downburst occurence in brazil

14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015

Downburst Occurrence in Brazil

Elias Galvan de Lima1, Acir Mércio Loredo-Souza1

11Department of Civil Eng., Faculty of Engineering Laboratório de Aerodinâmica das Construções - LAC, UFRGS, Bento Gonçalves 9500, Porto Alegre, Brazil

email: [email protected], [email protected]

ABSTRACT: Each year natural disasters are becoming more frequent in Brazil causing an increase in loss of life and properties. Many of these accidents are related with severe weather. Thus studies that help both in understanding and in predicting these phenomena are sorely needed to create better mitigation strategies and to also to the development of buildings prepared to resist against severe weather. Intense wind gusts are responsible for the major damage from severe thunderstorms and can mainly arise from phenomena such as downbursts or tornadoes. Downbursts are meteorological phenomena that may exhibit distinctive profiles of wind speed and turbulence when compared to those observed on extended pressure systems (EPS). These phenomena, which are the cause of many accidents throughout the world, is also found to be the cause of several accidents in Brazil, especially in the Amazon Basin region, in the Brazil South, Southeast and Northeast Regions The predictability of downbursts is also addressed in this study. It was found that the isolated analysis of the CAPE and CIN index for the occurrence of winds caused by downbursts is not efficient and must be considered a broad analysis to predict downbursts. Through the analyses of the literature it was found that downbursts must be considered on the design definition of buildings in Brazil specially when it comes to the regions specified above.

KEYWORDS: Downburst, Downburst Simulation, Brazil, CAPE, Thunderstorms.

1 INTRODUCTION The life loss and economic damage caused by severe weather events are becoming more expressive. Some researchers point

out that natural disasters are occurring due to climate variations or climate oscillations; other studies, however, indicate that the rapid expansion of urban boundaries especially in areas of risk is the reason to turn populations vulnerable.

Using data from the Emergency Events Database (EMDAT), it was observed that during 1900-2006 the most frequent causes for records of natural disasters around the world have been flooding and winds caused by severe storms, together these causes compose 66% of the total if only the records since 70s were analyzed this number would increase to 84% of the cases [1]. Furthermore, it was observed an increasing trend of climate-related disasters in a global scale and consequently the damage caused to the economy due to these events for the period between 1950 and 2012 [2].

According to the Paraná State Civil Defense between 1990 and 1999, half of the events of natural disasters were related to some occurrence of severe weather events (hail, gales and storms). In Santa Catarina State it was found that between 1976 and 2000, eighteen potentially tornadic episodes were identified (together with five other waterspouts events) and ten of these events were classified as completely developed [3]. The same authors highlighted that this number of occurrences of tornadoes was possibly higher than that one found because only the episodes that caused some material damage and/or loss of life come under review.

By Figure 1 it is notable that the distribution of frequency of occurrences of natural disasters of Brazil between the years 1900 and 2006 is higher in the South, Southeast and Northeast, precisely the areas most populated country, providing further evidence of the need for better understanding of meteorological phenomena that generate these severe events.

A number of observations of the depth of precipitation convective cores in the area corresponding to the domain of part of South America were made by [4] corresponding to the summer time between 1998 and 2006. Through Figure 2, the authors observed that in the Brazil South, Southeast and Midwest the developed of deep convective cells that can generate severe convective storms is much more intensive than in the Amazon Basin, it is possible to see by observing the yellow and red points on the cited Figure. On the other hand, the Amazon Region is largely covered by less deep convective cores, but these cores are present with greater frequency (see the green points), characterizing the high rainfall region.


Source: Marcelino (2007)

Figure 1. Distribution natural disasters occurrence in Brazil

(1900 - 2006) according to each region of the country.

Source: Romatschke e Houze (2010)

Figure 2. Places of occurrence of intense precipitation by convective processes. The period of analysis is related to the

summers of 1998 and 2006.

It was pointed out that atmospheric factors such as the establishment of a low-level circulation east of the Andes promotes the deep convection in regions of middle and subtropical latitudes of South America [5]. This system named Low Level Jet contributes to atmospheric destabilization through moisture transport from the Amazon Region especially during the austral Spring and Austral Autumn.

The occasional coupling between the South America Low Level Jet and the South Hemisphere High Level Jet is an important dynamic mechanism in the development of severe storms, a similar system is seen at North America and a simplified draw is showed at Figure 3 of the South America (a) and North America Case (b) [5].


Source: Nascimento (2005)

Figure 3. Commonly observed dynamic structure for the Subtropical Jet (SJ) and Polar Jet (JP) during periods of increased convective activity in South America (a) North and (b).

Many efforts are still necessary to finally improve the predictability of severe convective storms and is noticeable through that

few studies were made that specifically address the occurrence of downbursts in Brazil and this field needs further studies [5]. Among severe weather phenomena that offer risk to the society downbursts are highlight. This term was coined out by Fujita

in 1985 [6] and is described as a strong and dense column of cold air named downdraft that descends from storm clouds toward the ground and induces a strong burst of divergent winds, called outburst.

The first record of wind speed generated by a downburst is displayed at Figure 4, it was observed at the Air Base of the John F. Kennedy Airport (AAFB) in 1983 and cited by Fujita in 1985 [7]. This record was made at a height of 4.9 m in this case the reading should be taken from right to left. It is visible two peak velocities, and a 180° change in wind direction.

Source: Fujita (1985) cited by Damasceno Neto (2012)

Figure 4. Speed Registry and wind direction at AAFB in downburst event.

During the experiment Joint Airport Wind Shear (JAWS) it was summarized the downburst cycle of life [8]. At Figure 5, it is

initially observed the downdraft is associated with the core of precipitation, as suggested by [9] cited by [8] in this stage strong wind shear is not visible (T -5 min) and the entrainment causes scattering and developing of vorticity. No significant horizontal divergence is observed at the front edge of the downdraft.

When the leading edge approaches to the surface (T -2 minutes), the microburst starts a discrete horizontal and divergent scattering and becomes noticeable the wind shear by radar Doppler. This phenomenon is highly dangerous for aircraft and may only be visible in a short time (<1 min) before the full horizontal scattering (6).

When the microburst reaches the surface, core of stagnation is raised and the flow starts its development, the shear increases and an increase of the intensity of microburst can be observed and the flow becomes more organized.


The time at which the intensity reaches its maximum downburst is presented in accordance with what is observed in the experiment flow over a flat plate. This moment occurs about 5 minutes after the microburst touches the ground. Since then, the horizontal circular vortex, called rotor, develops and begins to move in the opposite direction to the place of stagnation.

Typically at the maximum point, the return flow above the rotor is not set yet, but about 10 minutes after the downdraft has touched the ground, the rotor will now be apparent complete (T + 10 min). As much as the downburst grows it becomes weaker, according with the Hydrostatic Equation, having lower peaks and lower shear speeds, tending to disappear.

Source: Hjelmfelt (1988)

Figure 5. The life cycle of a downburst.

At Figure 6 an ambient is shown with wind acting over a microburst causing a slope. These cases are very usual and are

observed with strong shear in the environment, they deserve special attention because they can generate the most intense wind gusts [8]. They usually show a reflectivity bow echo or spearhead reflectivity [8].

Source: Hjelmfelt (1988)

Figure 6. Outline of a downburst with background wind (not stationary).

The Figure 7 shows an ideal case where the structure of the flow is completely developed to its maximum extent [8] also is

showed the regions with maximum characteristic values based on observations. These values are normally used for validating models.


` ` Source: Hjelmfelt (1988)

Figure 7. Transverse section about the standard of a downburst with their maximum average values structure.

Studies that define the technical standards (such as the Brazilian Technical Standard NBR - 6123) and analyze the effects of

wind on buildings are widely developed in the wind tunnel designed specifically related to the aerodynamics of the structures and pollutant dispersion purposes. These tunnels specifically simulate the profile of wind speeds and turbulence characteristics of the Atmospheric Boundary Layer - ABL.

Technical standards in the most part of the countries address only the winds effects on buildings because of Extend Pressures Systems – EPS [10], these tests do not consider any difference when it comes to winds generated by Thunderstorms - TS. Thus, the differentiation lack between the various meteorological phenomena and the wind profiles of the ABL to determining the static and dynamic loads on structures generated by wind can result in errors expounding the structure to failure.

But there are some exceptions, some wind standards from countries like Australia, United States, South Africa and Canada, since 2008 began to registering specific recommendations for meteorological phenomena observed during TS, such as tornadoes and downbursts [7].

The main reason for the standards do not consider the winds forces generated by TS is the fact that in a 10 years return period, the maximum wind speeds occur from EPS events and not from TS phenomena, however for longer return period (50 years) these same maximum speeds occur due to TS events [11]. It means that TS and EPS winds are characterized by different probability functions and TS have a lower probability of occurrence [12] but the damages magnitude caused by TS events has been a constant concern of designers and engineers. Furthermore when it comes to the TS and EPS wind profile, they are always so different from each other that often in the downburst case, lower height buildings and short buildings are much more exposed [11].

Considering what the exposed above, specially after the 80s winds generated by TS have become the focus of several studies and during those years essential information was generated enhancing the detailed knowledge of the phenomenon, allowing the begin of the physics and numerical simulation of downbursts. Nowadays with more accurate equipment and with the increase of damages caused by severe storms this problem is slowly considered in the standards in order to prevent more accidents but much research is needed in order to have more and more secure and economic buildings

2 OBJECTIVES The occurrence of downbursts in Brazil has not been well explored yet, as there are few cases reported about the occurrence of

the phenomenon. By this way, this article aims to present the locations in Brazil that had reported the occurrence of this weather phenomenon as well as present an analysis of convective environments in which there were favorable conditions for the occurrence of downbursts.


3 RESULTS Downbursts are known as essentially visual phenomena [13], it means that the to have an idealized record of the phenomena it

should be verified by observers. However, considering the limitations of meteorological observations, the possible occurrence of downbursts can be evaluated through the analysis of meteorological variables on the occurrence region of the storm.

Convective parameters can be interesting tool to forecast a downburst occurrence, they are typically calculated from thermodynamic and kinematic atmospheric profiles or from numerical simulations [5]. Through the analysis of these indexes is possible to identify which areas are most favorable or unfavorable to the development of convective systems.

High values of the Convective Available Potential Energy Index - CAPE and low values of Convective Inhibitions Index - CINE, indicate the occurrence of favorable to the occurrence of downbursts environments [13]. CAPE values from 1000 to 2500 m2/s2 are already considered ideal to develop severe weather, values above 2500 m2/s2 indicate a pronounced instability and values above 4000 m2/s2 indicate extreme instability [5]. In the late 50’s it was identified that a dry layer between 700 hPa and 500 hPa indicate the existence of potential energy for the formation of a downward current [14] cited in [15]. Later, in the late 80’s the interaction between the entry of cold and dry air in the mid-tropospheric levels of a cloud with deep vertical development stimulate the generation of downdrafts with the potential to cause downbursts [8].

Through the Microburst and Severe Thunderstorms Project – MIST Project, it was documented various cases of occurrence of downbursts. The project was focused to identify the environment necessary for such events occur, it was found that the most propitious days for occurrence of downbursts is differed from those we observe storms in general [8]. It was concluded that analysis of equivalent potential temperature (θe) and the vertical wind profile aid in the differentiation between convective environments that can downbursts are observed [16].

Due to the entrainment of cold and dry air at high levels a region with very low equivalent potential temperature and extremely dry will be generated. It will ensure a negative buoyancy to keep the trajectory of downdraft leading it out of the cloud and causing at the ground a region called mesohigh. Furthermore a significant vertical wind shear ensures the storm develop even more significantly.

Through observation of microbursts near to Chicago and in South Florida it was found that in cases where the difference in θe between the surface value and the minimum value found in mid-tropospheric levels was greater than or equal to 20K there was great potential in the occurrence of downbursts. For values lower than 13K, could be intense storms but without downbursts [16].

It was suggested that the observation of gusts of winds greater than 10 m/s are initial conditions to suppose the occurrence of downbursts. The maximum observed was 17.2 m/s, however, these maxima do not explain certain falls and pull-outs of trees, suggesting that the force necessary to drop these elements require maximum speed around 25 m/s. Thus, it was suggested that due to space limitations from weather stations the wind gusts registered a rate at least 30% lower than the actually occurring [17]. The analysis and estimation of deep damage is discussed in [18].

It was found that gusts from these phenomena are usually accompanied by precipitation, an abrupt decrease of the equivalent potential temperature (θe) at the measurement level ranging between observed values of 4.00 K to 18.74 K also an instantaneous increase of average surface pressure in the 0.57 hPa 1.99 hPa and decay of specific humidity greater than 3.5 g / kg air must be identified [18]. The schedule of more frequent observation of downbursts in the study area was between 8 a.m. till 20 p.m. (local), with a mean in 13 p.m. (local), this time has greater local convective activity occurring. Figure 8 refers to a meteorological observation and addresses all the afore mentioned characteristics that define the occurrence of wet downbursts.

Others values of increase of pressure was found by other authors, by the experiment called Florida Area Cumulus Experiment – FACE, it was observed an increase in pressure in front of a region of the vortex downburst of 2.4 hPa [18]. Also it was observed an increase of 5 hPa pressure on the nose [13]. Finally it was observed an increase of 4 hPa pressure on the nose of a microburst of 2 km [20] cited in [17].

As noted, there is a range of values available in the literature that indicated thresholds of meteorological variables that could evidence the occurrence of downbursts on stormy environments. These data are shown in Table 1 in order to simplify the understanding of the relationship between them.

These table was used to observe the occurrence of these parameters in this two cases, the first occurred during the night of December 29, 2012 and the second occurred during the day in May 29, 2013. Both cases were chose due to severe weather action because the presence of Mesoscale Systems and both were intensively reported by media because the strong social impact on the states of Rio Grande do Sul (RS) and Santa Catarina (SC).

For these analyzes it was used hourly data from Automatic Weather Stations of Nacional Institute of Meteorology - INMET, scattered in the two affected states, also sounding form the cities of Porto Alegre and Santa Maria was taken.


a) b)

c) d)

e) Source: Gartang et al. (1998)

Figure 8 - Sequence of steps taken to 5 m in height above the treetops, on April 23, 1987 in the Ducke Weather Station -

Amazonas. (a) wind speed, (b) equivalent temperature, (c) precipitation, (d) specific humidity and (e) air temperature. Table 1. Interval values of meteorological variables that indicate the occurrence of downbursts on stormy environments.

Characteristics analysis

Range of values that characterize an atmopheric

environment that is observable the occurrence of Downbursts

Reference

Falling θe between surface and colder layer near the 700hPa (K) > 20 [16]

Wind Gusts (m/s) 10 (Minimum) [17]

25 (Mean) [8]

Drop in Equivalent Potential Temperature θe (K)

> 4 [17]

Drop in temperature (° C) > 5 [17] Increased Atmospheric Pressure

(hPa) > 4 [20]

Drop in Mixing Ratio (g / kg) >3,5 [17]

Convective Cloud Downdrafts as the Cause of Large Blowdowns in the Amazon Rainforest 205

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shown in Fig. 2 which show blowdowns _>30 ha to be rare events in this part of the basin. The characteristics of the observed downbursts in the vicinity of Manaus do, however, provide suffi- cient information to examine the physical processes which produce the downburst. The understanding of the physical processes produ- cing the downburst can then be used to determine why such processes are maximized in the regions where the large blowdowns are observed.

3. Physics of the Downburs t

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Air descending in the presence of liquid water in the cloud or in the precipitation plume will warm at a rate slower than the lapse rate of the air outside of the cloud as heat is lost to evaporating water. Descending motions in convective clouds begin to exceed ascending motions when the cloud reaches maturity. After maturity, descending motions will predominate, often initiated by precipitation loading (Browning, 1986). The descending volume of air may accelerate as a gravity current when the parcel of descending air emerges from the cloud into the environmental air. The acceleration of such a parcel, if isolated from mixing with its surroundings, is

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Fig. 5. Typical sequence of measurements taken 5 m above the rainforest canopy on April 23, 1987 at Ducke Station where (a) is the highest wind in m/s in any 1 rain period with a time resolution of the measurement of 1 sec, (b) is the minimum mean equivalent potential temperature, 0e, in K, (c) is the 1 min mean temperature in ~ (d) is the 1 rain mean specific humidity, q, in g/kg, and (e) is the 1 min mean rain rate with a 0.25 mm resolution. Each mean is compiled from 60 one second readings. Time constants on the instruments are >1 sec but < 5 sec


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Start of the precipitation along the downdrafts (mm/day) > 0,5 [17]

Meteorological phenomenon that originated the downburst

Supercells, squall lines and Derechos [13]

Reflectivity radar (dBZ) Dry > 15

[8] Wet < 65

The first case studied refers to a severe storm occurred on January 1, 2013. The atmospheric was in a strong instability

conditions both in Santa Maria, the same was observed in Porto Alegre. The clouds were with vertically developed with a deep dry air layer at mid tropospheric levels was identified, indicating favorable conditions for the development of strong downdrafts through the entrainment of dry air, this phenomena enhances the phenomenon of evaporative cooling creating a strong downdraft.

Through the application of the criteria set out in Table 1, Table 2 shows that 81% of objective parameters (underlined with a double line) of the cases are higher than suggested by the literature as a minimum to define the occurrence of downbursts.

The values of CAPE and CINE index recorded in Porto Alegre at 00Z of the day January 1, 2013 were 1132 J/kg and 137 J/kg, respectively and in Santa Maria was 1648 J/kg and 38.1 J/kg, in this case, the indexes were efficient on the characterization of atmospheric conditions, suggesting highly unstable environments.

Table 2: Data from the event occurred on the night of December 31, 2012 in several cities in Rio Grande do Sul.

The second case study observed occurred on May 29, 2013 were associated with a strong damage to the progress of a squall

line over the Rio Grande do Sul, which advanced through the southern state of Santa Catarina causing strong damage to the local population.

The data obtained in the second study is presented in Table 3, 50% of the values obtained (underlined with a double line) are higher than suggested in the literature to characterize the occurrence of downburst. This case showed less expression in the observations in comparison to the first one.

The variation of θe for the two cases at the times of the soundings on May 29, 2013 at 12 Z and 30 May 2013 to 00 Z were well below to the recommended for the occurrence of downbursts, however the data recorded at meteorological station of Morro da Igreja, in São Joaquim - SC and at in Santa Marta Lighthouse, in Laguna – SC showed values quite close to those suggested in the literature to define the downburst occurrence, with wind gusts of 100 km/h.

In São José dos Ausentes - RS the values obtained were quite impressive, as well as in Santa Maria - RS. The CAPE and CINE indices recorded in Porto Alegre to 12Z of the day May 31, 2014 was 1.61 J/kg and 408 J/kg, respectively and Santa Maria was 275.1 J/kg and 269 J/kg, respectively, which are quite below to the indicate to define an instable atmosphere.


Table 3: Data from the second study of multiple cases for the case occurred on May 29, 2013.

Reports regarding to the occurrence of downbursts in Brazil exists since the 90s, in Figure 9 we are to geographically locate all

the downbursts observed cases and also the places where it was observed favorable conditions for the occurrence of downbursts in this study.

Figure 9. Downbursts observed cases in Brazil. Red areas represent cases reported in the literature and purple areas indicate

cases suggested by this work.


4 SUMMARY AND DISCUSSIONS Studies of the dynamics of the atmosphere are relatively recent and better forecasts as being produced due to the advent of

technology and development of numerical techniques. The better data quality allows to obtain an increasing and precise amount of meteorological data and improve of the atmospheric numerical modeling with smaller resolutions, generating weather forecasts always more accurate.

To understanding the atmosphere highlights the needing to decrease the fragility of the society against extreme weather phenomena. Just like from floods, landslides and hail, intense winds generated from EPS or TS systems are phenomena that also affect different sectors of society and currently comprise a important focus within the field of wind engineering that seeks to analyze the effects of the winds generated by severe weather on buildings.

It was observed in this work that the wind generated from downbursts is often intense, devastating and compose an important challenge to be understood by aircraft due to frequent air accidents observed before the 80s. Through the studies led by Fujita regarding the training and development of downbursts, there was a marked reduction in cases of accidents, the result of an improvement in now casting techniques that are essential in predicting and identifying downbursts in the atmosphere.

Through this work, it was sought to highlight the importance of the separation of the winds generated by different meteorological phenomena, especially those generated by tornadoes. However very often the data available is in a homogeneous series showing the need to develop methods that seek to separate these winds the generation of more accurate statistics of each meteorological phenomenon.

Through the analysis undertaken and a literature review of the downbursts occurrence in Brazil, it was found that in the Western of Amazon Basin and less frequently in the Eastern of that area Downbursts are identified also in the South and Southeast part of the country this phenomena is observed with high frequency. Also it is highlighted the influence of the penetration of the trade winds over the northeast, along the lines of instabilities, and the downburst occurrence potential. However this is a preliminary study and further investigation are needed, revealing a suggestion for future studies.

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downburst occurence in brazil

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