IvanĪan-Picek, Branka (Meteorological and Hydrological Service, Zagreb, Croatia)
Lee cyclogenesis is mainly a synoptic scale process in which subsynoptic processes, such as frontal deformation and topographic blocking, play crucial role in triggering the larger in triggering the larger scale development.
Moreover, often exhibit mesoscale structures which may be the result of local forcing or instability. The pressure patterns can be modulated by the local orography, resulting in complex configurations and multiple centers. Genoa cyclones, for instance, often show two centers, one over the Ligurian sea and one over northern Italy with a deepening trough over the Adriatic sea.
Observational evidence and forecasting experience indicates that there is another case of completely independent mesoscale lee cyclogenesis related to frontal deformation apparently induced by the Dinaric Alps. The horizontal dimension of these vortexes is of the order of 100-200 km. Initially, they are shallow but, in the presence of cold air supply from north- east, they can grow in depth and intensity, resulting usually in stormy weather.
Associated phenomena are, in this case, strong winds (bora and jugo), large amounts of precipitation sometimes related to severe convection.
It is shown that the mesoscale development in the Adriatic is dependent on the synoptic situation, particularly on the strength of current around the eastern Alps, but its subsequent evolution may be at least partially independent of synoptic features. The main characteristic of the mesoscale field is the dipole structure of the orographically induced disturbance across the Dinaric Alps. Such a dipole structure is well known in larger scale Alpine lee cyclogenesis ans is comparable to the dipole configuration obtained as a difference between mountain/no-mountain numerical experiments. The topograhy of the Dinaric Alps accounts for cold air blocking and splitting flow which brings the cold air current to the northern and southern Adriatic, whereas only a part of the "bora" flow crosses the mountain entering the northern Adriactic as a downslope wind. The initiated mesoscale circulation contributes to further development of the low south of Dinaric Alps and the southerly or southeasterly wind "jugo" in advance of these low increases in strength. Thus, the orographic effects appear to play an important role in the mesoscale vortex generation practically in the same manner as the Alpine barriers influence the synoptic scale lee cyclogenesis.