This document summarizes a presentation given at the 4th American Association for Wind Engineering Workshop in Miami, Florida in August 2016. The presentation analyzed downburst occurrences in Brazil. Downbursts are strong downdrafts of air that induce powerful diverging winds at the surface. The document provides background on Brazil's climate and weather systems and discusses several downburst events that have occurred in Brazil, with damages. It also presents meteorological parameters that can help identify downbursts and suggests regions of Brazil that are most at risk for downburst occurrences, including the southern region, São Paulo state, western Amazon Basin, and northeast coast.

1. 4th American Association for Wind Engineering Workshop
Miami, Florida, USA
August 14 - 16, 2016
* Lead presenter
Analysis of Downburst Occurrences in Brazil
Elias Galvan de Lima a*, Acir Mércio Loredo-Souzab
aLAC Wind Tunnel - UFRGS, Porto Alegre, RS, Brazil, galvandelima@gmail.com
bLAC Wind Tunnel - UFRGS, Porto Alegre, RS, Brazil, lac@ufrgs.com
ABSTRACT:
Brazil's territory has shown an enormous potential to generate severe weather. Recently, due to
either urban expansion or climate variability, the hazards caused by thunderstorms became a
challenge that the population was not prepared to face. As a result, the government was forced to
adopt several costly and controversial emergency provisions, untimely revealing the need for more
effective resilience policies and the adoption of improved disaster mitigation strategies.
Although there are many hazards caused by the severe weather in Brazil, intensive downbursts
cases gained attention as a result of the severe destructive patterns not frequently seen before.
Downbursts are defined by Fujita as non-curl winds, presented as a strong widespread wind
outburst. They can be distinguished from other types of phenomena by their rapidly fluctuating
meteorological parameters, and their unique trail of damages left behind.
The importance of understanding the downburst incidence in Brazil becomes evident when
analyzing the losses and damages associated with them. The significant amount of accidents due
to extreme winds in the recent years suggests the need for a review of the Brazilian Wind Code,
which is presently being developed by the LAC Wind Tunnel in Porto Alegre, Brazil.
It is aimed to incorporate to this new version, besides advances the actual state of the art, an update
of climate data and implementation of thunderstorm winds analysis. This paper has the propose of
disseminate how extreme winds researches are being conducted in Brazil, and brings an overview
of the downburst identification process and suggests areas with major risk of occurrences,
including the Southern Region, the State of São Paulo, Western Amazon Basin, and the Northeast
Coast.
Keywords: Downbursts,Thunderstorms, Wind Code, Brazil.
1. INTRODUCTION
Brazil's territory has shown an enormous potential to generate severe weather. The extended
latitude and longitude range of its lands characterize the country with a very non-homogeny
climate (Reboita et al., 2010). Also, the atmospheric flow on South America has a singular
dynamic and several meteorological systems are found to have influence in the weather in Brazil,
as shown at Figure 1.
The Austral Spring Season has the biggest frequency of thunderstorms in Brazil, followed by the
Summer. In addition, due to the intensive heating during the day, thunderstorms have 5 times more
chances to happen during the afternoon than during the morning (Anselmo, 2015).

2. Figure 1. Weather systems at low and high troposphere that influence on the Climate of South America. At the low
tropospheric levels: Mesoscale Convective Complex (MCC), Northeast Trade Winds (NTW), Subtropical
Anticyclone of the South Atlantic (SASA), Intertropical Convergence Zone (ITCZ), Instability Lines (IL), Cold Front
(CF), Warm Front (WF), Subtropical Anticyclone of the South Pacific (SASP), Low Level Jet (LLJ) at east of the
Andes,Chaco Low (CL), Trade Winds from the Southeast (TWS), Low Pressure System(L), Region of Thermal Low
in Northwestern Argentina (LNA), Prefrontal Squall Line (PSL), Comma Cloud (CC), Cyclogenetic Regions (CR),
Convergence Zone South Atlantic (CZSA). At the high levels tropospheric, Cyclonic Vortices of High Tropical Levels
(CVHL), Boliva High (BH), Atmospheric Blockage Area (AB), Brazilian Northeastern Dug (BND), Subtropical Jet
(SJ), Polar Jet (PJ). AB and BND are systems which get set up only during the winter. Adapted from Reboita et al
(2010).
The weather conditions on the Southern of Brazil are mostly defined by frontal systems, mesoscale
convective complex (MCC) and instabilities lines (IL) and comma systems (CS) are commonly
found when atmospheric blocking conditions are set up (Reboita et al., 2010).
The northeastern region weather is mainly defined by the Intertropical Convergence Zone (ITCZ)
and physical heating and it is characterized by a wet coastal zone and drier continental area
(Reboita et al., 2012).
The northwestern states, the sea breeze and trade winds mostly define the precipitation patterns by
bring humidity from the oceans inside the continent. When combined with the Forest
evapotranspiration, humidity and energy are driven right to the south along the east side of Andes,
this process knew as “air rivers” define the Low Level Jets in South America and feed one of the
most active convective areas in the world at Brazilian Prata Basin, being responsible to strengthen
frontal systems and causing severe weather events (Nascimento, 2005).
Brazil weather is widely characterized by its wet climate and the high frequency of convective
thunderstorms all over its territory (Romatschke and Houze, 2010).Furthermore, climate
variabilities such as the El Niño - Southern Oscillation may contribute to increase the frequency

3. and intensity of severe weather at the Southern Area of the Country (Grimm and Pscheidt, 2004).
This large scale anomaly is pointed out as a booster factor for several intense events occurred in
southern region of Brazil (Grimm, Ferraz and Gomes, 1998), additionally Madden Julian
Oscillation (MJO), radioactively forced climate change (Tippett et al., 2015) and heat island
effects on extensive urban agglomerations might have influence.
Severe weather and floors are the most frequent and expensive types of natural disasters in the
World. Along the last two decades, the number of deaths have decreased significantly, however
more people have been affected by these events (Leaning and Guha-Sapir, 2013)
In Brazil, 84% of natural disasters are climate related. Also, 94% of the affected population for
this events, is located along the South, Southeast and Northeast States (Marcelino, Nascimento
and Ferreira, 2005). Furthermore, the lack of public actions for improvements on the Brazilian
Meteorological System and the low investments on a better operational strategy of forecast
services, the Brazilian population lives an imminent state of vulnerability against severe weather
events (Nascimento, 2005)
2. DOWNBURSTS IN BRAZIL
Recent accidents due to extreme winds in Brazil and the occurrence of strong downbursts
occurrence have put in check the safety of already existent buildings and reveal the necessity for
major studies and wind data analysis, leading to an update of the Brazilian Wind Code, which
considers actual data and thunderstorm winds analysis.
Downburst is a term first coined by Fujita (1985), it is described as a strong dense column of cold
air caused by a downdraft that descends towards the ground and induces a strong burst of divergent
winds, named outburst. A downburst may be classified as a microburst, when its outburst is not
larger than 4 km, and as macroburst, when it has a range larger than 4 km. The Fig. 2 shows the
typical lifetime of a microburst which usually is no longer than 10 min.
Figure 2. Typical downburst lifetime (Hjelmfelt, 1988).
Blowdowns are important disturbances in tropical forests environments such as Amazon. They are
responsible for opening clean space in the forest, allowing more water and light to reach the soil
allowing smaller trees species to grow up, beneficiating the regeneration of the ecosystem
(Garstang et al., 1998).
However, for urban environments, downbursts became a treacherous risk for the population, the
wind speed can strike down entire neighbourhoods, such as the case observed in Porto Alegre in
damages occurred to buildings, particularly to the façades. Fig. 1 shows some examples of
failures occurred during the event. This meteorological event offered an opportunity to
check and compare, in full-scale and under a downburst flow, the performance of buildings
previously tested in a conventional Boundary Layer Wind Tunnel.
Figure 1. Examples of damages occurred during the Porto Alegre January 29, 2016, downburst event.
2. DOWNBURSTS IN BRAZIL
2.1. Downbursts
Downburst is a term first coined by Fujita (1985) and is described as a strong dense column
of cold air caused by a downdraft that descends towards the ground and induces a strong
burst of divergent winds, called outburst. In space scale aspects, a downburst may be
classified as a microburst, which generates an outburst up to 4 km, and a macroburst, with a
range bigger than the previous value. Fig. 2 shows the typical lifetime of a microburst.
Figure 2. Typical downburst lifetime (Hjelmfelt, 1988)

4. January 29th of 2016, where major disturbance in the electricity system were observed, economical
activities were interrupted, and hundreds of buildings were partially or structurally damaged
(Loredo-souza et al., 2016)
Such destructive meteorological phenomena become a problem not only because its high speed
winds, but also due to its vertical variation. The velocity profile of a downburst event (in the ground
spreading process) differs from the typically observed atmospheric boundary layer (ABL) profiles
as indicated in Figure 3.
Figure 3. Schematic of an atmospheric boundary layer profile, on the left, and the velocity
profile in the outflow of a downburst,on the right (Bertsch and Ruck, 2015)
By analysing the rapidly fluctuating of meteorological parameters, it is possible to identify the
occurrence of downburst occurrence in thunderstorm environments. The main parameters are show
in the Table 1 and the will be applied for further investigations with an extensive set of data.
Table 1. Typical meteorological parameters values for a downburst occurrence
Characteristics
Typical Downburst Value
Range
Reference
Decrease between surface and colder
layer near the 700hPa (K)
> 20
(Atkins and Wakimoto,
1991)
Wind gust (m/s)
>10 (Minimum); 25 - 50
(significant damaging events)
(Garstang et al. 1998)
Effective decrease of instant
equivalent potential temperature (K)
> 4 (Garstang et al. 1998)
Temperature decrease (°C) > 5 (Garstang et al. 1998)
Dew point decrease (°C) - -
Atmospheric pressure increase (hPa) > 2.4 (Caracena and Maier, 1987)
Saturated instant mixing ratio
decreasing (g/kg)
>3.5 (Garstang et al. 1998)
Relative humidity decrease (%) - -
Registered precipitation along
downdrafts (mm/h)
> 0.5 (Garstang et al. 1998)
3. PRELIMINARY ANALYSIS
Previous studies suggested that the greatest frequency of heavy rainfall in the Brazilian Amazon
n terms of vertical variation, the velocity profile of a downburst event (in the ground
preading process) differs from the typically observed atmospheric boundary layer (ABL
profiles as indicated in Fig. 3. This may be paramount for very tall buildings, but for those
tudied in this work (not taller than 80m) this is not a big issue, although little is known
egarding the turbulence structure of the downburst flow.
Figure 3. Schematic of an atmospheric boundary layer profile, on the left, and the velocity profile in the
outflow of a downburst, on the right (Bertsch and Ruck, 2015)
2.1. The Porto Alegre event and previous cases in Brazil
A downburst event is a very unique meteorological phenomenon and several factors need to
be observed before confirming its occurrence. The severe weather event observed in Porto
Alegre can be defined as a macroburst mainly due to the pattern of destruction, but if rada
mages were available with a better and reliable resolution the event could be bette
understood. The satellite image of Fig. 4 shows a supercell well developed above the city a
he same time the strongest wind gusts were registered.
Figure 4. Satellite image of the supercell over Porto Alegre on 30/01/2016 00:45:51 UTC (REDEMET, 2016

5. is significantly correlated with the highest incidence of blowdowns (Nelson et al., 1994), it
indicates that a better understanding of the severe weather occurrence in Brazil may lead for a
clearer scenario of downburst occurrence over the Country.
Besides the northern region of Brazil, recent studies related intensive downburst activity on the
southern region, also it is commonly seen on the States of São Paulo and Mato Grosso do Sul
(Lima and Loredo-souza, 2015). Squall lines at the northeast region are also responsible to cause
downburst winds due to the winds (Garstang et al., 1998). The Figure 4 shows a review of the
downburst cases in Brazil until 2014.
Figure 4. Downbursts observed cases in Brazil. Red areas represent cases reported in the
literature and purple areas indicate cases suggested by this work (Lima and Loredo-souza, 2015).
4. FINAL COMMENTARIES
This paper aimed to disseminate part of extreme wind researches that are being conducted in
Brazil. It was shown a brief characterization of the severe weather in country and highlighted the
need for governmental actions to better prepare cities against extreme meteorological events.

6. Additionally, more data will be collected and analysed in order to incorporate a complete analysis
of thunderstorms winds to the Brazilian Wind Code.
5. REFERENCES
Anselmo, E. M. (2015) ‘Morfologia das tempestades Elétricas na América do Sul’, p. 128.
Atkins, N. T. and Wakimoto, R. M. (1991) ‘Wet Microburst Activity over the Southeastern United States:
Implications for Forecasting’, Weather and Forecasting, pp. 470–482. doi: 10.1175/1520-
0434(1991)006<0470:WMAOTS>2.0.CO;2.
Bertsch, A. and Ruck, B. (2015) ‘Interaction between convective downdrafts and inner city areas – a wind tunnel
study’,pp. 1–12.
Caracena, F. and Maier, M. W. (1987) ‘Analysis of a microburst in the FACE meteorological mesonetwork in
Southern Florida.’, Mon.Wea.Review, pp. 969–985. Available at: http://journals.ametsoc.org/doi/pdf/10.1175/1520-
0493(1987)115<0969:AOAMIT>2.0.CO;2.
Garstang, M., White, S., Shugart, H. H. and Halverson, J. (1998) ‘Convective cloud downdrafts as the cause of
large blowdowns in the Amazon rainforest’, Meteorology and Atmospheric Physics, 67(1–4), pp. 199–212. doi:
10.1007/BF01277510.
Grimm, A. M., Ferraz, S. E. T. and Gomes, J. (1998) ‘Precipitation Anomalies in Southern Brazil Associated with
El Niño and La Niña Events’, Journal of Climate, 11(11), pp. 2863–2880. doi: 10.1175/1520-
0442(1998)011<2863:PAISBA>2.0.CO;2.
Grimm, A. M. and Pscheidt, I. (2004) ‘Padrões Atmosféricos Associados a Eventos Severos de Precipitação no
Sul do Brasil Durante El Niño, La Niña e Anos Neutros’, in Congresso Brasileiro de Meteorologia, 2004. CBMet.
doi: 10.1017/CBO9781107415324.004.
Hjelmfelt, M. R. (1988) ‘Structure and Life Cycle of Microburst Outflows Observed in Colorado’, Journal of
Applied Meteorology,pp. 900–927. doi: 10.1175/1520-0450(1988)027<0900:SALCOM>2.0.CO;2.
Leaning, J. and Guha-Sapir, D. (2013) ‘Natural disasters, armed conflict, and public health.’, The New England
Journal of Medicine,pp. 1836–1842. doi: 10.1056/NEJMra1109877.
Lima, E. G. De and Loredo-souza, A. M. (2015) ‘Analysis of Downburst Occurrence in Brazil’, in 14th
International Conference on Wind Engineering.Porto Alegre - Brazil.
Loredo-souza, A. M., Lima, E. G. De, Rocha, M. M., Wittwer, A. R. and Oliveira, M. G. K. (2016) ‘Estudo
Comparativo entre Ensaios em Túnel de Vento e Danos em Estruturas Reais Causados porum Downburst’, in XXXVII
JornadasSudamericanasde Ingeniería Estructural.Assunción,Paraguay.
Marcelino, I. P. V. de O., Nascimento, E. de L. and Ferreira, N. J. (2005) ‘Tornadoes in Santa Catarina State
(southern Brazil): event documentation, meteorological analysis and vulnerability assessment’,
Lagavulin.Ltid.Inpe.Br,pp. 1–15. Available at: http://urlib.net/sid.inpe.br/iris@1912/2006/01.13.11.42.
Nascimento, E. L. (2005) ‘Previsão De Tempestades Severas Utilizando-Se Parâmetros Convectivos E Modelos
De Mesoescala:Uma Estratégia Operacional AdotávelNo Brasil?’, Revista Brasileira de Meteorologia,20, pp. 121–
140.
Nelson, B. W., Kapos, V., Adams, J. B., Oliveira, W. J. and Oscar, P. G. (1994) ‘Forest Disturbance by Large
Blowdowns in the Brazilian Amazon’, 75(3), pp. 853–858. Available at: http://www.jstor.org/stable/1941742.
Reboita, M. S., Gan, M. A., Rocha, R. P. Da and Ambrizzi, T. (2010) ‘Regimes de precipitação na América do
Sul: uma revisão bibliográfica’, Revista Brasileira de Meteorologia, 25(2), pp. 185–204. doi: 10.1590/S0102-
77862010000200004.
Reboita, M.S., Krusche, N., Ambrizzi, T. and Rocha, R. P. Da (2012) ‘Entendendo o Tempo e o Clima na América
do Sul’, Terrae Didática, 8(1), pp. 34–50.
Romatschke, U. and Houze, R. A. (2010) ‘Extreme summer convection in South America’, Journal of Climate,
23(14), pp. 3761–3791. doi: 10.1175/2010JCLI3465.1.
Tippett,M. K., Allen, J. T., Gensini, V. A. and Brooks, H. E. (2015) ‘Climate and Hazardous Convective Weather’,
Current Climate Change Reports, 1(2), pp. 60–73. doi: 10.1007/s40641-015-0006-6.