Positive lightning flashes are known to be more intense and cause more damage than negative flashes, although positive flashes only occur about 10% of the time. This study expounds on cloud microphysical aspects of thunderstorms and investigates the occurrence, timing and location of ice particles within thunderstorms and correlates this to the occurrence of positive cloud-to-ground lightning events. Satellite data obtained from the Meteosat Second Generation (MSG) satellite, were used to: 1) depict Cloud Top Temperatures (CTT) by considering Infra Red (IR) radiation with a wave length of 10.8μm, 2) compare results from the CTT with the Brightness Temperature Difference (BTD) calculated by subtracting IR10.8μm from IR8.7μm, 3) after the cloud particle phase was determined from the abovementioned comparison, the sum of cloud-to-ground lightning strokes over a 1-hour period around the time of a reported lightning fatality was compared with cloud microphysical properties and then 4) these results were further compared with the lightning polarity obtained from the South African Weather Service (SAWS) Lightning Detection Network (LDN) data set. Four case studies were identified to investigate from the many available case study dates. These four cases occurred on 3 separate days namely: 1)22 November 2007, iii 2) 10 February 2009 and 3) 29 October 2009. There were two fatal events reported on 22 November 2007 and therefore two cases studies were compiled for this one date. On 10 February 2009 over 250 insurance damage claims were honoured but no fatality was reported. The three case studies mentioned above were classified as primary case studies. The 29 October 2009 case was classified as a secondary, more generalized case which was chosen in order to test whether the results gained from the first three cases were indeed noteworthy. Results gathered from two of the three primary case studies showed that the fatalities occurred when the most intense part of the thunderstorm was to the east of the location where the lightning struck the victims, although actual storm properties were not considered as being particularly severe. The lightning data for the primary case studies showed that the percentage of cloud-to-ground lightning was within 10% of the total number of strokes recorded for 22 November 2007, and above 10% for 10 February 2009. In the one secondary case study of 29 October 2009 the percentage of positive lightning was only between 2% and 4% of the total number of strokes recorded, which was significantly lower than in the three primary cases. A significant difference in cloud microphysics between the primary and secondary cases was the possible occurrence of super-cooled liquid water found in Cumulonimbus (CB) clouds in the secondary case. This could have been a determining factor for the difference in percentage of positive lightning between the primary and secondary case studies. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Geography, Geoinformatics and Meteorology / unrestricted

This thesis portrays new information concerning the cloud-to-ground (CG) lightning flashes or ground flashes produced by thunderclouds. It emphasizes the importance of characterizing lightning studies as the relationship between lightning mechanisms, and of incorporating the influence of geographical location, latitude and storm type. Sweden, Malaysia and USA were chosen as the main locations for field experiments in 2009 to 2011 to gather a significant number of negative and positive CG flashes. This work provided data on a total of 1792 CG lightning flashes (1685 negative and 107 positive ones) from a total of 53 thunderstorms by monitoring both the slow and the fast electric field and the narrowband radiation field at 3 and 30 MHz signals simultaneously. This thesis is comprised of: (i) the relationship of the Low Positive Charge Region (LPCR) and Preliminary Breakdown Pulse (PBP) trains to the occurrence of negative CG, (ii) slow field changes generated by preliminary breakdown processes in positive and negative ground flashes, and (iii) the occurrence of positive and negative ground flashes. It was revealed that the PBP train appeared have a higher strength in the in Sweden. The strength of the PBP train was caused by the LPCR; in contrast,  weak PBP trains were characteristic in tropical countries constituting insignificant LPCR and needing little energy to break the “blocking” agent to allow the flash to propagate downward to the ground. The second contribution concerns the characteristics of the PBP train mentioned; this includes novel information for Malaysia. Further, it is stated that there are some different characteristics in the PBP trains in Johor, Malaysia and Florida, USA. The studies of slow field changes generated by preliminary breakdown processes clarifies unclear features concerning the starting position of slow field changes generated by preliminary breakdown processes in positive and negative ground flashes. It was found that the slow field changes did not occur before the initial process of the commencement of preliminary breakdown. Single-station electric field measurements incorporating narrowband radiation field measurement and high resolution transient recording (12 bits) with an accuracy of several nanoseconds, allows one to distinguish between the intracloud activities and the preceding processes of ground flashes. The results for the interstroke intervals, amplitude distribution of subsequent return-stroke (SRS) and the number of strokes per flash in the tropics, subtropics and northern regions were similar. Finally, a significant number of positive return-stroke (RS) electric fields provided statistically significant information on the characteristics of these strokes.

Negative cloud-to-ground (CG) lightning flashes transport negative charge from cloud to ground. Negative ground flashes typically involve various processes identified as preliminary breakdown, stepped leader, return stroke, dart leader, dart-stepped leader, subsequent return stroke, and cloud activity between strokes, such as regular pulse trains and chaotic pulse trains. These processes can be identified through their electromagnetic field signatures. The main focus of this thesis is to document the features and understand the origin of electromagnetic fields, especially the chaotic pulse trains, generated by lightning flashes. Electric field measurements have been used to study lightning flashes in Sweden. The equipment was a parallel flat plate antenna with an analog filter buffer circuit, connected to a digital high speed oscilloscope. Four simultaneous measurements were made: wideband measurement of the E-field (the vertical component) and its time derivative dE/dt, and two narrowband measurements of the E-field, centred around 3 MHz and 30 MHz. Fourier and wavelet transforms were used in the analysis of the measured data. The results show that preliminary breakdown pulses are stronger radiators at 3 MHz and 30 MHz than are the return strokes. A comparison of our results with those of previous studies obtained in different geographical regions clearly shows that the strength of preliminary breakdown pulses decreases with decreasing latitude. It is higher in the temperate regions (Sweden, for instance) and lower in the tropical regions. A comparison of the time derivatives of preliminary breakdown pulses and of the narrow bipolar pulses shows that the physical origin of these two types of pulse is different, even though they may have similar appearances in the broadband fields. This thesis introduces a new procedure to estimate the zero-crossing time of the lightning-generated radiation fields. The procedure is based on the fact that the time integral of the radiation fields generated by a discharge event whose duration is finite is equal to zero, and the zero-crossing time corresponds to the time when the peak of the integral is reached. In addition to tabulating the various statistical parameters and features of Chaotic Pulse Trains (CPTs), it is shown that these pulse trains are created by the simultaneous propagation of several dart-stepped leader type discharges in the cloud. Each dart-stepped leader type discharge generates a Regular Pulse Train (RPT), and these pulse trains combine randomly in time to generate CPTs. This conclusion is based on the results obtained by numerical simulations and by analysing the signatures of these pulse trains using Fourier and wavelet transformations. The results presented in this thesis show that electromagnetic fields, even those measured from a single station, can be used to extract information concerning the physical processes that gave rise to these fields.

Το θέμα της παρούσας διπλωματικής εργασίας είναι η μελέτη της συμπεριφοράς απαγωγέων υπερτάσεων (SPD), όταν αυτοί αποτελούν μέρος του εσωτερικού συστήματος αντικεραυνικής προστασίας για μια οικιακή εγκατάσταση. Αρχικά γίνεται μια αναφορά στα αίτια δημιουργίας κρουστικών υπερτάσεων στο δίκτυο διανομής, ενώ στη συνέχεια δίνεται έμφαση στις υπερτάσεις που προκαλούνται από άμεσα ή έμμεσα κεραυνικά πλήγματα στο σύστημα. Στα πλαίσια της μελέτης δημιουργήθηκε ένα μοντέλο προσομοίωσης που αναπαριστά ένα δίκτυο χαμηλής τάσης TN-C-S με δύο πανομοιότυπους οικιακούς καταναλωτές, όσον αφορά τη δομή της εσωτερικής ηλεκτρικής εγκατάστασής τους παρουσία ή μη διατάξεων αντικεραυνικής προστασίας. Σκοπός της προκειμένης μελέτης είναι να δειχθεί η βέλτιστη συνδεσμολογία των διατάξεων προστασίας μέσα σε εσωτερικές ηλεκτρικές εγκαταστάσεις έτσι ώστε να αποτρέπεται η ανάπτυξη επικίνδυνων τάσεων και ρευμάτων για τον άνθρωπο και τον εξοπλισμό της οικιακής εγκατάστασης. / The subject of this project is a study upon the dynamic performance of surge arresters as a part of the internal lightning protection system for a residential electrical installation. At first, a reference about the actions or the natural phenomena that cause surge overvoltages in low voltage systems is given in detail and is followed by an essential theoretical approach on the lighning phenomenon. Both the causes, the consequences and the conditions in which a lightning occurs are being analysed. Furthermore, the characteristics and the qualifications a Lightning Protection System needs to comply with, are given, according to the Greek Standard ΕΛΟΤ-1197 and the European Standards IEC 62305-2, IEC 61643-12. Along with writing this essay, a simulation model using Simulink-Matlab was produced. The model represents a LV TN-C-S system that distributes power to two households with identical internal electrical installation. The aim of the present study is to indicate the optimal connection of the surge protective devices (SPDs) in the domestic electrical installation in order to prevent the appearance of potentially dangerous overvoltages to the humans or to the household equipment.

Depuis une vingtaine d'années, la qualité de la prévision de la trajectoire des cyclones tropicaux a fortement progressé mais peu d'améliorations ont été apportées à la prévision de l'intensité. Cette thèse s'intéresse aux changements d'intensité des cyclones tropicaux du sud-ouest de l'océan Indien sous l'angle original de l'activité électrique.Une première étude climatologique s'appuyant sur les données du réseau de détection d'éclairs World Wide Lightning Location Network (WWLLN) a permis de valider la qualité de ces données et de montrer que les cyclones tropicaux pouvaient être une source importante de production d'éclairs à l'échelle du bassin. À partir de ce résultat, une étude ciblée a été menée en considérant la spécificité du bassin et trois régions ont été définis (océan ouvert, région à proximité de la côte est de Madagascar et Canal du Mozambique). L'activité électrique des cyclones tropicaux dépend de la région où ils se trouvent ainsi que de leur stade d'intensité. Les éclairs semblent être un marqueur des phases d'intensification et d'affaiblissement dans certains situations. La troisième étape de cette thèse a consisté à simuler de manière idéalisée un cyclone tropical mature et a tenté d'expliquer les processus physiques à l'origine de l'activité électrique. Le modèle reproduit bien le comportement sporadique des éclairs habituellement observé. Des bilans montrent que ce cyclone présente trois phases caractéristiques, chacune associée à un comportement dynamique, microphysique et électrique différent. Enfin, une deuxième étude purement numérique a tenté d'identifier des estimateurs de l'activité électrique. Pourcelà deux approches ont été adoptées : une étude globale et une étude par cellule. Dans les deux cas, les meilleurs estimateurs sont la masse totale de graupel, le volume d'updraft et le produit des flux des masses de glace précipitante et non précipitante. / Tropical cyclone track forecast has improved over the past two decades but little improvement have been done in intensity changes forecast. This thesis focuses on the intensity changes of tropical cyclones in the southwest Indian ocean through the original aspect of lightning activity. A climatology of lightning activity in the southwest Indian ocean using data from the World Wide Lightning Location Network (WWLLN) is first proposed. Results showed that the WWLLN was able to locate and capture the lightning activity in the basin and that tropical cyclones can be considered as a major source of lightning flashes in some oceanic parts of this region. From this result, study of lightning activity in tropical cyclones of the southwest Indian ocean has been done considering the basin configuration. Thus, three regions were defined: open ocean, region near the eastern coast of Madagascar and the Mozambique Channel. The location and intensity of lightning activity depend on the region and the intensity stage. Lightning flashes seem to be a proxy of intensification or weakening under some conditions. The third step of this thesis was to simulate in an idealized framework a mature tropical cyclone. The model was first able to reproduce the sporadic behavior of lightning activity as observed. Budgets have shown that the presence of three distinctive phases characterized by different dynamical, microphysical and electrical behaviors. Finally, another purely numerical work tried to identify some proxies of lightning activity according two approaches : a global analysis anda per-cell analysis. In both cases, the best proxies are the total graupel mass, the updraft volume and the product of precipitating and non-precipitating ice mass fluxes.