The formation of a dynamically unstable ridge at 500 hPa as a precursor of surface cyclogenesis in the central Mediterranean

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Abstract
An attempt is made to find the dynamical and physical characteristics of the atmospheric circulation which contributed to the development of a great storm in the vicinity of Greece on 5 October 1989. The surface cyclogenesis could be considered as a consequence of an upper‐tropospheric minor wave which formed upstream on the eastern flank of a blocking‐type anticyclone which was centred over Britain and dominated the atmospheric circulation over western Europe. This minor wave formed three days before the initiation of the surface cyclogenesis and could be explained by the theory of ‘a dynamically unstable ridge’. As the wave moved south‐eastwards, an elongated trough formed in the upper troposphere. The southernmost part of this elongated trough was disrupted by dynamical processes, leading finally to the formation of a cut‐off low. When the area of positive relative vorticity advection due to the low became superimposed upon a shallow frontal surface lying across southern Greece in the afternoon of 4 October 1989, a deep depression developed at the surface between Crete and Athens during the next 12 hours. To further elucidate the role played by the forcing conditions at the upper levels in the surface cyclogenesis, the potential vorticity has been analysed.

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