DOWNBURST CHARACTERISTICS

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2 The extratropical cyclones are low pressure systems, with diameters of around 1000km, which in the Southern Hemisphere rotate in the clockwise direction. When one of these systems intensifies during its trajectory, the intense pressure gradients that are established can produce important wind speeds close to the ground during several hours or even during a day or more. Away from the storm fronts that can be generated during these events, the flow is an Atmospheric Boundary Layer (ABL) flow. Severe storms are the other type of events that cause high winds in this region. These storms tend to form when warm humid air is driven upwards in an unstable atmosphere. The warm air can be elevated due to a unequal warming of the earth surface, due to topographic effects, to the encounter with a cold air mass, or to convergent surface winds. When the ascendant flow is fortified by one or more of these mechanisms, a severe storm can take place and cause quite intense wind speeds. These storms are usually accompanied by big hail, intense rain and violent gusts. This type of severe storms generate downbursts, which behave as jets that after impacting the surface, diverge and form a vortex ring, as shown in Figure 2. These downbursts with spatial scales from 40m to 40km and temporal scales of some minutes that can be quite intense, reaching wind speeds of up to 270km/h [1]. These downbursts generate important wind speeds, generally in the inferior region of the vortex ring, and cause damages at ground level. Most of these events occur between October and March in the Southern Hemisphere, and 80% during afternoon and night [2]. In this region, the storm translational direction is usually from southwest to northeast and from west to east, and the more intense gusts come from the southwest sector [2]. Figure 1 shows the region most affected by severe storms in South America, which includes Uruguay, as well as Paraguay, part of Argentine and part of Brazil. Figure 1: Region of occurrence of severe storms Figure 2: Downburst model DOWNBURST CHARACTERISTICS Unlike ABL flows, downbursts present a distribution of wind speed with height with a maximum wind speed between 30 and 100m, high correlation between the more intense wind speeds in distances around a kilometer [4], and flows that develop quite high and localized wind speeds, both spatially as temporally. Figure 3 compares the wind speed distributions with height for an ABL flow and for a typical downburst. This Figure shows that the downburst produced higher wind speeds between 2

3 10 and 120m in this case, for the same wind speed at 10m height. Additionally, Figure 4 illustrates how localized the damage produced by downbursts can be, showing fallen trees within a forest in an area that presents very delimited damages. In this Figure, the divergent pattern of the flow can also be noticed. Figure 3: Example of wind speed distribution for an ABL flow and a downburst Figure 4: Localized damages produced by downburst The affected area can present wind speed gradients similar to those indicated in Figure 5. The wind speed varies with proximity to the storm center and with the elapsed time from its beginning, and its distribution depends on the translational speed. Finally, Figure 6 shows the evolution of the horizontal component of the wind speed at 10m height during a downburst that occurred in Montevideo [7]. In this Figure, an increment in wind speed from 40 to almost 200km/h in less than 2 minutes can be observed, as well as the maintenance of wind speeds above 100km/h during 4 minutes, and a decrement to the original values again in few minutes. Although there are not many records of wind speeds during downbursts due to their relative small spatial and temporal scale, they usually present similar characteristics to those presented in this Figure: temporal scales of minutes, increments and decrements in wind speeds between 1 and 4 minutes, maximum horizontal wind speeds higher than 100km/h, and in cases where wind direction was additionally registered, important changes in wind direction. Figure 5: Wind speed distribution in a horizontal plane for a simulated downburst Figure 6: Wind speed record during a downburst in Montevideo, Uruguay 3

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