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

11. Damage Swaths

12. Vortex Ring

13. 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)

14. 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

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

16. Doppler radar view of a microburst