The dynamics of a case of cyclogenesis over the central Mediterranean associated with heavy precipitation, especially over the southeast Aegean Sea, have been investigated on the basis of the analyses of two meteorological parameters, namely, the isobaric absolute and relative vorticity and the isentropic potential vorticity. On the whole, the two approaches demonstrate the important role of the upper level dynamics in the initiation of the surface cyclogenesis and seem to identify the same features: interaction of a region of positive absolute vorticity advection ahead of a 500 hPa trough with a shallow frontal system, in the first approach, and an isentropic potential vorticity anomaly at the upper levels with a low level baroclinic zone in the second approach. Furthermore, the analysis of potential vorticity identified an intense low level potential vorticity anomaly, for which there are indications that this is associated with diabatic heating and is important for the deepening of the system. This makes potential vorticity analysis preferable for forecasters. Special emphasis is given to the upper-level circulation and the vorticity field over the major European region before the surface cyclogenesis commences, since it is considered to be crucial for the forecasting of the event.

A case of Saharan cyclogenesis associated with the equatorward intrusion of a trough at the end of the Atlantic storm track is investigated. It is shown that a potential vorticity anomaly resulting from a baroclinic wave life cycle associated with the polar jet interacts with the low-level baroclinicity over subtropical Africa beneath the subtropical jet. It is suggested that low-latitude synoptic-scale cyclogenesis events of this type can be triggered only by upper-level potential vorticity anomalies if they have a sufficient depth scale and that the interaction may be aided by the presence of low static stability associated with a well-mixed boundary layer. A weak cold front also forms during the cyclogenesis event associated with the convergence of the baroclinicity of the polar trough front with the baroclinicity of the subtropical heat low.

This study aims to investigate the day by day relationship between mesoscale circulation and air quality over the Metropolitan Athens for a period of thirteen years spatially and seasonally. Eleven distinct mesoscale patterns are remarkable based in a formulated methodology based on surface and upper air wind measurements. The air quality conditions were attributed into seven distinct classes for five main pollutants namely, O3, NO2, SO2, CO and Black Smoke (BS). It was found that the severe and bad air quality conditions over the Metropolitan Athens area are mainly associated with weak southerly flows. The most serious pollution problem in AMA is attributed to O3 in the warm period when is favoured even by intense northerly flow. Due to the complex topography and the distribution of industrial and anthropogenetic activities in the examined area, the relationship between the air pollutant’s concentration and mesoscale air circulation presents important spatial characteristics that are further investigated.

An attempt is made to investigate the role of the jet streaks upstream an upper tropospheric diffluent trough in its evolution. A proof of the relationship that controls the rate of change of the mean absolute vorticity in the region of the diffluent trough is provided. It is demonstrated that for certain value of the rate of the increasing mean regional vorticity, a cut-off low can develop in the upper troposphere, which is likely to trigger the initiation of surface cyclogenesis, under favourable lower tropospheric conditions. Therefore, it seems that this approach offers to the operational meteorologists a simple tool to evaluate the numerical weather prediction of a cyclonic development in southern Europe and to decide for its uncritical accept or rejection when the initial atmospheric conditions in northwest Europe are similar to those described above. This diagnostic tool is then applied to the case of cyclogenesis that occurred on the 29th January 1994 in Greece, in order to demonstrate its practical significance.