We examined changes in extreme temperature over the Greek area in relation to the geopotential thickness field (1000 to 500 hPa). Daily maximum and minimum temperature series from 20 Greek stations evenly distributed over Greece were used for the period 1958-2000. The atmospheric circulation over the Greek area was classified with the aid of a calendar of daily circulation types based on the geopotential thickness field of 1000 to 500 hPa for the same period using daily NCEP and HadAM3P datasets. There was a general increase in frequency of anticyclonic circulation types and a decrease in frequency of cyclonic types. In addition, there was an overall interannual increase (decrease) in maximum (minimum) extreme temperatures over the Greek area, with spatial and seasonal variations. The investigation of the temperature anomalies corresponding to each circulation type, along with the trends of the former, revealed that the increase in frequency of the anticyclonic types, which form positive anomalies, and their positive trends in the majority of the stations, can explain the upward trend of maximum temperature in summer. In contrast, the trend of decreasing minimum temperatures in the central and SW Greek area cannot only be interpreted by changes in frequency of circulation types. A statistical downscaling model for simulating temperature changes over Greece (both for NCEP and HadAM3P datasets) was developed following a circulation-type approach. This demonstrated high correspondence with the observed extreme temperature values in all seasons, with only slightly lower year-to-year variability. © Inter-Research 2006.

In this paper, an attempt is made to assess and evaluate the skill of the Hadley Center atmospheric General Circulation Model (HadAM3P) in generating successfully the frequency and intensity of severe cyclones (<1000 hPa) in the Mediterranean region. The cyclonic occurrence is studied in three regions of enhanced cyclonic activity: Gulf of Genoa, Southern Italy and Cyprus. It was found that the HadAM3P predicts a future decrease of the frequency of the severe cyclones at the SLP level, but the future cyclones will be more intense (deeper), especially at the 500 hPa level.

In this study, an attempt is made to evaluate winter precipitation data derived from the general circulation model (GCM) - HadAM3P (atmospherical circulation model - Hadley Center) and from the National Center for Environmental Prediction-National Center of Atmospheric Research (NCEP-NCAR) reanalysis project for the period 1960-1990. These data sets are compared on a seasonal basis (winter) with observational data derived from stations over Greece. More specifically, the suitability of their average distributions and their spatial and temporal variability is examined. The methods used are principal component analysis (PCA) and canonical correlation analysis (CCA). The connection between the precipitation data sets and five cyclonic circulation types, from a daily catalogue, is also studied. With respect to the spatial distribution of the mean precipitation amounts, NCEP data show better agreement with the observational data compared to the modeled data. However, both NCEP and HadAM3P values are generally underestimated. Moreover, it seems that the HadAM3P is able to capture and reproduce quite satisfactorily the relationship between the precipitation data and the circulation types, but inadequately simulates the impact of orography and the Aegean Sea. Finally, the links between the precipitation variability in Greece and the European 500 hPa variability given by CCA seem to be fairly well reproduced by the model. Copyright © 2006 Royal Meteorological Society.

In this study an attempt is made to investigate the aerosol spatial and size distributions at different heights over the Greater Athens Area (GAA), Greece, under sea breeze conditions and clear sky and to further discuss possible implications for aerosol characteristics. The data used are airborne measurements of aerosol collected during two flights that were performed within the context of the 1997 STAAARTE experimental campaign. The aerosol measurements cover particle diameters from 0.1 to 45.5 μm. The horizontal and vertical distribution revealed that higher concentrations exist within or just above the atmospheric boundary layer, while greater concentrations are observed over the sea compared to land at high altitudes. At all altitudes the number size distributions show dominant diameter ranges between 0.1 and 0.3 μm at all altitudes. The volume distributions are characterised by two modes, one in the accumulation and the other in the coarse particle regime. At lower altitudes, fresh combustion emissions more likely cause the predominance of the size range 0.1-0.3 μm while enhanced physical and chemical processes that favour the growth of smaller particles to larger sizes could also act. The relative humidity does not seem to affect the observed number size distributions at low altitudes, where relative humidity is below 70% while at 4000 m the distributions seem to change over the sea where the humidity increases. © 2006 Elsevier B.V. All rights reserved.

In this study an attempt is made to assess the indoor air quality status of clinics of the Athens University School of Dentistry with respect to VOCs and CC2. Measurements were collected for a period of approximately three months in three clinics with the aid of portable instrumentation. It was found that the levels of VOCs concentrations are high, being attributed to the use of acrylic substances and dental materials but also to the detergent products for cleaning the working surfaces. The VOCs levels depend on the number of occupants and the material use as well as on the number of open windows. The corresponding background values remain high in all clinics. The levels of the concentrations of CO2 are considered in general satisfactory as compared to the international standards, however, they can exceed the intervention limit at certain hours of the day, suggesting poor air renewal. The opening of the windows was found to improve the air quality status.

In this study, the synoptic scale atmospheric circulation prevailing during 1989-2004 air pollution episodes in the greater area of Thessaloniki (GTA) was examined. The episodes are defined on the basis of the main pollutants measured (SO2, PM10, black smoke, CO, NO2 and O 3) on an hourly basis, at five monitoring stations of the Municipal Air Quality Network of Thessaloniki. During this period, 95 air pollution episodes, which lasted 258 days in total, were detected. The results demonstrate that the air quality in Thessaloniki can be characterized as acute, with respect mainly to photochemical pollutants and suspended particulates (PM 10). It was found that air quality conditions are significantly associated with specific synoptic scale features that contribute to the formation of the local wind field. The high air pollution concentrations are mainly associated with anticyclonic conditions that appear at the highest frequency throughout the whole year and/or advection of warm air masses. These findings can help to understand the association of large scale with local scale flow over GTA that greatly promotes the occurrence of air pollution episodes. © by PSP 2006.

In this paper, an attempt is made to investigate the synoptic-scale atmospheric conditions and dynamic processes leading to the rejuvenation of depressions or cyclogenesis over the Eastern Mediterranean during the cold period of the year. A case study analysis is carried out when the Polar Front Jet (PFJ) is positioned to the south of its normal seasonal position and interacts with the Subtropical Jet Stream (SJS), which is positioned to the north of its normal seasonal position. It was found that the vigorous rejuvenation of a northeastwards moving Atlas mountains depression on 15 March 1998 over Cyprus region is associated with an increase of the hydrodynamic instability due to the juxtaposition of a polar front jet streak to the subtropical jet stream. Furthermore, the rejuvenation is related to the combined effect of the direct and indirect cross vertical circulations appearing on the right side of the entrance of a polar jet streak and the left side of a subtropical jet streak exit, respectively. © Springer-Verlag 2005.