A deep cyclone of African origin over the Western Mediterranean: diagnosis and numerical simulation

From 19 to 22 December 1979, a deep cyclone evolved over the Western Mediterranean. Gusty winds of more than 30 m/s, as well as a strong pressure decrease to about 990 hPa were recorded in Palma de Mallorca (Balearic Islands, Spain). ECMWF analyses are used for a diagnosis and numerical study of the case. Sensitivity experiments using the HIRLAM model are performed to assess the role of the surface sensible heat flux, latent heat release and orography on the genesis and evolution of the cyclone. At synoptic scale, the situation is governed by the instability of an upperlevel short wave. The cyclone developed within a notable baroclinic environment, which resulted from a cold advection from the northwest towards North Africa. The baroclinicity at first stages of the cyclogenesis is quantified by means of the Eady model. At latter stages, the evolution of the potential vorticity structures at high levels reveals a wide tropopause fold over the cyclone, as well as the presence of a strong anomaly associated with the low-level system. Sensitivity experiments reveal a notable cyclogenetic role of the latent heat release throughout the atmosphere in the deepening of the low, whereas no significant effect of the surface sensible heat flux is obtained for the simulation interval. On the other hand, an unusual cyclolytic role can be attributed to the northern ranges of the Mediterranean basin. Effectively, the low enlargement and deepening is constrained by a “wall effect”, which is a consequence of the interaction of the cyclonic flow and those northern mountainous systems.