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Maria A. F. Silva Dias

Abstract

This paper presents the development of tornado reports in Brazil since the middle of the twentieth century, both for the country as a whole and for the five regions of Brazil: the south, southeast, central-west, northeast, and north. No official tornado registry exists in Brazil so the reports come from various sources. Most of the tornadoes reported are from the south and southeast regions. The low number of reports for the central-west regions suggests, in view of the literature on intense storms and mesoscale convective complexes, that most tornadoes cases go unreported. The increase in tornado reports is compared to the evolution of population density and communications, with the latter represented by the evolution of local television stations and the popularization of the Internet. One particular event is a new Web site for volunteer tornado reports, which has completely changed the trends of tornado reports. Another possible cause for an increase in the number of tornado reports in the south and southern regions could be a shift in climate variability in this region in the 1970s, which has been reported by several authors. However, the increase in rainfall and extreme events reported by these studies point to an increase of as much as 40%, which is not compatible with the observed tenfold increase in tornado reports.

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Leila M. V. Carvalho and Maria A. F. Silva Dias

Abstract

Mesoscale cloud patterns are analyzed through the application of fractal box dimensions. Verification of fractal properties in satellite infrared images is carried out by computing box dimensions with two different methods and by computing the fraction of cloudy pixels for two sets of images: 174 are considered the “control series,” and 178 (for verification) are considered the “test series.” The main instabilities in the behavior of such dimensions are investigated from the simulation of circles filling space in several spatial distributions. It is shown that the box dimensions are sensitive to the increase of the area covered and to the spatial organization—that is, the number of cells, the spatial clustering, and the isotropy of the distribution of pixels. From a principal components analysis, the authors find six main patterns in the cloudiness for the control series. The three main patterns related to enhanced convection are the massive noncircular spread cloudiness, the highly isotropic distribution of cloud in several cells, and the most circular pattern associated with mesoscale convective complexes. The six patterns are separated into a cluster analysis, and the properties of each cluster are averaged and verified for the test series. This method is a simple and skillful procedure to recognize mesoscale cloud patterns in satellite infrared images.

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Júlia C. P. Cohen, Maria A. F. Silva Dias, and Carlos A. Nobre

Abstract

The environmental conditions associated with squall lines (SL) that were observed during the period of 13 April–13 May 1987 (GTE/ABLE-2B) originating at the northern coast of South America and propagating over the Amazon Basin are documented. The SL observed on 5–7 May are examined in more detail. The SL days had in common a stronger and deeper low-level jet when compared with the days without SL. Two possible explanations are found for the intensification of the low-level jet: propagating easterly waves in the tropical Atlantic, which eventually reach Manaus, and localized heat sources in the western Amazon. Both were observed on 5–6 May. It is suggested that numerical simulations should be performed to unravel the relative importance of each large-scale mechanism.

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Enio P. Souza, Nilton O. Rennó, and Maria A. F. Silva Dias

Abstract

A simple theory for convective circulations induced by surface heterogeneities is proposed. The theory is based on the thermodynamics of heat engines and provides a simple physical explanation for the general characteristics of circulations forced by surface inhomogeneities in sloping terrains. In particular, the theory is applied to a mesoscale circulation induced by deforestation. It predicts that the intensity of the mesoscale convective circulation forced by deforestation depends on the difference of the near-surface nonadiabatic temperature and humidity between the forest and cleared regions and on the depth of the convective boundary layer. The theory was successfully tested against observations made during a field experiment in the Amazon forest and a nearby clearing.

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Sandra I. Saad, Humberto R. da Rocha, Maria A. F. Silva Dias, and Rafael Rosolem

Abstract

The authors simulated the effects of Amazonian mesoscale deforestation in the boundary layer and in rainfall with the Brazilian Regional Atmospheric Modeling System (BRAMS) model. They found that both the area and shape (with respect to wind incidence) of deforestation and the soil moisture status contributed to the state of the atmosphere during the time scale of several weeks, with distinguishable patterns of temperature, humidity, and rainfall. Deforestation resulted in the development of a three-dimensional thermal cell, the so-called deforestation breeze, slightly shifted downwind to large-scale circulation. The boundary layer was warmer and drier above 1000-m height and was slightly wetter up to 2000-m height. Soil wetness affected the circulation energetics proportionally to the soil dryness (for soil wetness below ∼0.6). The shape of the deforestation controlled the impact on rainfall. The horizontal strips lined up with the prevailing wind showed a dominant increase in rainfall, significant up to about 60 000 km2. On the other hand, in the patches aligned in the opposite direction (north–south), there was both increase and decrease in precipitation in two distinct regions, as a result of clearly separated upward and downward branches, which caused the precipitation to increase for patches up to 15 000 km2. The authors’ estimates for the size of deforestation impacting the rainfall contributed to fill up the low spatial resolution in other previous studies.

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Luiz A. T. Machado, Maria A. F. Silva Dias, Carlos Morales, Gilberto Fisch, Daniel Vila, Rachel Albrecht, Steven J. Goodman, Alan J. P. Calheiros, Thiago Biscaro, Christian Kummerow, Julia Cohen, David Fitzjarrald, Ernani L. Nascimento, Meiry S. Sakamoto, Christopher Cunningham, Jean-Pierre Chaboureau, Walter A. Petersen, David K. Adams, Luca Baldini, Carlos F. Angelis, Luiz F. Sapucci, Paola Salio, Henrique M. J. Barbosa, Eduardo Landulfo, Rodrigo A. F. Souza, Richard J. Blakeslee, Jeffrey Bailey, Saulo Freitas, Wagner F. A. Lima, and Ali Tokay

CHUVA, meaning “rain” in Portuguese, is the acronym for the Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (GPM). The CHUVA project has conducted five field campaigns; the sixth and last campaign will be held in Manaus in 2014. The primary scientific objective of CHUVA is to contribute to the understanding of cloud processes, which represent one of the least understood components of the weather and climate system. The five CHUVA campaigns were designed to investigate specific tropical weather regimes. The first two experiments, in Alcantara and Fortaleza in northeastern Brazil, focused on warm clouds. The third campaign, which was conducted in Belém, was dedicated to tropical squall lines that often form along the sea-breeze front. The fourth campaign was in the Vale do Paraiba of southeastern Brazil, which is a region with intense lightning activity. In addition to contributing to the understanding of cloud process evolution from storms to thunderstorms, this fourth campaign also provided a high-fidelity total lightning proxy dataset for the NOAA Geostationary Operational Environmental Satellite (GOES)-R program. The fifth campaign was carried out in Santa Maria, in southern Brazil, a region of intense hailstorms associated with frequent mesoscale convective complexes. This campaign employed a multimodel high-resolution ensemble experiment. The data collected from contrasting precipitation regimes in tropical continental regions allow the various cloud processes in diverse environments to be compared. Some examples of these previous experiments are presented to illustrate the variability of convection across the tropics.

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Ariane Frassoni, Dayana Castilho, Michel Rixen, Enver Ramirez, João Gerd Z. de Mattos, Paulo Kubota, Alan James Peixoto Calheiros, Kevin A. Reed, Maria Assunção F. da Silva Dias, Pedro L. da Silva Dias, Haroldo Fraga de Campos Velho, Stephan R. de Roode, Francisco Doblas-Reyes, Denis Eiras, Michael Ek, Silvio N. Figueroa, Richard Forbes, Saulo R. Freitas, Georg A. Grell, Dirceu L. Herdies, Peter H. Lauritzen, Luiz Augusto T. Machado, Antonio O. Manzi, Guilherme Martins, Gilvan S. Oliveira, Nilton E. Rosário, Domingo C. Sales, Nils Wedi, and Bárbara Yamada
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