B. D. Katsoulis

Indications of the Urban Heat Island in Athens, Greece

Abstract The analysis of air temperature data for a period of 22 years in the meteorological network stations in the greater Athens area shows clearly the effect of the urban heat island due to the city. This effect appears with different intensity according to the season and to minimum and maximum temperatures. In particular, the analysis of the data pointed out the variations caused by natural processes and anthropogenic activities. The influence of the growth of the city on the mean minimum temperature is evident.

Some meteorological aspects of air pollution in Athens, Greece

SummaryThe purpose of this paper is to examine some meteorological aspects of air pollution in the Athens area, Greece. Over recent years much attention has been directed to the importance of spells of weather in providing conditions that give rise to inversion of temperature at or near ground level, and hence to risks of severe incidents of air pollution. So, emphasis is given to the importance of anticyclonic conditions, associated with pronounced stability during ground level radiation inversions, when the same sampling sites show high concentration of both SO2 and smoke. Fifteen years of surface and radiosonde data obtained from Hellinikon Airport Athens are used for a climatological base. The meteorology of high concentration episodes has also been examined and the synoptic characteristics for the onset and end of such episodes have been identified. Finally, trends of pollution episodes and calculation of an “air stagnation index” for Athens have been obtained, which give a predicting indication about the occurrence of high pollution concentration.

Global distribution of aerosol direct radiative forcing in the ultraviolet and visible arising under clear skies

A deterministic atmospheric spectral radiative transfer model, that uses comprehensive climatological data, is developed to compute the global distribution of mean monthly clear-sky total direct aerosol radiative forcing in the ultraviolet (UV) and visible, between 0.2–0.85 μm, at the top of the atmosphere (TOA), within the atmosphere and at the Earth’s surface for winter and summer conditions. The aerosol data were taken from the Global Aerosol Data Set (GADS), given for various fixed relative humidity values and for 11 wavelengths within the UV—visible range, both for natural and anthropogenic aerosols. We first derive global climatologies of extinction aerosol optical thickness (AOT), single-scattering albedo (ωaer) and asymmetry factor (gaer), for actual relative humidity values within the aerosol layer, based on the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project and the Tiros Operational Vertical Sounder (TOVS) datasets. We include the global distribution of cloud cover using the D2 data from the International Satellite Cloud Climatology Project (ISCCP), to define the clear-sky fraction at the pixel level for each month. Supplementary 10-yr climatological data for surface and atmospheric parameters were taken from NCEP/NCAR, ISCCP-D2 and TOVS. Our present analysis allows the aerosol radiative properties and forcings to vary with space, time and wavelength. The computed mean annual global AOT, ωaer and gaer values are found to be 0.08, 0.96 and 0.73, respectively, at 0.5 μm. On a mean monthly 2.5¼ pixel resolution, aerosols are found to decrease significantly the downward and the absorbed solar radiation at the surface, by up to 28 and 23 W m−2, respectively, producing a surface cooling at all latitudes in both winter and summer. Aerosols are found to generally increase the outgoing solar radiation at TOA (planetary cooling) while they increase the solar atmospheric absorption (atmospheric warming). However, the model results indicate that significant planetary warming, by up to 5 W m−2, can occur regionally, such as over desert areas, due to strong aerosol absorption. A smaller planetary warming (by up to 2 W m−2) is also found over highly reflecting ice- or snow-covered areas, such as Antarctica and Greenland, as well as over Eastern Europe, Siberia and North America. In general, the aerosol-induced surface cooling exceeds the induced atmospheric warming, except for regions characterized by strong aerosol absorption (e.g. deserts). On a mean annual global basis, natural plus anthropogenic aerosols are found to cool the Earth by 0.6 W m−2 (they increase the planetary albedo by 0.28%), to heat the atmosphere by 0.8 W m−2, while they decrease the downward and net surface solar radiation (surface cooling) by about 1.9 and 1.4 W m−2.

The changing rainfall regime in Greece and its impact on climatological means

This paper presents an analysis of rainfall amounts in Greece. For this purpose, monthly and annual rainfall data from 36 representative Greek weather stations were analysed statistically. Rainfall is represented by a standardised annual departure calculated as the ratio of the annual departure to the standard departure. The departures were calculated for two regional subdivisions. Clear evidence of changes in Greeces rainfall patterns over the past 25 years were found. The effect of changes on climatological rainfall means (‘normals’) is addressed and possible shifts in the rainy season were investigated. The secular variability of long time-series of annual rainfall amounts was also examined. In addition, the five-year running means of annual and monthly rainfall data and ‘normals’ were plotted. Comparisons of the means from the ‘old’ and ‘new’ 10-, 20- and 30-year, CLINO (Climatological Normal) period 1961–90 (the latest global standard normal period) and from the periods 1900–49 (‘old’ regime) and 1950–99 (‘new’ regime), as well as examination of the secular trend, shows that there is evidence that some parts of Greece have had shifts in their rainfall regime towards drier conditions. In general, rainfall amounts began to decline in the 1980s, a trend which has continued to the present. Most years between 1980 and 1999 experienced below to well-below ‘normal’ rainfall. Discrepancies between the ‘old’ and ‘new’ mean values for the amounts of rainfall and differences in the duration of rainy season suggest that ‘normal’ rainfall data for various water supply, planning, engineering and agricultural national projects should be calculated for the last 30 years. This is mostly true because of indications of a changing climate in Greece, a fact that requires more recent updating of the climatological ‘normals’. Copyright

Analysis of the long-term precipitation series at Athens, Greece

In this paper, the characteristics of the long-term precipitation series at Athens (1858–1985) have been statistically analyzed. This study covers both the history and the analysis of the data. The ten-year mean amounts, the monthly and annual amounts averaged over the intervals 1858–1890, 1891–1985, 1951–1980, 1858–1985, the mean number of hours of precipitation and the precipitation intensity are given. The analysis of long-term time series of climatic data (in particular precipitation) is a useful tool for the study of past climate. Different statistical techniques are used in order to depict monthly, seasonal and annual variations, as well as trends, periodicities and recurrence intervals of the amount, intensity and number of precipitation days. The analysis reveals many interesting characteristics. These characteristics of the precipitation regime are extended to a time scale from seasonal variation to a semi-secular trend. The study of such long-term series may be helpful not only in practical applications of rainfall, but also for explaining the possible physical or anthropogenic mechanisms of climatic fluctuations and tendencies. The series of precipitation at Athens is one of the longest in south-eastern Europe.

Spatial and Temporal Variation of Precipitation in Greece and Surrounding Regions Based on Global Precipitation Climatology Project Data

Abstract In this study, the spatial and temporal distribution of precipitation in the broader Greek area is investigated for the 26-yr period 1979–2004 by using monthly mean satellite-based data, with complete spatial coverage, taken from the Global Precipitation Climatology Project (GPCP). The results show that there exists a clear contrast between the more rainy western Greek area (rainside) and the drier eastern one (rainshadow), whereas there is little precipitation over the islands, particularly in the southern parts. The computed long-term areal mean annual precipitation amount averaged for the study area is equal to P = 639.8 ± 44.8 mm yr−1, showing a decreasing trend of −2.32 mm yr−1 or −60.3 mm over the 26-yr study period, which corresponds to −9.4%. This decrease of precipitation, arising primarily in winter and secondarily in spring, is the result of a decreasing trend from 1979 through the 1980s, against an increase during the 1990s through the early 2000s, followed again by a decrease up to t...

An objective definition of air mass types affecting athens, greece; The corresponding atmospheric pressure patterns and air pollution levels

Abstract This work aims at defining characteristic air mass types that dominate in the region of Athens, Greece during the cold (November‐March) and the warm (May‐September) period of the year and also at evaluating the corresponding concentration levels of the main air pollutants. For each air mass type, the mean atmospheric pressure distribution (composite maps) over Europe and the Mediterranean is estimated in order to reveal the association of atmospheric circulation with air pollution levels in Athens. The data basis for this work consists of daily values of thirteen meteorological and six pollutant parameters covering the period 1993–97. The definition of the characteristic air mass types is attempted objectively by using the methods of Factor Analysis and Cluster Analysis. The results show that during the cold period of the year there are six prevailing air mass types (at least 3% of the total number of days) and six infrequent ones. The examination of the corresponding air pollution concentration levels shows that the primary air pollutants appear with increased concentrations when light or southerly winds prevail. This is usually the case when a high pressure system is located over the central Mediterranean or a low pressure system lays over south Italy, respectively. Low levels of the primary pollutants are recorded under northeasterly winds, mainly caused by a high pressure system over Ukraine. During the warm period of the year, the southwestern Asia thermal low and the subtropical anticyclone of the Atlantic Ocean affect Greece. Though these synoptic systems cause almost stagnant conditions, four main air mass types are dominant and ten others, associated with extreme weather, are infrequent. Despite the large amounts of total solar radiation characterizing this period, ozone concentrations remain at low levels in central Athens because of its destruction by nitric oxide.

The relationship between synoptic, mesoscale and microscale meteorological parameters during poor air quality events in Athens, Greece

Abstract This paper presents an analysis of some aspects of the ambient carbon monoxide (and some other pollutants) data from 1984–1993 recorded at 10 appropriately distributed locations in the Athens Basin. A major purpose of this study is to assess the influence of meteorological conditions on the concentrations of pollutants. Appropriate statistical methods have been applied which relate CO to wind speed and traffic patterns. In addition, an analysis of the meteorological conditions during days in which concentration levels exceed standards is given. The main characteristics of the results are: (i) The variation of CO, O3, NOx concentrations are strongly affected by traffic, wind speed and height of the inversion. (ii) It is difficult to define a firm urban rate of change for CO since it varies considerably from year to year, and it is not possible to unequivocally attribute the entire observed decrease to emission limitations and meteorological influences. An analysis of the seasonal, weekday and weekend diurnal variations is also presented. (iii) Surface ozone, NOx and other pollutant data at several urban and suburban stations in Athens with variable record lengths, all show a slight increasing trend.

Assessment of the Air-Quality Over Urban Areas by Means of Biometeorological Indices. The Case of Athens, Greece

This article deals with the part of air pollution, which has a particular relevance to the objective assessment of the quality of urban air. The correct understanding of the pollution levels over an urban region is of great importance to both authorized government services and to the community. This is particularly true for high polluted urban regions such as the Athens basin; so, it is important to recognize the levels of atmospheric quality by means of an easily understandable manner even for non-specialists. Thus, in this study an attempt is made for the application of two different groups of air quality indices (AQI) (statistical and biometeorological) by utilizing air pollutants measured into Athens basin, in a network of 17 measuring stations, during the period 2001-2002. The calculations of the (AQI) are referred to data of all 17 measuring stations and concern levels of air-pollution concentrations to both short (daily) and long time periods. Then comparisons were made between the obtained statistical and biometeorological indices in order to identify whether or not there is any existence of consistency between them. The compositions of the calculated indices were also determined, as well as, the most important air-pollutant for them. This procedure was applied for each day of the week in order to reveal the weekly cycle of indices and perhaps to isolate air-quality differences between weekdays and weekends. Finally, the varying forms of both frequency distributions are mainly caused by the impact related concentrations ranges of single air-pollutants which are typical of air-quality indices. Especially, PM10 and O3 seem to have a stronger influence on the determination of values of air quality indices.

The wind energy potential of western Greece

Abstract In this study wind data have been used to determine the monthly and annual variations of the wind at 13 meterological stations in west Greece. An analysis of the available wind data for the Ionian Sea islands and the western coasts of Greece is carried out to ascertain its potential for wind energy development. The effect of the limited number of daily observations available on the accuracy of the mean wind speed and annual wind energy estimates is ascertained. The wind speed and direction distributions are represented with Weibull functions. Besides, a mass-consistent numerical mesoscale model has been used to give an overview of the wind prospecting and sitting problem, and an example of its use for Corfu (Kerkira), an island in the Ionian Sea, is given. The comparison of the accuracy of the stimulation results versus measured wind at an available site is quite encouraging even though it cannot be conclusive since only one station is available.