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 : Pressure gradient force per unit mass = p  y  z zz (p+  x)  y  z xx yy  x  y  z

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)

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

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

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 cyclone

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

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

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 gradient wind speed pressure gradient  p = p n -p 0 where p 0 is central pressure Holland (1980)

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 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

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

Meteorology of windstorms Downburst wind speed footprint : 15m/s Direction of storm

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 Static pressure Radial V r  0.5V  Vertical V v  0.62V 

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