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.

• Surface winds are called straight-line winds

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

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

Environmental Characteristics1. 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

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

Downburst/Microburst Structure

Weak environmental wind field– Downburst is symmetrical– Equal speed/damage on all sides

Downburst/Microburst Structure

Strong environmental wind field– Asymmetrical– Strongest wind is downwind of

stagnation cone– May produce a well-defined “foot”

shape to precipitation

Damage Swaths

Vortex Ring

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)

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

Detection of Microbursts

• Doppler radar velocities

• Terminal Doppler Weather Radar (TDWR)

• Algorithms detect microbursts in radar data

Doppler radar view of a microburst