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Meteorology of Windstorms. Wind loading and structural response Lecture 1 Dr. J.D. Holmes. Meteorology of windstorms. Types of storms producing extreme winds : . extra-tropical depressions (gales) synoptic scale 40-60° latitude. tropical cyclones (hurricanes, typhoons)
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Meteorology of Windstorms Wind loading and structural response Lecture 1 Dr. J.D. Holmes
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 (p+ x) y z y x x y z Pressure gradient force per unit mass =
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) = 2U 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 : (U2/r) per unit mass r = radius of curvature of isobars • Equation of motion : anti-cyclone cyclone Quadratic equations for gradient wind speed, U
Meteorology of windstorms • Gradient wind : • Solutions : anti-cyclone cyclone U is limited to for an anti-cyclone, but unlimited for a cyclone
Meteorology of windstorms • Friction at earth’s surface acts in opposite direction to flow new force balance - component towards low pressure region as height decreases • Frictional effects in ‘boundary layer’ : Direction change with height - Ekman spiral <30o
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-p0 where p0 is central pressure gradient wind speed
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 ….
cumulus cloud warm air cool air 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)
Meteorology of windstorms Max gust • Downburst anemometer record : 1 hour Wind speed Direction
15m/s 20 30 40 50 Direction of storm Meteorology of windstorms • Downburst wind speed footprint :
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 0.5V Vertical Vv 0.62V Static pressure
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’
