Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. Henry E. Fuelberg, Florida State University, College of Arts and Sciences, Dept. of Meteorology. Title and description from dissertation home page (viewed Sept. 24, 2004). Includes bibliographical references.

As a part of the European research program CAL, sprite observations were carried out from the OMP observatory in the French Pyrenees during the summer 2003. Images of the sprites were taken by two remotely controlled CCD cameras. The 23 July was considered particularly interesting because we then had access to data concerning both cloud-to-ground and intracloud lightning activity. This day was therefore chosen as the object of the present study. A large thunderstorm with two convective cores, one to the north and the other to the south, developed over the South of France during the late afternoon, and about two hours after sunset, the first sprite was detected. During a little more than three hours, 13 sprites were observed, 7 over the northern system and 6 over the southern system. The images enabled us to determine the azimuth angle of each sprite from the OMP observatory. 12 of the 13 sprites could be associated to positive cloud-to-ground flashes, and by putting together the sprite directions and the locations of the associated flashes on the radar images, we managed to get a rough idea of the position of the sprites in the storm system, and also to estimate their vertical and horizontal extent. Satellite images were included at this point of the study, and it appeared clear that sprites tend to occur over the stratiform region of the storm system in the area with the coldest (highest) cloud tops. The associated positive flashes were also within or close to this portion of the storm. The sprite occurrences were studied in relation to the cloud-to-ground and to the intracloud activity. We found that sprites seem to occur in a late stage of each storm system, when the rate of negative cloud-to-ground flashes has considerably decreased, and when the ratio of positive cloud-to-ground flashes is much higher then during the most active phase of the storm. Globally, the intracloud activity is also low during the sprite-producing periods, but sudden "bursts" of intracloud lightning could frequently be observed at the moment of the sprite. The peak current of the positive flashes was found to be rather weakly correlated to their sprite-generating capacity. The available Schumann resonance measurements seem to indicate that the charge moment is a much more adequate parameter in this respect. The areal coverage of the radar echo was calculated. The result supports the idea that sprite events tend to appear almost exclusively over large thunderstorm systems.

Thesis. 1978. M.S.--Massachusetts Institute of Technology. Dept. of Meteorology. / Microfiche copy available in Archives and Science. / Bibliography: leaves 159-160. / M.S.

No description available.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1981. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: p. 235-247. / by Earle Rolfe Williams. / Ph.D.

Neste trabalho foram estudadas as linhas de instabilidade (LI) que atingiram a RMSP, o desenvolvimento de tempo severo associados a estas e a possibilidade de utilizar alguns parâmetros de tempo severo na identificação destes eventos. Foram identificadas 185 linhas no período de 2002 a 2009 através da observação de imagens de satélite. Estas ocorrem durante todos os períodos do ano, embora a frequência seja maior nos meses de verão, em função da maior atividade convectiva nessa estação do ano. Foi identificado que 94 % das LI se deslocaram para leste enquanto apenas 6 % tiveram seu deslocamento para oeste. Testes sobre configurações mais adequadas para a simulação de LI com o modelo BRAMS indicaram que as parametrizações de convecção disponíveis no modelo não são capazes de representar os eventos de forma adequada, havendo a necessidade de aumento na resolução das simulações e forte dependência da parametrização de microfísica. Foi constatado que espaçamentos de grade de oito quilômetros são suficientes para uma boa representação dos eventos. Os testes também indicaram que, dentre as opções disponíveis no modelo, o uso da deformação Anisotrópica para o tratamento dos processos turbulentos é o mais indicado para a simulação de casos de LI. Os parâmetros convectivos mostraram-se bons indicadores de tempo severo na presença de sistemas de origem baroclínica, apresentando valores similares aos encontrados para o HN. Porém, em tempestades onde o efeito termodinâmico é o fator mais importante, como nas tempestades isoladas ou na presença de ZCAS os índices cinemáticos nem sempre são bons preditores de tempo severo. A análise dos parâmetros convectivos obtidos das radiosondagens lançadas na RMSP mostra que, em muitos casos observados de LI, não há valores indicativos de ocorrência de tempo severo, ressaltando a importância do uso de perfis obtidos da modelagem numérica, numa região de abrangência maior, para a obtenção desses índices. / A study about the squall lines that reached Metropolitan Area of São Paulo (MASP), the development of severe weather associated with these and the possibility of using some parameters in the identification of severe weather are presented. It was identified a total of 185 squall lines between 2002 and 2009 through the analysis of satellite images. These lines occur during all periods of the year, although the frequency of squall lines is higher in summer months, due to increased convective activity in this season. Also, it was identified that 94 % of the squall lines moved eastward while only 6% of then moved westward. Tests about the most appropriate configuration to the squall lines simulation with BRAMS model indicated that the convective parameterizations available in the model are not able to represent the events in an adequate manner, being necessary to use higher resolutions in the simulations, which creates a stronger dependence on microphysics parameterization. We note that grid spacing as low as eight kilometers are enough for a fair representation of the events. The tests also indicated that, among all the options available in the model for turbulent process the anisotropic deformation is the one that works better in simulating squall lines. The convective parameters proved to be good indicators of severe weather in the presence of baroclinic systems, with similar values to those found for the Northern Hemisphere. However in storms where the thermodynamic effect is the most important factor, as in isolated storms and in the presence of the Southern Atlantic Convergence Zone (SACZ), kinematics indices are not always good predictors of severe weather. The analysis of convective indices, obtained through radiosondes variables launched in the MASP, shows that in many squall line observed cases there is no indication of severe weather occurrence. This fact increases the importance of using the profiles provided by numerical modeling over a larger region to obtain such indices.

La question de la prévision des évènements fortement précipitants se produisant sur le bassin Méditerranéen est au coeur du programme international HyMeX (Hydrological cycle in Mediterranean EXperiment, http://www.hymex.org/) dont l'un des objectifs est d'améliorer la prévision et la prévention des risques hydrométéorologiques du bassin méditerranéen dans le contexte du changement climatique. Durant l'automne 2012, une campagne de mesures de deux mois dite "Période d'Observation Spéciale" (SOP1) a été menée afin de documenter les conditions propices à la formation et au développement des évènements convectifs de type cévenol souvent responsables de crues dévastatrices. Pendant cette SOP1 un dispositif instrumental sans précédent a été déployé avec notamment pour la première fois sur le sol français un imageur à haute résolution spatio-temporelle permettant d'observer les décharges électriques en trois dimensions : le Lightning Mapping Array (LMA). Cet instrument a été combiné aux radars du réseau ARAMIS de Météo-France, et plus précisément aux radars Doppler à diversité de polarisation dans le Sud-Est de la France qui offrent la possibilité d'obtenir des informations sur le type et la distribution des hydrométéores au sein des systèmes précipitants. La production d'éclairs étant le résultat d'une électrisation issue des interactions microphysiques (collisions entre graupels et cristaux de glace en suspension), une description détaillée des différents types d'hydrométéores présents dans les nuages convectifs est essentielle. Dans cette optique, les algorithmes d'identification des hydrométéores développés par Météo-France ont été évalués puis améliorés. Les résultats de cette étude ont montré que les restitutions entre les différents radars étaient plutôt cohérentes, à condition que l'information sur l'altitude de l'isotherme 0°C soit correcte. Ce travail a ensuite été complété par la création, via une méthode originale, de composites 3D d'hydrométéores permettant de décrire la microphysique majoritairement présente dans les systèmes convectifs observés pendant la campagne HyMeX. La deuxième partie de ce travail s'est basée sur l'exploitation de la synergie radar-LMA sur une ligne de grain observée durant la SOP1 de HyMeX. Les principales informations déduites de ce couplage ont mis en exergue l'importance des processus microphysiques intervenant dans l'électrisation du nuage d'orage, ainsi que l'impact du relief sur l'activité électrique globale du système convectif. Sur les quatre heures de données analysées du 24 Septembre 2012, le déclenchement et la propagation des éclairs ont majoritairement été observés dans les espèces microphysiques que sont le graupel, les cristaux de glace et dans une moindre mesure la grêle. Cette étude souligne également le rôle important de la topographie sur l'activité électrique et montre que le passage d'un faible relief peut dramatiquement influencer la distribution et l'intensité des éclairs dans les régions convectives. / The Hydrological cycle in Mediterranean Experiment (HyMeX, http://www.hymex.org/) is a 10-year research program focusing on the quantification and understanding of the water cycle in the Mediterranean at various time and spatial scales with particular emphasis on high-impact weather events. This study takes place within the framework of the first HyMeX field phase (HyMeX-SOP1), which was conducted in the autumn 2012. The unique and extensive dataset collected during this field campaign offers the possibility to further investigate the complex relationships between cloud microphysics and lightning at play within mesoscale convective systems observed in southern France. With this regard, the present study make the use of a Lightning Mapping Array (LMA) along with operational dual-polarization weather radar. The first instrument allows documenting the three-dimensional lightning activity, whereas the second has the ability to determine the type of hydrometeors within cloud systems. Since the production of lightning is the result of an electrification created by microphysical collisions between graupels and ice crystals in suspension, a highly detailed description of hydrometeor types within convective clouds is needed. With this respect, an improved version of Météo-France hydrometeor classification algorithm was developed and evaluated so as to be able to discriminate between a large number of microphysics species. Overall hydrometeor species retrieved from a pair of neighbouring radars within a common sampling area are consistent from one to another. This study has however pointed out the need to check the consistency related to the identification of 0°C isotherm derived from numerical weather prediction model outputs before to perform hydrometeor identification. As a follow up to this work, a novel interpolation method allowing the remapping of single-radar hydrometeor fields onto a common Cartesian grid was developed in order to get access the three-dimensional hydrometeor distribution within HyMeX convective systems. Another part of this work aims at combining LMA and polarimetric radar observations to infer relationships between the total lightning activity, microphysics, and kinematics within the intense bow-echo system observed above the complex terrain of southern France during HyMeX. Using the synergy between LMA and polarimetric radar data, it is underlined that microphysical processes involved in cloud electrification, along with the impact of the topography play at part onto the global lightning activity. Based on a 4h analysis on the 24 Setptember 2012, it is found that lighting initiation and propagation take preferentially place within graupel, ice and to a lesser extent hail regions. This study also highlights the important role of topography on lightning activity and shows that even a small hill can dramatically influence the distribution and intensity of lightning within convective areas.

This thesis deals with the physical modeling of the initiation and propagation of upward positive leader discharges from grounded structures during lightning strikes. It includes the analysis of upward leaders initiated under the influence of the electric field produced by a dominant negative cloud charge and due to the combined action of a negative thundercloud and a descending downward stepped negative leader. Thus, a self-consistent model based on the physics of leader discharges is developed for the evaluation of the attachment of lightning flashes to any kind of grounded structure. The predictions of the model have been found to be in good agreement with the results of laboratory long air gap experiments and with classical and altitude rocket triggered lightning experiments. Due to the high application level and predictive power of the developed model, several contributions to the physical understanding of factors influencing the initiation and propagation of upward positive leaders during thunderstorms have been made. For instance, it has been found that the initiation of upward connecting leaders is strongly affected by the average velocity of the downward stepped leader. Similarly, it is shown that the switching voltage impulses used in the laboratory do not “fairly approximate” the electric fields produced by a descending downward leader, as claimed by supporters of Early Streamer Emission (ESE) devices. Furthermore, it is found that the space charge layer created by corona at ground level significantly increases the thundercloud electric fields required to initiate upward lightning leaders from tall objects. On the other hand, it is also shown that the upward leader velocity depends on the downward leader average velocity, the prospective return stroke current, the lateral distance of the downward leader channel and the ambient electric field. By implementing the model to the analysis of complex structures, it has been observed that the corners of actual buildings struck by lightning coincide rather well with the places characterized by low leader inception electric fields. Besides, it has been found that the leader inception zones of the corners of complex structures do not define symmetrical and circular regions as it is generally assumed.

The NASA Kennedy Space Center (KSC) is situated along the east coast of central Florida, where a high frequency of lightning occurs annually. Although cloud-to-ground (CG) lightning forecasting using radar echoes has been thoroughly analyzed, few studies have examined intracloud (IC) and/or total (IC CG) lightning. In addition to CG lightning, IC flashes are of great concern to KSC launch operations. Four years (2006-2009) of summer (June, July, August) daytime (about 14-00 Z) Weather Surveillance Radar – 1988 Doppler data for Melbourne, FL were analyzed. Convective cells were tracked using a modified version of the Storm Cell Identification and Tracking (SCIT) algorithm and then correlated to CG lightning data from the National Lightning Detection Network (NLDN), as well as grouped IC flash data acquired from the KSC Lightning Detection and Ranging (LDAR) networks I and II. Pairs of reflectivity values (30, 35, and 40 dBZ) at isothermal levels (-10, -15, -20 and updraft -10 degrees C), as well as a vertically integrated ice (VII) product were used to optimize criteria for radar-based forecasting of both IC and CG lightning within storms. Results indicate that the best radar-derived predictor of CG lightning according to CSI was 25 dBZ at -20 degrees C, while the best reflectivity at isothermal predictor for IC was 25 dBZ at -15 degrees C. Meanwhile, the best VII predictor of CG lightning was the 30th percentile (0.840 kg m-2), while the best VII predictor of IC was the 5th percentile (0.143 kg m-2), or nearly 6 times lower than for CG! VII at both CG and IC initiation was higher than at both CG and IC cessation. VII was also found to be lower at IC occurrence, including at initiation, than at CG occurrence. Seventy-six percent of cells had IC initiation before CG initiation; using the first IC flash as a predictor of CG occurrence also statistically outperformed other predictors of CG lightning. Even though average lead time for using IC as a predictor of CG was only 2.4 minutes, when taking into account automation processing and radar scan time for the other methods, lead times are much more comparable.

In the Asian and other monsoon regions of the world most of the severe weather observed is local or mesoscale in nature. Forecasting convective storms or mesoscale systems in the monsoon regions, especially in the tropics, has always been a challenging task to operational meteorologists. Maldives Islands, being situated in the tropical Indian Ocean, are affected by monsoon depressions and tropical cyclones. Thunderstorms and the passage of squall lines are well known sources of heavy rainfall. However, due to the lack of professional people and necessary equipment the weather systems around these islands are seldom studied. Therefore the aim of this thesis is to investigate whether the small islands can create sufficient perturbations in the mesoscale environment to result in the development of convective systems. In this regard, two numerical models, Weather Research and Forecasting model (WRF version 2.2.1) and Regional Atmospheric Modelling System (RAMS version 6.0) were used in this study. Two experiments were performed using the WRF model. In the first experiment, a case study was investigated where the selected day experienced heavy rainfall and thunderstorms. In the second experiment, the same case study was used but with the topographical and surface properties removed in order to investigate the influence of the island in modifying the mesoscale environment. All the experiments were initialized using the re-analysis data from NECP. WRF was able to predict the large scale synoptic features with reasonable accuracy when compared to the observations. Development of the boundary layer and the downstream advection of the temperature anomaly generated by the island were well represented. However, the magnitude of the effects was shown to be weak, probably due to the influence of large scale synoptic features. Even though the model was able to predict the large scale features and some of the mesoscale features, it did not predict any storm development and underestimated the precipitation. Therefore, it was decided to idealize the storm development using the RAMS model. RAMS model was used in a two-dimensional framework. The model was initialized horizontally homogenous using a single sounding and six simulations were performed. The simulation results clearly depicted that the small island can generate its own circulation and influence the mesoscale environment. The daytime heating of the island and the downstream advection of the temperature anomaly in a moist unstable atmosphere could trigger a thunderstorm later in the day. The storm becomes mature approximately 40-80 km offshore. This also suggests that triggering of a storm on one side of an atoll could influence the islands on the downstream side. Sensitivity of storm development to the thermodynamics showed that even with an unstable atmosphere, enough moisture in the lower and mid-troposphere is needed to trigger the storm. Sensitivity to the change of SST showed that convective development was suppressed with a drop of 1 oC. However, this needs further investigation. Assessment of sensitivity to the size of the island showed that the time of triggering of the storm was later and the scale of influence was smaller with a smaller island.