Forces and Winds

1
Forces and Winds

• Lynn Gribble
• 6/1/09

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Stability Review Two ways to think about it.

• Absolutely unstable
• Gwet (6)ltGdry (10)ltGenv
• ie whether moist or dry, the parcel is ALWAYS
  warmer than the environment, therefore it WILL
  rise.
• Absolutely stable
• GenvltGwet (6)ltGdry(10)
• ie whether moist or dry, the parcel is ALWAYS
  cooler than the environment, therefore it WILL
  NOT rise.
• Conditionally unstable
• Gwet(6)ltGenvltGdry(10)

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Instrumentation

• Resistance thermometers - temperature sensors
  that exploit the predictable change in electrical
  resistance of some materials with changing
  temperature, almost invariably made of platinum
• Cooled mirror dewpoint hygrometers - use a
  chilled mirror and optoelectronic mechanism to
  detect condensation on the mirror surface.
• Aneroid barometer - uses a small, flexible metal
  box called an aneroid cell. The evacuated capsule
  is prevented from collapsing by a strong spring.
  Small changes in external air pressure cause the
  cell to expand or contract.

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Eco Tip 4 Local, Local, Local

• When Possible Buy Local.
• According to the Sierra Club, the average
  American meal travels more than 1500 miles from
  farm to plate.
• Pay attention to where food comes from and try to
  choose ones that come from close to home.
• By buying local, we all gain. Consumers get the
  freshest food possible, less CO2 and other
  pollution is emitted, and local sustainable
  farming is supported.

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More Physics Review (Forces)

• Newtons Laws
• 1 Inertia, An object in motion remains in
  motion, and an object at rest remains at rest
• 2 F ma (force is equal to mass times
  acceleration)
• 3 Action/Reaction
• Unit Newtons (kgm/s2 )

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Physics Review

• Vectors are quantities that have both magnitude
  and direction
• Vectors addition/subtraction
• An object undergoing constant motion, ie not
  accelerating, must have forces balanced. (SF0)
  the sum of the forces is equal to zero
• An accelerating object has unbalanced forces.

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Terminology

• Isopleths Lines of constant something
• Isotherms
• temperature
• Isobars
• pressure
• Isotachs
• Wind speed
• A group of Isopleths defines a gradient.

Figure 6.5
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Atmospheric Forces

• Gravitational Force directed downward and
  perpendicular to the ground (g 9.8m/s2 )
• Pressure Gradient Force results from pressure
  differences over distances
• PGF (-1/?)(?P/d)
• In words, PGF is due to the change in pressure
  over a certain distance.

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Atmospheric Forces (PGF)
Figure 6-4
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Atmospheric Forces

• Another way to view PGF is on Pressure Surfaces
• Waves

Figure 6-7
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Atmospheric Forces

• Coriolis an apparent force due to the rotation
  of earth
• Merry go-round analogy
• f 2earths rotation rate (O)sin(latitude)
• CF fv
• In words, coriolis force is a function of
  latitude and velocity (greater lat greater CF)
• Turns winds to right in NH
• Connection to tropical storms

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Atmospheric Forces

• Frictional Force caused by the roughness of
  Earths surface
• FF -kv
• Centrifugal Force due to the rotation of air
  around a high or low.
• Ball on a string analogy
• CENTF v2 /R

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Atmospheric Force Balances
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Atmospheric Force Balances
6-18
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More Balances

• Buys Ballots Law Low to the left
• Upper Level Winds can be approximated by
  Geostrophic Balance
• Lower Level Winds can be approximated by
  Guldberg-Mohn Balance
• Thermal Wind heating changed pressure gradient

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Convergence and Divergence
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Land and Sea Breezes

• Resultant of heat contrasts between land and sea
  due to heat capacity of water
• See diagram on board
• D\Media\Active_Figures\54_Direct_Thermal_Circ\A_5
  4.html

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Review Scales of Motion

• Coriolis Only works on large distance scales and
  time periods greater than or equal to a day.
• (relative orders of magnitude)
• Microscale lt1 km (clouds)
• Mesoscale 10-100 km (T-storms)
• Synoptic scale 1000 km (pressure systems)
• Planetary scale gt1000 km this is next!

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Hadley Cells (water world)

• ITCZ intertropical convergence zone
• Air flows north and south along tropopause and is
  turned by coriolis force (Subtropical Jet)
• Air that does not enter jet sinks(doldrums and
  desserts) and returns to equator. (trade winds)
• 4. Cold air sinks over poles. Equatorward air is
  turned by coriolis (polar easterlies)
• 5. Air rises at about 60N and joins polar front
  jet.

Figure 7-6
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Jet Streams

• Little friction at upper levels
• Conservation of angular momentum if distance
  from axis of rotation decreases, the velocity
  must increase to conserve angular momentum
  (figure skater)
• Ang Mom mvR (massvelocilyradius)
• Therefore as distance from the poles decease,
  velocity increases
• This creates subtropical jet _at_ 30N aloft
• Horse latitudes/doldrums _at_ sfc

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Jet Streams
7-7
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Jet Streams

• Polar Front
• Coriolis Force
• Seasonally Variant depending on heating contrasts
• Strongest in the spring and fall when difference
  between Temp _at_ equator and temp _at_30N is greatest

Figure 7.13
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Jet Streams
Figure 7.14
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ITCZ

• Another feature of Hadley circulations is the
  Inter-tropical Convergence Zone (ITCZ)
• Convergence of warm, moist, tropical air leads to
  a nearly constant band of clouds in this location.

Figure 7-10
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Poleward Transport of Energy
7-19
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Odds and Ends

• Zonal Flow when the jet stream flows directly
  east to west
• Meridional Flow when the jet stream curves and
  meanders north to south
• Omega Block zonal-gt meridional-gt zonal
• Rex Block split flow around an atmospheric
  obstacle
• Blocks cause stagnant weather patterns