ALPEX-SIMULATION

In a project ''ALPEX-Simulation'', sponsored by the ''Osterreichischer Fond zur Forderung der wissenschaftlichen Forschung (FWF), all eight cases of ALPEX-SOP cyclones were numerically simulated with a fine mes h isentropic model of the atmosphere. These numerical simulations in s ix-hourly intervals allow a deeper insight into the synoptics and dyna mics of the cyclogeneses in the Western Mediterranean, especially into the genesis of the two basic types of cyclones: the so-called ''Ubers tromungs''-type and ''Vorderseiten''-type. In the first phase of cyclo genesis of the ''Uberstromungs''-type, the blocking and flow splitting of the cold air due to the Alps and the canalization between the Alps and the Massif Central are important. Cold air flows cyclonically aro und the western part of the Alps, creating a vorticity maximum at the south western edge of the Alpine bow and leads also to an enhanced PV. In connection with warm air in the Mediterranean, a strong baroclinic zone is generated. The interaction between the arriving PV maximum in the upper troposphere and the enhanced PV at the bottom leads to cycl ogenesis in the Western Mediterranean. In the case of the ''Vorderseit en''-type warm air advection dominates with the exception of a shallow layer of cold air in the inner Po-Valley, which is shielded by the Al pine ridge. A well-pronounced PV maximum builds up and couples with th e PV maximum arriving at upper levels, even before the cold air, comin g from the north-west, has surrounded the Alps. The cold air only inte nsifies the development by raising the baroclinity. Therefore, the ''V orderseiten''-cyclogenesis is an orographically modified cyclogenesis, in the course of which the cyclonic development is triggered by the A lps, whereas the ''Uberstromungs''-cyclogenesis is an orographically i nduced cyclo-genesis i.e. a ''true'' lee cyclogenesis.