Download Unit 22

What aviation hazards are associated
with thunderstorms, and how can they
be avoided?
■ Where does turbulence occur around a
thunderstorm?
■ How do microbursts develop?
■ What conditions are favorable for the
formation of hail?
■ How does lightning affect flight?
■
Unit 22
Thunderstorm hazards
Turbulence
TS Aviation Weather Hazards
Due to strong up- and downdrafts and latent
heat release within the thunderstorm
■ Two different scales within thunderstorms
■
turbulence
■ downbursts / microbursts
■ gust fronts / outflow boundaries
■ tornadoes
■ airframe icing
■ hail
■ lightning
■ flashfloods
■
♦
large eddies such as the downdrafts and
updrafts
♦ small scale gusts produced by strong wind
shear on the edges of the vertical drafts.
■ In the clear air surrounding storms:
♦ gravity waves above storm tops
♦ mid/upper level eddies downstream of a storm
♦ below cloud base, due to updrafts, downbursts
& gust fronts
Turbulence regions near a TS
Airflow
Above and downstream from overshooting top
Within
cloud
Near upand
downdrafts
Beneath
overhanging
anvil on
downstream
side of storm
Downbursts and microbursts
Downburst - a strong downdraft which
includes an outburst of damaging winds on or
near ground
■ Leading edge of cold air called gust front
■ Microburst - a downburst with horizontal
dimensions less than 4 km
■
♦
■
Small means more shear
Caused by
♦
Near downburst
or microburst
Near gust front
Falling precipitation dragging air down with it
Evaporative cooling making air more dense
♦ Often augmented by violent storm dynamics
♦
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Microbursts
■
♦
♦
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they are small (large wind shear)
they develop suddenly (lasting less than 30 min)
Microbursts can be either wet or dry
After microburst touchdown, a vortex ring
spreads out over the ground
♦
■
Hazards - microbursts
Microbursts are dangerous to aviation because
Often made visible by dust
Vortex ring
Dry microbursts are largely due to evaporative
cooling within the virga below the cloud base
♦
A thunderstorm with a high cloud base and rain
evaporating before it reaches the ground can
produce severe DRY microbursts cloud base.
Photo Moller
After touchdown, a vortex ring spreads out over the
ground. Sometimes this ring is made visible by dust.
Gust front vortex along landing approach to Denver’s Stapleton Airport
Microbursts - cont'd
■
On 9 July 1982 at 2:15 pm, PANAM flight
759 took off at New Orleans.
♦
Soon after take-off, the plane crashed, killing all
people on board
♦ Crash was attributed to sudden change from
headwind to tailwind
♦ What should a pilot do if caught in a microburst?
45 kt
downburst
45 kt
tailwind
Flight path of plane
45 kt
headwind
Hazards - hail
Airframe icing
Stronger vertical motions
Þ greater potential for hail
■ Hail is recycled several
times within the cloud
before it falls out
■ Strong updrafts needed to
suspend large hailstones
and keep them within the
cloud for a long time
■
Strong updrafts can produce large
concentrations of large supercooled cloud
droplets, which freeze on impact
■ Ice accumulation potential is more severe
for large systems (squall lines and MCCs)
■
♦
♦
Difficult to fly out of or around these systems
Icing distorts airfoil shape, increases drag
and reduces lift
♦
■ Large hail can
A golfball-size hailstone
seriously damage
has a fall speed of 22 m/s
aircraft
(40 kt) so needs an updraft
(mainly airfoil and
of at least this speed
propellors).
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Distribution of large hail
deep
layer of
moisture in
the low levels
with drier air
in the mid &
upper levels
■ Lapse rates
with strong
conditional
instability at
lower levels
Baseball, anyone?
■ Need
Hazards - lightning
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Lightning kills more people in the USA every
year than tornadoes and hurricanes combined.
A lightning stroke is very hot (~30,000C), and it
triggers a sound wave in the air : the thunder.
Both cloud-to-ground and intra-cloud lightning
occurs.
In a strongly charged environment, the wings of
aircraft may start glowing: St Elmo's fire (a type
of corona discharge).
Lightning may disturb radio communications.
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