Chapter 6 Atmospheric Forces and Winds. Figure CO: Chapter 6, Atmospheric Forces and Wind Courtesy...

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Chapter 6 Atmospheric Forces and Winds

Transcript of Chapter 6 Atmospheric Forces and Winds. Figure CO: Chapter 6, Atmospheric Forces and Wind Courtesy...

Page 1: Chapter 6 Atmospheric Forces and Winds. Figure CO: Chapter 6, Atmospheric Forces and Wind Courtesy of RMS, Inc.

Chapter 6Chapter 6

Atmospheric Forces and Winds

Atmospheric Forces and Winds

Page 2: Chapter 6 Atmospheric Forces and Winds. Figure CO: Chapter 6, Atmospheric Forces and Wind Courtesy of RMS, Inc.

Figure CO: Chapter 6, Atmospheric Forces and Wind

Courtesy of RMS, Inc

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Figure UN01: Winds over France on Feb. 27-28, 2010

Data from Météo-France

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Figure UN02: Flooding in La Faute, France

© Regis Duvignau/Reuters/Landov

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Basics about Wind

• Wind direction is the direction from which the wind is blowing– A north wind blows from the north to the south– It is reported according to compass directions– Prevailing wind direction is the most frequent

direction• Wind speed

– Reported on U.S. weather maps in knots – 1 knot = 1.15 miles/hour = 0.5 meter/second– If wind > 15 knots and highly variable, the

weather report will include the wind gust, the maximum speed

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Figure 01: Wind directions in angles, compass headings.

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Forces

• Have magnitude (or strength) and direction

• Multiple forces can act on the same point– The resultant force is the net force– If two forces act in opposite directions, the net

force will have the direction of the stronger force and a strength equal to the difference of the two forces

– If two forces act at an angle to each other, the resultant force is along a diagonal and away from where the two forces are applied

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Figure 02: Force diagram.

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Figure 03: Graphical addition of force vectors.

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Forces and Movement

• A force applied to an object often results in movement

• An object’s velocity is the magnitude and direction of its motion

• The speed of the object, the distance traveled in a given amount of time, is the magnitude of the motion

• Acceleration is a change in an object’s velocity—magnitude, direction or both

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Forces cause the wind to blow

• Forces that act on air create horizontal wind

• A force acting through a distance does work

• Work is equivalent to energy• Ultimately, the sun provides the energy

that allows the winds to blow• Radiation causes temperature imbalances,

which lead to pressure imbalances and a force

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Newton’s second law of motion• Says that

– the sum of the forces = mass x acceleration– Or that acceleration = sum of forces/mass

• Helps scientist forecast changes in the wind direction and speed, or its acceleration• Requires that we specify which forces are acting and how strong they are• Is also called the Law of momentum• Momentum of an object is its mass x its velocity

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Gravity, the strongest force• Does not act horizontally, so does not influence the horizontal winds.• Does influence vertical air motions• Is directed downward toward the center of Earth• Is a very strong force• Keeps our atmosphere from escaping• Equals the mass x 9.8 m/s2

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The Pressure Gradient Force (PGF)

• The force that results from pressure differences over distances in a fluid• A pressure gradient is a change in pressure over a distance• PGF always directed from high to low pressure• Is stronger when isobars more closely spaced• Is stronger when the difference in pressure is greater over a particular distance• Determines the way air moves only if no other forces are acting

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Figure B01A: Fan blowing on paper

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Figure B01B: Air over plane wing, with lift and drag

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Figure 04: Pressure gradient force in highs and lows.

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The horizontal pressure gradient force

• Is always directed from high to low pressure

• Is stronger where the density is less—higher in the troposphere

• When stronger, causes stronger winds• Is always important in horizontal winds• Is not generally in the same direction the

wind blows, because other forces can act

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Figure 05: Surface weather map

From Plymouth State University Weather Center, [http://vortex.plymouth.edu/make.html.].

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Isobaric Charts

• Plot the altitude of a given pressure surface– Units of altitude are called geopotential meters

• Also called a constant-pressure chart– Common levels are 850, 700, 500, 250, and 200 mb

• Are useful for portraying horizontal pressure gradients above the ground

• The spacing between the lines of constant height is proportional to the PGF

• The winds in general blow parallel to the height contours, at right angles to the PGF

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Figure 06: 500-mb isobaric chart

From Plymouth State University Weather Center, [http://vortex.plymouth.edu/make.html.].

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Figure 07A: Isolines of constant height are proportional to the PGF

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Figure 07B: Isolines of constant height are proportional to the PGF

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Figure 07C: Isolines of constant height are proportional to the PGF

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Centrifugal Force/Centripetal Acceleration

• Centripetal acceleration is a change in direction even if the speed does not change

• From the point of view of an observer experiencing the centripetal acceleration, there is an apparent force called the centrifugal force

• The faster the speed and the tighter the curve, the larger is the centripetal acceleration

• The sign of the centripetal acceleration is positive for cyclones, negative for anticyclones, and always directed inward to the center

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Figure 08: Centrifugal force schematic

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The Coriolis Force• Deflects the wind to the right in the NH• Deflects the wind to the left in the SH• Is strongest at the poles• Is zero at the equator• Is stronger for stronger winds• Is weaker for weaker winds• Is zero for calm. It cannot start a wind

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Figure 09A: Curving path of ocean buoy

Adapted from Joseph et al., Current Science, 92 (2007).

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Figure 6.10: The centrifugal (CENTF) and Coriolis forces acting on an air parcel moving with respect to the rotating Earth

Modified from A. Persson, Bull. Amer. Meteor. Soc., 79 [1998]: 1378.).

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Figure 11A: Coriolis force at different latitudes.

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Figure 11B: variation of Coriolis force with latitude and wind speed

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Figure B02B: Carl-Gustaf Rossby

Courtesy of University of Chicago News Office

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The Friction Force• Acts in the direction opposite to the direction the wind is blowing• Acts to slow down the wind• Is most important at Earth’s surface• Gets stronger when the winds are stronger• Is not important above the boundary layer (the lowest 1 km in the atmosphere)• The rougher the surface and the stronger the wind the greater is the friction force

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Figure 12: Frictional force diagram

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Why force-balances are important

• Force-balances simplify Newton’s second law of motion by limiting the number of forces• Force-balances describe winds that come close to describing the observed winds• Even though the forces are balanced, the wind need not be calm• The PGF is important in every force balance• Only the PGF can set calm air into motion

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Figure T01: Some Atmospheric Force-Balances

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Hydrostatic Balance• Gravity (downward) balances the Vertical Pressure Gradient Force (upward)• Does not apply inside cumulus clouds, because buoyancy is important there• Does apply generally in the atmosphere• Limits vertical motions to be much weaker than horizontal winds

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Figure 6.13: Air parcel in hydrostatic balance

Reproduced from Lester, P., Aviation Weather, Second edition. With permission of Jeppsen Sanderson, Inc. Not for Navigation Use. Copyright © 2010 JeppesenSanderson, Inc.

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More on Hydrostatic Balance

• The pressure gradient force is stronger when the air is less dense• The density of air is less when the air Temperature is higher• Pressure decreases upward less rapidly when the air has a higher temperature• Hydrostatic balance helps explain the sea breeze and other thermal circulations

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Pressure levels on weather maps

• The atmosphere is very close to hydrostatic balance• This means that the height of a particular pressure level is roughly equivalent to the pressure at a related height level• An altimeter is a barometer with a height scale• Upper-level weather maps are labeled in m• Winds on a weather map are strong when the height contours are close together, weak where they are farther apart

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Geostrophic Balance

• Is a balance between the horizontal pressure gradient force and the Coriolis force• Ignores the friction force• Has isobars that are straight lines• Does not mean that the wind is calm• Has a wind called the geostrophic wind• Winds on weather maps above the surface are close to the geostrophic wind• Blows with lower pressure (height) on the left (NH)

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Figure 14: Geostrophic balance

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The Geostrophic Wind• Is a wind in geostrophic balance• Is parallel to the isobars• In the NH has low pressure on the left• In the SH has low pressure on the right• In the NH the wind blows clockwise around high pressure centers and counterclockwise around low pressure centers• In the SH CW flow around lows and CCW flow around highs

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Figure 15: Geostrophic wind in highs and lows

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Gradient Balance and the Gradient Wind

• Gradient balance is between the PGF, the Coriolis force and the centrifugal force

• Gradient balance allows curving wind patterns called the gradient wind

• The centrifugal force is always outward– Around a low the centrifugal force opposes

the PGF and the resulting flow is subgeostrophic

– Around a high the centrifugal force adds to the PGF and the resulting flow is supergeostrophic

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Figure 16: As in Figure 6-15, except now we also include the centrifugal force, leading to gradient balance.

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Adjustment to Geostrophic Balance

• Initially there is an imbalance of forces• Air parcels move toward lower pressure

(PGF)• As soon as there is a wind, the Coriolis

force acts• Parcels oscillate towards a balance

between the PGF and the Coriolis force• Adjustment takes minutes to hours• Adjustment is temporary and incomplete

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Figure 17: Wavy path of parcel adjusting to balance

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Guldberg-Mohn Balance

• Is a balance between the PGF, the Coriolis force, and friction

• Friction slows the wind and the Coriolis force weakens

• The winds blow across the isobars at an angle toward low pressure (away from high pressure)– Between 15° and 30° over water– Between 25° and 50° over land

• Friction damps oscillations during adjustment to balance

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Figure 18: Guldberg-Mohn balance

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Figure 19: A numerical simulation of how varying amounts of friction affect the adjustment to Guldberg–

Mohn balance.Modified from Knox, J., and Borenstein, S., J. Geoscience Ed., 46 [1998]: 190–192.

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Figure B03A: Chart of wind speeds and max wave heights

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Figure 21: The isobars at the surface drawn over a satellite image of a cyclone

Image created by Prof. Joshua Durkee, Western Kentucky University, using GREarth software.

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The Thermal Wind

• The thermal wind relates temperature and winds to each other

• The winds are more westerly as you go up wherever it’s colder toward the poles

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Putting horizontal and vertical winds together

• At the surface, the wind blows across the isobars into low-pressure areas– At the center of the low-pressure area the air must rise– Low-pressure areas are usually cloudy and wet

• At the surface, the wind blows across the isobars out of high-pressure areas– At the center of the high-pressure area the air must sink– High-pressure areas are usually clear and dry

• These patterns are the result of Guldberg-Mohn balance

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Figure 22: Schematic of pressure levels when air is heated

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Figure 23: Cross-section of winds at various pressure levels

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Figure 24A: How surface wind patterns

induce vertical wind

motions

Figure 24B: How surface wind patterns induce vertical wind motions

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Figure 24A: How surface wind patterns induce vertical wind motions

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Figure 24B: How surface wind patterns induce vertical wind motions

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The thermal circulation• The sea breeze is a thermal circulation• A thermal circulation has both horizontal and vertical air motions• The horizontal pressure gradient force is most important in a thermal circulation• Upward air motions occur in the warmer air column of the circulation; downward air motions occur in the cooler air column

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The sea breeze

• Is a daytime circulation• Depends on differential heating at the surface between land and water• Has the warmer, rising air column over the land, which absorbs more incoming solar radiation• Has the cooler, sinking air column over the water, which absorbs less radiation• Air flows from warmer to cooler column aloft• Air flows from cooler to warmer column at the surface

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Figure 25: Sea breeze schematic

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Figure 26A: Satellite image of sea breeze

Courtesy of SSEC, University of Wisconsin-Madison

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Figure 26B: Satellite image of sea breeze

Courtesy of SSEC, University of Wisconsin-Madison

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Figure 26C: Satellite image of sea breeze

Courtesy of SSEC, University of Wisconsin-Madison

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Figure 26D: Satellite image of sea breeze

Courtesy of SSEC, University of Wisconsin-Madison

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The sea breeze and the land breeze

• As solar heating diminishes in the late afternoon, the sea breeze weakens• At night, differential cooling occurs• The cooler, s inking air column is over land, where radiational cool ing is more rapid than over the water• The warmer, rising air column is over the water• The land breeze develops at night

– Air f lows towards the land aloft– Air f lows towards the water at the surface

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Figure 27: Schematic of land breeze

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Scales of motion in the atmosphere

• Describe the size and lifetime of wind patterns in the atmosphere• Determine which forces are most important to forming the wind patterns• Are largest when the lifetimes are longest• Are smaller when the lifetime is shorter• Have a variety of names and definitions

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More on scales of motion• Microscale: <1 km in diameter

– PGF, centrifugal, friction forces are important• Mesoscale: Between 1 and 1000 km in size

– PGF, centrifugal, friction, and Coriolis Force for largest sizes• Synoptic scale: At least 1000 km in size

– Balance between PGF and Coriolis Force dominates• Planetary scale: Roughly 10,000 km in size

– Balance between PGF and Coriolis Force dominates