Review of last lecture • 2 types of mesoscale convective systems • Structure of MCCs • Structure of squall lines: four components • Derechos
Downbursts: Introduction • Downbursts are gusts of wind that can reach speeds in excess of 270km/hr (165mph), and are potentially deadly. • Three common types: • Derechos (1000 km) • Haboobs (10-100 km) • Microbursts (1 km)
Video: Microburst http://www.youtube.com/watch?v=NT3YbuGHfC4
Microburst • is a very localized column of downdraft (sinking air) in a thunderstorm that is less than 2.5 miles in scale. • produces damaging divergent and straight-line winds at the surface as high as 150mph • is similar to, but distinguishable from, tornadoes, which generally have convergent damage. • can produce dangerous situations at airports, as they impede air travel. • 3 types: dry, wet, hybrid
Development of microburst Three stages: Contact, Outburst, Cushion
Dry microburst Very dry boundary layer topped by a moist layer Primarily driven by cooling beneath the thunderstorm cloud base due to rain evaporation and ice sublimation Little or no rain. Often associated with high-based thunderstorms. Sometimes associated with fair weather
Dry microburst: visual identification • Virga in the sky: defined as wisps or streaks of water or ice particles falling out of a cloud but evaporating before reaching the earth's surface as precipitation. (NOAA 2001) • Blowing dust/dust rings at surface • Very good indicators of dry microburst potential
Wet microburst A nearly saturated layer topped by an elevated dry layer primarily driven by entrainment of mid-level dry air and precipitation loading. accompanied by heavy precipitation at the surface.
Wet microburst: visual identification • A rain foot may be a visible sign of a wet microburst.
Fatal crashes or aircraft incidents due to microbursts A BOAC Canadair C-4 (G-ALHE), Kano Airport - 24 June 1956. A Malév Ilyushin Il-18 (HA-MOC), Copenhagen Airport – 28 August 1971. Eastern Air Lines Flight 66 Boeing 727-225(N8845E), John F. Kennedy International Airport – 24 June 1975 Pan Am Flight 759 Boeing 727-235 (N4737), New Orleans International Airport – 9 July 1982 Delta Air Lines Flight 191 Lockheed L-1011 TriStar (N726DA), Dallas/Fort Worth International Airport – 2 August 1985 Martinair Flight 495 McDonnell Douglas DC-10 (PH-MBN), Faro Airport – 21 December 1992 USAir Flight 1016 Douglas DC-9 (N954VJ), Charlotte/Douglas International Airport – 2 July 1994 Goodyear Blimp GZ-20A (N1A, "Stars and Stripes"), Coral Springs, Florida – 16 June 2005 Bhoja Air Flight 213 Boeing 737-200 (AP-BKC), Islamabad International Airport, Islamabad, Pakistan- April 20 2012
Causes of atmospheric turbulence Thermals - Heat from the sun makes warm air masses rise and cold ones sink. Jet streams- Fast, high-altitude air currents disturb the air nearby. Mountains - Air passes over mountains and causes wave and turbulence on the other side. Wake turbulence- Near the ground a passing plane or helicopter sets up small, chaotic air currents, or Microbursts – dry, wet and hybrid.
Video: Haboobs http://www.youtube.com/watch?v=LGDvBk4knF0
Haboobs • Very strong horizontal winds over desert regions create sandstorms called haboobs (from the Arabic word meaning “wind”). • Occur regularly in arid regions throughout the world.
Factors contributing to dust storms Desertification Drying of global soil moisture
Desertification • Caused mainly be human activities and climate change • Is one of the most significant global environmental problems • About a billion people are under threat
Drying of global soil moisture PDSI: Palmer drought severity index
Summary • 3 types of downbursts (derechos, haboobs, microbursts) • 3 types of microbursts (wet, dry, hybrid). • 4 causes of atmospheric turbulence. • Haboobs (dust storms). Global desertification. Drying of global soil moisture
Works cited http://www-frd.fsl.noaa.gov/mab/microburst/ http://www.cimms.ou.edu/~doswell/microbursts/Handbook.html