Chapter 21 Downbursts

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

2. 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) • < 4km diameter • less than 5 min • winds up to 270 km/h (170mph) • macroburst • > 4km diameter • 5-30 min • winds up to 188 km/h (117mph)

3. 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

4. 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)

5. 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)

6. 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

7. 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)

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

9. Wet or Dry??

10. Dry Microburst

11. Dry Microburst

12. Dry Microburst

13. Wet Microburst

14. Wet Microburst

15. Wet Microburst

16. Wet Microburst

17. Wet Microburst

18. 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 H

19. 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 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!!)

20. 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!!

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

22. Detection of Microbursts

23. Detection of Microbursts divergence convergence

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

25. 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