The accurate prediction of warm-season convective systems, and the heavy rainfall and severe weather associated with them, remains a challenge for numerical weather prediction models. This study looks at one such circumstance in which back-building convection forms perpendicular to, and above the cold pool region behind strong bow echoes. We refer to this phenomenon as a "bow and arrow" because, on radar imagery, the two convective lines resemble an archer's bow and arrow. The "arrow" can extend over hundreds of kilometers and can cause damage from high winds, hail and flooding. Events of this nature pose a particular challenge to forecast because they require an accurate forecast of the earlier convection and the effects of that convection on the environment. In this study, radar and surface observations of four events are presented to identify common environmental conditions prior to the development of the back-building convection. Additionally, simulations of three cases using the Weather Research and Forecasting (WRF) model are analyzed in an attempt to understand the mechanisms responsible for initiating and maintaining the convective line. Due to coarse resolution, observational analyses are only useful for inspection of the synoptic-scale. Model output from numerical simulations is utilized to examine the mesoscale in the vicinity of the convective arrow. Several environmental characteristics are evident in each of the studied cases. Strong southwesterly flow (inducing warm air advection and gradual isentropic lifting), in addition to directional and speed convergence into the convective arrow region possibly contribute to convection initiation. Horizontal wind speed shear and increased wind speed in the area surrounding the arrow may be associated with the linear orientation of the arrow. It seems as though when these ingredients are combined with thermodynamic instability, there is a greater possibility of formation and maintenance of a convective arrow behind a bow echo.

Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-07-12T19:42:05Z No. of bitstreams: 1 RAFAELLA DE ARAÚJO AIRES VILAR – DISSERTAÇÃO (PPGMET) 2016.pdf: 3521923 bytes, checksum: 8abe82b0df1c51917eb7f25334055e37 (MD5) / Made available in DSpace on 2018-07-12T19:42:05Z (GMT). No. of bitstreams: 1 RAFAELLA DE ARAÚJO AIRES VILAR – DISSERTAÇÃO (PPGMET) 2016.pdf: 3521923 bytes, checksum: 8abe82b0df1c51917eb7f25334055e37 (MD5) Previous issue date: 2016-02-26 / Capes / A formação de tornados é um fenômeno pouco frequente no Centro-Oeste do Brasil. O registro de um tornado em Brasília-DF em 01 de outubro de 2014 evidencia a importância da utilização de métodos que permitam compreender melhor os vários mecanismos atmosféricos dominantes em episódios de tempo severo e monitorar esse fenômeno meteorológico com alto potencial de destruição. Este estudo teve como propósito analisar a estrutura vertical da atmosfera que prevaleceu no desenvolvimento de condições atmosféricas adversas e identificar parâmetros convectivos adequados para determinação de padrões atmosféricos que favoreceram o desenvolvimento de tempo severo. A técnica estatística Análise de Componentes Principais (ACP), índices de instabilidade e imagens de radar meteorológico foram os principais métodos de análise utilizados. Os resultados mostram que a combinação entre o alto grau de instabilidade atmosférica, temperaturas elevadas e vento intenso acompanhado de rajadas foi determinante para o desenvolvimento do tornado. Entretanto, os valores dos índices de instabilidade diferem dos limiares normalmente utilizados como indicadores da formação de tornados. Linhas de instabilidade em forma de arco (“bow echoes”) detectadas pelo radar durante o período de chuva intensa com danos em superfície são os indícios mais fortes do tornado que atingiu a região. Registros fotográficos de linhas de energia, telhados, árvores e carros danificados pelo vento (95 km/h) também são apresentados. / The frequency of tornado formation in Central-West Brazil is low. A tornado observed in Brasília-DF on 01 October 2014 highligths the importance of using methods that allow a better understanding of the various atmospheric mechanisms that dominate in severe weather episodes, and the monitoring of this meteorological phenomenon with high destructive potential. The aim in this study was to analyze the vertical structure of the atmosphere that dominated the development of adverse atmospheric conditions and to identify convective parameters that are adequate for determining atmospheric patterns that favor severe weather development. The statistic technique Principal Component Analysis (PCA), instability indices and meteorological radar images were the main analysis methods employed. The results show that the combination of high degree of atmospheric instability, high temperatures and intense wind with gusts was determinant for tornado development. However, values of the instability indices differ from those normally used as indicators of tornado formation. Arc-shaped squall lines (“bow echoes”) detected by radar during the period of intense rainfall with damages at the surface are the strongest evidence of the tornado that hit the region. Photos of power lines, roofs, trees and cars damaged by the winds (95km/h) are presented also.