Transcript Downburst Definition - OU Cooperative Institute for

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
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
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