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# 03 lect1meteorology

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### Transcript

• 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
• 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 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
Includes centrifugal force : (U 2 /r) per unit mass r = radius of curvature of isobars
• Equation of motion :
• anti-cyclone
• 11. Meteorology of windstorms
• 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
Funnel-shaped vortex created in thunderstorms
• 26. Meteorology of windstorms
‘ Supercell’ - intense convective cell
• 27. Meteorology of windstorms
• 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
• 29. Meteorology of windstorms
• Regions exposed to tornadoes (ASCE 7-95):
• 30. Meteorology of windstorms