POTENTIAL VORTICITY ANALYSIS OF THE STORM EVENT OF THE 9-10 NOVEMBER ALGERIAN FLOOD

We analyse, from a Potential Vorticity (PV) perspective, the generation and the evolution of a cyclone associated to the severe flood event that struck Algeria on November 10, 2001. The generation of severe weather events in the extratropical regions is often related to the formation of a PV tower throughout the troposphere, resulting from the merging of a diabatic PV anomaly with a PV intrusion originating from the polar lower stratosphere. The MIMOSA contour advection transport model (Fierli, 2002) reproduces a filamentary structure of PV at the 300 K potential temperature surface, originating from high latitudes and rolling up to form a cyclone above North Africa on November 9 2001. Severe storm were recorded the following day on the coast of Balearic Island (Romero, 2002). Moreover TOMS total ozone data shows the presence of a filamentary structure of high ozone concentration, originating above Scandinavia and extending southward as far as Morocco and Algeria. This suggests that the cyclonic structure is a polar low (Tripoli et al., this issue). PV from ECMWF analyses, a high-resolution transport model, and 3D trajectories is used to follow the polar low evolution and decay. Moreover we estimate the influence of the stratospheric intrusion on the cyclogenesis.