Wind: Small-Scale and Local Systems

1. Wind: Small-Scale and Local Systems

2. RECAP • Coriolis Force: • Results from the rotation of the planet. • Maximum at the poles and no effect at the equator. • Acts perpendicular to the direction of motion: changes the direction of the wind but not the wind magnitude. • The force is proportional to the wind velocity: larger force on stronger winds and vice versa • In the NH deflects the wind to the right. • In the SH deflects the wind to the left.

3. RECAP • Geostrophic winds • the pressure force balanced by the Coriolis force. • the wind is parallel to the isobars. • westerly winds in both NH and SH • Gradient winds • pressure force not balanced by the Coriolis force. • Cyclones (L), around centers of low pressure • Counterclockwise in NH, clockwise in SH • Anticyclones (H), around centers of high pressure • Clockwise in NH, counterclockwise in SH • Surface winds (today): • Balance between the pressure gradient force, the Coriolis force and the air friction. • The wind crosses the isobars. • Vertical air motion (today): convergences and divergences.

4. Surface Winds-a balance of three forces • In the boundary layer (~1km thick) friction is important! • Friction is acting opposite the direction of the velocity -> friction reduces the wind speed -> the Coriolis force becomes weaker -> it cannot balance the pressure force. • The wind starts to blow across the isobars towards the low pressure • The angle between the direction of the wind and the isobars is on average 30 deg (Buys-Ballot’s law). It depends on the topography.

5. Buys-Ballot’s Law • Turn your back to the wind, then turn clockwise 30 deg. The center of low pressure is on your left. W. Ferrel Buys Ballot James Coffin

6. Is this a surface or a high-altitude map?Which hemisphere is this? Surface map in the Northern hemisphere

7. Vertical Air Motion: Convergences and Divergences • Near a center of low surface pressure there is a convergence of air -> the air is forced to rise and then diverge at higher altitudes. The opposite takes place near a center of high surface pressure.

8. Hydrostatic Equilibrium • On average gravity is balanced by the pressure gradientforce -> hydrostatic equilibrium • Small deviations from hydrostatic equilibrium result in small vertical winds (a few cm/s)

9. Begin Chapter 9

10. Scales of motion • Microscale: short-lived eddies, form around obstructions • Mesoscale: a few – 100 km in size, may last last minutes to hours. Local winds, thunderstorms, tornadoes, small tropical storms… • Synoptic scale: thousands of kms, may last days or weeks. • Planetary (global) scale. • Macroscale: synoptic + planetary scales.

12. Friction and turbulence • Friction of air flow mostly due to turbulence. • Mechanical turbulence: eddy motions due to obstructions. Wind gusts. • Strong wind speeds • Rough or hilly landscape • Thermal turbulence: due to thermals • Steep lapse rate, unstable atm.

13. Eddies: big and small • Size and shape of eddies depend on the size and shape of the obstacle and the speed of the wind • Eddies downwind from a mountain • Roll eddies (rotors) • Mountain wave eddy

14. The force of the wind • Strong crosswinds create dangerous traffic conditions • Near hills parallel to the road • On high exposed bridges • Especially hazardous for tall vehicles (trucks, SUVs)

15. Microscale winds: some examples snow roller sand dune ripples ripples desert pavement wind-sculptured trees snow fence snow dune

16. Wind direction: the direction from which the wind is blowing. Other names: Onshore versus offshore winds Upslope versus downslope winds Expressing wind direction: In degrees In terms of compass points Determining wind direction and speed

17. The influence of prevailing winds • Prevailing wind: the wind most commonly observed at a given location over some time period. • Importance for city planning, building a house… • Representation: wind rose (the percentage of time the wind blows from a particular direction).

18. Wind instruments • Measuring wind speed: anemometer • Measuring wind direction • Wind sock • Weather cock • Wind vane

19. Thermal circulations • Due to uneven heating of the surface. Example: • South area heats up, North area cools • Warmer southern air aloft moves north towards low pressure • It then cools and sinks • Surface pressure to the North increases • Surface wind from N to S • The surface air warms up and rises. • The process continues

20. Sea breeze Land breeze

21. Valley breeze Mountain breeze

22. Seasonally changing winds: the monsoon • The monsoon is a periodic sea/land breeze on a very large scale • Surface winds in a thermal circulation: from Cold to Warm • Dry season (winter monsoon, December – February): winds from the cold land towards the warmer seas • Wet season (summer monsoon, June-September): winds from the colder seas towards the warmer continent

23. Desert winds • Dust storms and sandstorms • Dust devils (whirlwinds) • Form on clear hot days over a dry surface (need some convection) • No preference for cyclonic or anticyclonic • Begin at the surface (tornadoes “touch down”)