03 lect1meteorology

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03 lect1meteorology

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

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