Wind loading and structural response Lecture 1 Dr

Wind loading and structural response Lecture 1 Dr. J. D. Holmes Meteorology of Windstorms

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

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)

Meteorology of windstorms • Pressure gradient force : z p y z x y (p+ x y z Pressure gradient force per unit mass = x) y z

Meteorology of windstorms • Coriolis force : • an apparent force due to the rotation of the earth (S. Pole) AA = U ( t)2 = (½)a ( t)2 a=2 U

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) = 2 U sin = f. U = angle of latitude = angular velocity of the earth f= 2 sin Coriolis force is zero where and f are zero i. e. at Equator

Meteorology of windstorms • Geostrophic wind : • Balance between pressure gradient and Coriolis forces • Approximates wind speed in upper atmosphere • Ugeostrophic =

Meteorology of windstorms • Geostrophic wind : • Flow parallel to isobars • Anti-clockwise rotation around low pressure centre in N. Hemisphere

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

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

Meteorology of windstorms • Gradient wind : • Solutions : anti-cyclone U is limited to for an anti-cyclone, but unlimited for a cyclone

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

Meteorology of windstorms • Characteristics of hurricanes : • Known as ‘typhoons’ in S. China sea and elsewhere as ‘tropical cyclones’ • 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

Meteorology of windstorms • Characteristics of hurricanes : (S. Hemisphere) • 3 -dimensional vortex structure with ‘eye’ of calm winds

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’)

Meteorology of windstorms • Wind field of hurricanes :

Meteorology of windstorms • Wind field of hurricanes :

Meteorology of windstorms • Saffir-Simpson scale for hurricanes :

Meteorology of windstorms • Profiles of pressure and gradient windspeed : pressure Holland (1980) pressure gradient p = pn-p 0 gradient wind speed where p 0 is central pressure

Meteorology of windstorms • Profiles of pressure and gradient wind : pressure profile gradient wind speed profile Cyclone ‘Tracy’ 1974

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 ….

Meteorology of windstorms • Downbursts : • Macrobursts : > 4 km in horizontal extent • Microbursts : < 4 km in horizontal extent cumulus cloud warm air cool air • Largest documented wind speed : 67 m/s (130 knots), Andrews Air Force Base, Maryland, 1983 (anemometer record : Fig. 1. 9 in book)

Meteorology of windstorms • Downburst anemometer record : Max gust 1 hour Wind speed Direction

Meteorology of windstorms • Downburst wind speed footprint : 15 m/s 20 Direction of storm 30 40 50

Meteorology of windstorms • Tornadoes : Funnel-shaped vortex created in thunderstorms

Meteorology of windstorms • Tornadoes : ‘Supercell’ - intense convective cell

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

Meteorology of windstorms • Tornadoes - Fujita scale:

Meteorology of windstorms • Regions exposed to tornadoes (ASCE 7 -95):

Meteorology of windstorms • Tornado wind field: • Can be modelled as a Rankine (combined) vortex: Tangential velocity Radial Vr 0. 5 V Static pressure Vertical Vv 0. 62 V

Meteorology of windstorms • Tornado damage footprint :

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’

End of Lecture 1 John Holmes 225 -405 -3789 JHolmes@lsu. edu