Chapter 21Chapter 21DownburstsDownbursts

Guest Lecturer: Pedro MuleroGuest Lecturer: Pedro Mulero

Atmospheric and Oceanic Atmospheric and Oceanic SciencesSciences

What is a Downburst?What is a Downburst?

Strong downdraft that develops and intensifies Strong downdraft that develops and intensifies within the lower part of a cumulus cloud or within the lower part of a cumulus cloud or thunderstormthunderstorm

Associated with strong straight-line winds as the Associated with strong straight-line winds as the downward-directed air crashes onto the grounddownward-directed air crashes onto the ground

Two types (based on length scale):Two types (based on length scale): microburst (most common)microburst (most common)

< 4km diameter< 4km diameter less than 5 minless than 5 min winds up to 270 km/h (170mph)winds up to 270 km/h (170mph)

macroburstmacroburst > 4km diameter> 4km diameter 5-30 min5-30 min winds up to 188 km/h (117mph)winds up to 188 km/h (117mph)

Mechanisms of Formation Mechanisms of Formation (1)(1)

Evaporation (most important)Evaporation (most important) very dry (low RH) air below the cloud very dry (low RH) air below the cloud

enhances evaporationenhances evaporation latent heat of evaporation/melting is latent heat of evaporation/melting is

transferred from the unsaturated air into transferred from the unsaturated air into the droplets/ice crystals or hail the droplets/ice crystals or hail

Colder, denser air sinking at great Colder, denser air sinking at great speedsspeeds

Mechanisms of Formation Mechanisms of Formation (2)(2)

Precipitation dragPrecipitation drag force associated with the push of million force associated with the push of million

of raindrops of air downwardof raindrops of air downward Colder, denser air accelerating Colder, denser air accelerating

downward even faster!! (with speeds of downward even faster!! (with speeds of 64-96 km/h or 40-60 mph)64-96 km/h or 40-60 mph)

Favorable Environment for Favorable Environment for Downburst Formation (1)Downburst Formation (1)

Large environmental lapse rate below Large environmental lapse rate below the cloud (close to dry adiabatic lapse the cloud (close to dry adiabatic lapse rate)rate) found in areas where air is ‘well-mixed’ found in areas where air is ‘well-mixed’ conduction-dry convection mechanism conduction-dry convection mechanism

mixes air well up to several km and mixes air well up to several km and increases the lapse rate (especially in the increases the lapse rate (especially in the early afternoon when surface heating early afternoon when surface heating reaches a maximum)reaches a maximum)

Favorable Environment for Favorable Environment for Downburst Formation (2)Downburst Formation (2)

Dry air below cloud baseDry air below cloud base the drier, the greater the evaporation ratethe drier, the greater the evaporation rate

Shallow moist air layer near surfaceShallow moist air layer near surface moist air is less dense than dry airmoist air is less dense than dry air

Below-freezing temperatures in much of Below-freezing temperatures in much of the cloud (capable of ice-crystal formation)the cloud (capable of ice-crystal formation) more heat supplied by the air (latent heat of more heat supplied by the air (latent heat of

evaporation plus latent heat of fusion/melting) evaporation plus latent heat of fusion/melting) producing further cooling producing further cooling

Wet or Dry??Wet or Dry??

Dry microburstsDry microbursts No measurable No measurable

precipitationprecipitation Not easy to detect Not easy to detect

except maybe when except maybe when ground is dryground is dry

More common in the More common in the West and Great PlainsWest and Great Plains

Environment: thick dry Environment: thick dry air layer below cloud air layer below cloud base (inverted-V base (inverted-V sounding)sounding)

Wet or Dry??Wet or Dry??

Wet microburstsWet microbursts measurable measurable

precipitation (0.01” precipitation (0.01” or more)or more)

more easily seenmore easily seen EnvironmentEnvironment

dry air above the dry air above the cloud, lower cloud cloud, lower cloud basebase

Wet or Dry??Wet or Dry??

Dry MicroburstDry Microburst

Dry MicroburstDry Microburst

Dry MicroburstDry Microburst

Wet MicroburstWet Microburst

Wet MicroburstWet Microburst

Wet MicroburstWet Microburst

Wet MicroburstWet Microburst

Wet MicroburstWet Microburst

Microbursts and AirplanesMicrobursts and Airplanes

What provides the What provides the lift to aircrafts?lift to aircrafts? the shape of the the shape of the

wings!wings! But how? But how?

Bernoulli principleBernoulli principle air speeds up over air speeds up over

top of the wing: top of the wing: low pressurelow pressure

high pressure high pressure under the bottom under the bottom of the wingof the wing

pressure gradient pressure gradient force upward (lift) force upward (lift)

L

H

Microbursts and AirplanesMicrobursts and Airplanes

Lift depends on the air moving across the Lift depends on the air moving across the wing: wing: Lift speed of plane relative to ground + actual windLift speed of plane relative to ground + actual wind

If you want to lift the airplane... increase the If you want to lift the airplane... increase the ground-relative speed!ground-relative speed!

If you want to descend... reduce the ground-If you want to descend... reduce the ground-relative speed, but don’t forget to lower the relative speed, but don’t forget to lower the wing flaps!!wing flaps!!

if actual wind is ‘headwind’, lift increasesif actual wind is ‘headwind’, lift increases if actual wind is ‘tailwind’, lift decreases if actual wind is ‘tailwind’, lift decreases

(here’s when it gets dangerous!!)(here’s when it gets dangerous!!)

Danger to Airplanes!!Danger to Airplanes!!

A ‘critical’ speed A ‘critical’ speed across the wings across the wings must be maintained must be maintained to remain in the airto remain in the air

When the speed is When the speed is below the critical below the critical value a potentially value a potentially dangerous situation dangerous situation unfolds,...,especiallunfolds,...,especially near the ground!!y near the ground!!

Detection of MicroburstsDetection of Microbursts

Terminal Terminal Doppler Doppler Weather Radar Weather Radar (TDWR)(TDWR) microbursts, microbursts,

wind shear, wind shear, severe windssevere winds

Detection of MicroburstsDetection of Microbursts

Detection of MicroburstsDetection of Microbursts

divergence convergence

Detection of MicroburstsDetection of Microbursts

Low-level Low-level Wind-Shear Wind-Shear Alert System Alert System network of network of

anemometeranemometers that detect s that detect wind shearwind shear

Forecasting MicroburstsForecasting Microbursts

Wind Index (WI) – identifies the Wind Index (WI) – identifies the location and magnitude of strong location and magnitude of strong winds associated with microburstswinds associated with microbursts ELR below melting levelELR below melting level height of melting levelheight of melting level moisture content in the lowest kmmoisture content in the lowest km moisture content at melting levelmoisture content at melting level