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This special issue of Climatic Change contains a series of research articles documenting co-ordinated work carried out within a 3-year European Union project ‘Prediction of Regional scenarios and Uncertainties for Defining European Climate change risks and Effects’ (PRUDENCE). The main objective of the PRUDENCE project was to provide high resolution climate change scenarios for Europe at the end of the twenty-first century by means of dynamical downscaling (regional climate modelling) of global climate simulations. The first part of the issue comprises seven overarching PRUDENCE papers on: (1) the design of the model simulations and analyses of climate model performance, (2 and 3) evaluation and intercomparison of simulated climate changes, (4 and 5) specialised analyses of impacts on water resources and on other sectors including agriculture, ecosystems, energy, and transport, (6) investigation of extreme weather events and (7) implications of the results for policy. A paper summarising the related MICE (Modelling the Impact of Climate Extremes) project is also included. The second part of the issue contains 12 articles that focus in more detail on some of the themes summarised in the overarching papers. The PRUDENCE results represent the first comprehensive, continental-scale intercomparison and evaluation of high resolution climate models and their applications, bringing together climate modelling, impact research and social sciences expertise on climate change.

Interest in atmospheric ultraviolet radiation (UV) research has increased considerably since the discovery of the Antarctic ozone “hole” and after indications of the existence of a similar kind of potential in northern high latitudes. UV measuring and research activities have grown accordingly, including development of new instruments, improvement in quality assurance/quality control methodologies, as well as development of models. Modeling methodologies have been applied for development of spaceborne techniques for estimating the global distribution of solar UV radiation. The European Conference on Atmospheric UV Radiation (ECUV), which was held on June 28 to July 2, 1998 in Helsinki, Finland, brought together 160 scientists from all parts of the world. The Journal of Geophysical Research (JGR) ECUV special issue summarizes 32 papers presented at the conference. New UV trend analyses based on spectral and broadband measurements and modeled UV data for the most recent decades indicate increases of UV-B radiation in various parts of Europe. Additional observational and modeled information on the impact of cloudiness, total ozone, ozone profiles, aerosols, and albedo on ground level UV irradiance has been gathered. The regional UV albedo has been studied over snow-covered surfaces, vegetation, and sea areas by aircraft-based, ground-based, and modeled methods. Further instrument development, improved QA/QC practices, and the application of data correction methods have already taken place but are also clearly needed in the future. Different methods for estimating UV irradiance using spaceborne information have been developed. Additional efforts to improve our understanding of the impact of cloudiness, albedo, and aerosol on UV are expected to improve spaceborne UV retrievals in the future. In Europe the European Commission and national funding agencies have been supporting research projects aimed at understanding the UV climatology in Europe through UV instrument development and intercomparisons, UV database establishment, and UV process, trend, and scenario studies.

SUMMARY A statistical evaluation of the air pollution situation in Athens for the year 1982 is presented. This analysis refers to NOz, NO, O3 and SO, concentration levels measured at the National Observatory of Athens (NOA) with a fully automatic monitoring station. The seasonal, weekly and daily variations of the above pollutants are in agreement with the corresponding activities in the city which are responsible for these emissions. Auto- and cross-correlation techniques are applied to the pollutants for January and July. It is found that all pollutants exhibit a well defined oscillatory behaviour because of their diurnal peaks in both months, except for NO2 in January whose auto-correlation function shows no regular shape. For the same reason the January NO-NO2 cross-correlation function behaves in the same way; the pairs NO-03 and N02-03 exhibit a clear oscillatory pattern. The spectral estimate method is also employed to the above pollutants for the two months in question. The energy content of 03, for instance, in July shows to be concentrated around a frequency corresponding to a periodicity of 24 hours.

The long-term precipitation series (1926–1990) from a 10 rain gauge station network in the Marathon area, Greece, have been statistically examined for trends. Application of the Mann—Kendall rank statistic test to the annual time series revealed a significant decreasing trend in the precipitation over the area. Similar decreasing trends are observed in neighbouring regions to the Marathon area (Athens basin and Beotia). The differences in precipitation trends during the transitional seasons among the three areas seem to be caused by the urbanization process. The seasonal variations and the periodicities of precipitation in the study areas also are examined. Finally, the spatial distribution of precipitation and the relation of precipitation amounts to the station height in the Marathon area are discussed.

Field measurements of wind, air temperature and humidity were taken at the eastern part of the Attika district in June 1991, to examine the topographic influences exerted on the local sea breezes. These influences are due either to the elongated Evia island, faced by the northern half of Attica coastline some tens of kilometers offshore, or to the coast-parallel range of Hymettos mountain, rising steeply 12 km onshore. The instrumentation consisted mainly of three tethered meteorological balloons released at characteristic sires (i.e., the coast, a location between shoreline and mountain foot and the mountain top) and three autographic ground-based anemometers operating at selected locations. Data from the ground-based and upper air stations of the Hellenic National Meteorological Service, as well as the diurnal weather maps were also obtained and analyzed. Observations were made under different synoptic wind and the latter was found to determine remarkably the significance of the topographic effects. A preliminary two-dimensional numerical approach was also made concerning the sea breeze capability to reach the Hymettos mountain top in the case of a weak opposing geostrophic flow.

Research related to the atmospheric effects of aircraft emissions has become increasingly important during the last decade within the EC Environment research programmes and the national programmes by the Member States of the European Union. The aim is to assess the global effects on climate and the ozone layer resulting from the increasing air traffic. Currently, the EC research activities are supported by the ‘Global Change, Climate and Biodiversity’ Key Action of the Environment and Sustainable Development programme. Coordination of EC and nationally funded projects occurs through the CORSAIRE (Coordination of Research for the Study of Aircraft Impact on the Environment) cluster. The results of the projects involved in CORSAIRE contribute to aviation related atmospheric assessments and provide a solid scientific basis for any future policy or regulatory action. Finally, the perspective of the European research, which makes a major contribution to the worldwide efforts in this field, are described.

Based on an extensive wind dataset over complex terrain, the commonly used small-angle approximation σv ≈ σθV is studied and found to overestimate over all wind speeds and σθ values observed. This should be anticipated due to the assumptions necessary to derive the approximation. Overestimation (of 10%–30%) is also observed in the small σθ range. The three parameters involved are further discussed to gain better understanding of the behavior of the approximation under different conditions. The standard deviation of wind direction σθ is shown to vary inversely with wind speed not only under stable, but also under convective conditions, reaching a site-dependent constant value at high wind speeds. The dependence of the ratio of the mean longitudinal wind component to the scalar mean wind speed on wind speed and σθ is examined, as well as that of the relevant standard deviations (σu,σVa). While the former obtains small values in the high-σθ, or low-wind range, or both, estimated values of the latter...

The identification of katabatic flows and their characteristics observed on a simple slope (the western side of Hymettos mountain) for a six-month period (January to June, 1990) are presented. This is the first application of data obtained from an 84 m high meteorological research tower recently erected at the National Center for Scientific Research “Demokritos” in Athens, Greece. The tower is described with respect to construction and instrumentation. The topography of the region and the observational site are also described. Criteria for the identification of katabatic flows are presented, while the frequency of occurrence of such flows is estimated. Finally, two case studies are analysed in order to reveal the special characteristics of the katabatic flows.

The three-dimensional compressible non-hydrostatic mesoscale model ADREA, developed at NCSR “Demokritos” for wind field and dispersion predictions in complex terrain, is used to perform two-dimensional simulations of orographically induced precipitation. The model makes use of a simple cartesian grid by allowing blocked areas in the grid volumes and grid faces which are crossed by the irregular topography (the so-called porous-medium concept). Additionally, since we are seeking both simplicity and low computational requirements, only one prognostic equation for the whole of the water substance is implemented. The model equations are based on the theory developed for multicom-ponent/multiphase mixtures and give full account of the effects arising from the relative motion between water particles and moist air. The evaluation of the feasibility of this approach is obtained by comparing model predictions with measurements and analytical results concerning orographic precipitation episodes in the mountain barrier of Sierra Nevada.

Accurate and reliable predictions and an understanding of future changes in the stratosphere are of major importance to our understanding of climate change. Simulating the interaction between chemistry and climate is of particular importance, because continued increases in greenhouse gases and a slow decrease in halogen loading are expected. These both influence the abundance of stratospheric ozone. In recent years a number of coupled chemistry climate models (CCMs) with different levels of complexity have been developed. They produce a wide range of results concerning the timing and extent of ozone-layer recovery. Interest in reducing this range has created a need to address how the main dynamical, chemical, and physical processes that determine the long-term behavior of ozone are represented in the models and to validate these model processes through comparisons with observations and other models. A set of core validation processes structured around four major topics (transport, dynamics, radiation, and stratospheric chemistry and microphysics) has been developed. Each process is associated with one or more model diagnostics and with relevant datasets that can be used for validation. This approach provides a coherent framework for validating CCMs and can be used as a basis for future assessments. Similar efforts would benefit other model communities allowing development in our understanding of the various processes as models increase their degree of complexity.

International audience; Between November 1999 and April 2000, two major field experiments, the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) and the Third European Stratospheric Experiment on Ozone (THESEO 2000), collaborated to form the largest field campaign yet mounted to study Arctic ozone loss. This international campaign involved more than 500 scientists from over 20 countries. These scientists made measurements across the high and middle latitudes of the Northern Hemisphere. The main scientific aims of SOLVE/THESEO 2000 were to study (1) the processes leading to ozone loss in the Arctic vortex and (2) the effect on ozone amounts over northern midlatitudes. The campaign included satellites, research balloons, six aircraft, ground stations, and scores of ozonesondes. Campaign activities were principally conducted in three intensive measurement phases centered on early December 1999, late January 2000, and early March 2000. Observations made during the campaign showed that temperatures were below normal in the polar lower stratosphere over the course of the 1999-2000 winter. Because of these low temperatures, extensive polar stratospheric clouds (PSC) formed across the Arctic. Large particles containing nitric acid trihydrate were observed for the first time, showing that denitrification can occur without the formation of ice particles. Heterogeneous chemical reactions on the surfaces of the PSC particles produced high levels of reactive chlorine within the polar vortex by early January. This reactive chlorine catalytically destroyed about 60% of the ozone in a layer near 20 km between late January and mid-March 2000, with good agreement being found between a number of empirical and modeling studies. The measurements made during SOLVE/THESEO 2000 have improved our understanding of key photochemical parameters and the evolution of ozone-destroying forms of chlorine.

Air quality has become a major issue during the last decades for a great number of cities and a matter of concern for both citizens and scientists. The quality of life has deteriorated in urban regions due to high pollution levels and a large number of people suffer from pollution effects on health. The prediction of air pollution episodes is very useful in that it enables the local authorities to give a warning against high pollutant concentrations or to take limitation measures on the emission sources. hi the past, efforts for pollution level forecasting have been reported using mathematical models (simulating the physical process) or statistical methods (Zannetti, 1990). The mathematical model operation requires high computational capacity, meteorological data and emission inventory. The required meteorological data are very difficult to collect and process, while the creation of an emission inventory is not an easy task.

The EURIDICE Code System, a joint effort by the CEA-France and DEMOKRITOS-Greece, performs wind field calculations and atmospheric dispersion analysis in meso and/or micro-scale level, over terrains of any complexity. Within the EURIDICE framework, the wind field is calculated by the ADREA code, which is a time dependent, 3-D compressible, non-hydrostatic code, treating the ground as a porous medium. The concentration of airborne and deposited pollutants is calculated by the MC31 code which solves the diffusion-advection equation by a MONTE-CARLO method. The modelling approach used in both codes permits the capability of utilizing detailed topography description, approaching reality: the calculational cell can include many ground surface elements of arbitrary size with their own identities (orientation, area, albedo, sunshade, soil type, roughness, deposition velocity, moisture etc). This is obtained by the DELTA code which provides the topography description using adjacent triangular surfaces in number and size depending only on the accuracy required. DELTA also calculates rain water runoff trajectories and accumulation points on the ground. For illustration purposes, the EURIDICE Code system has been used to simulate the wind field in the Greater Athens Area (GAA) and calculate the dispersion of a radioactive pollutant released from a point source located in the suburban area of the city.

A programme of ground-based stratospheric and total NO2 column measurements was instituted at the Laboratory of Atmospheric Physics (40.5° N, 22.9° E) in August 1985. We present here the results of the first two years of measurements with a modified Canterbury filter photometer, details of which are given in the text. The stratospheric NO2 column, obtained at twilight during low local NO2 levels, shows the seasonal variation with monthly mean values of about 6×10-15 molec. cm-2 in the summertime to about 2.2×10-15 molec. cm-2 in the wintertime. These measurements compare well with measurements obtained with different instruments by other groups at similar latitudes (about 40° N) but in different places. Also, the asymmetry of the evening-to-morning stratospheric NO2 over Thessaloniki was found to be on the average equal to 1.58. Total NO2 column over Thessaloniki has a pronounced seasonal variation with amplitude of 0.68 matm. cm which can be explained partly from measured local NO2 sources which discharge in the mixing layer and partly from photolysis of the NO2 reservoir species.

Surface ozone data at several urban and one suburban station in Greece with variable record lengths, all show increasing trends during the period 1980–1987. Greece with its prolonged hot and sunny summers is particularly susceptible to photochemical production of ozone. The increasing summertime trends in monthly mean surface ozone values are statistically significant and, depending on station, they range from 8.5 ppb/year up to 13 ppb/year. The trends are larger in the suburban location than at urban sites. NOx ozone precursors also exhibit an increase of about 35% per year, which is about the same for all seasons. The large trends found in surface ozone, are among the largest in Europe.

Spectrophotometric and solar photometric measurements of solar UV and visible radiation are used to obtain estimates of total columnar amounts of SO2, O3 and NO2 over major Thessaloniki. Routine measurements of these gases as well as of total oxidants/ozone, are grouped under representative meteorological and solar flux conditions. The diurnal and seasonal variability of air pollutants in this part of Greece and their causes are discussed.