Curved Flow and the Gradient Wind

Surface winds

How Winds Generate Vertical Air Motion

Winds Aloft and Geostrophic Flow

3/13/03

Outline for Lecture 13

Factors Affecting Wind

(a) No pressure gradient,no wind.

(b) Unequal heating creates pressure differences aloft which causes air to flow.

(c) The transfer of air aloft(from the land to the sea)creates a surface high overthe sea which results in a flow of air from sea to land (a.k.a. a sea breeze).

Formation of A Sea Breeze

Sea Breeze

Returning air

steep pressure gradient(fast winds)

shallow pressure gradient(slow winds)

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distance

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Pressure Gradient Force

Named after the French Scientist Gaspard Gustave Coriolis

free moving objectsare deflected to the rightof their path in the NorthernHemisphere (to the left oftheir path in the SouthernHemisphere) because of theEarth’s rotation.

It depends on an object’sspeed—higher speed means stronger Coriolis Force.

The Coriolis Force

Coriolis deflection of winds blowing eastward at different latitudes

Coriolis force also increases with increasing

Latitude…strong over poles,

no effect over equator.

The Coriolis Force

Strong

Middling

None

Weak

Friction• Friction acts at the

surface.winds at the surface aren’t as strong as those at higher altitudes

• Air is a little viscous, so the layer next to the surface is also affected, but not as much.A

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Balance pressure gradient force Coriolis Force: Geostrophic Flow.

Winds Aloft and Geostrophic Flow

Geostrophic winds:•go in a straight line•go parallel to the isobars•have speeds proportional to the pressure gradient force.

Geostrophic Flow

• WHAT!?!?• pressure difference starts wind • wind gets going a little, starts being deflected by

Coriolis force• wind goes faster in response to pressure

difference, gets deflected more by Coriolis Force• Eventually, the two balance

Newton’s Laws of Motion (condensed version)

II. F = ma

I. An object in motion tends to stay in motion (unless acted upon by an external force)

(that’s pretty much it.)

Put down the pencils

Let’s break down the forces

Just a Pressure Gradient

Start with just a pressure gradient

Then wind blows straight from High P to Low P

And accelerates as it goes

H L

View from top

Pressure 1017 1015 1013 1011 1009 1007 1005 return

Just a Pressure Gradient

Wind blows straight from High to Low, eventually evening out the pressure and stopping the wind.

H L

Pressure 1014 1012 1010 1008

Just a Pressure Gradient

The wind would blow at the same speed regardless of altitude. H L

Pressure 1013 1011 1009 1007 1005 return

Now add Friction

Then wind still blows straight from High P to Low P, but it doesn’t get moving as fast as soon, especially near the ground

H L

View from top

Pressure 1017 1015 1013 1011 1009 1007 1005 back up

Friction and Pressure Gradient

The pressure difference also evens out eventually, though it might take a bit longer.

H L

Pressure 1014 1012 1010 1008

Friction and Pressure Gradient

Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere.

H L

Pressure 1013 1011 1009 1007 1005

Friction and Pressure Gradient

Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere.

H L

Pressure 1013 1011 1009 1007 1005 back up

Pressure Gradient and Coriolis

Forget friction.

The wind starts out straight, but as soon as it starts building up speed, the Coriolis force turns it a bit to the right.

H L

The wind can’t accelerate any more over here because it’s going parallel to the isobars

This is the Geostrophic Flow

Pressure Gradient and Coriolis

The Coriolis force limits the wind speed by redirecting it AND it prevents wind from blowing straight from H to L

H L

Pressure Gradient and Coriolis

Since the wind never really reaches the low, the pressure difference is maintained, and the low never fills!

H L

Reality: Pressure Gradient, Coriolis Force, and Friction

Coriolis Force turns the wind some, friction slows the wind some, and the result is roughly a 30º angle between isobars and wind.

H L30º

Reality: Pressure Gradient, Coriolis Force, and Friction

The wind doesn’t blow straight from High to Low, but it does eventually get in there and even out the pressure difference, so H and L don’t last forever without a source of energy

H L30º

Reality: Pressure Gradient, Coriolis Force, and Friction

Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere.

H L

Pressure 1013 1011 1009 1007 1005

Reality: Pressure Gradient, Coriolis Force, and Friction

Coriolis Force is turning the wind toward us in the right part of the picture. H L

Pressure 1013 1011 1009 1007 1005

Reality: Pressure Gradient, Coriolis Force, and Friction

Since the Coriolis Force depends on wind speed, its effect decreases toward the ground where the wind speed is slower.

H L

Pressure 1013 1011 1009 1007 1005

Coriolis

Friction

Pencils are now allowed

Geostrophic winds are up high and go straight:only Coriolis and Pressure Gradient Forces are important.

Friction is important down low: below about 1500 meters.

How do the Coriolis and Pressure Gradient forces change?

Coriolis Force

Coriolis Force

Wind speed

Latitude

Pressure Gradient

Wind flows from high to low pressure.

“Isobaric packing” P.G.F.

Coriolis Force

Winds Aloft and Geostrophic Flow

Wind direction is directly linked to the prevailing pressure pattern.

Dutch meteorologist Buys Ballott, 1857

Buys Ballott’s Law states: In the Northern Hemisphere if you stand with your back to the wind, lower pressure will be found toyour left and higher pressure will be found to the right.

Best when there are no frictional forcesor topography involved .

Winds Aloft and Geostrophic Flow

Actual flow around pressure systems are never this regular (because of small changes in the pressure field).

Cyclonic Flow Anticyclonic Flow

Curved Flow and the Gradient Wind

Surface winds—high pressure(anticyclone)

H

COLDDRY

WARMMOIST

Surface winds—low pressure(cyclone)

L

COLDDRY

WARMMOIST

Variations in height are analogousto variations in pressure. High height fields correspond to high pressure fields.

500mb chart

Upper Level Weather Chart

Troughs and Ridges

TROUGH

RIDGE RIDGE

Troughs and Ridges

• An elongated region of low pressure (trough) or high pressure (ridge)

• Tend to be quite common at higher altitudes

• At the surface, a trough is usually a fairly weak feature

Around a surface low pressure center, a net inward transport of aircauses a shrinking of the area occupied by the mass. This is knownas horizontal convergence.

How Winds Generate Vertical Air Motion

“Upper level support” is important in cyclone development

Airflow Associated with Cyclones and Anticyclones

Wind speeds and isobars

Slack winds Slack winds

STRONG WINDS

The tighter they’re packed, the stronger the wind

• Friction: air flow from ocean to land (upward motion)air flow from land to ocean (downward motion)

•Mountain ranges

Factors that promote vertical airflow

Wind roses provide a method of representing prevailing winds byindicating the percentage of time the wind blows from various directions

Wind Measurement

Go Terps!ACC Tournament this Weekend