Chapter 21 Downbursts Guest Lecturer: Pedro Mulero Atmospheric and Oceanic Sciences

What is a Downburst? Strong downdraft that develops and intensifies within the lower part of a cumulus cloud or thunderstorm 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 Associated with strong straight-line winds as the downward-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 min less than 5 min winds up to 270 km/h (170mph) winds up to 270 km/h (170mph) macroburst macroburst > 4km diameter > 4km diameter 5-30 min 5-30 min winds up to 188 km/h (117mph) winds up to 188 km/h (117mph)

Mechanisms of Formation (1) Evaporation (most important) Evaporation (most important) very dry (low RH) air below the cloud enhances evaporation 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 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 Colder, denser air sinking at great speeds

Mechanisms of Formation (2) Precipitation drag Precipitation drag force associated with the push of million of raindrops of air downward force associated with the push of million of raindrops of air downward Colder, denser air accelerating downward even faster!! (with speeds of km/h or mph) Colder, denser air accelerating downward even faster!! (with speeds of km/h or mph)

Favorable Environment for Downburst Formation (1) Large environmental lapse rate below the cloud (close to dry adiabatic lapse rate) Large environmental lapse rate below the cloud (close to dry adiabatic lapse rate) found in areas where air is ‘well-mixed’ 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) 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) Dry air below cloud base Dry air below cloud base the drier, the greater the evaporation rate the drier, the greater the evaporation rate Shallow moist air layer near surface Shallow moist air layer near surface moist air is less dense than dry air moist air is less dense than dry air Below-freezing temperatures in much of the cloud (capable of ice-crystal formation) 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 more heat supplied by the air (latent heat of evaporation plus latent heat of fusion/melting) producing further cooling

Wet or Dry?? Dry microbursts Dry microbursts No measurable precipitation No measurable precipitation Not easy to detect except maybe when ground is dry Not easy to detect except maybe when ground is dry More common in the West and Great Plains More common in the West and Great Plains Environment: thick dry air layer below cloud base (inverted-V sounding) Environment: thick dry air layer below cloud base (inverted-V sounding)

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

Wet or Dry??

Dry Microburst

Wet Microburst

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

Microbursts and Airplanes Lift depends on the air moving across the wing: Lift depends on the air moving across the wing: Lift speed of plane relative to ground + actual wind 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 lift the airplane... increase the ground-relative speed! If you want to descend... reduce the ground- relative speed, but don’t forget to lower the wing flaps!! If you want to descend... reduce the ground- relative speed, but don’t forget to lower the wing flaps!! if actual wind is ‘headwind’, lift increases if actual wind is ‘headwind’, lift increases if actual wind is ‘tailwind’, lift decreases (here’s when it gets dangerous!!) if actual wind is ‘tailwind’, lift decreases (here’s when it gets dangerous!!)

Danger to Airplanes!! A ‘critical’ speed across the wings must be maintained to remain in the air 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!! When the speed is below the critical value a potentially dangerous situation unfolds,...,especially near the ground!!

Detection of Microbursts Terminal Doppler Weather Radar (TDWR) Terminal Doppler Weather Radar (TDWR) microbursts, wind shear, severe winds microbursts, wind shear, severe winds

Detection of Microbursts

divergenceconvergence

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

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