Chapter 21 Downbursts

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Chapter 21Downbursts

• Guest Lecturer Pedro Mulero
• Atmospheric and Oceanic Sciences

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What is a Downburst?

• 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)
• microburst (most common)
• lt 4km diameter
• less than 5 min
• winds up to 270 km/h (170mph)
• macroburst
• gt 4km diameter
• 5-30 min
• winds up to 188 km/h (117mph)

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Mechanisms of Formation (1)

• Evaporation (most important)
• 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

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Mechanisms of Formation (2)

• Precipitation drag
• 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)

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Favorable Environment for Downburst Formation (1)

• Large environmental lapse rate below the cloud
  (close to dry adiabatic lapse rate)
• 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)

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Favorable Environment for Downburst Formation (2)

• Dry air below cloud base
• the drier, the greater the evaporation rate
• Shallow moist air layer near surface
• moist air is less dense than dry air
• Below-freezing temperatures in much of the cloud
  (capable of ice-crystal formation)
• more heat supplied by the air (latent heat of
  evaporation plus latent heat of fusion/melting)
  producing further cooling

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Wet or Dry??

• Dry microbursts
• 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)

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Wet or Dry??

• Wet microbursts
• measurable precipitation (0.01 or more)
• more easily seen
• Environment
• dry air above the cloud, lower cloud base

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Wet or Dry??
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Dry Microburst
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Dry Microburst
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Dry Microburst
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Wet Microburst
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Wet Microburst
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Wet Microburst
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Wet Microburst
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Wet Microburst
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Microbursts and Airplanes

• What provides the lift to aircrafts?
• the shape of the wings!
• But how?
• Bernoulli principle
• air speeds up over top of the wing low pressure
• high pressure under the bottom of the wing
• pressure gradient force upward (lift)

L
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Microbursts and Airplanes

• Lift depends on the air moving across the wing
• 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
  ground-relative speed, but dont forget to lower
  the wing flaps!!
• if actual wind is headwind, lift increases
• if actual wind is tailwind, lift decreases
  (heres when it gets dangerous!!)

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Danger to Airplanes!!

• 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!!

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Detection of Microbursts

• Terminal Doppler Weather Radar (TDWR)
• microbursts, wind shear, severe winds

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Detection of Microbursts
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Detection of Microbursts
divergence
convergence
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Detection of Microbursts

• Low-level Wind-Shear Alert System
• network of anemometers that detect wind shear

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Forecasting Microbursts

• Wind Index (WI) identifies the location and
  magnitude of strong winds associated with
  microbursts
• ELR below melting level
• height of melting level
• moisture content in the lowest km
• moisture content at melting level