bow echo.
► Mallorcan orography explains tornadoes location.