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03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
03 lect1meteorology
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03 lect1meteorology

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  • 1. Meteorology of Windstorms Wind loading and structural response Lecture 1 Dr. J.D. Holmes
  • 2. Meteorology of windstorms
    • Types of storms producing extreme winds :
    • extra-tropical depressions (gales)
        • synoptic scale 40-60° latitude
    • tropical cyclones (hurricanes, typhoons)
        • synoptic scale 5°-30° latitude oceans and coast
    • thunderstorms (downbursts, tornados)
        • meso scale 0°-50° latitude local convection
    • downslope winds (chinook, Santa Ana)
        • meso scale certain topographic situations thermally driven
  • 3. Meteorology of windstorms
    • Forces acting on air in horizontal motion :
    • Pressure gradient - pressure differences created by non -uniform solar heating
    • Coriolis force - (apparent) force due to the earth’s rotation
    • Centrifugal force - air moving with small radius of curvature
    • Frictional forces - due to earth’s surface -significant at lower heights (boundary layer)
  • 4. Meteorology of windstorms
    • Pressure gradient force :
    Pressure gradient force per unit mass = p  y  z  z (p+  x)  y  z  x  y  x  y  z
  • 5. Meteorology of windstorms
    • Coriolis force :
    • an apparent force due to the rotation of the earth
    AA  =  U (  t) 2 a = 2 U  = ( ½)a (  t) 2 (S. Pole)
  • 6. Meteorology of windstorms
    • Coriolis force :
    • acts to the right of the direction of motion in the northern hemisphere, and to the left of the velocity vector, in the southern hemisphere
    • Coriolis force per unit mass (acceleration) = 2U  sin 
    • =
     = angle of latitude  = angular velocity of the earth f= 2  sin  Coriolis force is zero where  and f are zero i.e. at Equator f U
  • 7. Meteorology of windstorms
    • Geostrophic wind :
    • Balance between pressure gradient and Coriolis forces
    • Approximates wind speed in upper atmosphere
    • U geostrophic =
  • 8. Meteorology of windstorms
    • Geostrophic wind :
    • Flow parallel to isobars
    • Anti-clockwise rotation around low pressure centre in N. Hemisphere
  • 9. Meteorology of windstorms
    • Cyclonic systems :
    • Anti-clockwise rotation in Northern Hemisphere
    Clockwise rotation in Southern Hemisphere
    • direction of Coriolis force determines direction of rotation in cyclones including hurricanes
    Near the equator (  < 5° N), hurricanes cannot form
  • 10. Meteorology of windstorms
    • Gradient wind :
    Includes centrifugal force : (U 2 /r) per unit mass r = radius of curvature of isobars
    • Equation of motion :
    • anti-cyclone
    Quadratic equations for gradient wind speed, U cyclone
  • 11. Meteorology of windstorms
    • Gradient wind :
    • Solutions :
    • anti-cyclone
    U is limited to for an anti-cyclone, but unlimited for a cyclone cyclone
  • 12. Meteorology of windstorms
    • Frictional effects in ‘boundary layer’ :
    • Friction at earth’s surface acts in opposite direction to flow
    • new force balance - component towards low pressure region as height decreases
    Direction change with height - Ekman spiral <30 o
  • 13. Meteorology of windstorms
    • Characteristics of hurricanes :
    • Can exist between 5  and 40  latitude 
    • full strength between 10  and 30  latitudes
    • Require ocean temperature greater than 26  Celsius (79°F)
        • taken to higher latitudes by warm ocean currents
    • 3-dimensional vortex structure with ‘eye’ of calm winds
    • Known as ‘typhoons’ in S. China sea and elsewhere as ‘tropical cyclones’
  • 14. Meteorology of windstorms
    • Characteristics of hurricanes :
    • 3-dimensional vortex structure with ‘eye’ of calm winds
    (S. Hemisphere)
  • 15. Meteorology of windstorms
    • Variation of wind speed/direction at a point in a hurricane :
    Wind speed Direction (Figure 1.7 in book shows low wind speed in ‘eye’)
  • 16. Meteorology of windstorms
    • Wind field of hurricanes :
  • 17. Meteorology of windstorms
    • Wind field of hurricanes :
  • 18. Meteorology of windstorms
    • Saffir-Simpson scale for hurricanes :
  • 19. Meteorology of windstorms
    • Profiles of pressure and gradient windspeed :
    pressure gradient wind speed pressure gradient  p = p n -p 0 where p 0 is central pressure Holland (1980)
  • 20. Meteorology of windstorms
    • Profiles of pressure and gradient wind :
    pressure profile gradient wind speed profile Cyclone ‘Tracy’ 1974
  • 21. Meteorology of windstorms
    • Thunderstorm winds :
    • Severe winds in small areas generated by strong downdrafts and tornadoes
    • Conditions for generation of downdrafts
      • High wind shear
      • Supply of warm moist air at ground level
      • Uplift mechanism
    • Melting hail cools surrounding air
      • initiates downdraft
      • augmented by evaporating rain
    • Extreme winds are dominated by thunderstorm downdrafts in many locations : Argentina, South Africa, Central U.S., Singapore ….
  • 22. Meteorology of windstorms
    • Downbursts :
    • Macrobursts : > 4 km in horizontal extent
    • Microbursts : < 4 km in horizontal extent
    • Largest documented wind speed : 67 m/s (130 knots), Andrews Air Force Base, Maryland, 1983
    • (anemometer record : Fig. 1.9 in book)
    warm air cool air cumulus cloud
  • 23. Meteorology of windstorms
    • Downburst anemometer record :
    Wind speed Direction Max gust 1 hour
  • 24. Meteorology of windstorms
    • Downburst wind speed footprint :
    15m/s 20 30 40 50 Direction of storm
  • 25. Meteorology of windstorms
    • Tornadoes :
    Funnel-shaped vortex created in thunderstorms
  • 26. Meteorology of windstorms
    • Tornadoes :
    ‘ Supercell’ - intense convective cell
  • 27. Meteorology of windstorms
    • Tornadoes :
    • Narrow width - typically 100 metres can be up to 1000 m
    • Can travel long distances before dissipation - up to 50 km (30 miles)
    • Most common in mid-West of U.S. (esp. Oklahoma, Kansas)
    • also occur in Argentina, South Africa, India, Russia, Australia
  • 28. Meteorology of windstorms
    • Tornadoes - Fujita scale:
  • 29. Meteorology of windstorms
    • Regions exposed to tornadoes (ASCE 7-95):
  • 30. Meteorology of windstorms
    • Tornado wind field:
    • Can be modelled as a Rankine (combined) vortex:
    Tangential velocity Static pressure Radial V r  0.5V  Vertical V v  0.62V 
  • 31. Meteorology of windstorms
    • Tornado damage footprint :
  • 32. Meteorology of windstorms
    • Downslope winds :
    • Thermally driven - several different phenomena
    • usually occur on the lee slopes of mountains, or in valleys
    • U.S. : chinook winds (Colorado)
    • Santa Ana winds (California)
    • Affect small areas
    • Dealt with in wind loading codes as ‘special wind regions’
  • 33. End of Lecture 1 John Holmes 225-405-3789 JHolmes@lsu.edu

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