Rozprawy doktorskie na temat „Mesoscale convective cloud systems”

Elevated convection occurs when convection originates from above the boundary layer. The interaction of an elevated storm with the stable layer beneath it often generates features such as waves and bores that maintain the convection. The Convective Storm Initiation Project (CSIP) took place in the UK in 2005. Only one case of elevated convection was observed during CSIP, in which several mesoscale convective systems (MCSs) formed. One MCS remained elevated and wave-lifted throughout the observation period. Another elevated MCS observed during IOP 3 was associated with Kelvin-Helmholtz billows. The billows and the elevated convection appeared to interact. The aim of this thesis is to use high-resolution numerical models to investigate the processes occurring in the elevated MCSs observed during CSIP. The thesis is presented in two parts. In the first part a simulation is performed using the Weather Research and Forecasting (WRF) model. The model reproduces the wave-lifted elevated convection in the early stages of the simulation but, unlike the observations, the simulated convection becomes surface-based and gravity current-lifted. The sensitivity of the simulated MCS to surface heat fluxes and diabatic cooling processes is explored. Surface heating and advection are shown to increase the buoyancy of the boundary layer air and enhance the transition to surface-based convection. Diabatic cooling processes are shown to maintain the simulated MCS in two ways: they strengthen the descent of the rear-inflow jet, generating a wave, and they also strengthen the undercurrent via cold outflow from the north of the storm. In the second part of this thesis the Met Office Large Eddy Model is used to investigate the interaction between Kelvin-Helmholtz billows and elevated convection. It is shown that there is a strong coupling between the updraughts and downdraughts in the billows and convective clouds.

L’étude des propriétés et processus microphysiques caractérisant la phase glace permet de mieux définir le rôle des nuages dans le cycle de l’eau et sur bilan radiatif de l’atmosphère. Les modèles atmosphériques et les codes d’inversion des données de télédétection utilisent des paramétrisations établies à partir de mesures in situ. Ces mesures servent également des besoins industriels en lien avec la problématique du givrage en aéronautique. L’étude présentée se base sur les données de deux campagnes aéroportées réalisées dans le cadre de la collaboration internationale HAIC-HIWC, ciblant les zones à fort contenu en glace que l’on peut observe rau sein des systèmes convectifs à méso-échelle (MCS) tropicaux. Sur la question des relations « masse-diamètre » (m - D) d’abord, une nouvelle approche est présentée. Basée sur la résolution d’un problème inverse, elle permet de restituer la masse des cristaux à partir de mesures colocalisées classiques en s’affranchissant de la traditionnelle hypothèse de loi puissance, et montre que cette dernière ne permet pas de représenter correctement les propriétés massiques de populations de cristaux hétérogènes (morphologie et tailles différentes) typiques des MCS. La variabilité horizontale des distributions de tailles permet d’étudier le vieillissement de l’enclume d’un point de vue microphysique et de souligner le rôle essentiel du processus d’agrégation dans l’élimination des petits cristaux apportés dans la haute troposphère par la convection profonde et dans la formation d’agrégats supra-millimétriques, précurseurs glacés des précipitations stratiformes. Les relations m - D restituées permettent d’identifier des régimes microphysiques distincts et ouvre la voie aux développement d’une paramétrisation de la masse volumique des hydrométéores en fonction de critères environnementaux
The detailed characterization of ice cloud microphysics is key to understand their role in theEarth’s hydrological cycle and radiation budget. The developement of atmospheric models and remote sensingalgorithms relies on parametrisations derived from in situ measurements. These measurements are also usedby the aviation industry to handle the problem of ice crystal icing. This PhD work presents an analysis of themass and size properties of ice crystals observed in high ice water content areas embedded in tropical mesoscaleconvective systems (MCS) during two airborne field campaigns of the HAIC-HIWC international project.A new approach is developped to derive mass-size relationships (m - D) from size distributions and icewater contents. The retrieval is formulated as an inverse problem which waives the power law constraint, aclassical assumption that proves to be an oversimplification when applied to heterogeneous populations of iceparticules typical of MCS anvils.The horizontal variability of size distributions and the aging of MCS anvils is described in terms of microphysicalprocesses. The importance of the aggregation growth process is emphasized as it efficiently removessmall ice particles brought into the upper troposphere by deep convection and significantly contributes to theformation of large agregates, precusor of the stratiform precipitations. The analysis of mass properties revealsthat distinctive microphysical regimes may be identified from the m-D relationship retrieved in various conditions.It paves the way toward a statistical model of the effective density of ice particles as a function of environmentalparameters

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1996.
Includes bibliographical references (p. 109-112).
by Marja Helena Bister.
Ph.D.

Approved for public release; distribution is unlimited
A better understanding of the role mesoscale convective systems (MCS) play in the formation stages of tropical cyclones will increase the ability to predict their occurrence and motion. This thesis employs high-resolution satellite imagery to observe the interaction between MCSs and their environment. Specifically, thirteen cases of tropical disturbances that eventually developed into tropical cyclones are analyzed to determine the role of MCSs in increasing the system organization. Following two conceptual models developed during the Tropical Cyclone Motion (TCM-93) mini-field experiment, each tropical cyclone is classified according to the relative importance of MCS activity to its development. Both conceptual models are verified through analysis and a third model is created to account for tropical cyclone developments that share features of the previous two models. An alternate approach is proposed for determining tropical system organization using only visible and infrared satellite imagery

The mesoscale organization and structure of convective clouds is thought to be rooted in the thermodynamic properties of the atmosphere and in the turbulent to mesoscale dynamics of the flow. Such structure may contribute to the transition between shallow and deep convection. The thermodynamic state of the boundary layer is forced by the amount of surface fluxes from below. Conversely, landscape patterns and land-cover heterogeneity may equally give rise to focused regions for deep convection triggering, in particular when patch sizes exceed 10 km. Since the convective boundary layer has a mediating function between the surface and deep storm clouds, the connection between surface and upper atmosphere is not straightforward. It is generally believed to involve local erosion of the capping inversion layer, the build-up of a moist energy supply, gradual humidification of the lower-free troposphere that reduces dry air entrainment into burgeoning deeper clouds, and thermal mesoscale circulations that can generate moisture convergence and locally forced ascent. To what extent microscale realistic surface heterogeneity and an interactive surface response matter to shallow and deep convection and its organization remains an open question. In this dissertation, we describe the coupling of a physiology-based vegetation model (HYBRID) and of a sea surface flux algorithm (COARE) to the cloud-resolving Active Tracer High-resolution Atmospheric Model (ATHAM). We investigate the full diurnal cycle of convection based on the example of the Hector storm over Tiwi Islands, notably the well-characterized event on 30th November 2005. The model performs well in terms of timing and cloud dynamics in comparison to a range of available observations. Also, ATHAM-HYBRID seems to do well in terms of flux partitioning. Whilst awaiting more thorough flux validation, we remain confident that the interactive surface response of both HYBRID and COARE is suited for the purpose of simulating convective-scale processes. We find the storm system evolution in 3D simulations to be robust with respect to differences in surface configuration and initialization. Within our 3D sensitivity runs, we could not identify a strong dependence on either realistic surface heterogeneity in the island landscape or on the interactive surface response. We conclude that in our case study at least, atmospheric (turbulent) dynamics likely dominate over surface heterogeneity effects, provided that the bulk magnitude of the surface energy fluxes, and their partitioning into sensible and latent heat (Bowen ratio), remain unaltered. This is consistent with 2D sensitivity studies, where we find model grid-spacing and momentum diffusion, governing the dynamics, to have an important influence on the overall evolution of deep convection. Fine grid-spacing is necessary, as the median width of updraught cores mostly does not exceed 1000 m. We associate this influence with the dry air entrainment rate in the wake of rising parcels, and with how resolution and diffusion act on coherent structures in the flow. In 2D sensitivity studies with differences in realistic heterogeneities of surface properties, we find little evidence for a clear deterministic influence of these properties on the transition between shallow and deep convection, in spite of largely different storm evolutions across the various runs. In these runs, we tentatively ascribe triggering to stochastic features in the flow, without discarding the relevance of convergence lines produced by mesoscale density currents, such as the sea breeze and cold pool storm outflows.

Thesis (M.S. in Meteorology and Physical Oceanography) Naval Postgraduate School, March 1998.
"March 1998." Thesis advisor(s): Russell L. Elsberry, Patrick A. Harr. Includes bibliographical references (p. 91-93). Also available online.

Mesoscale convective systems (MCS) are difficult to forecast due to their inherent unpredictability and small scale. Here the impact of model resolution on MCS structure and downstream forecast evolution are determined. Four case analyses of European MCSs using output from a 12-km grid-spacing model reveal a consistent potential vorticity (PV) structure with a positive PV anomaly in the mid troposphere (5 PVU) and negative PV anomalies above and to either side of it (-1 PVU). Convection-permitting models produce stronger MCS PV anomalies than convection-parametrizing models in a case study from July 2012. These differences persist after coarse graining to 100-km grid spacing and are largest in the upper troposphere. The effect of poor representation of MCSs on forecasts is investigated by adding MCS perturbations, calculated as differences from coarse-grained convection-permitting (4.4- km grid spacing) model output, to convection-parametrizing (25-km grid spacing) deterministic and six-member ensemble (with operational initial condition perturbations) model forecasts. Upper-level MCS perturbations have more impact than those at middle levels, though using all levels yields the greatest impact. For the first 30 hours differences grow on the convective scale related to the MCS and a developing UK cyclone, despite perturbation damping. Subsequently, differences grow rapidly onto the synoptic-scale and by five days impact the entire northern hemisphere. The MCS perturbations systematically affect the ensemble forecasts though the differences are smaller than those generated by the initial condition perturbations. MCS perturbations slow the eastward movement of Rossby waves due to ridge amplification. A downstream cyclone deepens by up to 3 hPa after five days and forecast errors (compared to analysis) in mean-sealevel pressure are reduced in most members (by up to 2%). Thus, perturbing convection-parametrizing models to include PV anomalies associated with MCSs produces alternative realisations to those generated by initial condition perturbations and so could be useful operationally.

Neste estudo sao analisados os sistemas convectivos de mesoescala (SCMs) subtropicais, de origem continental e oceanica, observados na America do Sul durante o South American Low Level Experiment (SALLJEX), realizado no verao de 2002/2003. Nesta analise sao avaliadas a distribuicao espacial, variabilidade diurna, ciclo de vida, deslocamento e parametros morfologicos dos SCMs continentais e oceanicos, sendo tambem investigadas as origens das fontes de umidade que contribuem para a genese desses sistemas. Outros aspectos explorados sao a caracterizacao das condicoes sinoticas com atencao a identificacao dos fatores pre-condicionantes e mecanismos de gatilho a conveccao; a avaliacao das condicoes dinamicas e termodinamicas observadas durante a genese do sistema convectivo e sua evolucao ao longo do ciclo de vida. Alem disso, sao realizadas simulacoes numericas dos SCMs com enfoque na verificacao da qualidade da previsao de precipitacao em funcao da capacidade do modelo em reproduzir as condicoes atmosfericas essenciais a geracao da conveccao. Os resultados mostram que os sistemas convectivos continentais sao mais numerosos que os oceanicos, e em ambas as regioes, SCMs com maior duracao apresentam taxas de expansao maiores nas primeiras horas do seu ciclo de vida. O jato de baixos niveis (JBN) afeta a formacao de sistemas continentais, porem, seu efeito sobre os SCMs oceanicos e minimo. O oceano Atlantico tropical, a regiao Amazonica e o oceano Pacifico subtropical, sao as principais regioes de origem da umidade para a genese dos sistemas convectivos, contudo, a atuacao dessas fontes depende da regiao de formacao do sistema e das condicoes sinoticas. O aquecimento diurno da camada limite convectiva, o jato de baixos niveis, o cavado em 500 hPa e a circulacao transversa sao os pre-condicionantes identificados na formacao dos SCMs continentais noturnos. Em relacao aos mecanismos de gatilho, os principais fatores sao os sistemas frontais e o fluxo catabatico dos Andes. Sistemas convectivos que apresentam ciclo de vida mais longo sao gerados em um ambiente em que se identificam mais de um fator pre-condicionante e de gatilho atuando de forma conjunta. Alem disso, o cisalhamento vertical do vento em baixos niveis parece ser um dos diferenciais na fase inicial dos SCMs de maior duracao. Os resultados das simulacoes numericas sugerem que quando as caracteristicas sinoticas e de mesoescala que atuam como pre-condicionantes ou mecanismos de gatilho sao mais bem definidas, o desempenho do modelo melhora sensivelmente, pois a simulacao consegue reproduzir com mais precisao as condicoes ambientais observadas durante a genese do sistema convectivo. Assim, sistemas com maior duracao tendem a ser mais bem simulados.
In this study, we analyze continental and oceanic mesoscale convective systems (MCS\'s) observed in the subtropics of South America during the South American Low Level Experiment (SALLJEX), held in the summer of 2002/2003. This analysis evaluated the spatial distribution, diurnal variability, life cycle, displacement and morphological parameters of the continental and oceanic MCSs, and also investigated the origins of moisture sources that contribute to these systems genesis. Other issues considered are the characterization of the synoptic conditions and the identification of preconditioning and trigger mechanisms, and evaluation of dynamic and thermodynamic conditions observed during the convective system genesis and its evolution over the life cycle. Moreover, MCSs numerical simulations are made focused on the model capability to replicate the essential atmospheric conditions to the convection generation. The results show that the continental convective systems are more numerous than the oceanic ones, and in both regions, MCS\'s with longer duration have higher growth rates in the early hours of their life cycle. The low level jet (LLJ) affects the formation of the continental systems, but its effect on oceanic MCS\'s is minimal. The tropical Atlantic Ocean, the Amazon region and the subtropical Pacific Ocean, are the main moisture source to the genesis of convective systems, however, the incidence of these sources depends on the convective system genesis area and the synoptic conditions. The daytime heating of convective boundary layer, the low level jet, the trough at 500 hPa and transverse circulation are the preconditioning processes identified in the formation of nocturnal MCS\'s with continental origin. For trigger mechanisms, the main factors are the frontal systems and the katabatic flow from the Andes Mountain. Convective systems with longer life cycles are generated in an environment in which several preconditioning and trigger mechanisms act jointly. Moreover, the vertical wind shear at low levels appears to be one of the differences in the initial phase of long living MCSs. The results of numerical simulations suggest that when the characteristics of synoptic and mesoscale that act as pre-conditions or triggering mechanisms are better defined, the model\'s performance improves significantly, because the simulation can more accurately reproduce the environmental conditions observed during the genesis of the convective system. Thus, systems with longer duration tend to be better simulated.

O presente trabalho verifica as principais características dos sistemas convectivos de mesoescala (SCMs), com origem continental e oceânica que, em pelo menos um momento do seu ciclo de vida, tiveram trajetória sobre a bacia Amazônica, durante um ano e meio de realização do projeto Green Ocean Amazon (GOAmazon). A análise incluiu a verificação da distribuição espacial, variabilidade diurna, ciclo de vida, deslocamento e áreas médias nas diferentes fases do ciclo de vida. Foi criada uma climatologia utilizando 14 anos de dados para comparar os resultados obtidos durante o GOAmazon. Para os SCMs que se formaram próximos às estações do GOAmazon foram realizadas análises das condições sinóticas, dinâmicas e termodinâmicas observadas durante a gênese e ao longo do ciclo de vida. Os resultados mostram que o número de ocorrências de SCMs continentais é de 7053 por ano. Em 2014 a ocorrência foi de 56,3% deste valor e em 2015 foi de 58% da climatologia para a mesma época do ano. Os SCMs ocorridos durante o GOAmazon também apresentaram menores tempos de vida, deslocamentos médios e velocidades médias. A evolução do ciclo de vida é muito similar para os SCMs de curta e longa duração, com poucas horas de diferença entre a mesma fase. O tempo que os sistemas de curta duração levam em média para alcançar a fase de maturação é de 2 a 3 horas enquanto que os SCMs de longa duração levam 5 a 6 horas. Durante o projeto GOAmazon este tempo foi igual a climatologia para SCMs de curta duração, porém variou entre 3 a 4 horas para SCMs de longa duração. A velocidade média, direção de propagação e deslocamentos médios variam de acordo com a época do ano e ao longo de toda bacia Amazônica. Os deslocamentos médios são maiores durante o inverno. A densidade média mensal de SCMs revela regiões preferenciais de gênese. São elas: 1) corrente abaixo da Cordilheira dos Andes, entre 10ºS e 20ºS/70ºW a 75ºW; 2) confluência do rio Tapajós com o rio Amazonas, por volta de 2,5ºS/54ºW; 3) sobre a Serra da Pacaraima, no Planalto das Guianas, em aproximadamente 5ºN/60ºW; 4) Serra do Imeri, no Planalto das Guianas, em 0º/65ºW e; 5) no norte do Mato Grosso, em torno de 10ºS/55ºW. Durante o projeto GOAmazon as anomalias negativas de densidade de SCMs ocorreram espalhadas ao longo de toda a bacia, com algumas regiões pontuais de maior ocorrência de sistemas. Os SCMs oceânicos ocorrem preferencialmente no período de inverno ao norte da bacia Amazônica. A frequência de ocorrência é baixa (em média 4 sistemas por mês), no entanto, eles possuem grandes áreas durante sua fase de maturação, grandes tempos de vida e deslocamentos. Como a maioria apresenta gênese muito próxima a costa, o desenvolvimento destes sistemas ocorre majoritariamente sobre a bacia Amazônica. Durante o projeto GOAmazon sua ocorrência foi muito menor comparado a climatologia e suas características médias diferentes. A análise detalhada para os 21 casos em que os SCMs ocorreram próximos às estações do GOAmazon mostrou que a combinação entre os ventos alísios direcionados para a bacia Amazônica e sistemas frontais que se aproximaram da região Tropical foram fundamentais na manutenção dos SCMs com longo ciclo de vida. Durante a ocorrência de SCMs com grandes áreas, os valores de cisalhamento foram mais altos comparados aos outros casos. Durante a maior parte dos anos 2014 e 2015 ocorreram padrões anômalos na circulação atmosférica, impulsionados por anomalias na temperatura da superfície do mar no oceano Pacífico Equatorial, o que justificaria a menor ocorrência, tempos de vida e deslocamento dos SCMs. De acordo com a literatura revisada, este é o primeiro trabalho que realiza uma análise climatológica anual da ocorrência de SCMs através de dados de alta resolução temporal e espacial com pouquíssimas falhas usando uma delimitação geográfica da bacia Amazônica, isto é, considerando somente os SCMs que em pelo menos um momento do seu ciclo de vida interagiram com a bacia Amazônica.
In the present study, we analyzed the continental and oceanic mesoscale convective systems (MCSs) that occurred over the Amazon Basin, during one year and six months of Green Ocean Amazon Project (GOAmazon). The analysis included the spatial distribution, diurnal variability, lifecycle, displacement and morphological parameters of the MCSs. A climatology using 14 years data was developed to compare the results obtained during the GOAmazon. A synoptic, thermodynamic and dynamic analysis was made for 21 MCSs that occurred next to the GOAmazon data collection sites. The climatology results show 7053 continental MCSs occurring along the year. In 2014, the occurrence was 56.3% of that value and during the period analyzed in 2015 it was 58%. The MCSs occurred during the GOAmazon also presented shorter lifecycles, displacements and speeds compared to climatology. The lifetime evolution of short lived and long lived MCSs present few hours of difference between the same phase. The time from genesis to maturation phase of short lived systems is 2 to 3 hours and for those long lived the time is 3 to 4 hours. The mean speed, direction and displacement are greater during the winter. The average density reveal preferential regions of genesis. They are: 1) downstream Andes Mountain, among 10ºS and 20ºS/70ºW and 75ºW; 2) confluence of Amazon and Tapajós Rivers, near to 2.5ºS/54ºW; 3) Pacaraima Mountains at Guyana Shield, in approximately 5ºN/60ºW; 4) Imeri Mountains at Guyana Shield, in 0º/65ºW and; 5) between north of Mato Grosso state and south of Pará state, around 10ºS/55ºW. During the GOAmazon the negative density anomalies occurred spread along the Amazon Basin, with some points of greater occurrence. The oceanic MCSs occurred preferentially in the winter season in the northeast of Amazon Basin. Their frequency of occurrence is fewer than continental, in average four MCSs per month. Nevertheless, they have large areas during their maturation phase, longer lifecycles and displacements. Most of them have genesis next to the land and their development is over the Amazon Basin. During GOAmazon their occurrence was fewer than the climatology and the features were different. The analysis for the 21 cases in which MCSs occurred next to GOAmazon stations showed that a combination of trade winds driven to Amazon Basin and frontal systems close to Tropical region were important for keeping the long lived MCSs. During occurrence of large area systems, the wind shear was greater than during other events. In most of 2014 and 2015, anomalous patterns in the atmospheric circulation, triggered by anomalous sea surface temperature in the Equatorial Pacific Ocean, occurred and this may justify the fewer occurrence of MCSs, lifecycle and displacement in that period. From our knowledge of the literature, this is the first work that makes an annual climatological analysis of MCSs occurrence through high temporal and spatial data and very few missing data using a geographical delimitation of Amazon basin. That is, considering only the MCSs that in one moment of their lifecycle, at least, interact with the Amazon Basin.

Os sistemas convectivos da região Amazônica possuem características microfísicas peculiares, que variam de um caráter convectivo marítimo (estação chuvosa) a continental (estação de transição seca-chuvosa). Essas características modulam a eletrificação desses sistemas, porém ainda não se sabe quais são os processos dominantes que intensificam o número de descargas elétricas de uma estação para outra: efeito dos aerossóis, termodinâmico, grande-escala ou topografia? Para responder à essa pergunta, o objetivo deste trabalho foi identificar e quantificar a importância de cada um desses efeitos na eletrificação dos sistemas convectivos da Amazônia. A metodologia foi baseada em análises de dados observacionais do experimento de campo DRYTOWET e em um modelo numérico com parametrizações de transferências de cargas e descargas elétricas. A análise do ciclo anual das descargas elétricas do tipo nuvem-solo (CGs) mostrou que a atividade elétrica dos sistemas precipitantes da região sudoeste da Amazônia aumenta durante a transição da estação seca para a estação chuvosa (Agosto a Setembro), associada aos sistemas convectivos com maior desenvolvimento vertical que acontecem nesse período. Com o estabelecimento da estação chuvosa (Novembro a Março), o número de CGs diminui porém a atividade elétrica ainda se mantêm. A porcentagem desses totais de CGs que tinham polaridade positiva (+CGs) tem média de 12% durante todo o ano, aumentando drasticamente para até 25% em Setembro, durante a transição entre as estações secas e chuvosa. Esse aumento da %+CGs ocorreu simultaneamente ao aumento da poluição atmosférica provocada pela queima de biomassa das pastagens realizada pelos fazendeiros locais, que as preparam para a agricultura e pecuária durante o início das primeiras chuvas. Por outro lado, o aumento da %+CGs das tempestades também ocorreu preferencialmente sobre a área de pastagem do estado de Rondônia. Através da análise de dados de radar dos sistemas precipitantes que ocorreram durante o experimento DRYTOWET, foi constatado que as tempestades positivas (tempestades que produzem mais de 50% de +CGs em 50% de seu tempo de vida) se formaram em ambientes mais secos e com alturas do nível de convecção por levantamento (NCL, altura da base da nuvem) maiores do que as demais tempestades (tempestades negativas), durante todo o experimento mas com maiores diferenças durante o final da estação seca (Setembro-Outubro). Com altura da base da nuvem mais elevada, a espessura da camada quente (ECQ - base da nuvem até a isoterma de 0oC) diminui, aumentando assim a velocidade das correntes ascendentes através de um melhor processamento da energia potencial disponível para convecção (CAPE) devido a um menor entranhamento. O aumento da velocidade das correntes ascendentes dentro da nuvem resulta em tempestades mais profundas e mais intensas. O efeito do aumento do NCL é uma característica das regiões com vegetação de pastagem, onde a razão entre o calor sensível e latente na superfície é maior do que as áreas florestadas, aumentando a altura da camada limite planetária. As diferenças de concentração total e distribuição de tamanho dos aerossóis devido ao aumento da poluição durante a transição entre as estações seca e chuvosa não foram conclusivas quanto a um possível efeito na distribuição de hidrometeoros das tempestade positivas e negativas, uma vez que o ciclo diurno da concentração dos aerossóis acompanha o ciclo diurno da camada limite planetária, que também regula o efeito da ECQ. Simulações numéricas com um modelo 1D de nuvem, acoplado à parametrizações de transferências de cargas elétricas entre hidrometeoros e raios, mostraram que a estrutura termodinâmica da atmosfera foi a maior responsável pela eletrificação das tempestades simuladas, aumentando a velocidade das correntes ascendentes. O efeito do aumento do número de aerossóis, que inibe da fase quente da nuvem e conseqüentemente fortalece a da fase fria da nuvem fornecendo mais vapor e gotículas de nuvem para essa região, provocou a diminuição da quantidade de granizo nas tempestades simuladas e o aumento de partículas agregadas menores, como os flocos de neve e graupel, diminuindo a freqüência de raios.
Amazonian convective systems have unique microphysical characteristics, varying from a maritime convective behavior (rainy season) to a continental behavior (wet-dry transition season). These characteristics modulate the electrification of these systems, however it is still not well understood which are the dominant processes that intensify the frequency of lightning from one season to another: aerosol effect, thermodynamics, large-scale variability, landscape or topography? To answer this question, the objective of this study was to identify and quantify the importance of each one of these effects on the electrification of convective systems over the Amazon. The methodology was based on the analysis of observational data from the field experiment DRYTOWET and a numerical model with charge transfer parameterizations and lightning discharges. The cloud-to-ground (CG) lightning discharges annual cycle presented that the electrical activity of the southwestern Amazonian precipitating systems increased during the transition between the dry and wet seasons (August to September), in association with the convective systems deepening. With the establishment of the wet season (November to March), the number of CGs decreased but the electrical activity continued. The mean annual percentage of cloud-to-ground lightning of positive polarity (+CGs) was 12%, increasing drastically to 25% in September during the transition between the dry and wet seasons. This percentage of +CGs raise happened simultaneously with the increase in the atmospheric pollution due to the pasture biomass burning, held by local farmers to prepare the soil for agriculture and livestock during the begging of first rains. On the other hand, the increase in %+CGs also occurred preferentially over pasture areas of Rondonia state. Through the analysis of radar precipitating systems that occurred during the field campaign DRYTOWET, it was noted that positive thunderstorms (storms that produced more than 50% of +CGs over 50% of their life time) were initiated in drier and higher lift condensation levels (cloud base height) environments than other storms (negative thunderstorms) during all the field experiment, especially in the end of the dry season (September-October). A higher cloud base height is associated with a shallower warm cloud depth (cloud base height to the 0oC isotherm) and consequently less entrainment, increasing the updrafts due to a more efficient processing of the convective available potential energy (CAPE). This increase in updrafts inside the clouds results in deeper and stronger thunderstorms. The higher cloud base heights is a characteristic from pasture regions, where the ratio between sensible and latent heats at surface is greater than forested areas, which increases the top of the planetary boundary layer. The differences in the aerosol total concentration and size distribution, due to the increase in the atmospheric pollution during the transition between the dry and wet seasons, were inconclusive in a possible aerosol effect in the strength of positive and negative thunderstorms, once the aerosol concentration diurnal cycle follows the cycle of the planetary boundary layer, that also regulates the warm cloud depth effect. Numerical simulation of an 1D cloud model, coupled with charge transfer between the hydrometeors, showed that the thermodynamic structure was the main responsible feature for cloud electrification, increasing the updraft velocities. The pollution effect was masked in simulations, which inhibit the warm precipitation and consequently strength the the mixed and cold regions of the cloud, was responsible for an increase in the number of smaller aggregated particles, like snow flakes and graupel, decreasing the electrification and lightning frequency.

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The happening of Mesoscale Convective Systems (MCS) over a region causes impacts on the surface, that is usually associated with heavy precipitation, strong wind gusts and hail. This phenomenon is common in southern Brazil reach the Rio Grande do Sul (RS) and causes damage to its economy. Therefore the MCS study that reaching of RS is of great importance, as well as allowing a greater understanding of weather and climate of the region also provide knowledge to decision makers. Thus, the aim was to study the seasonal physics features of MCS whose genesis and maintenance happened southward of 20 º S, with life cycle of at least 6 h, with spontaneous initiation and normal dissipation and that reached the region covering the RS from 2004 to 2008, using the FORTRACC method (Forecasting and Tracking of Active Cloud Clusters). Channel 4 Geostationary Operational Environmental Satellite (GOES) images from 2004/01/01 to 2008/12/31, with 4 km x 4 km spatial resolution in your subsatellite point and ½ hour temporal resolution were used. These images were provided by DSA/CPTEC/INPE (Satellite Division and Environmental Systems/Center for Weather Forecast and Climate Studies/National Institute for Space Research) and served as a database for the use of the FORTRACC method. The results showed that: i) the MCS that affected the RS occurring majority in the warmest quarter of the year (JFM), ii) in the four quarters of the study period most of the MCS that affected the RS (64.7%) had lifetime between 6 and 12 h, iii) MCS with longer life cover larger areas, and iv) the minimum temperature (Tir) had lower mean values observed in the MCS in OND and JFM, v) in all periods of the year around 6.4% of the MCS showed a more linear format, approximately 64.0% have irregular shape and about 29.6% had more circular shape, vi) lower values of Tir are related to MCS with shape close to circular (eccentricity equal to or greater than 0.5) and longer duration, vii) the three phases of the life cycle of MCS that affected the RS can occur in a distributed manner throughout the day, viii) in all periods of years the births of MCS that affected RS occur predominantly in and around the grill that covers the RS, with a tendency to start over on the west side of the grid on the mainland; ix) in all quarters there is a tendency for the mean trend preferential be from west to east.
A passagem de Sistemas Convectivos de Mesoescala (SCM) sobre uma região causa impactos em superfície, pois geralmente está associada à precipitação intensa, fortes rajadas de vento e granizo. Esse fenômeno bastante comum na região sul do Brasil afeta o Rio Grande do Sul (RS) e causa prejuízos a sua economia. Portanto o estudo dos SCM que atingem o RS é de grande importância, pois além de permitir um maior entendimento do tempo e do clima da região também fornece subsídios aos tomadores de decisão. Assim, o objetivo deste trabalho foi estudar as características físicas sazonais dos SCM cuja gênese e manutenção ocorreu ao sul de 20ºS, que apresentaram ciclo de vida de no mínimo 6 h, que tiveram nascimento espontâneo e dissipação normal e que afetaram a região que cobre o RS durante o período de 2004 a 2008, utilizando a técnica ForTrACC (Forecasting and Tracking of Active Cloud Clusters). Foram utilizadas as imagens brutas do satélite GOES (Geostationary Operational Environmental Satellite) do canal 4, com resolução espacial no seu ponto subsatelite de 4 km x 4 km e resolução temporal de ½ hora, do período de 01/01/2004 a 31/12/2008. Essas imagens foram fornecidas pela Divisão de Satélites Ambientais do Centro de Previsão de Tempo e Estudos Climáticos do Instituto Nacional de Pesquisas Espaciais (DSA/CPTEC/INPE) e serviram como base de dados para a utilização da técnica ForTrACC. Os resultados para o período de estudo mostraram que: i) os SCM que afetaram o RS apresentaram maior número de ocorrência no período mais quente do ano (JFM); ii) nos quatro trimestres do período de estudo a maioria dos SCM que afetou o RS (64,7%) apresentaram tempo de vida entre 6 e 12h; iii) SCM com maior tempo de vida cobrem áreas maiores; iv) a temperatura mínima (Tir) apresentou valores médios menores nos SCM observados em JFM e OND; v) em todos os períodos do ano em torno de 6,4% dos SCM apresentaram formato mais linear, aproximadamente 64,0% apresentaram formato irregular e em torno de 29,6% apresentaram formato mais circular; vi) valores menores de Tir estão relacionados à SCM com formato mais próximo do formato circular (excentricidade igual ou maior do que 0,5) e com maior duração; vii) as 3 fases do ciclo de vida dos SCM que afetaram o RS podem ocorrer de forma distribuída ao longo do dia; viii) em todos os períodos do ano os nascimentos dos SCM que afetaram o RS ocorrem predominantemente dentro e em torno da grade que cobre o RS, com tendência a iniciarem mais no lado oeste da grade sobre o continente; ix) em todos os trimestres há uma tendência de a trajetória média preferencial ser de oeste para leste.

La formation des systèmes convectifs est un processus complexe qui s'étend de l'échelle synoptique, avec la mise en place de circulations favorisant la convection, à la micro-échelle, avec les processus de formation et de croissance des hydrométéores. C'est aux échelles les plus fines que se concentre cette thèse dont l'objectif est d'étudier l'apport d'une microphysique complexe sur l'occurrence et la morphologie d'évènements fortement précipitants. La microphysique évaluée est celle du schéma LIMA, de type bulk à deux moments, capable de prendre en compte l'évolution d'une population d'aérosols multimodale et le traitement pronostique de son interaction avec les nuages et les précipitations. Dans un premier temps, l'apport de la microphysique de LIMA est évalué en comparaison à la microphysique bulk à un moment du schéma ICE3, moins sophistiqué et actuellement opérationnel à Météo-France dans le modèle AROME. Afin de mesurer l'apport de ce nouveau schéma sur la simulation de cas fortement précipitants tels que ceux qui touchent régulièrement le sud-est de la France à l'automne, deux cas d'étude de la campagne HyMeX ont été simulés avec Meso-NH et comparés aux nombreuses observations disponibles. Si l'évaluation des cumuls de précipitations montre un impact modéré de l'un ou l'autre des schémas microphysiques, l'écart est plus marqué en terme de composition et de structure des systèmes convectifs : la microphysique à 2 moments développe une structure verticale plus réaliste et introduit plus de variabilité sur les champs microphysiques. L'évaluation a aussi identifié des biais dans le schéma, notamment une surestimation des diamètres de gouttes de pluie. Des pistes d'amélioration de la microphysique de LIMA ont alors été proposées et évaluées sur les mêmes cas. Des tests de sensibilité à l'initialisation de la population d'aérosols ont ensuite été menés. Il s'avère que les aérosols n'affectent pas uniquement les hydrométéores primaires (gouttelettes d'eau nuageuse et cristaux de glace) mais aussi les autres hydrométéores, engendrant des impacts sur le développement des systèmes convectifs simulés, en termes de composition nuageuse et de précipitations. Les simulations avec une population d'aérosols réaliste initialisée à partir des analyses CAMS ont montré un impact modéré sur les cumuls de précipitations mais une amélioration plus significative de l'évolution temporelle du système (intensification, dissipation) et de la composition nuageuse, réduisant le diamètre des gouttes de pluie sur les cas d'étude
The triggering and growth of Convective systems is a complex process that extends from the synoptic scale, with the establishment of atmospheric circulations promoting convection, to the microscale, with the formation and growth processes of hydrometeors. This PhD focuses on these finest scales and investigates the contribution of complex cloud-microphysics to the occurrence and morphology of heavy precipitation events. The two-moment microphysical scheme LIMA evaluated in this study takes into account the evolution of a multimodal aerosol population and the prognostic treatment of its interaction with liquid and ice clouds and precipitation. First, the contribution of LIMA is evaluated in comparison to the ICE3 one-moment bulk microphysical scheme, which is less sophisticated and currently operational in the AROME model at Météo-France. In order to measure the performance of this new scheme, two case studies of the HyMeX campaign were simulated with the Meso-NH model and compared to a wide variety of available measurements. The assessment of cumulative precipitation shows a moderate impact of each of these microphysical schemes, but the difference is more pronounced in terms of convective systems composition and structure: the two moment microphysics develops a more realistic vertical structure and introduces more microphysical variability. The evaluation also identified biases in the scheme (such as an overestimation of rain drop diameters). Some improvements to the implementation of LIMA were proposed and evaluated on the same cases. Then, the scheme is used to perform a sensitivity test to the aerosol population on the same case studies. Tests on the concentration of idealized populations have shown that aerosols do not only affect primary production of cloud droplets and ice crystals but also precipitating hydrometeors, causing impacts on the development of simulated convective systems in terms of cloud composition and generated precipitation. Simulations based on a realistic aerosol population initialized from CAMS analyses also showed a moderate impact on cumulative precipitation, but a more significant improvement on the temporal evolution of the system (intensification, dissipation) and cloud composition, leading to a reduction of rain drop diameters in the studied cases

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In this work it is presented the study of 6 years of detected lightning data in association to the occurrence of mesoscale convective systems in the region located between 15ºS - 27ºS and 35ºW - 55ºW. The Lightning data were provided by FURNAS Centrais Elétricas S.A. for the period 2002-2007. Geostationary satellite images, enhanced in the infrared channel, were used to identify the cases of interest utilizing -50ºC as cloud top level temperature threshold for convective clusters. For these days, lightning detected in the study region should attain a minimum value of 1000 during 1 hour in some moment of occurrence during the identified cases. From the 565 days that satisfied this criterion, it was selected 25 cases occurred in 33 days during the years of 2003, 2005 (from October to December due to satellite images limitations), 2006 and 2007. For the period of occurrence of each one of these 25 cases, the detected lightning were counted during an interval of 15 minutes before and after the time of the satellite images used in each case to put in evidence the lightning associated to the observed convective clusters. The quantity of detected lightning in a convective cluster were related to its evolutive cycle.
Neste trabalho é apresentado o estudo de 6 anos de dados de descargas elétricas atmosféricas (DEA) detectadas em associação com a ocorrência de sistemas convectivos de mesoescala na região compreendida entre 15ºS e 27ºS e 35ºW e 55ºW. Os dados de DEA foram cedidos por FURNAS Centrais Elétricas S.A. para o período de 2002-2007. As imagens de satélite geoestacionário, realçadas no canal infravermelho, foram usadas para a identificação dos casos de estudo utilizando o valor limiar de -50ºC para temperatura dos topos das nuvens convectivas. Para estes dias, as DEA detectadas na região de estudo deveriam atingir o valor mínimo de 1000 no intervalo de 1 hora em algum momento de ocorrência dos casos identificados. Dos 565 dias contabilizados desta forma, foram selecionados 25 casos de estudo, ocorridos em 33 dias dos anos 2003, 2005 (de outubro a dezembro, por limitações nas imagens de satélite), 2006 e 2007. Para o período de ocorrência de cada um destes 25 casos, foram contabilizadas as DEA no intervalo de 15 minutos antes e depois do horário das imagens de satélite utilizadas em cada caso, para colocar em evidência as DEA associadas aos aglomerados convectivos observados. Ficou evidenciado que a quantidade de DEA detectadas em um aglomerado convectivo esteve relacionada diretamente com o ciclo evolutivo do mesmo.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
Este trabalho teve como objetivo principal analisar uma Linha de Instabilidade (LI) e um Complexo Convectivo de Mesoescala (CCM) e as condições atmosféricas que conduziram à sua formação, desenvolvimento e geração de descargas elétricas atmosféricas. Estes sistemas ocorreram entre os dias 17 e 19 de outubro de 2007, na região Sudeste do Brasil. A análise dos casos de estudo foi feita em quatro períodos: pré-formação, ocorrência da LI, período de transição e ocorrência do CCM. Para a obtenção dos resultados, a base de dados foi composta por dados de descargas atmosféricas e dados simulados, obtidos com o modelo WRF. As simulações com o modelo WRF foram feitas com três grades aninhadas, de resoluções horizontais de 36, 12 e 4km, que permitiram extrair os campos meteorológicos à grande escala, mesoescala e microescala, respectivamente, bem como o cálculo e análises de alguns parâmetros meteorológicos. Os dados de descargas atmosféricas foram obtidos pela RINDAT (Rede Integrada Nacional de Detecção de Descargas Atmosféricas), que foram sobrepostas às imagens de satélite e aos campos meteorológicos. Dentre os resultados obtidos, observou-se que a região onde os sistemas se formaram e desenvolveram estava propícia para a ocorrência de tempestades, apresentando grande umidade, confluência de ventos, índices de instabilidades com valores que atestavam o potencial de desenvolvimento de tempestades. As condições de grande escala associadas à ocorrência da LI e do CCM apresentaram diferenças e semelhanças, mas em ambos os casos, a atividade elétrica máxima esteve associada ao período mais favorável ao desenvolvimento de correntes ascendentes intensas.
This study had as main objective to analyze a Squall Lines (SL) and a Mesoscale Convective Complex (MCC) and the weather conditions that conducted to its formation, development and generation of lightning. These systems occurred between 17 and 19 October 2007, in Southeastern Brazil. The analysis of the case studies was done in four periods: pre-initiation, occurrence of SL, transition period and occurrence of MCC. To obtain the results, the database consisted of data lightning and simulated data, obtained using the WRF model. The simulations with the WRF model were made with three nested grids with horizontal resolutions of 36, 12 and 4 km, which allowed extracting the meteorological fields to large-scale, mesoscale and microscale, respectively, and an evaluation and analysis of some meteorological parameters. The lightning data were obtained by RINDAT (Rede Integrada Nacional de Detecção de Descargas Atmosféricas), which were superimposed on satellite images and meteorological fields. Among the results, it was observed that the region where the systems were formed and developed was favorable to the occurrence of storms, with high humidity, confluence of winds, instability indices with values that attested to the potential development of storms. The large-scale conditions for the occurrence of SL and MCC showed differences and similarities, but in both cases the maximum electrical activity was associated with more favorable period for the development of intense updrafts.

This study is mainly concentrated on examining the positive cloud-to-ground lightning activity associated with Mesoscale Convective Systems. Six MCS events which occurred during the O.K. PRE-STORM program in 1985 are studied. Data indicating the location and polarity of the cloud-to-ground lightning flashes from a lightning location network are analyzed in conjunction with the low-level echo patterns as obtained from radar. Spatial and temporal characteristics of positive cloud-to-ground flashes are identified from the data analysis. For all cases examined, positive cloud-to-ground flashes were found most commonly in the stratiform regions of the MCSs examined, and their frequency tended to peak during the later stages of the storm lifecycle. Two mechanisms responsible for the occurrence of positive cloud-to-ground lightning flashes with the above spatial and temporal characteristics are discussed. Based on the laboratory results, a 1-D charge generation model is developed. The model results show that in-situ charging is unlikely to be the dominant mechanism for charge generation in the stratiform region under normal atmospheric conditions. Sensitivity studies show, however, that in-situ charging processes strongly depend upon the liquid water, graupel and snow contents in the cloud. Under favorable atmospheric conditions, in-situ charging may lead to a significant charge generation. Hence, we cannot completely dismiss in-situ charging mechanism. Analysis of wind fields from dual-Doppler radar in combination with vertical profile of electric fields indicates that charge advection from the convective region to the stratiform region of MCSs may be a potential mechanism responsible for the occurrence of positive cloud-to-ground lightning flashes in the stratiform region.
Graduation date: 1989

Deep convective clouds play an important role in global energy balance through vertical transport of water vapor, momentum and energy, altering radiation and also influence hydrological cycle via precipitation. These clouds are organized mainly at Synoptic scale (~1000 km), Mesoscale (~100 km) and storm-scale (~10 km) and involve interactions from micro-scale (e.g., cloud condensation nuclei and droplets) to planetary scale. Physical processes associated with such clouds are the largest sources of uncertainty in atmospheric weather and climate models. Clouds involve rich physics and therefore, studying and understanding of convective clouds is an important research area in weather and climate sciences. In present work, the mesoscale and storm scales of convective cloud systems are addressed using spacebrone and ground based Doppler weather (conventional and polarimetric) radars. The work started with the analysis of cloud systems over Tibetan Plateau. These cloud systems are observed to be very deep in nature. After finding the underestimation of radar reflectivity especially in convective regime, analysis is further extended in entire latitudinal belt of 38°N-38°S. The coincident data collected with the precipitation radar (PR) onboard TRMM (Tropical Precipitation measuring Mission) satellite and profiling radar (CPR) onboard CloudSat satellite is used. It is shown the PR measures properties of convective part but it misses portions of the anvil part of mesoscale convective cloud systems (MCSs). CPR measures the full spatial extent of MCSs however its reflectivity values are very low due to the strong attenuation suffered by the radar beam while passing through a precipitating convective cloud. CPR beam gets attenuated severely during convective rain episodes especially below 6 km height. While going by their technical specification, we can expect substantial overlap in the radar reflectivity factor for convective clouds, very little overlap is observed. One should be very careful while drawing conclusions on the cloud characteristics measured with the PR and CPR. I felt that it is better to study the cloud properties using ground radars, hence, most of the results reported in the thesis are based on ground radar data. As part of the Continental Tropical Convergence Zone (CTCZ) program of the Ministry of Earth Sciences, Govt. India, India Meteorological Department (IMD) made available its Doppler weather radar (DWR) data for the years 2012 and 2013 to researchers within the country. Using IMD DWR data, life cycle of monsoonal MCSs over Indian monsoon zone and properties of storms embedded within MCSs are studied at five locations, namely, Kolkata, Hyderabad, Nagpur, Patiala and Delhi. Stages in the lifecycle of MCSs have been explored including convective area and precipitation fractions. It was observed that intense precipitation within an MCS is confined to several pockets having areas much smaller than that of an MCS. Those convective cells are called storms in this work. Storm is a precipitating convective cloud having a volume of more than 50 km3 of connected pixels with radar reflectivity factor of at least 30 dBZ. The results of storm properties are reported for the first time using the DWR data for the Indian subcontinent. It is observed that the growth phase of an MCS is characterized by a rapid increase in the number of storms. An MCS can have more than one growth and decay phases during its lifetime. MCS may contain few or large number of storms depending upon geographic location and life phase. Average area of storms varies from less than 20 to more than 250 km2, while average storm height lies typically between 6 and 10 km. In one extreme case, it is found even 17 km. Average convective precipitation fraction (CPF) is 40% or less, highest at Kolkata, Hyderabad, Patiala and Delhi (~40%) and the least at Nagpur (13%). Average convective area fraction (CAF) is less than 15% at all locations. The maximum CAF and CPF can go higher up to 45% and 90% respectively. The most intense convective clouds are observed over Patiala and Delhi where 30 dBZ radar echoes are found above 10 km. These locations lie near Himalayan foothills. According to previous studies, this region experiences intense convective systems due to high degree of potential instability caused by high moisture flux from low-level air flow from Arabian Sea to over foothills of Himalayas which interacts with extra moisture supplied from soil wetted from earlier precipitation. The vertical structure of MCSs is different at each radar location. These differences appear remarkably below 5 km and above 10 km altitudes. The final part of the thesis is based on the analysis of data from a polarimetric DWR located at Delhi. Using polarimetric DWR radar reflectivity data at Delhi (a land Indian region), the three prominent features of an MCS (Severe precipitation (below 4 km), melting band (at ~4 km) and anvil structures at higher altitudes (~12 km) are captured in vertical distributions of convective and stratiform echoes. Convective clouds are very deep over the Delhi region, many of them extended beyond 16 km. The typical storm duration is an hour while few exceed 3½ hours. Storms those have large average areas and long durations contribute more in precipitation amount. The average precipitation rate (R) of storms is estimated in between 5 and 34 mm hr-1. The total accumulated precipitation (Pacc) derived from polarimetric variable (Kdp) is as large as 250 mm in 6 days at Delhi. The cloud liquid water content (M) is derived using horizontal radar reflectivity (Zh, property of cloud volume observed by the radar beam which is which is proportional to the 6th moment of the diameter of hydrometeors) and specific differential phase (a measure of phase difference between horizontally and vertically polarized waves). The mean of cloud liquid water content derived from Zh (~1 gm m-3) is just half of that derived from Kdp. However, their maximum bound (~4.2 gm m-3) are found similar but have different frequencies. At each altitude, values of M vary largely which reflects the natural variability in clouds. One of the important finding is that the Pacc and M estimates are found to be more realistic when derived using polarimetric variable.

Thesis (M.S.)--University of Wisconsin--Madison, 1998.
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碩士
國立臺灣大學
大氣科學研究所
107
In this study, Rankine vortices that represent idealized Mesoscale Convective Vortex (MCV) with maximum wind speeds at different levels embedded in a quiescent tropical environment are studied using Vector Vorticity Model (VVM), a three-dimensional, cloud resolving model. We aim to evaluate how different extent of the potential temperature profile modulates the evolution of the initial vortex. Under balanced thermodynamics, positive (negative) potential temperature anomaly will be present above (below) maximum level of potential vorticity. If such profile is optimally placed, the corresponding stabilization is theorized to enhance “bottom heavy” mass flux profile, inducing convergence at lower level, further promoting column saturation and precipitation, contributing to the spin-up of the low-level vortex. The experiment with a Rankine vortex where maximum wind (vmax) located at z = 4.5 km, is the earliest to undergo cyclogenesis, leading other runs by approximately 7 hours or more. The vortex with vmax at sea level or above 7.5km, does not develop after 144 hours. The time required to reach cyclogenesis is proportional to the difference between the level in which vmax is located below or above 4.5 km, where z = 4.5 km seems to be the optimal height to promote genesis. Based on cyclogenesis occurs in 144 hours or not, we can categorize them into developing sets (DS) and non-developing sets (NDS). To analyze the stabilization of thermodynamic environment, we define stability index (SI) based on the difference of saturated moist entropy (s*) between upper and lower troposphere. Smaller (larger) values of SI indicates higher (lower) environmental stability. 48 hours prior to the genesis, the SI of DS systematically decreases, and during the period 0-12 hours prior to the genesis, the median SI of DS is around 5JK^-1kg^-1 lower than that of NDS. The saturation fraction (SF) in DS also shows a systematic increase prior genesis. Joint-PDF of SI and SF confirms the fact that SF is inversely proportional to SI, demonstrating stabilization is accompanied by column saturation. In an environment with higher saturation fraction is more likely to produce large, organized convection. Convective structures that are highly rotational in cyclogenetic environments are phrased as “Vortical Hot Towers (VHTs)” and the increase in number and merger of VHTs plays a notable role as a source of vorticity convergence and heating during genesis. Here we identify the size, height and other characteristics of clouds by connecting cloud grids together as cloud objects using a six-connected segmentation algorithm. After cloud objects are labeled, VHTs are then filtered using cloud thickness exceeding 10 km as the criteria. We then compare the size distribution of these VHTs between the DS and NDS. In the DS, probability density of large VHTs with volumes over 10^4 km3 within a 100×100 km^2 square box around the vortex center steadily overpasses NDS, which is a sign of aggregation and upscale growth 24-36 hours prior to the genesis. Finally, we show that the generation of “bottom-heavy” mass flux profile is more likely in low SI environments by comparing the probability distribution of a bottom-heavy index in low SI and high SI environments. The tendency to generate large VHTs and bottom-heavy mass flux profiles, promotes processes such as organized heating and stretching, which intensifies the incipient vortex.

Very small fractional area (0.1%) occupied by the cumulonimbus (Cb) clouds belies their importance in Earths hydrological cycle and climate. For example, Riehl and Malkus (1958) estimated that the vertical transport of energy needed for the global energy balance can be accomplished by 1500 to 5000 active, undiluted Cb clouds (i.e., hot towers). Cb clouds feed hydrometeors to the anvil cloud region in mesoscale convective system (MCS). Applications such as the estimation of the vertical profile of latent heating, cumulus parameterizations, satellite rainfall retrievals, inferring the probability of lightening, etc., require information on the vertical distribution of hydrometeors in convective clouds (e.g., Xu and Zipser, 2012). Knowledge of the vertical structure of Cb clouds near individual cloud scale becomes necessary for validating cloud resolving model results. However, information on the vertical structure of convective clouds at horizontal scales comparable to that of a deep convective cloud is not available over most regions in the tropics. The PR provides an unprecedented long time series of data on the 3D structure of precipitating clouds in the tropics. The TRMM PR equivalent radar reflectivity factor (Ze) data product 2A25 version 6 is the main data used in the study. The present thesis work primarily focuses on the properties of convective clouds at the PR pixel scale. TRMM, operational since December 1997, is a non-sunsynchronous satellite with 350 inclination and samples the tropics several times a day (e.g., Kummerow et al., 1998, 2000). The PR works in Ku band (13.8 GHz or 2.2 cm wavelength), and its scan, consisting of 49 beams, had a width of 215 km when launched and _250 km after August 2001. The beam width is 0.710; nearby beams are separated by 0.710, giving a maximum scan angle of 170 (Kummerow et al., 1998). There are 80 levels in the vertical, each having 250 m resolution with the lowest level being the Earths ellipsoid. The height corresponding to different vertical levels in the 2A25 data set is the distance measured along the PR beam. Hence, corrections to pixel heights along different beams have been applied. Present thesis presents the vertical structure of radar reflectivity factor in tall cumulonimbus towers (CbTs) and intense convective clouds (ICCs) embedded in the South Asian monsoon systems and other tropical deep cloud systems. CbT is defined referring to a reflectivity threshold of 20 dBZ at 12 km altitude and is at least 9 km thick. ICCs are constructed referring to reflectivity thresholds at 8 km and 3 km altitudes. Cloud properties reported here are based on 10 year climatology. It is observed that the frequency of occurrence of CbTs is highest over the foothills of Himalayas, plains of northern India and Bangladesh, and minimum over the Arabian Sea and equatorial Indian Ocean west of 900E. The regional differences depend on the reference height selected, namely, small in the case of CbTs and prominent in 6􀀀13 km height range for ICCs. Land cells are more intense than the oceanic ones for convective cells defined using the reflectivity threshold at 3 km, whereas land versus ocean contrasts are not observed in the case of CbTs. Compared to cumulonimbus clouds elsewhere in the tropics, the South Asian counterparts have higher reflectivity values above 11 km altitude. One of the main findings of the present thesis is the close similarity in the average vertical profiles of CbTs and ICCs in the mid and lower troposphere across the ocean basins, while differences over land areas are larger and depend on the reference height selected. Foothills of the Western Himalayas, southeast South America and Indo-Gangetic Plain contain the most intense CbTs, while equatorial Africa, foothills of the Western Himalayas and equatorial South America contain the most intense ICCs. Close similarity among the oceanic cells suggests that the development of vigorous convective cells over warm oceans is similar and understanding gained in one region is extendable to other areas. South Asia contains several areas where the seasonal summer monsoon rainfall is influenced by the orography. One of the fundamental questions concerning the orographic rainfall is the nature of the associated precipitating clouds in the absence of synoptic forcing. It is believed that these are shallow and mid-level clouds, however, there is not much information in the literature on their vertical structure. Chapter 4 explores the vertical structure of active shallow (SC) and mid-level clouds (MLC) in Southeast Asia which are associated with the orographic features. Shallow and mid-level clouds have been defined such that their tops lie below 4.5 km and between 4.5 and 8 km, respectively. Only those TRMM PR passes are considered for active shallow and mid level cloud, which consists less than 5% deep cloud (_ 8 Km), compared to shallow cloud (_ 4.5 km) and mid level cloud (4.5 and _ 8 km). The reflectivity and height thresholds with constraint on percentage of deep clouds, ensure that we only captures the intense and isolated shallow and mid level clouds, away from deep cloud. The Western Ghats contains the highest fraction of the shallow clouds followed by the adjacent eastern Arabian Sea, while the Khasi hills in Meghalaya and Cardamom Mountains in Cambodia contain the least fraction of them. Average vertical profiles of shallow clouds are similar in different mountainous areas while that of mid-level clouds show some differences. Below 3 km, cloud liquid water content of the mid-level clouds is the highest over the Western Ghats and the eastern Arabian Sea. The average cloud liquid water content increases by 0.19 gm m􀀀3 for SCs between 3 km and 1.5 km, while the corresponding increase for MLCs is around 0.08 gm m􀀀3. MCS has a life cycle consisting of formative, intensifying, mature and dissipating stages. From the maximum projection of reflectivity on longitude and latitude plane from the 3D reflectivity fields, CS is defined as the common area of connected pixels with Ze _ 17 dBZ and polarized corrected temperature (PCT) _ 250 K, with atleast 500 km2. A CS is considered in subsequent analysis if its area detected in the Ze projection is at least 50% of its area seen in the PCT imagery. An algorithm is applied to obtain the phase of evolution. The algorithm is based on the average vertical profile of CSs and the reflectivity peak altitude (Hmax). An index namely reflectivity difference (RD) and Hmax is used to identify the phases of evolution. A close similarity has been observed during different phases in average vertical profiles as well as in CFAD. Growing or intensifying stage consists the highest reflectivity below the 2 km altitude. Mature phase does not show the much variation in Ze below the freezing level, whereas in the decaying stage, shows the largest regional differences in this layer of the atmosphere. Melting band signature is most pronounced in the decaying stage. Fraction of convective area decreases as CSs go through its life cycle, except over Atlantic Ocean during winter.