Wind SmallScale and Local Systems RECAP Coriolis Force

• RECAP Coriolis Force: ♦ Results from the rotation of the planet. ♦

• • RECAP Geostrophic winds ♦ the pressure force balanced by the Coriolis

• • Surface Winds-a balance of three forces In the boundary layer (~1

• James Coffin W. Ferrel Buys Ballot Buys-Ballot’s Law Turn your back to

Is this a surface or a high-altitude map? Which hemisphere is this? Surface map

• Vertical Air Motion: Convergences and Divergences Near a center of low surface

• • Hydrostatic Equilibrium On average gravity is balanced by the pressure gradient

• • • Scales of motion Microscale: short-lived eddies, form around obstructions Mesoscale:

Friction and turbulence • • • Friction of air flow mostly due to turbulence.

• • Eddies: big and small Size and shape of eddies depend on

• • The force of the wind Strong crosswinds create dangerous traffic conditions

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

• • • Determining wind direction and speed Wind direction: the direction from

• • • The influence of prevailing winds Prevailing wind: the wind most

• • Wind instruments Measuring wind speed: anemometer Measuring wind direction ♦ Wind

Thermal circulations • Due to uneven heating of the surface. Example: ♦ South area

• • Seasonally changing winds: the monsoon The monsoon is a periodic sea/land

• • Desert winds Dust storms and sandstorms Dust devils (whirlwinds) ♦ Form

Slides: 23

Download presentation

Wind: Small-Scale and Local Systems

• 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.

• • 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. • •

• • Surface Winds-a balance of three forces In the boundary layer (~1 km 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.

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

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

• 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.

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

Begin Chapter 9

• • • Scales of motion Microscale: short-lived eddies, form around obstructions Mesoscale: a few – 100 km in size, may 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.

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.

• • 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

• • 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)

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

• • • Determining wind direction and speed 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

• • • 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).

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

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

Sea breeze Land breeze

Valley breeze Mountain breeze

• • 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

• • 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”)