The average pressure distribution at mean sea level and the vertical structure of synoptic scale surface cyclones (with central pressure less than 1000 hPa) that occur in the Mediterranean region is studied for a 40 year period (1958-1997) on a seasonal and daily basis. The cyclonic occurrences are studied in three regions of enhanced cyclonic activity: gulf of Genoa, Southern Italy and Cyprus. The cyclones are identified with the aid of an objective method based on grid point values, available every 6 hours. The analysis revealed different characteristics of the cyclones that occur in the three regions, reflecting the different mechanisms that are responsible for their occurrence in each region. For the Genoa region the cyclone pressure minimum is located over the gulf, associated with orographic forcing, while surface dynamics occur further south. Over Southern Italy, the pressure minimum covers a wide area, whilst the surface dynamics are found to act in the same region, becoming more important in winter and spring. The pressure minimum of cyclones over Cyprus is located over the land during winter and spring and is influenced by surface dynamics and orography.

An experimental campaign was carried out on a small Greek island that is characterized by complex terrain: its aim was to study the local characteristics of the vertical structure of the atmospheric boundary layer (ABL). The instrumentation was installed close to the shoreline and consisted of a 13-m-high meteorological mast instrumented at three levels, and a high-range vertical monostatic sodar. Tethered balloon flights were carried out for 3 days under different atmospheric conditions. The analysis of the available data revealed interesting features of the vertical structure of the atmosphere over the island, with the development of a convective internal boundary layer (IBL) within the first 150 m above the ground, while the marine boundary layer (MBL) formed at higher altitudes, up to 450 m. Buoyant oscillations appear within the MBL in the form of gravity waves with frequencies of 7 min. Theoretical calculations of the IBL height verified the experimental results. During the night, a complex wind flow forms in the lower 250-300 m, resulting from the development of katabatic flows and topographic channeling.

The large-scale dynamical processes associated with an exceptionally dry and warm autumn during 1993 in Greece were studied. On the regional scale, it was found that low-frequency eddies prevailed over Europe in the form of a block with large-scale descending motion persisting throughout the whole troposphere. Moreover, the subtropical jet was unusually positioned much farther north over the Balkan peninsula and oscillated between this position and one over the central mainland of Greece where it interacted with the southern part of the polar front jet. On the global scale, it was found that this type of atmospheric circulation over Europe was closely related with a region of latent heat release in convective clouds over the tropics between the longitudes 100° and 120°E, and the formation of a steady divergence system at 150 hPa over the same region, strongly affecting the global rotational flow. © 2002 Elsevier Science Ltd. All rights reserved.

The effect of the synoptic-scale atmospheric circulation on the urban heat island phenomenon over Athens, Greece, was investigated and quantified for a period of 2 yr. employing a neural network approach. A neural network model was appropriately designed and tested for the estimation of the heat island intensity at 23 stations during the examined period. The day-by-day synoptic-scale atmospheric circulation in the lower troposphere for the same period was classified into eight statistically distinct categories. The neural network model employed as an input the corresponding synoptic categories in conjunction with four meteorological parameters that are closely related to the urban heat island. It was found that the synoptic-scale circulation is a predominant input parameter, affecting considerably the heat island intensity. Also, it was demonstrated that the high pressure ridge mostly favors the heat island phenomenon and categories characterized by intense northerly component winds are responsible for its nonappearance or termination.