lecture UMD Atmospheric and Oceanic Science

					     Outline for Lecture 13


Factors Affecting Wind
 Winds Aloft and Geostrophic Flow
  Curved Flow and the Gradient Wind
   Surface winds
    How Winds Generate Vertical Air Motion
                                         3/13/03
Formation of A Sea Breeze
                  (a) No pressure gradient,
                  no wind.

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

                  (c) The transfer of air aloft
  Returning air   (from the land to the sea)
                  creates a surface high over
                  the sea which results in a
                  flow of air from sea to land
  Sea Breeze
                  (a.k.a. a sea breeze).
                  Pressure Gradient Force
                steep pressure gradient   shallow pressure gradient
                (fast winds)              (slow winds)
pressure




           p


                        n                           n
                     distance
                 The Coriolis Force
Named after the French Scientist Gaspard Gustave Coriolis

                                         free moving objects
                                         are deflected to the right
                                         of their path in the Northern
                                         Hemisphere (to the left of
                                         their path in the Southern
                                         Hemisphere) because of the
                                         Earth’s rotation.

                                         It depends on an object’s
                                         speed—higher speed means
                                         stronger Coriolis Force.
                  The Coriolis Force
Coriolis deflection of winds blowing eastward at different latitudes

                                                 Coriolis force also
                 Strong
                                          increases with increasing
                                                         Latitude…
                 Middling                        strong over poles,
                                             no effect over equator.
                 Weak

                  None
                     Friction
• Friction acts at the
  surface.
  winds at the surface
  aren’t as strong as
  those at higher
  altitudes



                             Altitude
• Air is a little viscous,
  so the layer next to the
  surface is also
  affected, but not as
  much.
        Winds Aloft and Geostrophic
                             Flow Force: Geostrophic Flow.
 Balance pressure gradient force Coriolis




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


II. F = ma
          (that’s pretty much it.)
        Put down the pencils
Let’s break down the forces
              Just a Pressure Gradient
Start with just a                View from top
   pressure
   gradient
Then wind blows
   straight from
   High P to Low
                    H                                L
   P
And accelerates
   as it goes

           Pressure 1017 1015 1013 1011 1009 1007 1005   return
            Just a Pressure Gradient
Wind blows
straight from
High to Low,
eventually      H                          L
evening out
the pressure
and stopping
the wind.
          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               View from top
blows straight
from High P to
Low P, but it
doesn’t get  H
moving as fast
                                                  L
as soon,
especially near
the ground
        Pressure 1017 1015 1013 1011 1009 1007 1005   back up
      Friction and Pressure Gradient
The pressure
difference
also evens out
eventually,      H                         L
though it
might take a
bit longer.


          Pressure 1014   1012   1010   1008
  Friction and Pressure Gradient
Friction slows
the wind at the
ground—its
effects decrease
as you go up in
                       H                               L
the atmosphere.



                   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
                       H                               L
the atmosphere.



                   Pressure 1013 1011 1009 1007 1005   back up
      Pressure Gradient and Coriolis
Forget friction.
The wind starts
out straight, but
as soon as it

speed, the
                 H
starts building up                                            L
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
                     This is the Geostrophic Flow
      Pressure Gradient and Coriolis
The Coriolis
force limits the
wind speed by
redirecting it
AND it
prevents wind
               H                   L
from blowing
straight from
H to L
     Pressure Gradient and Coriolis
Since the wind
never really
reaches the
low, the
pressure
difference is
                 H                L
maintained,
and the low
never fills!
         Reality: Pressure Gradient,
         Coriolis Force, and Friction
Coriolis Force
turns the wind
some, friction
                                   30º
slows the wind
some, and the
result is roughly
                    H                    L
a 30º angle
between isobars
and wind.
         Reality: Pressure Gradient,
         Coriolis Force, and Friction
The wind doesn’t
blow straight
from High to
Low, but it does                   30º

out the pressure
                H
eventually get in
there and even                           L
difference, so H
and L don’t last
forever without a
source of energy
    Reality: Pressure Gradient,
    Coriolis Force, and Friction
Friction slows
the wind at the
ground—its
effects decrease
as you go up in
                       H                               L
the atmosphere.



                   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
                     H                              L
ground where
the wind speed Friction
is slower.
                Pressure 1013 1011 1009 1007 1005
Pencils are now allowed
     Winds Aloft and Geostrophic Flow
  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                 Pressure Gradient

Wind speed   Coriolis Force               Wind flows from
                                          high to low pressure.
Latitude     Coriolis Force        “Isobaric packing”     P.G.F.
    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 to
your left and higher pressure will be found to the right.

Best when there are no frictional forces
or topography involved .
  Curved Flow and the Gradient Wind

Cyclonic Flow                                     Anticyclonic Flow




 Actual flow around pressure systems are never this regular (because of small changes in
 the pressure field).
Surface winds—high pressure
        (anticyclone)COLD
                   DRY




           H
  WARM
  MOIST
  Surface winds—low pressure
           (cyclone)
COLD
DRY




             L
                         WARM
                         MOIST
   Upper Level Weather Chart




500mb chart
                Variations in height are analogous
                to variations in pressure. High
                height fields correspond to high
                pressure fields.
  Troughs and Ridges

RIDGE
                       RIDGE




          TROUGH
         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
How Winds Generate Vertical Air Motion




Around a surface low pressure center, a net inward transport of air
causes a shrinking of the area occupied by the mass. This is known
as horizontal convergence.
Airflow Associated with Cyclones and
           Anticyclones




  “Upper level support” is important in cyclone development
          Wind speeds and isobars
                  STRONG WINDS
Slack winds                                            Slack winds




          The tighter they’re packed, the stronger the wind
Factors that promote vertical airflow
 • Friction:
 air flow from ocean to land (upward motion)
 air flow from land to ocean (downward motion)

 •Mountain ranges
              Wind Measurement
Wind roses provide a method of representing prevailing winds by
indicating the percentage of time the wind blows from various
directions
        Go Terps!
ACC Tournament this Weekend

				
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posted:3/25/2011
language:English
pages:40