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Downbursts

Downbursts. Downburst/Microburst Definition. A downburst is an area of strong, often damaging winds produced by a convective downdraft over a horizontal area between less than 1 and 10 km A microburst is a downburst that covers an area less than 4 km, with peak winds that last 2–5 minutes.

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Downbursts

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  1. Downbursts

  2. Downburst/Microburst Definition • A downburst is an area of strong, often damaging winds produced by a convective downdraft over a horizontal area between less than 1 and 10 km • A microburst is a downburst that covers an area less than 4 km, with peak winds that last 2–5 minutes. • Surface winds are called straight-line winds

  3. Why are they important? • Eastern Airlines Flight 66 crashed 24 June 1975 • August 1983 near miss at Andrews AFB • Fast winds with rapidly shifting directions are bad for planes trying to take off or land • There are typically 50–100 downbursts each year during the convective season

  4. Downdraft Formation Two main mechanisms: • Evaporation • Cools the air; cold air sinks • Rain that evaporates before reaching the ground is called virga • Cold air can descend as fast as 40–60 m.p.h. • Drag force • Falling precipitation drags air down with it, creating fast descending air • One raindrop is inconsequential but many drops have a large effect on air flow

  5. Environmental Characteristics 1.Role of Stability • Want parcels to be cooler than the environment all the way to the ground • Easy to do in an unstable atmosphere, harder to do in a conditionally unstable atmosphere

  6. Environmental Characteristics 2. Amount of dry air • Lower relative humidity leads to higher evaporation  more cooling 3. Moisture near the surface • Moist air is less dense, which leads to larger density differences when this air encounters descending air 4. Below freezing temperature in the cloud • More energy is necessary to sublimate ice than to evaporate water

  7. Downburst/Microburst Structure Weak environmental wind field • Downburst is symmetrical • Equal speed/damage on all sides

  8. Downburst/Microburst Structure Strong environmental wind field • Asymmetrical • Strongest wind is downwind of stagnation cone • May produce a well-defined “foot” shape to precipitation

  9. Damage Swaths

  10. Vortex Ring

  11. Two Types of Microbursts • Wet microbursts • Measurable precipitation • Easily observable (you can see the rain) • Dry microbursts • No measurable precipitation • Difficult to detect visually (dust may be the only indication of a microburst)

  12. Effect on Airplanes • On takeoff • Increased lift at slow speeds • Lose lift on other side of downdraft • On Landing • Increased lift entering the downdraft • Decreased lift exiting the downdraft

  13. Detection of Microbursts • Doppler radar velocities • Terminal Doppler Weather Radar (TDWR) • Algorithms detect microbursts in radar data

  14. Doppler radar view of a microburst

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