Transcript MET 2204 METEOROLOGY

MET 2204
METEOROLOGY
Presentation 4: Wind
Presented by Mohd Amirul for AMC
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Recapitulate
• Lets recap what we have done last week:
– What is the Atmosphere?
– Composition of Atmosphere
– Layers of Atmosphere
– Variation in Atmosphere
– Air stability
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Presentation Outline
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Introduction
Convection
Gradient Wind
General Circulation
Friction
Jet Stream
Local and Small Winds
Wind Shear
Wind Pressure Systems and Weather
Conclusion
Presented by Mohd Amirul for AMC
Part 1
Part 2
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Learning Outcomes
• At the end of this session, student should be
able to:
– Explain the concept of winds around the earth
surface and upper winds.
– Understand the relation of winds to pressure
patterns and movement of weather system.
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Introduction
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What is the WIND??
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Cont.
Difference in
Temperature
Difference in
Pressure
WIND
How??
If the temperature increase, the molecule inside the
box will move rapidly. Then there are more and rapid
collision happen between the molecule and wall.
Therefore the pressure will increase. From this
pressure difference the air will moving and it will
create the WIND.
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Convection
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Cont.
• Convection : transmits heat by transporting
groups of molecules from place to place.
• Convection occurs in fluids such as water and
air, which move freely.
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Cont.
• Two surface are heated
unequally
– Warm air
• less dense and lifting to the
top
• Spreads and cool
– Cold air
• more dense and drawn to the
ground
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Cont.
• The horizontal air flow
in convective current is
WIND. (sometime
called ad ADVECTION)
Convective current
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General Circulation
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Pressure Gradient Force (PGF)
Idea:
pressure
steep pressure gradient
(fast winds)
shallow pressure gradient
(slow winds)
p
n
distance
n
Cont.
• Pressure difference- create the FORCE and to
drive the wind. The force we call as PGF.
• PGF: The difference in atmospheric pressure
per unit distance
• Wind move from HIGHER PRESSURE to LOWER
PRESSURE by PGF.
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Cont.
Usage of ISOBAR on determine wind speed,
strength, and direction.
• On the Isobars:
– When there is difference in pressure the air begins
to moving from higher pressure to lower pressure.
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Cont.
• Closer the spacing of isobars, stronger the
PGF. Therefore stronger and faster the WIND.
• Widely space isobar – create light/slow wind
• Closely space isobar – create strong/fast wind
• Therefore general idea of wind speed can get
from isobar.
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Cont.
• Surface pressure
– Low in warm region (equatorial)
– High in cold region (poles)
• Because of PRESSURE DIFFERENCE- create PGF
• Therefore, PGF develop from poles to
equatorial.
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Cont.
• Imagine that earth didn’t rotate. Therefore
PGF is the only force acting on WIND.
– Circulation happen between two hemisphere
(Circulation Process):
Convective
Current
• Cold air SINK at poles
• Because dense air flows from poles to equator,
wind will blow straight from poles to equator.
• Because less dense air rising, warm air force upward at
the equator.
• High level wind blow directly toward the poles.
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Cont.
• However above circulation is distorted since
the earth does ROTATE!!!!
• So the PGF is not only force acting on the
WIND.
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Coriolis Force (Geostrophic Force)
• Idea:
– Imagine the rotation of the
compact disk when the song
played. Try draw a line during the
rotation. Then stop the disk.
– Now you can see the line you draw
is deflected away. Not a straight
LINE!!
– This is the concept of Coriolis
Force.
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Cont.
• Coriolise Force (Geostrophic Force) is the force
caused by rotation of the earth
• It will make a moving mass of air turn to the
right at northern hemisphere
• It will make a moving mass of air turn to the
left at southern hemisphere.
• The angle of deflection/turn directly
proportional to wind speed.
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Cont.
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Cont.
• In this course we will concentrate on Northen
Hemisphere where mass of air will deflect to
the right.
• Coriolis Force varies from equator to poles:
– Maximum at poles and minimum at equator.
• So it will effect wind direction everywhere
except at the equator.
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Differences of Coriolis and PGF
Pressure Gradient Force
• Force created by different
pressure
• Wind blow perpendicular to
isobar
Coriolis Force
• Apparent deflection result
from earth rotation
• Wind blow parallel to isobar
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Geostrophic Wind
• PGF drives wind perpendicular to isobars
(higher pressure to lower pressure).
• GF (coriolis force) drives wind to the right until
defelected to 90˚ and become perpendicular
to isobars.
• At this time, GF and PGF is BALANCE!!!
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Cont.
• Geostrophic Wind is the wind that blows
when the GF is balance with PGF.
• Geostrophic wind directly proportional to the
PGF.
• Geostrophic wind speed increase as latitude
decrease. (Wind speed becomes faster close
to the equator)
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How Geostrophic wind occur??
1. pressure difference starts wind
2. wind gets going a little, starts
being deflected by Coriolis force
3. wind goes faster in response to
pressure difference,
4. gets deflected more by Coriolis
Force
5. Eventually, the two balance
Geostrophic winds:
•go in a straight line
•go parallel to the isobars
•have speeds proportional to the pressure gradient force.
Cont.
• But within the 15 degrees latitude from the
equator GF is ZERO.
• For the wind to be geostrophic it must occur:
– Above the friction layer
– At a latitude greater than 15 degrees
– When the pressure situation is not changing
rapidly
– With the isobars STRAIGHT and PARALLEL.
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Gradient Wind
• The gradient wind occurs when the isobars are
curved. The force makes the wind follow a
curved path parallel to the isobars.
• Gradient wind is due to a combination of 3
forces:
– PGF
– GF (Coriolis Force)
– Cyclostrophic Force
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Cont.
• Cyclostrophic force acts towards the center of
pressure system when the isobars are curved.
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Friction
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Cont.
• Idea:
– Consider two boxes of wood contact with each
other. Try move each of those boxes in the
opposite direction. It will slow the movement or
hardly to move smoothly because of FRICTION
FORCE.
• Friction act as the resistance when one body is
moved in contact with another.
V
F
r
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Cont.
• Same goes with wind.
• There will be a friction between wind and terrain. It will
slower the wind.
• Friction Layer: Greatest near and right at the surface. Usually
confined to lowest 1000m (3000 ft).
• The friction increase when:
– Rougher the terrain
– Stronger the wind speed
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Cont.
• Friction acting OPPOSITE in wind direction
• Friction increase will result decrease in GF.
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Cont.
• Therefore, the friction will result:
GF and PGF no longer in balance!!
• Friction on sea less than from land.
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PGF + GF + Friction
Chronology
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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
View from top
H
L
Pressure 1017 1015 1013 1011 1009 1007 1005
return
Just a Pressure Gradient
The wind would
blow at the
same speed
regardless of
altitude.
H
L
return
Pressure 1013 1011 1009 1007 1005
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
View from top
L
Pressure 1017 1015 1013 1011 1009 1007 1005
back up
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
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.
The wind can’t accelerate any more over here
because it’s going parallel to the isobars
H
L
This is the Geostrophic Wind
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
30º
L
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
Coriolis Force
depends on
wind speed, its
effect
decreases
toward the
ground where
the wind speed
Friction
is slower.
H
L
Pressure 1013 1011 1009 1007 1005
Jet Stream
• In the Troposphere, wind will be stronger
(increase in speed) as the height increase.
• The speed/strength of the wind will be
increase to the maximum near tropopause.
• This maximum wind will concentrate in the
narrow band and we call as the JET STREAM.
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Cont.
• Jet stream: narrow band of strong wind at
level near the tropopause.
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Local and Small Scale Wind
(Minor Wind system)
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Cont.
• Before this we dealt with general circulation
and major wind system.
• But now, how the local terrain (e.g. mountain,
valley, sea) can affect local weather?
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Cont.
• Local winds: Winds that blow over short
distances
• Caused by unequal heating of the earth’s
surface within a small area
• Form only when no winds are blowing from
far away
Mountain and Valley Winds
• During daytime
– Air contact with the mountain slope heated from
solar radiation.
– This air will become warmer than air at the same
altitude which are not in contact to the ground
(farther from the slope)
– The colder air (denser air) will going downward. It
will force warmer air near the ground up to the
mountain slope.
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Cont.
– This movement of the air will be called as VALLEY
WIND (because air flowing up out of the valley).
• During night:
– Air contact with the mountain slope cooled by
terrestrial radiation and becomes heavier (dense)
than surrounding air.
– It will sinks along the slope
– We call the wind as MOUNTAIN WIND (flow like
water down the mountain slope)
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Cont.
• Mountain Wind stronger than Valley Wind
(especially in winter)
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Katabatic Wind
• Katabatic Wind: Any wind blowing down an incline
when inclination is influential in causing the wind.
• Therefore, mountain wind is the katabatic wind.
• Originated from cold (dense air) spills down slopping
terrain displacing warmer air (less dense air ahead of
it).
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Land and Sea Breezes
• Land surface warm and cool more rapidly than
water surface.
• During daytime:
– Land will be warmer than sea
– Wind will blow from cool (high pressure) to warm
land(low pressure)
– Therefore, it will blow from sea (cooler) to land
(warmer).
– We call this wind as SEA BREEZE.
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Cont.
• During night
– Wind reverses
– Wind will blow from cool land(high pressure) to
warmer water (lower pressure)
– We call this wind as LAND BREEZE.
• Land and sea breezes develop only when the overall
pressure gradient (change of pressure) weak.
– Because wind with a stronger pressure gradient mix the air
so rapidly. Thus, local temperature and pressure gradient
don’t develop along the shore line.
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Cont.
Land and Sea Breeze
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Wind Shear
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Cont.
• Idea: Try rubbing 2 objects against each other. It will
create the friction.
– If 2 objects are SOLID, they will have no exchange
of mass.
– If 2 objects are FLUID, they will creates eddy(whirl)
along common shallow mixing zone.
• This shallow layer induce (produce) eddies and
mixing is a SHEAR ZONE.
*We will discuss more on wind shear and relate with the
turbulence in Turbulence chapter.
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Cont.
Wind Shear
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Cont.
• Wind shear is defined as a change in wind
speed and/or direction within a short
distance. it can be in either a horizontal or
vertical direction, or both. this change causes
a shearing or tearing effect, which causes
great concern for pilots and airline industries.
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Wind, Pressure System, and
Weather.
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Cont.
• Rising air
– Low pressure
• Bad weather
• Conducive to cloudiness and precipitate
– High Pressure
• Good weather
• Because it breaking up the cloudiness and make it
scatter.
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Cont.
• Stagnant air
– When downward motion of air prevent the
convection and any upward motion.
– Form a stratus (a large dark cloud), fog, haze and
smoke.
• Warm air
– Pressure decrease more slowly.
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Cont.
• Remember:
• Low Pressure- Bad Weather
• High Pressure- Good Weather
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Conclusion
• Wind is the moving air from higher pressure to lower
pressure.
• Isobar provides general information of wind speed
and direction.
• Wind speed and direction influenced by the pressure
gradient force, coriolis force and friction.
• Two major wind systems are gradient wind and
geostrophic wind.
• Jet stream: narrow band of strong wind at level near
the tropopause.
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Cont.
• The local terrain might affect the local wind.
• Wind shear is defined as a change in wind
speed and/or direction within a short
distance.
• Low pressure of air will create bad weather
while good pressure of air will create good
weather.
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Key Points
•
•
•
•
•
•
•
•
What is the Wind?
Concept of Covection
How the air move?
Use of Isobar
The force acting on the wind: PGF, GF, Friction
Major Wind: Geostrophic and Gradient Wind
Jet Stream
Local and Minor Surface Wind: mountain and Valley; Katabati;
Land and Sea.
• Wind Shear
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End of Presentation #4
5 Minutes for Q/A session
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Quiz 2
1. What is the air? What is the atmosphere? Is it
same?
2. Why tropopause is important in meteorology?
3. What is the solar and terrestrial radiation?
4. Differentiate heat and temperature.
5. How density of air vary with altitude?
6. Define humidity.
Duration: 20 minutes
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