The climatology of (severe) thunderstorm days is investigated on a pan-European scale for the period of 1979–2017. For this purpose, sounding measurements, surface observations, lightning data from ZEUS (a European-wide lightning detection system) and European Cooperation for Lightning Detection (EUCLID), ERA-Interim, and severe weather reports are compared and their respective strengths and weaknesses are discussed. The research focuses on the annual cycles in thunderstorm activity and their spatial variability. According to all datasets thunderstorms are the most frequent in the central Mediterranean, the Alps, the Balkan Peninsula, and the Carpathians. Proxies for severe thunderstorm environments show similar patterns, but severe weather reports instead have their highest frequency over central Europe. Annual peak thunderstorm activity is in July and August over northern, eastern, and central Europe, contrasting with peaks in May and June over western and southeastern Europe. The Mediterranean, driven by the warm waters, has predominant activity in the fall (western part) and winter (eastern part) while the nearby Iberian Peninsula and eastern Turkey have peaks in April and May. Trend analysis of the mean annual number of days with thunderstorms since 1979 indicates an increase over the Alps and central, southeastern, and eastern Europe with a decrease over the southwest. Multiannual changes refer also to changes in the pattern of the annual cycle. Comparison of different data sources revealed that although lightning data provide the most objective sampling of thunderstorm activity, short operating periods and areas devoid of sensors limit their utility. In contrast, reanalysis complements these disadvantages to provide a longer climatology, but is prone to errors related to modeling thunderstorm occurrence and the numerical simulation itself.

During the last 10 years, the Institute for Environmental Research and Sustainable Development of the National Observatory of Athens has developed and operates a network of automated weather stations across Greece. The motivation behind the network development is the monitoring of weather conditions in Greece with the aim to support not only the research needs (weather monitoring and analysis, weather forecast skill evaluation) but also the needs of various communities of the production sector (agriculture, constructions, leisure and tourism, etc.). By the end of 2016, 335 weather stations are in operation, providing real‐time data at 10‐min intervals. This paper provides information about the logistics of this network, including real‐time applications of the collected data as well as information on the quality control protocols, the construction of the station data and metadata repository and the means through which the data are made available to users.

[1] In the frame of this paper, 1-year of lightning data from the experimental network ZEUS operated by National Observatory of Athens is analyzed. The area of interest is the Mediterranean and surrounding countries. At a first stage, the ZEUS data are compared with the data provided by the UK Met Office long-range lightning detection network for the warm period of the year. It was found that although ZEUS system underestimates the nighttime and early morning activity, during the rest of the day ZEUS system detects increased number of lightning compared to ATD that suffers from saturation when the number of flashes exceeds a certain threshold. Furthermore, the possible relationship between lightning and elevation, terrain slope and vegetation is investigated. The analysis showed that during spring and summer there is a positive relationship of lightning activity with elevation, while this feature is not evident during the rest of the year. The lightning activity was found to be positively correlated with the elevation slope throughout the year except winter. As it concerns the vegetation cover, it was found that over bareground the lightning ''yield'' is low during the whole year, while the inverse is true for woodland areas. During the warm period of the year, due to drying of the Mediterranean surfaces, the forested and wooded areas that keep soil moisture present increased lightning ''yield'' in contrast with the shrubland areas that, for the same period, present a decreased lightning yield.Citation: Kotroni, V., and K. Lagouvardos (2008), Lightning occurrence in relation with elevation, terrain slope, and vegetation cover in the Mediterranean,

Abstract. This paper introduces the development of a database of high-impact weather events that occurred in Greece since 2001. The selected events are related to the occurrence of floods, flash floods, hail, snow/frost, tornados, windstorms, heat waves and lightning with adverse consequences (excluding those related to agriculture). The database includes, among others, the geographical distribution of the recorded events, relevant meteorological data, a brief description of the induced impacts and references in the press. This paper further offers an extensive analysis of the temporal and spatial distribution of high-impact weather events for the period 2001–2011, taking into account the intensity of weather conditions and the consequent impact on the society. Analysis of the monthly distribution of high-impact weather events showed that they are more frequent during October and November. More than 80 people lost their lives, half of which due to flash floods. In what concerns the spatial distribution of high-impact weather events, among the 51 prefectures of the country, Attica, Thessaloniki, Elia and Halkidiki were the most frequently affected areas, mainly by flash floods. Significant was also the share of tornados in Elia, of windstorms in Attica, of lightning and hail events in Halkidiki and of snow/frost events in Thessaloniki.

Abstract. In the framework of this paper, one-year of lightning data from the experimental network ZEUS operated by the National Observatory of Athens is compared to collocated data provided by the LINET detection network. The area of comparison is limited to a part of Central-Western Europe, where LINET data exhibits the highest data quality, permitting thus to be used as the validation dataset. The location error of ZEUS was calculated to be ∼6.8 km, while the detection efficiency was ∼25%, with a characteristic underdetection during nighttime. Moreover, the analysis revealed that ZEUS is also capable to detect not only cloud-to-ground but also intra-cloud strokes. Analysis of a specific case study revealed that the spatial distribution of ZEUS was very close to that of LINET, although the total number of strokes as seen by ZEUS is much lower than the one from LINET. The overall analysis permitted to assess the main characteristics of ZEUS network, information considered of paramount importance before the use of ZEUS data for a variety of observational and modeling work.

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The Etesians (northern sector winds), which blow over the Aegean Sea during summer, affect human activities in the area. The numerous islands of the Aegean and especially Crete (a mountainous island in the southern Aegean oriented perpendicular to the surface flow) seem to play an important role in the modification of the wind field during the Etesians. The Crete mountain ranges, surrounded as they are by water, are an excellent example of a major isolated topographic feature which significantly modifies the regional airflow and pressure; however, this modification can hardly be defined due to the lack of observing stations over the sea. For this reason, the available land surface and ship synoptic observations are used, together with ERS scatterometer wind data in order to identify the regions over the Aegean where the wind reaches its maximum intensity, and to assess the influence of Crete on the wind field. Moreover, numerical modelling is used to provide some further insight on the orographically disturbed wind flow. Sensitivity tests performed with the hydrostatic model BOLAM show that the interaction of the Etesian wind flow with the mountains of Crete produces deceleration of the Etesians up to almost 120 km upstream, the leftward deflection of the air as it approaches the mountains, and the associated intensification of the flow east of the island.