Transcript Chapter 21 Downbursts - UW-Madison Department of

Chapter 21
Downbursts
Guest Lecturer: Pedro Mulero
Atmospheric and Oceanic Sciences
What is a Downburst?
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Strong downdraft that develops and intensifies within
the lower part of a cumulus cloud or thunderstorm
Associated with strong straight-line winds as the
downward-directed air crashes onto the ground
Two types (based on length scale):
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microburst (most common)
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< 4km diameter
less than 5 min
winds up to 270 km/h (170mph)
macroburst
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> 4km diameter
5-30 min
winds up to 188 km/h (117mph)
Mechanisms of Formation (1)
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Evaporation (most important)
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very dry (low RH) air below the cloud
enhances evaporation
latent heat of evaporation/melting is
transferred from the unsaturated air into the
droplets/ice crystals or hail
Colder, denser air sinking at great speeds
Mechanisms of Formation (2)
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Precipitation drag
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force associated with the push of million of
raindrops of air downward
Colder, denser air accelerating downward
even faster!! (with speeds of 64-96 km/h or
40-60 mph)
Favorable Environment for
Downburst Formation (1)
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Large environmental lapse rate below the
cloud (close to dry adiabatic lapse rate)
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found in areas where air is ‘well-mixed’
conduction-dry convection mechanism mixes
air well up to several km and increases the
lapse rate (especially in the early afternoon
when surface heating reaches a maximum)
Favorable Environment for
Downburst Formation (2)
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Dry air below cloud base
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Shallow moist air layer near surface
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the drier, the greater the evaporation rate
moist air is less dense than dry air
Below-freezing temperatures in much of the
cloud (capable of ice-crystal formation)
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more heat supplied by the air (latent heat of
evaporation plus latent heat of fusion/melting)
producing further cooling
Wet or Dry??
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Dry microbursts
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No measurable
precipitation
Not easy to detect except
maybe when ground is dry
More common in the West
and Great Plains
Environment: thick dry air
layer below cloud base
(inverted-V sounding)
Wet or Dry??
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Wet microbursts
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measurable
precipitation (0.01” or
more)
more easily seen
Environment
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dry air above the
cloud, lower cloud
base
Wet or Dry??
Dry Microburst
Dry Microburst
Dry Microburst
Wet Microburst
Wet Microburst
Wet Microburst
Wet Microburst
Wet Microburst
Microbursts and Airplanes
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What provides the lift
to aircrafts?
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the shape of the
wings!
But how?
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Bernoulli principle
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air speeds up over
top of the wing: low
pressure
high pressure under
the bottom of the
wing
pressure gradient
force upward (lift)
L
H
Microbursts and Airplanes
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Lift depends on the air moving across the wing:
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Lift speed of plane relative to ground + actual wind
If you want to lift the airplane... increase the
ground-relative speed!
If you want to descend... reduce the groundrelative speed, but don’t forget to lower the
wing flaps!!
if actual wind is ‘headwind’, lift increases
if actual wind is ‘tailwind’, lift decreases (here’s
when it gets dangerous!!)
Danger to Airplanes!!
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A ‘critical’ speed
across the wings must
be maintained to
remain in the air
When the speed is
below the critical
value a potentially
dangerous situation
unfolds,...,especially
near the ground!!
Detection of Microbursts
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Terminal Doppler
Weather Radar
(TDWR)
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microbursts, wind
shear, severe
winds
Detection of Microbursts
Detection of Microbursts
divergence
convergence
Detection of Microbursts
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Low-level
Wind-Shear
Alert System
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network of
anemometers
that detect
wind shear
Forecasting Microbursts
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Wind Index (WI) – identifies the location
and magnitude of strong winds associated
with microbursts
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ELR below melting level
height of melting level
moisture content in the lowest km
moisture content at melting level