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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)
pre
ssu
re
distance
p
nn
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
ltitu
de
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