| Περίληψη: | Lightning is considered as one of the most severe weather phenomena, affecting the environment and the society. Thunderstorms and the associated lightning are related to deaths, injuries, damage in properties, forest fires, floods and power system failures. Consequently, lightning and its spatiotemporal distribution are of great importance to our society.
This thesis focuses on the climatology of the lightning occurrence in the Mediterranean region and its relation with cyclone and thunderstorm intensity. For this analysis, we used 10-year (2005-2014) lightning data form ZEUS long-range lightning detection system, operated by National Observatory of Athens. These 10-year lightning observations were used in conjunction with cyclone tracks in order to study the relation of cyclones and lightning. Moreover, we applied a clustering method to the cloud-to-ground lightning stroke dataset in order to analyze the climatology of Mediterranean thunderstorms.
In the first part of this thesis, the spatiotemporal variability of the cloud-to-ground (CG) lightning activity and the effect of elevation, terrain slope, vegetation cover and convective available potential energy (CAPE) on the distribution of the CG lightning strokes in the Eastern Mediterranean for the years 2005–2014 were examined. Results showed that lightning depends on the diurnal cycle of insolation and the underlying topographic features of the region. The spatial lightning distribution confirms that lightning occurs mainly over continental areas. In spring and summer, CG lightning activity is dominant over the land while in winter and autumn, it is dominant over the marine areas. Lightning activity is maximum in June and minimum in January. Continental lightning activity displays strong diurnal variation, with a lightning peak in the late afternoon, while marine lightning exhibits minimal diurnal variation, with morning hours are slightly enhanced. Furthermore, it is found that the orography and the terrain slope affect the distribution of lightning especially in winter and autumn. During spring and summer, the forested areas have an increased preference of lightning occurrence. The number of CG lightning strokes and CAPE have a linear relation, indicating that CAPE values could be used as a proxy for the occurrence of lightning.
The second part of this thesis focused on the relation of intense Mediterranean cyclones with lightning activity. A 10-year data set of intense Mediterranean cyclones was used for a twofold objective: first to quantify the cyclone’s contribution to lightning occurrence in the region and second to investigate potential connection of lightning with cyclones intensity. For this reason, we used cyclone tracks, lightning observations and reanalysis from the European Centre for Medium Range Weather Forecasts, for the 10-year period of 2005–2014. Results showed that the cyclone's contribution to lightning occurrence is of the order of less than 10% over the Mediterranean Sea with several hot spots, where cyclone contributions might reach 20 to 30%. We found that the intense cyclones, which are associated with lightning activity close to their centre, constitute about one third of the total number of tracked cyclones, forming two cyclone groups: associated with and without deep convection. Analysis of the vertical profiles of ice and liquid water concentration in the proximity of the two groups' cyclone centers revealed that the first group presents about 35% more ice and 15% more liquid cloud content within the upper and lower atmospheric levels, respectively. The first group is also related to approximately three times greater values of CAPE in average. Further analysis showed no significant differences between the intensities of the two cyclone groups suggesting that deep convection may not be a major mechanism for the occurrence of intense Mediterranean cyclogenesis. Finally, we found that cyclones that are associated with lightning present highest lightning activity about 6 hours prior to the cyclones maximum intensity.
The third and final part of this thesis was devoted to the analysis of thunderstorm climatology and the thunderstorm's physical characteristics in the Mediterranean area. For the needs of this analysis, a clustering algorithm was developed and applied to the CG lightning stroke data, to identify thunderstorms from 2005-2014 in the study area. Results showed the diurnal cycle of insolation and the topographic features determine the spatiotemporal distribution of annual thunderstorms days. The majority of thunderstorms occur over land in spring and summer, while winter and autumn thunderstorms tend to occur over the sea. On average winter thunderstorms are less frequent, have less mean CG lightning intensity, are more long-lasting, have greater cluster areas and are moving faster than summer thunderstorms. Regardless the season, the mean propagation direction of the thunderstorms is from south-west to north-east. Furthermore, we found that the higher the duration of a continental thunderstorm, the earlier in the day it is expected to occur. The intensity of the thunderstorm and the associated precipitation have a linear relation over the Mediterranean region, where the rain yield was estimated to about 1.8 108 kg of rainfall per CG lightning.
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