Transcript Document

Chapter 6
Atmospheric Forces and
Winds
Figure CO: Chapter 6, Atmospheric Forces and Wind
Courtesy of RMS, Inc
Figure UN01: Winds over France on Feb. 27-28, 2010
Data from Météo-France
Figure UN02: Flooding in La Faute, France
© Regis Duvignau/Reuters/Landov
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
Figure 01: Wind directions in angles, compass
headings.
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
Figure 02: Force diagram.
Figure 03: Graphical addition of force vectors.
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
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
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
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
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
Figure B01A: Fan blowing on paper
Figure B01B: Air over plane wing, with lift and drag
Figure 04: Pressure gradient force in highs and lows.
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
Figure 05: Surface weather map
From Plymouth State University Weather Center,
[http://vortex.plymouth.edu/make.html.].
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
Figure 06: 500-mb isobaric chart
From Plymouth State University Weather Center,
[http://vortex.plymouth.edu/make.html.].
Figure 07A: Isolines of constant height are proportional to the PGF
Figure 07B: Isolines of constant height are proportional to the PGF
Figure 07C: Isolines of constant height are proportional to the PGF
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
Figure 08: Centrifugal force schematic
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
Figure 09A: Curving path of ocean buoy
Adapted from Joseph et al., Current Science, 92 (2007).
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.).
Figure 11A: Coriolis force at different latitudes.
Figure 11B: variation of Coriolis force with latitude and
wind speed
Figure B02B: Carl-Gustaf Rossby
Courtesy of University of Chicago News Office
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
Figure 12: Frictional force diagram
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
Figure T01: Some Atmospheric Force-Balances
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
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 Jeppesen
Sanderson, Inc.
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
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,
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)
Figure 14: Geostrophic balance
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
Figure 15: Geostrophic wind in highs and lows
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
Figure 16: As in Figure 6-15, except now we also
include the centrifugal force, leading to gradient
balance.
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
Figure 17: Wavy path of parcel adjusting to balance
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
Figure 18: Guldberg-Mohn balance
Figure 19: A numerical simulation of how varying
amounts of friction affect the adjustment to Guldberg–
Modified from Knox, J., and Borenstein, S., J. Geoscience
Ed.,
46
Mohn
balance.
[1998]: 190–192.
Figure B03A: Chart of wind speeds and max wave heights
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.
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
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
Figure 22: Schematic of pressure levels when air is
heated
Figure 23: Cross-section of winds at various pressure levels
Figure 24A:
How surface
wind patterns
induce
vertical wind
motions
Figure 24B:
How surface
wind patterns
induce vertical
wind motions
Figure 24A: How surface wind patterns induce vertical wind motions
Figure 24B: How surface wind patterns induce vertical wind motions
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
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
Figure 25: Sea breeze
Figure 26A: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison
Figure 26B: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison
Figure 26C: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison
Figure 26D: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison
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, sinking air column is over land, where
radiational cooling is more rapid than over the
water
• The warmer, rising air column is over the water
• The land breeze develops at night
– Air flows towards the land aloft
– Air flows towards the water at the surface
Figure 27: Schematic of land breeze
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
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