Thermal comfort under non-steady-state conditions primarily deals with rapid environmental transients and significant alterations of the meteorological conditions, activity, or clothing pattern within the time scale of some minutes. In such cases, thermal history plays an important role in respect to time, and thus, a dynamic approach is appropriate. The present study aims to investigate the dynamic thermal adaptation process of a human individual, after his transition from a typical indoor climate to an outdoor hot environment. Three scenarios of thermal transients have been considered for a range of hot outdoor environmental conditions, employing the dynamic two-node IMEM model. The differences among them concern the radiation field, the activity level, and the body position. The temporal pattern of body temperatures as well as the range of skin wettedness and of water loss have been investigated and compared among the scenarios and the environmental conditions considered. The structure and the temporal course of human energy fluxes as well as the identification of the contribution of body temperatures to energy fluxes have also been studied and compared. In general, the simulation results indicate that the response of a person, coming from the same neutral indoor climate, varies depending on the scenario followed by the individual while being outdoors. The combination of radiation field (shade or not) with the kind of activity (sitting or walking) and the outdoor conditions differentiates significantly the thermal state of the human body. Therefore, 75 % of the skin wettedness values do not exceed the thermal comfort limit at rest for a sitting individual under the shade. This percentage decreases dramatically, less than 25 %, under direct solar radiation and exceeds 75 % for a walking person under direct solar radiation. © 2014 ISB.

The development mechanisms of an explosive cyclone over Central Mediterranean are examined, by relating the cloud patterns in the infrared, water vapour and visible channels of the Meteosat Second Generation (MSG) satellite images, to the surface-upper air operational analyses and thermodynamic parameters, including potential and geostrophic vorticity analyses, potential and equivalent potential temperature, static stability and thermal heat fluxes. The specific case study derived from an updated climatology of Mediterranean explosive cyclones for the 2002-2010 period, being performed with the aid of the University of Melbourne Cyclone Tracking scheme (MS algorithm) and ERA-INTERIM datasets. It was found that during the ordinary cyclogenesis the increased mid-upper level relative humidity over Northern Algeria along with the enhanced moisture gradient in the area between Portugal and Spain; indicate the existence of a jet -streak, with the dry zone to be located on the cold side of the jet axis. The descent of the dry air is more pronounced within the left exit region of the jet streak in the poleward portion of an upper confluence zone. Six hours later, the eastward movement of the cloud pattern in the Northern African coasts significantly resembles the structure of a baroclinic leaf, being related to the early stages of surface frontogenesis, due to the deformation process within a strong wind field. During the time of rapid deepening, when the surface cyclone propagated from the Northern African coasts towards the area of Sicily - Gulf of Taranto, the development is characterised by the transition from the baroclinic leaf structure to the comma cloud one and finally the formation of a bent-back occlusion. © 2013 SPIE.

The dynamic and kinematic characteristics of surface explosive cyclones in the Mediterranean are examined and compared to those of ordinary cyclones. The cyclone detection is performed with the tracking algorithm developed in the University of Melbourne, using the 1° × 1° ERA-40 mean sea level pressure dataset for a 40 year period. It is verified that the explosive cyclogenesis in the Mediterranean is mainly a maritime phenomenon, occurring along the northern Mediterranean coast during the cold season. On the contrary, the ordinary cyclogenesis exhibits significant maxima in both continental and maritime environments throughout the year. The explosive cyclones are characterized by longer lifetime and greater propagation speed. They are larger and deeper in the eastern Mediterranean, whereas the ordinary cyclones are deeper in the western and larger in the eastern Mediterranean. The trend analysis revealed that both explosive and ordinary cyclones become less frequent in the Mediterranean basin, while there is a tendency for deeper ordinary cyclones over North Africa and shallower over the Aegean Sea and Cyprus. © 2013 Springer-Verlag Berlin Heidelberg.

The development mechanisms of an explosive cyclone over Central Mediterranean are examined, by relating the cloud patterns in the infrared, water vapour and visible channels of the Meteosat Second Generation (MSG) satellite images, to the surface-upper air operational analyses and thermodynamic parameters, including potential and geostrophic vorticity analyses, potential and equivalent potential temperature, static stability and thermal heat fluxes. The specific case study derived from an updated climatology of Mediterranean explosive cyclones for the 2002-2010 period, being performed with the aid of the University of Melbourne Cyclone Tracking scheme (MS algorithm) and ERA-INTERIM datasets. It was found that during the ordinary cyclogenesis the increased mid-upper level relative humidity over Northern Algeria along with the enhanced moisture gradient in the area between Portugal and Spain; indicate the existence of a jet -streak, with the dry zone to be located on the cold side of the jet axis. The descent of the dry air is more pronounced within the left exit region of the jet streak in the poleward portion of an upper confluence zone. Six hours later, the eastward movement of the cloud pattern in the Northern African coasts significantly resembles the structure of a baroclinic leaf, being related to the early stages of surface frontogenesis, due to the deformation process within a strong wind field. During the time of rapid deepening, when the surface cyclone propagated from the Northern African coasts towards the area of Sicily - Gulf of Taranto, the development is characterised by the transition from the baroclinic leaf structure to the comma cloud one and finally the formation of a bent-back occlusion. © 2013 SPIE.

The influence of microscale and mesoscale meteorology on the local scale variation of air temperature cannot be correctly simulated by the coarse resolution Global Climate Models. The scope of this work is to develop a hybrid dynamic-statistical downscaling procedure and quantify its predictive ability to estimate air temperature variability at finer spatial scales. The study focuses on the warm period of the year (June - August) and the method is applied to eight sites located in Greece with different topographical characteristics. The two-step methodology initially involves the dynamic downscaling of coarse resolution climate data via the RegCM4 regional climate model and subsequently the statistical downscaling of the modeled outputs by training site-specific artificial neural networks (ANN). The RegCM4 model is employed to enhance the representativity of the dataset, while the ANNs are used as function approximators to model the relationship between a number of atmospheric predictor variables and the observed air temperature time series. An insight of the ANN transfer function is obtained by examining the relative contribution of each input variable. The performance of the methodology is evaluated and the results indicate significant improvement from the inclusion of the ANN models in downscaling air temperature. © 2013 Global NEST Printed in Greece. All rights reserved.

The vertical structure of surface cyclonic tracks affecting the eastern Mediterranean region is studied on a climatological basis for the cold period of the year. The dataset used is the 1° × 1° ERA-40 Reanalysis for a 40-year period (1962-2001). The vertical tracking of surface cyclonic tracks was performed with the aid of the Melbourne University Vertical Tracking Algorithm. It was found that about 83 % of the cyclones were extended up to the 500-hPa level and almost 65 % up to 200-hPa level, implying that the cyclones are in general well organized. The surface tracks that originate within the examined area exhibit the smallest vertical extension, intensity, radius, and depth compared to the cyclones originating in the other sectors. Moreover, the 500-hPa counterparts for the said cyclones are mainly located to the north-west or south-west of the surface cyclone position, consistent with the baroclinic character of the Mediterranean cyclones. The zonal (eastward) component of motion predominates both at the surface and at 500 hPa. © 2012 Springer-Verlag.

The paper presents a semi-supervised weather classification method based on 850-hPa isobaric level maps. A preprocessing step is employed, where isolines of geopotential height are extracted from weather map images via an image processing procedure. Α feature extraction stage follows where two techniques are applied. The first technique implements phase space reconstruction, and yields multidimensional delay distributions. The second technique is based on chain code representation of signals, from which histogram features are derived. Similarity measures are used to compare multidimensional data and the k-means algorithm is applied in the final stage. The method is applied over the area of Greece, and the resulting catalogues are compared to a subjective classification for this area. Numerical experiments with datasets derived from the European Meteorological Bulletin archives exhibit an up to 91 % accurate agreement with the subjective weather patterns. © 2013 Springer-Verlag Wien.

Classification of weather maps at various isobaric levels as a methodological tool is used in several problems related to meteorology, climatology, atmospheric pollution and to other fields for many years. Initially the classification was performed manually. The criteria used by the person performing the classification are features of isobars or isopleths of geopotential height, depending on the type of maps to be classified. Although manual classifications integrate the perceptual experience and other unquantifiable qualities of the meteorology specialists involved, these are typically subjective and time consuming. Furthermore, during the last years different approaches of automated methods for atmospheric circulation classification have been proposed, which present automated and so-called objective classifications. In this paper a new method of atmospheric circulation classification of isobaric maps is presented. The method is based on graph theory. It starts with an intelligent prototype selection using an over-partitioning mode of fuzzy c-means (FCM) algorithm, proceeds to a graph formulation for the entire dataset and produces the clusters based on the contemporary dominant sets clustering method. Graph theory is a novel mathematical approach, allowing a more efficient representation of spatially correlated data, compared to the classical Euclidian space representation approaches, used in conventional classification methods. The method has been applied to the classification of 850. hPa atmospheric circulation over the Eastern Mediterranean. The evaluation of the automated methods is performed by statistical indexes; results indicate that the classification is adequately comparable with other state-of-the-art automated map classification methods, for a variable number of clusters. © 2012 Elsevier B.V.

The scope of the present study is to assess the influence of meteorology on different diameter particles (PM 10, PM 2.5, PM 2.5-10) during a 53months long experimental campaign at an urban Mediterranean area. Except for the investigation of the wind, temperature and relative humidity role, day by day synoptic conditions were classified over the Attica peninsula in order to explore as well, the role of the synoptic scale atmospheric circulation. The strong dependence of the aerosols character on their various sources, not only explain the different diameter particles behavior and their differentiation with the inorganic pollutants but also highlights the need for an effective emission policy. High PM 10 and PM 2.5-10 concentrations found to be closely related to the southwesterly regime, suggesting long range transport from the ’polluted’ south sector while the general prevalence of the secondary particles generation revealed the health hazard. PM 2.5 showed a weaker correlation than the bigger particles with both the circulation patterns and the parameters’ fluctuations. Temporal pollutants variations were clearly governed by the emissions patterns while the low wind speed was not necessarily a good indicator of high concentration levels. Finally it was found that only during the open/close anticyclonic days and the southwesterly wind regime the morning levels were continuously higher than those of the night. © 2012 Elsevier B.V.

Synoptic climatology relates the atmospheric circulation with the surface environment. The aim of this study is to examine the variability of the surface meteorological patterns, which are developing under different synoptic scale categories over a suburban area with complex topography. Multivariate Data Analysis techniques were performed to a data set with surface meteorological elements. Three principal components related to the thermodynamic status of the surface environment and the two components of the wind speed were found. The variability of the surface flows was related with atmospheric circulation categories by applying Correspondence Analysis. Similar surface thermodynamic fields develop under cyclonic categories, which are contrasted with the anti-cyclonic category. A strong, steady wind flow characterized by high shear values develops under the cyclonic Closed Low and the anticyclonic H-L categories, in contrast to the variable weak flow under the anticyclonic Open Anticyclone category. © 2012 Springer-Verlag.

An attempt is made to explore the vertical structure of the surface explosive cyclones in the Mediterranean on a climatological basis during the cold period of the year in order to get a better insight in the interaction between the upper and lower levels responsible for the genesis and evolvement of the phenomenon. The vertical profile of the explosive cyclones was examined with the aid of the vertical tracing software of the University of Melbourne Cyclone Tracking Algorithm, using the 1 × 1° spatial resolution of ERA-40 reanalysis data. It was found that about 57 % of the track steps of surface explosive cyclones extend up to 500 hPa. The north-westward tilting of the surface cyclones with height during the stage of explosive cyclogenesis, with a mean distance of 350 km between mean sea and 500 hPa levels, confirms the importance of baroclinicity. About 45 % of the surface explosive cyclones reached their maximum depth before their 500 hPa counterparts, implying the role of surface processes. © 2012 Springer-Verlag.

The objective of this study was to explore the ability of 3 regional climate models (RCMs), CNRM-Aladin, C4I-RCA3 and KNMI-RACMO along with their parental global climate models (GCMs), ARPEGE and ECHAM5, to represent the relationship between large-scale atmospheric circulation and climate extremes in the Mediterranean region. Subsequently, an evaluation and inter-comparison of these 3 RCM/GCM couples for the present climate was performed. For this purpose, the Regularised Canonical Correlation Analysis (RCCA) was employed and 4 extreme climate indices of temperature and precipitation were used to define extreme events over the study region. The evaluation of these relationships was carried out against gridded observational and reanalysis datasets. It was found that the observed upper air large scale patterns related to climate extremes in the Mediterranean are not very well reproduced by the RCM/GCM couples in all seasons. In addition, in many cases, the coupled models display patterns of extreme climate indices which are not consistent with the accompanied upper level circulation. Furthermore, all coupled models display substantial deficiencies in simulating precipitation extremes. In the case of summer data, the ability of all 3 models is limited, possibly because the strength of the large-scale atmospheric flow decreases, the control exerted by the lateral boundary conditions is weaker, and the nested models are mainly governed by local processes. © Inter-Research 2011.

A comparison of two objective climatologies of explosive cyclones in the Mediterranean region is performed. The results are derived from two different mean sea-level pressure reanalysis data resolutions, but from the same assimilation model, in order to quantify the pure impact of higher resolution on the identification and characteristics of explosive cyclones, when the assimilation model is the same. The explosive cyclones were identified with the aid of the Melbourne University automatic cyclone finding and tracking scheme over a 40-year period, using the 6-hourly analyses of ERA-40 mean sea-level pressure (MSLP) on: (a) 2.5 × 2.5 and (b) 1 × 1 latitude-longitude grid. The comparison of the two datasets revealed the significant role of the increase in spatial resolution of MSLP data on the identification and tracking process, and the number of the explosive cyclones in the high-resolution dataset is almost four times greater than the respective one in the lower resolution dataset. However, the comparison of explosive cyclone characteristics, including spatial and temporal variations of explosive deepening, revealed differences in the geographical distribution of the location of the maximum explosive deepening and average explosive cyclone Laplacian of the central pressure. These differences are due to the identification in the higher resolution set of smaller scale and secondary explosives along the strongly baroclinic northern Mediterranean boundaries, south of the Alps and the Pyrenees. Explosive deepening appears a bias to the daytime period from 12 to 18 Coordinated Universal Time (UTC) for both datasets, which is more prominent in the LR dataset. Statistically significant difference of pressure tendency between the two datasets appear for the daytime period from 06 to 12 UTC, accounting for better representation of orographic forcing in the HR dataset. © 2011 Springer-Verlag.

An objective climatology of explosive cyclones is performed over a 40-year period (1962-2001), for the Mediterranean region with the aid of the Melbourne University automatic cyclone finding and tracking scheme. The major temporal and spatial characteristics of Mediterranean explosive cyclones are investigated, including frequency, movement, generation and dissipation, scale, depth, and intensity. It was verified that the explosive cyclogenesis in the Mediterranean is a rather rare phenomenon that occurs mainly from November to March. Explosive cyclones preferentially form along the northern Mediterranean coast, with maximum density in the Ligurian and Aegean Seas. The cyclogenesis mechanism that predominates in their development is characterized by an interaction of a short and an open long wave in a pre-existing development (referred to as the KF mechanism). There are distinct differences between Western Mediterranean and Eastern Mediterranean explosive cyclones. Although explosive cyclones tend to form in the Western Mediterranean, their scale and depth is greater in the Eastern Mediterranean. Explosive cyclones more likely move from the main cyclogenetic areas in the Western Mediterranean along specific eastward paths over southern Italy and the Ionian Sea where they dissipate or they generate in Eastern Mediterranean and move north-eastwards to the Black Sea or eastwards to Cyprus and Middle East. © 2010 Royal Meteorological Society.

An experimental campaign has been conducted in the frame of the European Commission funded network of excellence ECATS, at Athens International Airport (AIA), Greece, from 13 to 25 September 2007. In order to quantify the background air quality and study the influence of meteorological conditions and airport emissions to the broader region, the analysis was extended to the greater Messogia Plain of Attica, Greece. Mixing height was estimated taking into account measurements from remote sensing and surface based single point instrumentation. According to the analysis, the air quality over Messogia Plain is mainly controlled by the intensity and the direction of the background flow. Under low background wind conditions, the development of local flows (sea and land breeze cells) over the greater area preserves high concentrations of air pollutants which are mainly attributed to the airport emissions, the local activities and traffic. When the background flow is strong, the diurnal cycle of all concentrations was significantly reduced by more than 50%, due to the advection and the subsequent mixing of the lower atmosphere. In order to identify and quantify the various processes linked with meteorology and air quality, the Hilbert-Huang transform was selected to be applied on the air-quality time-series data. The calculated Hilbert spectra of the main pollutants showed that meteorology plays a prescriptive role on the evolution of air pollutants, determining the influence of the local scale characteristics at each station. © 2011 Elsevier Ltd.

In this study, an updated and extended climatology of cyclonic tracks affecting the eastern Mediterranean region is presented, in order to better understand the Mediterranean climate and its changes. This climatology includes intermonthly variations, classification of tracks according to their origin domain, dynamic and kinematic characteristics, and trend analysis. The dataset used is the 1962–2001, 2.58 3 2.58, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The identification and tracking of the cyclones was performed with the aid of the Melbourne University algorithm. It was verified that considerable intermonthly variations of track density occur in the eastern Mediterranean, consistent with previous studies for the entire Mediterranean, while further interesting new features have been revealed. The classification of the tracks according to their origin domain reveals that the vast majority originate within the examined area itself, mainly in the Cyprus area and the southeastern Aegean Sea, while the tracks that originate elsewhere most frequently enter from the west. Deeper cyclones follow the southwest track originating from the area between Algeria and the Atlas Mountains. A greater size characterizes the westerly tracks (southwest, northwest, and west), while the northwest tracks propagate faster over the study area. A negative trend of the track frequency was found on an annual basis that can be mostly attributed to the winter months, being associated with variations in the baroclinicity. This negative trend is more prominent for the westerly and northeasterly tracks, as well as for those originating in the northern part of the examined area.

This paper evaluates the role of meteorological factors on the configuration of the complex aerosol’s character at a suburban area of Athens, Greece. To this effort, 4-month period measurements of hourly PM2.5 aerosol samples were employed, being collected with the aid of a conventional R&P TEOM (model 1400a) at Aghia Paraskevi. The simultaneous daily PM10 and PM2.5.10 concentrations are also discussed. Then, the entire daily data set was divided in 5 different classes at similar proportion by one decimal point for each one of assessed meteorological parameter, temperature, relative humidity and wind speed. The present study discusses not only the variations of the recorded levels of the three PM fractions and their seasonality but also the inter-fraction relationship for all possible matching of the aerosol’s pairs and the PM ratios in connection with the differentiation of the meteorological parameters’ values. PM10 and PM2.5-10 appeared to have the same character as in all the cases they proved to be strongly correlated. It was found that temperature enhanced the concentrations of the PM fractions and especially those with bigger diameter while relative humidity seemed to favour the formation of fine particles. Wind direction appeared to play a key role on the recorded PM profile. The transportation from the ’polluted’ Athens center proved to be the major mechanism which deters the ’clean atmosphere’ of the suburban area. © by PSP.

A case of abrupt and significant increase of surface ozone was examined, which is recorded during the night of October 9th, 2003, over the area of the Thriassion Plain - Greece, being accompanied by the occurrence of a wind outbreak during the same period. As this increase cannot be explained by the ordinary diurnal evolution of tropospheric ozone concentration, the possibility of downward stratospheric ozone transport was investigated. It was indeed found that the case is associated with tropopause folding and subsequent stratospheric air intrusion in the upper troposphere over Greece. With the aid of mesoscale modeling, being also supported by the hydraulic theory, it was found that the intense wind outbreak is associated with the development of disturbances in the lee side of the neighbouring mountain ranges. These disturbances contribute to the free troposphere - atmospheric boundary layer interaction, facilitating the transportation of stratospheric air close to the surface, thus resulting to the observed tropospheric ozone increase. © 2010 Gebrüder Borntraeger, Berlin, Stuttgart.

In this study, an updated and extended climatology of cyclonic tracks affecting the eastern Mediterranean region is presented, in order to better understand the Mediterranean climate and its changes. This climatology includes intermonthly variations, classification of tracks according to their origin domain, dynamic and kinematic characteristics, and trend analysis. The dataset used is the 1962-2001, 2.5° × 2.5°, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The identification and tracking of the cyclones was performed with the aid of the Melbourne University algorithm. It was verified that considerable intermonthly variations of track density occur in the eastern Mediterranean, consistent with previous studies for the entire Mediterranean, while further interesting new features have been revealed. The classification of the tracks according to their origin domain reveals that the vast majority originate within the examined area itself, mainly in the Cyprus area and the southeastern Aegean Sea, while the tracks that originate elsewhere most frequently enter from the west. Deeper cyclones follow the southwest track originating from the area between Algeria and the Atlas Mountains. A greater size characterizes the westerly tracks (southwest, northwest, and west), while the northwest tracks propagate faster over the study area. A negative trend of the track frequency was found on an annual basis that can be mostly attributed to the winter months, being associated with variations in the baroclinicity. This negative trend is more prominent for the westerly and northeasterly tracks, as well as for those originating in the northern part of the examined area. © 2010 American Meteorological Society.

The objective of this study is to investigate the linkage of large-scale upper air circulation over the greater European area with intense precipitation events over Eastern Mediterranean and then to estimate potential changes in the atmospheric patterns in the future, under global warming conditions. For this purpose, results from the regional climate model HadRM3P and Global Circulation Model HadAM3P have been used for the present period 1960-1990 (control run) and the future period 2070-2100 based on the B2a IPCC emission scenario. For the identification of the precipitation extremes the Simple Daily Intensity Index (SDII) was employed. Our analysis has shown a notable relation of extreme events with the East Atlantic and Scandinavia teleconnection patterns, as well as the Eastern Mediterranean Pattern (EMP) during the wet period. In the future, similar patterns are found, with different magnitude and position, following the projected changes in atmospheric circulation over Europe.

The relationship of prolonged dry spells in Eastern Mediterranean with large-scale surface and upper circulation is investigated on seasonal basis with the aid of the Singular-Value Decomposition Analysis (SVDA) for the period 1958-2000. The study was based on daily precipitation data of 56 stations, evenly distributed over Eastern Mediterranean region. Extreme dry spells are defined using the CDD index (maximum number of consecutive dry days). It was found that teleconnection patterns centered over Northern Atlantic and northern Europe seem to affect the duration of the longest dry spells over the Eastern Mediterranean, while surface synoptic scale systems in Northern Africa play a substantial role. The SVDA results compare well with the corresponding results of Canonical Correlation Analysis (CCA), mainly for the surface circulation during winter and summer. © Springer-Verlag 2009.

The objective of this study is to investigate the relationship of orography over Greece with prolonged dry spells, as represented by the maximum number consecutive dry days (CDD index) during an extremely dry summer. For this purpose a simulation experiment was conducted with the aid of the regional climate model RegCM3.1 using a spatial resolution of 10 km. It was shown that a significant precipitation regime formed over the mountainous areas of continental Greece and Crete during this dry summer, due to orographically forced precipitation, consequently influencing the length of dry spells. Furthermore, the CDD appears spatial variations over the maritime areas, despite the zero or insignificant precipitation. The sensitivity test that was performed with the elimination of orography demonstrated the important role that orography plays in the distribution of CDD, since significant lengthening of extreme dry spells was found over the mountainous areas. © Author(s) 2010.

The relative contribution of the main mechanisms that control indoor air quality in residential flats was examined. Indoor and outdoor concentration measurements of different type pollutants (black carbon, SO 2, O 3, NO, NO 2,) were monitored in three naturally ventilated residential flats in Athens, Greece. At each apartment, experiments were conducted during the cold as well as during the warm period of the year. The controlling parameters of transport and deposition mechanisms were calculated from the experimental data. Deposition rates of the same pollutant differ according to the site (different construction characteristics) and to the measuring period for the same site (variations in relative humidity and differences in furnishing). Differences in the black carbon deposition rates were attributed to different black carbon size distributions. The highest deposition rates were observed for O 3 in the residential flats with the older construction and the highest humidity levels. The calculated parameters as well as the measured outdoor concentrations were used as input data of a one-compartment indoor air quality model, and the indoor concentrations, the production, and loss rates of the different pollutants were calculated. The model calculated concentrations are in good agreement with the measured values. Model simulations revealed that the mechanism that mainly affected the change rate of indoor black carbon concentrations was the transport from the outdoor environment, while the removal due to deposition was insignificant. During model simulations, it was also established that that the change rate of SO 2 concentrations was governed by the interaction between the transport and the deposition mechanisms while NO X concentrations were mainly controlled through photochemical reactions and the transport from outdoors. © 2009 Springer Science+Business Media B.V.

The sensitivity of regional air quality modeling simulations to boundary conditions over Greece is investigated, for various synoptic conditions. For this purpose, a global to mesoscale model-chain is developed and applied, coupling the individual models’ simulations. The global chemical transport model GEOS-CHEM, applied in a one-way nested procedure, is used to drive the regional UAM-V chemical dispersion model with time-varying lateral and top boundary conditions. The results of the coupling procedure are compared with the MINOS campaign measurements at Finokalia (Southern Greece) during the period from 1 to 16 August 2001 which is mainly characterized by an interchange of two synoptic types, High-Low and Long Wave trough. The comparison between the simulation results and the measurements reveals that the coupling procedure captures satisfactorily the range of observed CO concentrations at the southern part of Greece. The most severe deviations are observed under strongly variable atmospheric circulation, when no distinct synoptic circulation is allowed to be established in the area. Regarding O3, the highest, though underestimated, surface concentrations are simulated under Long Wave trough conditions due to the influence of the ozone inflow predicted by GEOS-CHEM at the western boundary of the innermost domain and/or under enhanced NOy emissions arriving at Finokalia from urban and ships plumes. © 2009 Elsevier Ltd. All rights reserved.

The objective of this study is to investigate the impact of the eastern Mediterranean teleconnection pattern (EMP) on the present and future climate of the eastern Mediterranean during winter. For the present climate, daily precipitation and maximum and minimum surface temperature station data are employed for the period of 1958-2003. For the future climate, datasets of the same parameters are derived from the Hadley Centre Regional Climatic Model (HadRM3P) for the period of 2070-2100, using two Intergovernmental Panel on Climate Change (IPCC) emission scenarios for the evolvement of the future atmospheric concentrations of greenhouse gases. The investigation of the impact was based on the regularized canonical correlation analysis (RCCA), while qualitative estimations were performed for each phase of the pattern. It was found that the pattern indeed affects the mean winter patterns of temperature, precipitation, and their extreme events with inverse impacts between the two phases. More specifically, a positive phase of EMP is associated with a decrease in temperatures and an increase in precipitation, while the opposite occurs during the negative phase of EMP. In the future, the present impact according to each phase persists and intensifies in most of the cases. However, results are quite different between the two scenarios, because of the different estimated future shift of the EMP poles. © 2009 American Meteorological Society.

In this study, an assessment of indoor air quality (IAQ) and thermal comfort in the Athens Traffic Control Tower (ATCT) offices of Hellinicon building complex, which is mechanically ventilated, is presented. Measurements of PM 10, PM 2.5, TVOCs and CO 2 concentrations were performed during three experimental cycles, while the Thom Discomfort Index was calculated to describe the employees’ feeling of discomfort. The aim of the first cycle was to identify the IAQ status, the second to investigate the effectiveness of certain measures taken, and the third to continuously monitor and control IAQ. During the first two cycles, daily spot measurements of TVOCs and CO 2 were performed at various indoor locations and at the respective outdoor air intake positions, in addition with mean 24-h spot measurements of indoor PM 10 and PM 2.5. Results revealed that pollution levels vary according to the occupancy and the kind of activity. Following that, an automated system (IMAS) was designed and employed to continuously monitor indoor and outdoor CO 2, TVOCs, temperature and relative humidity. The ultimate scope was to control the IAQ and offer acceptable comfort conditions to the employees, whose work is of special nature and extremely demanding. Intervention scenarios were formulated and applied to the system to improve indoor conditions, when and where necessary. Regarding the third cycle, 1-year measurements collected from the system to examine its effectiveness. While it was shown that discomfort may be attributed to co-existence of unsatisfactory thermal comfort conditions and IAQ, usually the sole predominant factor of discomfort feeling is thermal comfort. © Springer Science+Business Media B.V. 2008.

Weather maps refer to meteorological data that characterize the atmospheric circulation in a region. The classification of weather maps into categories becomes an important task for understanding regional climate. Towards this goal, manual and semiautomatic techniques have been used, requiring manpower and supervision. In this paper, we propose a machine vision based method for the classification of weather maps into distinct classes. The chain code descriptor is applied to extract the feature of isobaric lines and we introduce the Double-Side Chain Code (DSCC) histogram for feature representation. Handling DSCC histograms as multidimensional vectors, the A:-nearest neighbors (k- NN) algorithm classifies the objects to an appropriate number of classes, based on closest training set in the feature space. This method provides an automated and more ’objective’ classification scheme, applying straightforward to the input weather map’s image. © 2009 IEEE.

During the peak of an extensive heat wave episode on 23-25 July 2007, simultaneous thermophysiological measurements were made in two non-acclimated healthy adults of different sex in a suburban area of Greater Athens, Greece. Based on experimental measurements of mean skin temperature and metabolic heat production, heat fluxes to and from the human body were calculated, and the biometeorological index heat load (HL) produced was determined according to the heat balance equation. Comparing experimental values with those derived from theoretical estimates revealed a great heat stress for both individuals, especially the male, while theoretical values underestimated heat stress. The study also revealed that thermophysiological factors, such as mean skin temperature and metabolic heat production, play an important role in determining heat fluxes patterns in the heat balance equation. The theoretical values of mean skin temperature as derived from an empirical equation may not be appropriate to describe the changes that take place in a non-acclimated individual. Furthermore, the changes in metabolic heat production were significant even for standard activity. © 2008 ISB.

This study explores the influence of air gaseous pollutants-aerosols and solar zenith angle (SZA) on the spectral diffuse-to-direct beam Edλ/Ebλ irradiances ratio. It does so using ground-based spectroradiometric measurements taken over the Athens atmosphere during May 1995. It was found that the spectral Edλ/Ebλ ratio decreases rapidly with increasing wavelength and regression curves of the form EEdλ/Ebλ = aλ-b fitted the experimental data. These curves are strongly modified by aerosols-air pollutants, aerosol optical properties, and SZA. The log-log plot of Edλ/Ebλ versus λ reveals a significant departure from linearity, which is likely to be associated with aerosol physical properties and SZA effects. The effect of atmospheric turbidity, as expressed through the aerosol optical at 500 nm and SZA on the spectral Edλ/ Ebλ ratio, is investigated in detail for two discernible atmospheric conditions observed in the urban Athens atmosphere. The first case includes different atmospheric turbidity levels under the same SZA, while the second corresponds to different SZA values under the same turbidity levels. It was found that the correlation between Edλ/Ebλ and spectral aerosol optical depth can be a useful tool in determining the aerosol optical properties and aerosol types composition. © Springer-Verlag 2009.

Air pollution episodes in urban coastal areas follow certain pre-determined patterns, being associated with certain local meteorological conditions and emission of primary pollutants. In this study, the synoptic and local scale atmospheric circulation that prevails during air pollution episodes in a coastal major city in Greece, Thessaloniki, is examined for a period of 15 years (1989-2004). The study signifies the importance of studying air pollution meteorological patterns between coastal areas with different terrain characteristics. For Thessaloniki, it was found that the episodes occur mainly during the cold period of the year, while four types of synoptic scale circulation were recognized (I, II, III, IV) and five patterns of the local scale circulation (A1, A2, B1, B2 and B3). The highest percentage of episodes is associated with the presence of an anticyclone over the northern Greece (types I and IV), being characterized by weak or very weak surface pressure gradient intensity, according to the position and extension of the anticyclone. Moreover, a temperature increase of at least 1°C during the previous 3 days is required in the lower troposphere. Consistent with the synoptic conditions, the development of the sea breeze plays a crucial role in the occurrence of the episodes, even in the cold period of the year, when the sea breeze can still develop with smaller frequency and intensity. Finally, it was found that a small number of episodes is related with the advection of polluted air masses from the industrial area in the northwest of the city and from the Eordaia area in the west, which is the largest lignite producing area of Balkans. © Springer-Verlag 2008.

Objective: To quantify the amount of Total Volatile Organic Compounds (TVOCs) emitted from five dental substances commonly used in a dental clinic. Method: An assessment of the indoor air quality in two clinics of the Athens University Dentistry Faculty was conducted. Results: It was found that TVOC emissions from Kalocryl and Bacillol reached extremely high values affecting the air in the dental clinics at short and long distances from the source. Six pollutants TVOCs, CO2, PM10, PM25, SO 2 and NOx were measured and indoor sources associated with dental activities were identified. Very high concentrations of TVOCs were found which exceeded the acceptable levels by a factor of 20, associated with the nature of the dental activities and the ventilation conditions. The CO 2 concentration levels were high and the indoor conditions were considered unsatisfactory, associated with the number of occupants and the ventilation conditions. PM concentrations were high due to the nature of the dental activities and they exceeded the outdoor levels by a factor of 5 to 6. The concentrations of the gaseous pollutants NOx were low while the SO 2 concentrations were hardly detectable. © 2008 FDI/World Dental Press.

Air quality in two large athletic halls with different ventilation (natural and mechanical) was investigated in relation to outdoor pollution and meteorological conditions. Simultaneous measurements of O3, NO, NO2 and SO2 were performed in the halls at two heights (at the arena and spectators’ seats) and outdoors. BTX concentrations at the spectators’ seats and outdoor NMHCs, CH4, PM10 and CO concentrations were also measured. Analysis of diurnal variations of the pollutants’ concentrations, cross correlation analysis of the concentration time-series and principal component analysis were applied to the collected data. Results revealed that outdoor pollution significantly affected indoor air quality of both halls. However, this effect was different for the two buildings, depending on the ventilation types, the wind direction prevailing at the areas and the kind of indoor activity recorded. It was found that the latter parameters controlled the pollutants concentration levels in the halls and their response to the changes of the outdoor pollution levels. Temperature and pollution stratification enhancing during athletic events were also evident in both halls but with different characteristics observed, such as the spatial distribution of the indoor pollutants. The airflow patterns prevailing in each hall, imposed by the ventilation operation schemes were important factors. © 2007 Elsevier Ltd. All rights reserved.

The aim of this study is to estimate future potential changes in duration of extreme dry and wet spells and rainfall intensity in Eastern Mediterranean. For this purpose, daily precipitation amounts, deriving from the regional climate model of UK Hadley Centre HadRM3P have been used for the present (1960-1990) and the future period 2070-2100 on 0.44° × 0.44° latitude by longitude grid. Future data are based on B2 IPCC emission scenario. For the identification of precipitation extremes three climatic indices were employed: a) CWD (Maximum number of consecutive wet days), b) CDD (Maximum number of consecutive dry days) and c) SDII (quotient of precipitation amount of wet days and the number of wet days of the period). They were calculated for the present and future period, on a seasonal and annual basis. A general future tendency was found towards drier Eastern Mediterranean, with reduced rainfall intensity. Longer dry spells are expected in all seasons, except autumn, with the largest increase in the southern part of the area. Extreme wet spells will shorten everywhere during all seasons, except autumn. Precipitation intensity was found reduced for all seasons and mostly for summer in South Aegean Sea. © 2008 Global NEST Printed in Greece. All rights reserved.

The objective of this study is to estimate the duration, frequency and intensity of precipitation extreme episodes in Cyprus, in relation with the two phases of the Eastern Mediterranean teleconnection Pattern (EMP), during winter for the period 1958–2005. A standardised teleconnection index was employed to determine the phases (positive and negative) and the strength of the EMP. The identification of the precipitation extremes was performed with the aid of four climatic indices. It was found that during the positive phase of the pattern, the length of dry periods reduces while that of wet periods increases, being followed by increase of frequency of extreme wet days and precipitation intensity. On the contrary, during the negative phase, the dry spells become longer in accordance with shortening of the wet spells, decrease of the number of extreme wet days and precipitation intensity.

Airborne particulate matter (PM10, PM2.5, PM1) and volatile organic compounds (benzene, toluene, m,p-xylene, o-xylene) samples were collected during winter and summer seasons of 2005 at two sites, representing an urban and a suburban region of the Greater Athens Area. Urban site traffic emissions were the major contributor to the concentration of PM2.5, PM10, toluene, and xylenes, while benzene and PM1 concentrations were presented in significant spatial variations. K+, Na+, Mg2+, Ca2+, NO3 -, Cl- and SO4 2 - ions were analyzed for the chemical characterization of the collected PM samples. The results showed that Na+ cations and SO4 2 - anions were the dominant species, during winter and summer, respectively, in both sites. The analysis of the synoptic scale and mesoscale atmospheric circulation during the experimental periods demonstrated that the meteorological conditions play a key role, not only in the variation but also in the distribution of the ionic concentrations at the three fractions of particulates and the dominant character (alkaline/acidic/neutral) of the particulates at the two sampling sites. © 2008 Elsevier Ltd. All rights reserved.

The capability of the general circulation model(GCM) Hadley Center atmospheric general circulation model 3P (HadAM3P) to reproduce the mean pattern and the frequency of circulation types concerning the 500 hPa geopotential height fields and 1000-500 hPa thickness fields over Europe and the Mediterranean region are evaluated against the NCEP/NCAR dataset for the period 1960-1990. Daily catalogues of the circulation types describing the regional circulation over Greece at the 500 hPa and 1000-500 hPa thickness fields are constructed based on the two datasets, and the corresponding seasonal frequencies are also estimated. The evaluation is performed for the mean seasonal fields of the two parameters, as well as for those derived for each circulation type separately. It is demonstrated that the HadAM3P model generally succeeds in reproducing the two fields over the examined area, while their seasonal variability is underestimated. The mean patterns of the circulation types are well represented by the model but the seasonal simulated frequencies of the circulation types are not in good agreement with the national centers for environmental predictions (NCEP) ones. The systematic errors of the frequency estimation and the variability in some anticyclonic or cyclonic types should be considered seriously, because it can introduce some degree of uncertainty in the results of a downscaling method based on a circulation type approach. Nevertheless, the links between the precipitation, temperature and circulation types frequency seem to hold in the GCMs. It is therefore safe to use the frequencies of circulation types as predictor in a downscaling technique in order to obtain future precipitation and temperature. Copyright © 2007 Royal Meteorological Society.

Significant changes in the regional climate of coastal and island stations in and bordering the Aegean during 1961-2002 are identified. Here we start with a very large number of parameters describing the surface temperature climate measured at 9 Greek island and coastal stations, and highlight those parameters which show regionally significant trends. Statistical significance of trends is assessed over multiple sliding time-windows of data. For each parameter and each time-window, a regional summary is produced using two quantities: the statistical significance of the trend in the regionally averaged parameter, and the number of stations which individually show significant trends in that parameter. The relationship between these two quantities provides a simple way of highlighting those climate parameters for which the region shows a trend, which is spatially relatively homogeneous, and those for which the region may need to be divided into sub-regions. Some very detailed characteristics of trends in daily surface temperatures are summarized in a concise way. Notably, the established cooling during the 1960s is shown to be especially strong for November and also December. Some trends were also shown to be significant only in terms of temperatures of unseasonably warm or cool days. For example, cooling in May temperatures in the late 70s/early 80s occurred most clearly for the low-temperature range-appearing as a decrease in the temperature reached by unseasonably cold days. This behaviour was inhomogeneous across the region, with significant cooling at only a subset of the stations. Such results could be used by subsequent analysis to focus on particular parts of the data, rather than using averaging to reduce the data. Copyright © 2008 Royal Meteorological Society.

In this study an attempt is made to investigate the impact of the urban heat island (UHI) effect on the energy consumption of a building using both, an accurate, extensively validated, transient simulation model and a neural network one. The energy consumption is calculated for a typical building during the summer period at 20 different sites of the Greater Athens area with the aid of the TRNSYS model. It was found that the UHI affects significantly the thermal behaviour of the building, implying much lower cooling load cost and energy cost in the suburban areas. A neural network model was then appropriately designed and tested for the estimation of the energy consumption, employing as an input, the UHI intensity. The results of both methods were tested and compared and it was found that there is a good agreement between the cooling rate values calculated by the TRNSYS programme and those derived by the neural model. Moreover, the employment of the neural model helped in quantifying the contribution of each input parameter in the calculation of the cooling rate, and it was demonstrated that the UHI effect is a predominant parameter, affecting considerably the energy consumption of a building in the Mediterranean region. © 2006 Elsevier Ltd. All rights reserved.

A statistical downscaling technique based on artificial neural network (ANN) was employed for the estimation of local changes on seasonal (winter, spring) precipitation and raindays for selected stations over Greece. Empirical transfer functions were derived between large-scale predictors from the NCEP/NCAR reanalysis and local rainfall parameters. Two sets of predictors were used: (1) the circulation-based 500 hPa and (2) its combination along with surface specific humidity and raw precipitation data (nonconventional predictor). The simulated time series were evaluated against observational data and the downscaling model was found efficient in generating winter and spring precipitation and raindays. The temporal evolution of the estimated variables was well captured, for both seasons. Generally, the use of the nonconventional predictors are attributed to the improvement of the simulated results. Subsequently, the present day and future changes on precipitation conditions were examined using large-scale data from the atmospheric general circulation model HadAM3P to the statistical model. The downscaled climate change signal for both precipitation and raindays, partly for winter and especially for spring, is similar to the signal from the HadAM3P direct ouput: a decrease of the parameters is predicted over the study area. However, the amplitude of the changes was different. Copyright © 2006 Royal Meteorological Society.

In this study, an attempt is made to investigate possible teleconnection patterns of atmospheric circulation, centered over eastern Mediterranean (EM) with the aid of gridded NCEP/NCAR daily values of geopotential heights for the period 1958-2003. For this purpose, two approaches have been used: correlation analysis and rotated principal component analysis (PCA) on a seasonal and monthly basis. A teleconnection pattern between the EM and northeastern Atlantic was identified at 500 and 300 hPa in winter, which will be referred to as the Eastern Mediterranean Pattern (EMP), appearing as an independent mode of the upper circulation. The pattern also exists in autumn but is substantially weakened with its dipole centers being shifted eastwards. Significant monthly variations were found in the location, strength and structure of the pattern. The employment of a standardized index demonstrated that the negative phase of the EMP prevails throughout the year with the maximum frequency at wintertime. Copyright © 2006 Royal Meteorological Society.

The purpose of this work is to assess, both experimentally and theoretically the status of air quality in a dentistry clinic of the Athens University Dentistry Faculty with respect to chemical pollutants and identify the indoor sources associated with dental activities. Total VOCs, CO2, PM10, PM2.5, NOx and SO2 were measured over a period of approximately three months in a selected dentistry clinic. High pollution levels during the operation hours regarding CO2, total VOCs and Particulate Matter were found, while in the non-working periods lower levels were recorded. On the contrary, NOx and SO2 remained at low levels for the whole experimental period. These conditions were associated with the number of occupants, the nature of the dental clinical procedures, the materials used and the ventilation schemes, which lead to high concentrations, far above the limits that are set by international organizations and concern human exposure. The indoor environmental conditions were investigated using the Computational Fluid Dynamics (CFD) model PHOENICS for inert gases simulation. The results revealed diagonal temperature stratification and low air velocities leading to pollution stratification, accompanied by accumulation of inert gaseous species in certain areas of the room. Different schemes of natural ventilation were also applied in order to examine their effect on the indoor comfort conditions for the occupants, in terms of air renewal and double cross ventilation was found to be most effective. The relative contribution of the indoor sources, which are mainly associated with indoor activities, was assessed by application of the Multi Chamber Indoor Air Quality Model (MIAQ) to the experimental data. It was found that deposition onto indoor surfaces is an important removal mechanism while a great amount of particulate matter emitted in the Clinic burdened severely the indoor air quality. The natural ventilation of the room seemed to reduce the levels of the fine particles. The emission rates for the fine and coarse particulates were found to be almost equal, while the coarse particles were found susceptible to deposition onto indoor surfaces. © 2007 Elsevier B.V. All rights reserved.

In the present study, an attempt is made to assess the atmospheric boundary-layer (ABL) depth over an urban area, as derived from different ABL schemes employed by the mesoscale model MM5. Furthermore, the relationship of the mixing height, as depicted by the measurements, to the calculated ABL depth or other features of the ABL structure, is also examined. In particular, the diurnal evolution of ABL depth is examined over the greater Athens area, employing four different ABL schemes plus a modified version, whereby urban features are considered. Measurements for two selected days, when convective conditions prevailed and a strong sea-breeze cell developed, were used for comparison. It was found that the calculated eddy viscosity profile seems to better indicate the mixing height in both cases, where either a deep convective boundary layer develops, or a more confined internal boundary layer is formed. For the urban scheme, the incorporation of both anthropogenic and storage heat release provides promising results for urban applications. © Springer Science+Business Media, B.V. 2007.

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.

Assessment of the reliability of NCEP-NCAR reanalysis data has been the subject of much recent climatological research. In this study, an attempt is made to evaluate the reanalysis data for maximum and minimum temperatures over Greece for the period 1958-2000. Considering that available observations for Greek stations are not included as input in the NCEP-NCAR reanalysis data, the evaluation was made using gridded observational data derived from long time series for the examined area, using an objective scheme. The reanalysis data were compared with respect to the intra-annual variability for extreme years, the inter-annual variability of the corresponding temperature anomalies, and their ability to represent cold and warm spells. Also, the correlation patterns between the two datasets of daily and monthly values were examined. In general, it was found that there was good agreement between the two datasets, although some regional and seasonal differences existed and these were associated with topographical features not correctly represented by the reanalysis model resolution, such as land-sea distribution and orography. © Springer-Verlag 2004.

The objective of this study was to investigate the evolution of pollutant concentrations generated by smoking in a controlled indoor environment. For this purpose, a small flat in the centre of Athens, Greece was equipped with NO x, O 3 and SO 2 continuous measuring instruments and portable analysers for spot measurements of TVOCs and CO 2, while two volunteer smokers remained inside and smoked as normal inhabitants. The results indicated that when windows are kept closed and smoking takes place NO x, CO 2 and TVOCs concentrations increase by an order of 3, 4 or 10 times, respectively, and decrease returning to initial levels after 1 or 2 h. © 2004 Elsevier B.V. All rights reserved.

This study has been carried out in order to evaluate the performance of CALPUFF modelling system in an area of complex topography. For this reason a comparison was performed between numerical results produced by this modelling system and those derived by two Eulerian models (UAM-V and REMSAD), together with experimental measurements. The meteorological input parameters, produced by the meteorological model CALMET are also examined through comparison with observations and to MM5 model results. Emphasis is given to the mixing height calculation, due to its importance in the prediction of pollutant concentrations. It was found that the overall performance of the CALPUFF is satisfactory and the results derived are compatible with these produced by the Eulerian models, especially under unstable atmospheric conditions. The CALPUFF model’s performance is improved significantly when vertical meteorological profiles produced by MM5 are additionally employed. Copyright © 2005 Inderscience Enterprises Ltd.

A new methodology is presented for the model estimation of an import-export pollution balance across country borders based on the classification of the synoptic scale atmospheric circulation. As an application, the transboundary transport of SO2 and NOx across the Greek borders is investigated for the year 1995. The annual total inflow-outflow fluxes were derived by weighting the model results with the annual relative frequency of the corresponding synoptic type appearance. The results of the new modelling method are similar to those following the day-by-day calculation of the inflow-outflow fluxes for the whole year 1995 using the same models. The advantage of the new methodology is that it is definitely less time-consuming than the day-by-day calculations. Hence, the proposed methodology can be employed as a practical tool for several applications requiring the implementation of complex models over longer time periods. © 2003 Elsevier Ltd. All rights reserved.

An attempt is made to examine the relationship of the surface circulation prevailing over Europe with the corresponding surface and 500 hPa over Greece by correlating Lamb weather types for western Europe and Hess and Brezowsky (HB) types for central Europe with those derived from a new classification scheme for the Greek area. It was found that it was difficult to formulate rules controlling the frequency distributions of the circulation types over the Greek area in relation to the circulation over western and central Europe. However, statistically significant correlation was found between certain types with high frequency, which is greater between Lamb and HB types with the surface circulation types over the Greek area, compared with 500 hPa circulation types. For the most correlated pairs, seasonal composites of mean sea-level pressure and 500 hPa geopotential height anomalies demonstrated that the formation of the circulation types over the Greek area depends on the extent, intensity of the anticyclonic or cyclonic centres, air mass characteristics, and stability profile in the lower troposphere over the regions examined, but especially over the central and eastern Mediterranean. Copyright © 2004 Royal Meteorological Society.

An attempt is made to examine rainfall variability over the Greek area in relation to 500 hPa atmospheric circulation. Daily precipitation series from 22 evenly distributed Greek stations have been used for the period 1958-2000, along with the classification scheme of daily circulation types at 500 hPa for the same period. The seasonal frequency and the trends of circulation types have been calculated. It was found that there is a general positive trend of anticyclonic circulation types and a negative one for cyclonic types. The seasonal trends of rainy days and the precipitation totals have also been calculated and analysed. A general decreasing tendency of winter rainfall is observed; the decreasing trend during autumn and spring is less significant. Concerning the frequency and intensity of rainfall per circulation type, a decreasing tendency becomes evident for the majority of the stations during winter, whereas there is an increasing tendency during autumn. A multiple regression-cross-validation model was developed using the seasonal frequency of circulation types as predictors and the seasonal rainfall totals as predictants. Only the winter modelled precipitation shows a good agreement with the observed precipitation, whereas for the other seasons the agreement is relatively poor. This could be caused by the influence of different factors that are not captured by the classification scheme used. The proposed model could serve as a circulation-based downscaling method that could be further applied to geopotential data available from general circulation models in order to study regional climatological consequences of future climate scenarios. Copyright © 2004 Royal Meteorological Society.

A methodology to estimate annual transboundary atmospheric transport is proposed, taking into account different background meteorological conditions. For this purpose, the 850 hPa synoptic-scale atmospheric circulation over the area examined was classified on a daily basis into 14 distinct synoptic types, using a database for an extended period of 40 years. The corresponding frequency of appearance of the synoptic types for a normal year was calculated. For selected days representing the synoptic types during this year, the inflow-outflow fluxes of NOx and SO2 were estimated through a balancing box with the aid of a MEMO/TRAPPA modelling system. According to the proposed method, the total inflow-outflow fluxes were calculated using the lateral fluxes for each representative day, being weighted by the annual relative frequency of appearance of the synoptic scale weather types. It was demonstrated that this approach provides similar results to the intensive method based on day-by-day calculations. Furthermore, this approach is easier to use, since it is based on the simulations of selected days, and it allows the investigation of the influence of different types of atmospheric circulation on the inflow-outflow budget. Copyright © 2003 Royal Meteorological Society.

The synoptic scale atmospheric circulation prevailing during hail days in Northern Greece is examined for a period of 26 years (1976-2001) during the warm season. The initial objective of this classification scheme is to serve as a predictive tool for hail forecasting and permit a specification of greater or lesser hail risk, within the forecast activities of the Greek National Hail Suppression Program. The atmospheric circulation was classified into seven synoptic types and their frequency distribution was examined on an annual and monthly basis. Southwesterly flow (SW) and Shortwave trough (SWT) appear to be the most conducive synoptic types for hailstorm activity. Furthermore, the thermodynamic environment favoring the hailstorm development and evolution was examined using selected instability indices. The frequency distribution of the instability indices for a period of 11 years (1991-2001) in association with the synoptic types demonstrated their performance as a forecasting aid for thunderstorm activity to occur in a given synoptic environment.

The horizontal and vertical distributions of volatile organic compounds (VOCs) and ozone (O3) concentrations within the lower troposphere over the greater Athens area. Greece, under sea-breeze conditions were studied. Furthermore, an attempt was made to explain the dynamic and chemical mechanisms responsible for the formation of these distributions. Measurements were collected using a specially instrumented Falcon 20-E5 research aircraft, ground-based meteorological instrumentation, and a network of air quality monitoring stations within the context of the Scientific Training and Access to Aircraft for Atmospheric Research Throughout Europe (STAAARTE) 1997 experimental campaign. Relatively high ozone values (55-100 ppbv) were identified within the first 300-400 m above ground, and significantly reduced values were found over the depth of the atmospheric boundary layer. High values of VOC concentrations [150-350 ppbCarbon (C)] were observed near the ground as well as within the first 300 400 m above ground. At higher altitudes, of 1400-1600 m, VOC concentrations remained relatively high (100-200 ppbC). It was demonstrated that the sea-breeze circulation plays a major role in the formation of the above-mentioned concentration levels and that chemical transformations explain specific characteristics.

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 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.

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.

A climatological study of the spatial and temporal distribution of mean relative geostrophic vorticity over the Mediterranean region is examined for a period of 40 years (1958-1997). Two major cyclonic vorticity maxima are found in the Mediterranean at low levels, one covers the Gulf of Genoa and south Italy and the second is located over the southeastern Aegean Sea with different diurnal and seasonal characteristics, controlling cyclone formation. At upper levels, a large scale cyclonic vorticity zone covers the whole Mediterranean region with notably increased values at 300 hPa, being associated with jet stream dynamics, especially in the eastern Mediterranean. The seasonal changes of the horizontal vorticity distribution substantially reduce with height while the diurnal changes become unimportant at upper levels. Furthermore, the investigation of the vertical structure separately for the main cyclonic centres in the Mediterranean implies the different mechanisms that are reponsible for cyclone formation in the western and eastern basin. Copyright © 2001 Royal Meteorological Society.

We investigated the short-term effects of synoptic and mesoscale atmospheric circulation types on mortality in Athens, Greece. The synoptic patterns in the lower troposphere were classified in 8 a priori defined categories. The mesoscale weather types were classified into 11 categories, using meteorologic parameters from the Athens area surface monitoring network; the daily number of deaths was available for 1987-1991. We applied generalized additive models (GAM), extending Poisson regression, using a LOESS smoother to control for the confounding effects of seasonal patterns. We adjusted for long-term trends, day of the week, ambient particle concentrations, and additional temperature effects. Both classifications, synoptic and mesoscale, explain the daily variation of mortality to a statistically significant degree. The highest daily mortality was observed on days characterized by southeasterly flow [increase 10%; 95% confidence interval (CI), 6.1-13.9% compared to the high-low pressure system), followed by zonal flow (5.8%; 95% CI, 1.8-10%). The high-low pressure system and the northwesterly flow are associated with the lowest mortality. The seasonal patterns are consistent with the annual pattern. For mesoscale categories, in the cold period the highest mortality is observed during days characterized by the easterly flow category (increase 9.4%; 95% CI, 1.0-18.5% compared to flow without the main component). In the warm period, the highest mortality occurs during the strong southerly flow category (8.5% increase; 95% CI, 2.0-15.4% compared again to flow without the main component). Adjusting for ambient particle levels leaves the estimated associations unchanged for the synoptic categories and slightly increases the effects of mesoscale categories. In conclusion, synoptic and mesoscale weather classification is a useful tool for studying the weather-health associations in a warm Mediterranean climate situation.

Sea-surface temperature (SST) variations of the oceans surrounding southern Africa are associated with seasonal rainfall variability, especially during austral summer when the tropical atmospheric circulation is dominant over the region. Because of instabilities in the linear association between summer rainfall over southern Africa and SSTs of the tropical Indian Ocean, the skilful prediction of seasonal rainfall may best be achieved using physically based models. A two-tiered retro-active forecast procedure for the December-February (DJF) season is employed over a 10-year period starting from 1987/1988. Rainfall forecasts are produced for a number of homogeneous regions over part of southern Africa. Categorized (below-normal, near-normal and above-normal) statistical DJF rainfall predictions are made for the region to form the baseline skill level that has to be outscored by more elaborate methods involving general circulation models (GCMs). The GCM used here is the Centre for Ocean - Land - Atmosphere Studies (COLA) T30, with predicted global SST fields as boundary forcing and initial conditions derived from the National Centres for Environmental Prediction (NCEP) reanalysis data. Bias-corrected GCM simulations of circulation and moisture at certain standard pressure levels are downscaled to produce rainfall forecasts at the regional level using the perfect prognosis approach. In the two-tiered forecasting system, SST predictions for the global oceans are made first. SST anomalies of the equatorial Pacific (NIÑO3.4) and Indian oceans are predicted skilfully at 1- and 3-month lead-times using a statistical model. These retro-active SST forecasts are accurate for pre-1990 conditions, but predictability seems to have weakened during the 1990s. Skilful multi-tiered rainfall forecasts are obtained when the amplitudes of large events in the global oceans (such as El Niño and La Niña episodes) are described adequately by the predicted SST fields. GCM simulations using persisted August SST anomalies instead of forecast SSTs produce skill levels similar to those of the baseline for longer lead-times. Given high-skill SST forecasts, the scheme has the potential to provide climate forecasts that outscore the baseline skill level substantially. Copyright © 2001 Royal Meteorological Society.

In this study an attempt is made to investigate comprehensively the dynamics of a case of cyclogenesis over the Aegean Sea within the context of the potential vorticity. At early stages the cyclogenesis is manifested by a large scale development at the upper levels over Adriatic Sea and Yugoslavia associated with an upper tropospheric potential vorticity anomaly. At later stages a smaller scale development was generated over Aegean Sea associated with a low-level potential vorticity anomaly and a surface warm anomaly. By means of a two-dimensional potential vorticity inversion it is demonstrated that the scale, the position and the strength of the involved anomalies contribute to the surface development, however, the low-level potential vorticity anomaly seems to constitute the most significant feature, more likely to be associated with condensation.

An experimental campaign was conducted at the lee side of a 1 km high steep mountain in order to study the development of strong downslope winds under favorable conditions, using combined remote and insitu instrumentation. The examination of the upstream atmospheric conditions reveals that the development of strong downslope winds is favored by a mean state critical layer or a significant decrease in static stability (such as at the top of a temperature inversion) at the proper height above the mountain top. Strong downslope winds could occur even for wind directions with a deviation of 60° off the axis perpendicular to the ridgeline, as long as the cross-mountain wind has a significant value (at least 7 ms-1). The developed disturbances are associated with intense downdrafts of the order of 4-5 ms-1 within the first 600 m above ground and characteristic vertical turbulent structures that were observed by sodar. The same phenomenon is observed not only during nighttime but also under unstable and neutral conditions of the atmospheric boundary layer (ABL) albeit with weaker intensity. Fourier analysis of the vertical velocity field demonstrated that the typical time period of intense disturbances was about 5 min. Further evidence is also provided for the application of the hydraulic-like theory under real atmospheric conditions. Copyright 2000 by the American Geophysical Union.

In this study an attempt is made to investigate the upper-tropospheric downstream development over north-west Europe, which leads to surface cyclogenesis in the central Mediterranean. A case study is analysed to demonstrate that the upper-tropospheric downstream development could be closely related to the upper-tropospheric frontogenesis that appears upon the north-eastern flank of a blocking high. The frontogenesis is characterised by a jet streak within a strongly baroclinic zone and a tropopause folding associated with cold stratospheric air intrusion into the troposphere. According to this interpretation, the eddy ageostrophic divergence of eddy geopotential fluxes (dispersion and spreading of eddy kinetic energy), other than friction dissipation and barotropic conversion to the mean flow, is mainly responsible for the loss of kinetic energy from a decaying depression of synoptic scale that has passed the mature stage. This dispersed eddy kinetic energy accumulates in the vicinity of the aforementioned jet streak where it is transferred downstream and further triggers the generation or rejuvenation of a new disturbance.

National and international authorities recommend a variety of air- quality standards that should not be exceeded in local and regional scales currently. With this work a uniform indexing scale is introduced which characterises several urban pollutants in a simple and comparable manner. The ’indicators’ proposed are implemented at the Athens Metropolitan Area (AMA) which is an area with serious pollution problems. Hourly data from all available monitoring stations are analysed during 1983 and 1995. This analysis demonstrates that the status of air quality in Athens can be characterised as acute with regards to photochemical pollutants while strong spatial and temporal variability is encountered for all pollutants. National and international authorities recommend a variety of air-quality standards that should not be exceeded in local and regional scales currently. With this work a uniform indexing scale is introduced which characterizes several urban pollutants in a simple and comparable manner. The ‘indicators’ proposed are implemented at the Athens Metropolitan Area (AMA) which is an area with serious pollution problems. Hourly data from all available monitoring stations are analyzed during 1983 and 1995. This analysis demonstrates that the status of air quality in Athens can be characterized as acute with regards to photochemical pollutants while strong spatial and temporal variability is encountered for all pollutants.

Studies of the energetics of synoptic-scale systems and similar kinds of investigation have traditionally used a Eulerian framework. In this study, the energetics of a synoptic-scale system have been considered using a quasi-Lagrangian method, in order to isolate the disturbance under consideration within a volume which moves together with the system at each stage of its development. Applying a Lagrangian framework implies that the dimensions of the computational area can be framework on the basis of predetermined criteria. In this study, an area surrounding a depression as shown on the surface analysis, has been selected. This area moves together with the centre of the depression. The energetics results obtained using such a quasi-Lagrangian scheme are compared to those obtained by using a Eularian framework. The synoptic-scale system studied here is a wintertime frontal depression, the greatest development of which occurred in the central Mediterranean on 7 December 1991. This depression moved east accompanied by significant temperature changes, heavy precipitation and gale force winds.

In this study day by day synoptic conditions are classified over the Attica peninsula for a period of sixteen years. Eight synoptic categories which are demonstrated to be statistically distinct are selected with respect to the atmospheric circulation in the lower troposphere. Furthermore, a methodology is proposed to classify the mesoscale patterns for the same period on the basis of surface wind measurements, and this distinguishes eleven distinct mesoscale categories. In general, the frequency analysis reveals that the anticyclonic circulation dominates while the weak mesoscale flows prevail, with preference in May and June. A day by day cross tabulation of the synoptic flow patterns with the mesoscale categories is then performed in order to identify the association of the synoptic conditions with the mesoscale flow regime. It was found that the synoptic conditions at the level of 850 hPa are closely related to the observed surface local flows and therefore it is suggested that the synoptic categories can be used for the identification of the most favourable mesoscale atmospheric circulation.

In this study an attempt is made to examine and analyse the mean characteristics of the katabatic flows at the western slope foot of a 1024m high knife edge mountain using a meteorological tower and three surface meteorological stations. In addition, the frequency distribution of the occurrence of the katabatic flow over one year period is studied along the characteristics of the flow arriving in the neighbouring urban area at a distance of 1.5km. It was found that the katabatic flow occurs mainly in autumn and spring with the highest frequency in April. The flow is generally characterised by small depth as it is affected substantially by the background flow. The expected direction of the katabatic wind dominates mainly at the level of 7 m, where the influence of the background flow is minimised. At the level of 18m the wind direction shifts, due to the interaction of the katabatic wind with the background flow. The katabatic flow can penetrate at a distance of 1.5 km being substantially weakened.

The daily relationship between synoptic circulation in the lower troposphere and air quality conditions is demonstrated over the Athens Metropolitan Area (AMA) for the period between 1983 and 1995. This approach utilises eight distinct synoptic categories at the isobaric level of 850 hPa and air quality indicators with respect to O3, NO2, CO, SO2 and black smoke (BS). It is demonstrated that high concentrations of photochemical pollutants and SO2 are closely related to the synoptic scale circulation while CO and BS show a weaker correlation. The most favourable synoptic pattern for the accumulation of high concentrations is the open anticyclonic circulation and for the occurrence of extreme events the closed anticyclone. NO2 and BS present the highest frequency of severe events. The seasonal analysis reveals that the bad and extreme air quality conditions due to O3 and NO2 prevail in the warm season while during the cold season CO and SO2 events prevail. The synoptic category representing a combination of high and low pressure systems, commonly associated with an intense wind flow over the Aegean Sea in the warm period, seems to favour the accumulation of photochemical pollutants, especially O3, over the AMA. The converse relationship was found for the SO2 case. During cold periods, the synoptic category characterised by ’zonal flow’ seems to favour the development of bad conditions for NO2 and particularly CO.

The objective of this study is to identify and establish the day-by-day relationship between mesoscale circulation and the air quality status over the Metropolitan area of Athens for a period of 13 years and to further investigate its temporal and spatial variability. Eleven distinct mesoscale patterns are identified using a formulated methodology based on surface wind measurements. The air quality conditions are classified into seven distinct classes according to the method of the Air Quality Indicators for five main pollutants namely, O3, NO2, SO2, CO and black smoke (BS). It was found that severe and bad air quality conditions over specific parts of the examined area are associated with the weak mesoscale patterns of southern direction or calm conditions. The good and moderate conditions are established mainly under northerly airflows. The most serious pollution problem favored even by intense northerly flow is attributed to O3 during the warm period in the northern zone. High concentrations of O3, NO2 and SO2 in the warm period are remarkably related to the pure sea breeze circulation, especially in the central and northern zone.

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.

An attempt is made to identify and investigate the structure, behaviour and synoptic characteristics of cyclogenesis over the Aegean Sea. A simple, realistic and easily used methodology of identification, in an analytical form, was established. This combines two types of criteria: synoptic and dynamic/thermodynamic. Six synoptic categories that could favour cyclogenesis over the Aegean Sea are distinguished according to the synoptic criteria. Grid-point values of geopotential height and temperature are used to calculate the dynamic/thermodynamic criteria, which are used to identify potential cases of cyclogenesis with respect to the thermodynamic prerequisites and the time continuance. It is demonstrated that cyclogenesis over the Aegean Sea is not as rare as was believed. The majority of cases result from the prevalence of a south-westerly flow, a long-wave trough and a closed system over the Aegean Sea. The cyclogenesis appears to occur primarily in the cold period of the year, mainly from October to May, and to have a small intensity and lifetime.

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.

The prediction of propagation and motion of thunderstorm activity, especially when heavy rainfall accumulation amounts could occur, is a matter of great importance. The objective of this study is to examine the possibility of using the environmental wind data for the problem of assessing predictability of the propagation and movement of thunderstorms in northern Greece. The propagation of radar thunderstorm echoes (radar echo reflectivity maxima) and cell movement (fine radar echo patterns) were determined for the spring and summer thunderstorms in 1992 and 1993 (April to September). Radar data were used by examining and analyzing digitally recorded plan position indicator for the entire time that the radar was operated. Multicell and single-cell thunderstorms were identified and thunderstorm propagation and cell motion were calculated for each. Environmental winds at standard levels of 850, 700, and 500 hPa were considered and, additionally, a mean 0- to 6-km layer density-weighted wind was also examined. In northern Greece, winds and radar thunderstorm echoes rarely moved from the clockwise sector 070° to 220° (ENE - SW). The movement of thunderstorm cells was not only steered by the 700-hPa level wind but was also well represented by the wind at the 500-hPa level and by the average wind in the layer 0-6 km. Using standard levels, cell motion can be determined by adding 5° to 500-hPa wind direction and reducing the 500-hPa wind speed by 30%. With the use of the mean 0- to 6-km density-weighted wind, cell motion can be represented by adding 5° to the 0- to 6-km wind direction and increasing 35% the 0- to 6-km wind speed. The propagation of multicell and single-cell thunderstorm echoes was very similar, in spite of initial expectations, and was approximately equal to the cell motion, suggesting that the new cells grew on all sides of existing multicell thunderstorms. The majority of northern Greece thunderstorms do not propagate significantly and their motion is substantially translational and similar to cell motion. However, in cases of severe thunderstorms, propagation was indicated. Characteristic cases of storm propagation are presented and vertical wind shear is investigated as a particularly important factor in influencing storm structure and evolution and the resulting storm propagation.