Download - Lecture10 oct15-bb
Lecture 10
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Pressue gradient forceCoriolis force
Geostrophic wind
Pressure gradient
force (PGF)
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Forces
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What creates wind?
Wind is a result of different forces acting on a parcel of air.
air moving must follows Newton’s second lawF = maF = force (a push or pull)m = mass of an object (parcel)a = acceleration (speed up or slow down)if there are many forces, the acceleration of wind will be
1 2 3 ... nF F F Fa
m
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What are these forces in the atmosphere?
Pressure gradient force Coriolis force Centripetal force Friction
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A force has two properties
MagnitudeDirection
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Pressure gradient
force (PGF)
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Pressure gradient force (PGF)a change in pressure over a
given distance, i. e.pressure differencepressure gradient = =
distance distancehigh lowP P
1020 1016 4
100 100
mb mb mb
km km
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Pressure gradient force (PGF)estimate the magnitude of PGF from a map
pressure differencepressure gradient = =
distance distancehigh lowP P
if the isobars are close together, the PGF is large
if the isobars are far apart, the PGF is small
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Pressure gradient force (PGF)magnitude and direction
PGF magnitude is calculated by formula (or assessed by spacing of isobars
pressure gradient = distancehigh lowP P
PGF direction is always directed from high pressure to low pressure and is always perpendicular to the isobars
H L
1012 mb 1004 mb
1008 mb
100 km
PGF = 8mb/100km
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Pressure gradient force (PGF)magnitude and direction
L
1008 mb
mb
1000
H1012
1008 mb
mb
Q: What is the direction of PGF surrounding an area of low pressure?
Q: What is the direction of PGF surrounding an area of high pressure?
Q: Is the magnitude of PGF larger surrounding the High or the Low?
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200mb
200mb
Q: What direction should the winds aloft at 5 km be blowing based on PGF?
500 mb
700 mb 500 mb
700 mb
5 km
Q: Actually, prevailing wind
direction is from west to east, what
does cause this discrepancy?
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PGF
Coriolis force (CF)
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Coriolis force (CF) due to earth’s rotation
Example
Nonrotating Rotating
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On nonrotating platform A, the thrown ball moves in a straight
line. On platform B, which rotates
counterclockwise, the ball continues to move
in a straight line. However, to anyone on the platform B, the ball
appears to deflect to the right of its intended path.
CF direction
Northern hemisphere Deflection to right
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60oN
30oN
0o
30oS
60oS
Northern hemisphere
Southern hemisphere
N
S
CF directionas air moves from high pressure to low pressure in the northern (southern) hemisphere, it is deflected to the right (left) by the Coriolis force
Northern hemisphereDeflection to right
Southern hemisphereDeflection to left
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60oN30oN
0o
30oS60oS
N
S
CF magnitude 2 sinCF V
Deflectionto right
Deflectionto left
CF =0, sin(0o)=0sin(30o)=0.5
sin(60o)=0.87CF=maximum, sin(90o)=1
CF =0 as V=0
CF is large as a parcel moves faster
The Coriolis force can only change a parcel's direction, it CAN NOT
affect its speed.
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CF acts on objects not rigidly attached to the earthCF acts on objects not rigidly attached to the earth
Q: if a high speed train travels from LA to NY, will the CF act on the train?
Q: Does the CF act on the ocean currents?
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Answer is NoAnswer is No
Answer is Yes Answer is Yes
Geostrophic Wind
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200mb
200mb
500 mb
700 mb 500 mb
700 mb
5 km
Q: Actually, prevailing wind direction is from west to east, what does cause this discrepancy?
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PGF
Parcel motion aloftQ: Where will the parcel be at later
time? Position #1 or #2?
Z= 5 km
W
N
S
E
500 mb
475 mb
450 mb
425 mb
1 2
Initially at rest
PGF
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Parcel motion aloftQ: Where will the parcel be at later time? Position #1 or #2?
Z= 5 km
W
N
S
E
500 mb
475 mb
450 mb
425 mb
1 2
Initially at rest
PGF
CF
PGFWind
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Parcel motion aloft
Q: What cause CF increase?
Z= 5 km
W
N
S
E
500 mb
475 mb
450 mb
425 mb
Initially at rest
PGF
CF
PGFWind
CF
PGFWind
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Parcel motion aloftthe parcel now is moving from west to
east, not from south to north.
Z= 5 km
W
N
S
E
500 mb
475 mb
450 mb
425 mb
Initially at rest
PGF
CF
PGFWind
CF
PGFWind
CF
PGF Wind
the PGF is equal and
opposite to the CF
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Geostrophic Wind when the isobars are straight, parallel lines, and the
only two forces acting on a parcel are the PGF and the
CF, then the wind is called geostrophic wind• PGF and CF are equal in magnitude and
opposite in direction• Geostrophic winds are always parallel to
the isobars
Surface500 mb
500 mb
475 mb
CF
PGF Wind
H
L
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