Transcript ATM OCN 100 Summer 1999

Weather 101 and beyond
Edward J. Hopkins
Dept. of Atmospheric &
Oceanic Sciences
Univ. of Wisconsin-Madison
Midwest Hot Air Balloon Safety Seminar
“Hot Aireventure”
Oshkosh 3 March 2001
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Boundary Layer
 Where
we live
 Extends from surface to ?
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Concerns of Balloonists
 The
Winds
 The Surface
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WIND
 Why
Winds?
– Local Thermal Effects
– Large Scale Dynamic Effects
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High Pressure Systems
 Circulation
 Consequences
 Types
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The Surface
 The
“Obvious”
– Obstacles to take-off and landing
(e.g., trees, power lines, animals)
 The
Surface and the Winds
– Affects the Boundary Layer wind
flow
– Can produce local wind regimes
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Relative Surface Roughness
Classification
Smooth
Landcaspe
Snow covered fields
Roughly Open
Very Rough
Closed
Chaotic
Lansn al
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Quiz
 Which
way do winds blow
around:
High
Low
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Features in a Surface Low
(Convergence & Ascent)
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Features in a Surface High
(Sinking & Divergence)
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January Temperatures Madison, WI (1981-90)
TEMPERATURE (deg F)
26
24
22
20
18
16
14
12
10
0
3
6
9
12
15
18
21
24
HOURS CST
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January Wind Speeds Madison, WI (1981-90)
AVERAGE WIND SPEED [mph]
14
12
10
8
6
4
2
0
3
6
9
12
15
18
21
24
HOURS CST
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July Temperatures - Madison, WI
(1981-90)
TEMPERATURE (deg F)
85
80
75
70
65
60
55
0
3
6
9
12
15
18
21
24
HOURS CST
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July Wind Speeds Madison, WI (1981-90)
AVERAGE WIND SPEED [mph]
12
10
8
6
4
2
0
3
6
9
12
15
18
21
24
HOURS CST
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Daily Heating
2.5
Normalized Heat Flux
2
1.5
1
0.5
0
6:00
12:00
18:00
23:00
5:00
-0.5
-1
-1.5
-2
Time
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ALTITUDE [km]
NORMALIZED DENSITY PROFILE
US STANDARD ATMOSPHERE 1976
80
60
40
20
0
0.0%
20.0%
40.0%
60.0%
80.0%
100.0%
PERCENT OF SEA LEVEL DENSITY
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U.S. STANDARD ATMOSPHERE
See Fig. 1.9 Moran & Morgan (1997)
Altitude [km]
140.
120.
100.
80.
Thermosphere
Mesopause
60.
Stratopause
Stratosphere
Tropopause
Troposphere
40.
20.
0.
-100.
Mesosphere
-50.
0.
50.
100.
Temperature [deg C]
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Weather Satellites and the Space
Science & Engineering Center
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BASIC CONCEPTS Air Pressure
(con’t.)
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Explaining Differences in
Air Pressure
 Low
Pressure
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 High
Pressure
23
Display of Pressure Differences on
a Weather Map - Isobars
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AIR PRESSURE CLIMATOLOGY
(con’t.)
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AIR PRESSURE CLIMATOLOGY
(con’t.)
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AIR PRESSURE CLIMATOLOGY
(con’t.)
ALTITUDE [km]
VERTICAL PRESSURE PROFILE
US STANDARD ATMOSPHERE, 1976
50
40
30
20
10
0
50% of surface
0
200
400
600
800
1000
AIR PRESSURE [millibars]
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D. VARIATION OF OBSERVED AIR
TEMPERATURE WITH HEIGHT
 Temperature
–
–
lapse rates
Rate of cooling with height
Units: degrees per meter or feet or kilometers
 Layer
nomenclature
– lapse
– inversion
– isothermal
where ...
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LAPSE CONDITIONS
Altitude [km]
Temperature decreases with height
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.
5.
10.
15.
20.
Temperature [deg C]
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INVERSION CONDITIONS
Altitude [km]
Temperature increases with height
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.
5.
10.
15.
Temperature [deg C]
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ISOTHERMAL CONDITIONS
Altitude [km]
Temperature remains constant with height
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.
5.
10.
15.
Temperature [deg C]
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ENERGY TRANSPORT: CONVECTION (con’t.)
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UNSTABLE CONDITIONS
Compare Environment with DALR
Warmer parcel continues upward
2500
2000
1500
1000
500
0
0
5
10
15
20
25
30
-500
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BEAUFORT WIND FORCE SCALE
[Modern version, Source: Federal Meteorological Handbook I]
Scale Description
0
1
2
3
4
5
Calm
Land & Sea Observations
Smoke rises vertically.
Sea surface is like mirror.
Light air
Smoke, but not wind vane, shows direction of
wind.
Slight ripples on sea.
Light breeze Wind felt on face, leaves rustle, wind vanes
move.
Small, short wavelets.
Gentle breeze Leaves and small twigs moving constantly,
small flags extended.
Large wavelets, scattered whitecaps.
Moderate
Dust and loose paper raised, small branches
breeze
moved.
Small waves, frequent whitecaps.
Fresh breeze Small leafy trees swayed.
Moderate waves.
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Knots
<1
1-3
4-6
7-10
11-16
17-21
34
BEAUFORT WIND FORCE SCALE (con’t.)
Scale Description
Land & Sea Observations
6
Strong
Large branches in motion, whistling heard in
utility wires. Large waves, some spray.
breeze
Knots
22-27
7
Near gale
Whole trees in motion.
White foam from breaking waves.
28-33
8
Gale
Twigs break off trees.
Moderately high waves of great length.
34-40
9
Strong gale Slight structural damage occurs. Crests of
waves begin to roll over. Spray may impede
visibility.
Storm
Trees uprooted, considerable structural
damage.
Sea white with foam, heavy tumbling of sea.
Violent
Very rare; widespread damage.
Unusually high waves.
storm
Hurricane
Very rare; much foam and spray greatly reduce
visibility.
10
11
12
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41-47
48-55
56-63
>63
35
ASOS Wind Instruments
Wind Vane (left) & Cup Anemometer (right)
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Aerovane
Measures wind speed & direction
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B. EXPLANATIONS of
ATMOSPHERIC MOTION
 Practical
Problems
 Historical Concepts
 Forces of Motion & Newton's Laws
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C. DESCRIBING ATMOSPHERIC
MOTION
 Reasons
for Atmospheric Motions:
– Buoyancy Effects
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or
Dynamic Effects
39
C. DESCRIBING ATMOSPHERIC
MOTION
 Complications
involved with
Atmospheric Motion:
– Spherical planet;
– Rotating planet &
non-inertial frame of reference.
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DESCRIBING ATMOSPHERIC MOTION
(con’t.)
 Three-Dimensional
Equation of Motion
for the Atmosphere
– A vector equation;
– Entails specification of all forces per unit
mass (i.e., equivalent to acceleration);
– All forces do not act alone;
– Vector sum of individual forces
equals net force.
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Numerical Weather Prediction
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Numerical Weather Prediction
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Numerical Weather Prediction
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An example of an equation of motion
NASA
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FORCES ASSOCIATED WITH
ATMOSPHERIC MOTION
 Following
forces influence motion
of air parcels:
– Pressure Gradient Force
– Gravitational Force or Gravity
– Coriolis Effect or "Force"
– Frictional Force or Friction
– Centripetal Force
or more specifically --
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PRESSURE GRADIENT FORCE
 Generated
by differences in pressure
within a fluid element;
 Responsible for initiation of
all air motion;
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PRESSURE GRADIENT FORCE (con’t.)
 A 3-dimensional
vector that has:
 Magnitude
of pressure gradient force
vector depends:
– directly upon difference in pressure over a given
distance (i.e., slope or grade equals “pressure
gradient”).
 Direction
of pressure gradient force
vector is:
– from High pressure to Low pressure,
– along steepest direction of pressure gradient.
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PRESSURE GRADIENT FORCE (con’t.)
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PRESSURE GRADIENT FORCE (con’t.)
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GRAVITATIONAL FORCE or GRAVITY
 Produced
by mutual physical attraction
between massive bodies;
 Gravity refers to acceleration;
 Acts continuously, regardless of motion;
 A vector quantity that has:
–Direction – toward center of earth.
–Magnitude ~ 9.8 m/s2 (32 ft/s2)
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GRAVITATIONAL FORCE or GRAVITY (con’t.)
 Magnitude
of gravity vector depends
upon:
– Mass of earth & object;
– Distance between two objects;
(inverse square relationship).
– [NOTE: Issac Newton quantified relationship]
– Usually gravity is assumed 32 ft/s2 = 9.8m/s2 .
 Direction
of gravity vector is
– toward vicinity of earth’s center
(i.e., essentially downward).
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CORIOLIS EFFECT or FORCE
 Produced
by earth’s rotation;
 A “fictitious force” used to explain apparent
deflection of moving object on a rotating
frame of reference;
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CORIOLIS EFFECT or FORCE (con’t.)
Speed is dependent upon latitude:
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CORIOLIS EFFECT or FORCE (con’t.)
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CORIOLIS EFFECT or FORCE (con’t.)
 Produced
by earth’s rotation;
 A “fictitious force” used to explain apparent
deflection of moving object on a rotating
frame of reference;
 Acts only after motion is initiated;
 Can only modify direction of motion;
 A 3-dimensional vector,
but consider only horizontal
component described by:
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CORIOLIS EFFECT or FORCE (con’t.)
 Magnitude
of horizontal Coriolis force
vector depends upon:
– Rotation rate of earth
(Direct relationship);
– Speed of object;
(Direct relationship)
– Latitude
(specifically, sine of latitude).
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CORIOLIS EFFECT or FORCE (con’t.)
 Direction
of horizontal component of
Coriolis force vector:
– Causes a deflection of moving object to right
of direction of motion in Northern Hemisphere;
but
– Deflects moving object to left of intended
motion in Southern Hemisphere.
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FRICTIONAL FORCE or FRICTION
 Produced by “viscosity”
(interactions of moving fluid elements with one
another or with a boundary surface) due to:
– random molecular motions;
– large random turbulent motions of fluid
associated with either:
thermal turbulence
mechanical turbulence
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FRICTIONAL FORCE (con’t.)
 Acts
only after motion is initiated;
 Acts to retard motion;
 Magnitude of friction force vector depends
upon:
– Speed of motion of fluid;
– Type of surface, e.g., “surface roughness”;
– Temperature structure of fluid.
 Direction of friction force vector is
– opposite motion vector.
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CENTRIPETAL FORCE
 Produces curved motion;
 Opposite the “centrifugal force”;
 Acts only after motion is initiated;
In reality, a net force
Used to describe imbalance of
other forces in curved motion;
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CENTRIPETAL FORCE VECTOR (con’t.)
 Centripetal
force vector is described by:
 Magnitude of centripetal force vector
depends upon:
– Speed of instantaneous motion
(a direct relationship);
– Radius of curvature
(an inverse relationship).
 Direction
of centripetal force vector is
– inward toward center of curvature.
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SUMMARIZING
 A 3-D
Equation of Motion for Atmosphere (in
word form):
Net force = Pressure gradient force
+ gravitation force
+ Coriolis force + friction.
 Notes:
– The above is a vector equation!
– Since a unit mass is used, force
is equivalent to an acceleration.
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Isobars - lines of equal barometric pressure
- use sea level corrected pressure
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Demonstrating
Buys -Ballot Rule
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Demonstrating
Buys -Ballot Rule
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BUYS-BALLOT RULE
 Empirical
relationship stated by Dutch
meteorologist Buys-Ballot in 1850’s;
 With your back to wind, Low pressure is to
your left in Northern Hemisphere;
 However, in Southern Hemisphere, Low is
to your right ;
 Mathematically proved by American
meteorologist Wm. Ferrel in 1850’s.
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ASSUMPTIONS
For convenience, assume that:
 Winds are nearly horizontal;
 Atmosphere is in nearly
“hydrostatic balance”
i.e., air parcels do not accelerate
upward or downward;
 Define
motion in terms of
horizontal & vertical components.
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B. HORIZONTAL EQUATION OF
ATMOSPHERIC MOTION
 The
3-D vector Equation of
Atmospheric Motion can be written in
terms of horizontal and vertical
components:
Net force =
Horizontal Pressure gradient force
+ Vertical Pressure gradient force
+ gravity + Coriolis force + friction.
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HYDROSTATIC BALANCE CONCEPT
 Earth’s
atmosphere remains and is
essentially in “hydrostatic balance”.
 This balance is between the vertically
oriented vector quantities:
– gravity, and
– acceleration due to vertical component of
pressure gradient force.
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HYDROSTATIC BALANCE CONCEPT
See Fig. 9.11 Moran & Morgan (1997)
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THE VERTICAL PRESSURE
GRADIENT FORCE
 Magnitude
of Vertical Pressure Gradient
force vector is:
– a function of both air density & vertical
component of pressure gradient.
 Direction
of Vertical Pressure Gradient
force is:
– always pointed upward, from high pressure
(near surface) to low pressure (aloft).
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HYDROSTATIC BALANCE CONCEPT
(con’t.)
 Assume
that acceleration of gravity is
essentially constant with altitude;
 Air pressure ALWAYS decreases with
increased altitude in atmosphere;
 But, rate of pressure decrease with altitude
depends upon density of air column:
– Decrease is more rapid in cold, dense air
column than in warm, less dense column.
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VERTICAL PRESSURE GRADIENTS Dependency on density (temperature)
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VERTICAL PRESSURE GRADIENTS
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HYDROSTATIC BALANCE CONCEPT
(con’t.)
 In
summary, acceleration vectors of
gravity and vertical pressure gradient are
equal in magnitude, but opposite in
direction:
FNet, V = 0 = FPG,V + g
or FPG,V = - g
(A vector summation).
 A balance exists between these vertically
oriented vector quantities, meaning no
net vertical force nor acceleration!
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THE HORIZONTAL PRESSURE
GRADIENT FORCE
 Parcels
are accelerated in horizontal
direction from High to Low pressure.
 Direction of force & resulting accelerating
motion is perpendicular to isobars on a
surface weather map.
 Magnitude of acceleration is inversely
proportional to isobar spacing.
–
(i.e., greater horizontal pressure gradient force with tightly
packed isobars).
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HORIZONTAL PRESSURE GRADIENT
FORCE (con’t.)
Direction is from High to Low pressure!
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HORIZONTAL PRESSURE
GRADIENT FORCE (con’t.)
See Fig. 9.1 Moran & Morgan (1997)
Magnitude depends on isobar spacing!
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AS A RESULT  The
3-D vector Equation of
Atmospheric Motion can be rewritten:
 Horizontal Component:
Net horizontal force =
Horizontal Pressure gradient force +
+ Coriolis force + friction;
FNet, H = FPG,H + FCor + FFriction
(A vector summation).
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AS A RESULT (con’t.)
 Vertical Component:
Vertical Pressure gradient force + gravity
Since:
Net vertical force = 0
= Vertical Pressure gradient force + gravity
FPG,V + g = 0.
(A vector summation).
(a statement of
Hydrostatic Balance Assumption).
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Recall
VERTICAL PRESSURE GRADIENTS Dependency on density (temperature)
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C. FLOW RESPONDING TO
PRESSURE GRADIENT FORCE LOCAL WINDS
 Assumptions:
– Only Pressure gradient force operates due to
local pressure differences;
– Horizontal flow.
–  Net force = pressure gradient force
 Examples:
– Sea-Land Breeze Circulation
– Mountain-Valley Breeze Circulation
– City-Country Circulation
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Sea (Lake) Breeze
(Graphics from UIUC WW2010)
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REASONS FOR LAND-SEA
TEMPERATURE DIFFERENCES
 Water
–
Smaller temperature response for heat added
 Water
–
is transparent
Sunlight penetrates to depth
 Water
–
is a fluid
Mixing warm water downward
 Water
–
has higher heat capacity
surface experiences evaporation
Evaporative cooling
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Sea (Lake) Breeze (con’t.)
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Sea (Lake) Breeze (con’t.)
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Sea (Lake) Breeze (con’t.)
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Sea (Lake) Breeze (con’t.)
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Sea (Lake) Breeze (con’t.)
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Sea (Lake) Breeze (con’t.)
(Lake)
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Sea (Lake) Breeze (con’t.)
See Fig. 12.2 A Moran & Morgan (1997)
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Land Breeze
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Land Breeze (con’t.)
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Land Breeze (con’t.)
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Land Breeze (con’t.)
See Fig. 12.2 B Moran & Morgan (1997)
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Mountain Breeze
See Fig. 12.14 Moran & Morgan (1997)
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Valley Breeze
See Fig. 12.14 Moran & Morgan (1997)
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D. STRAIGHT-LINE, BALANCED,
FRICTIONLESS MOTION
- “GEOSTROPHIC FLOW”
 A powerful
conceptual model
involving horizontal motion on
rotating planet;
 Background & Word Derivation:
– Named by Sir Napier Shaw in 1916:
“Geo” = earth + “strephein” = to turn.
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“GEOSTROPHIC FLOW” (con’t.)
 Assumptions
– horizontal flow (FPG,V + g = 0);
– balanced flow
(FNet, H = 0);
– no friction
(FFriction = 0);
– straight line flow (with straight isobars)
(FCentripetal = 0);
– parallel and equally spaced isobars
(FPG,H = constant).
 Initiation
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of Geostrophic Flow
100
Geostrophic Adjustment
See Fig. 9.12 Moran & Morgan (1997)
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“GEOSTROPHIC FLOW” (con’t.)
 Resultant
Geostrophic Flow
– Balance between horizontal
components of pressure gradient &
Coriolis forces, or
0 = FPG,H + FCor (A vector summation).
 Geostrophic
Wind vector (Vg)
can be described as:
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“GEOSTROPHIC FLOW” (con’t.)
 Direction
of Vg vector is:
– parallel to isobars, with
Low pressure to left
(in Northern Hemisphere);
 Magnitude
of Vg vector is related:
– Directly to pressure gradient;
– Inversely to Coriolis force (i.e., latitude).
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“GEOSTROPHIC FLOW” (con’t.)
 Implications
of Geostrophic
Balance
– Geostrophic wind (Vg) is:
a
hypothetical wind
a balance between
–horizontal pressure gradient
(isobar spacing)
–latitude (or Coriolis effect)
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Geostrophic Wind
See Fig. 9.12 Moran & Morgan (1997)
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E. BALANCED FLOW
in FRICTION LAYER
 The
Nature of Friction
 The Friction Layer
 The Effect of Friction upon the
Geostrophic Wind
 Assumptions
– Same as for geostrophic wind case,
except FFriction is not zero.
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FLOW IN FRICTION LAYER (con’t.)
 Resultant
Motion
0 = FPG,H + FCor + FFriction (A vector
summation).
– Magnitude of flow is less than
geostrophic wind.
– Direction of flow is turned at angle
across isobars toward Low pressure in
either hemisphere.
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Flow in Friction Layer
See Fig. 9.15 Moran & Morgan (1997)
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FLOW IN FRICTION LAYER (con’t.)
 Variations
of Near-Surface Winds
with Height
– Wind speeds reach zero at surface
& increase to geostrophic at top of
friction layer;
– Wind direction at lower levels turned
more toward Low, then become parallel
to isobars;
– The result, a wind spiral is formed.
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F. CURVED, HORIZONTAL BALANCED
MOTION “GRADIENT FLOW”
 Assumptions
– horizontal flow ( FPG,V + g = 0);
– no friction (FFriction = 0);
– curved flow (with curved isobars)
(FCentripetal = FNet, H );
– concentric and equally spaced
isobars
(FPG,H = constant).
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Curved Flow
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“GRADIENT FLOW” (con’t.)
 Resultant
flow without Friction
FCentripetal = FPG,H + FCor
(A vector
summation).
 Two
cases:
– Cyclonic Flow
(around a low pressure cell)
– Anticyclonic Flow
(around a high pressure cell)
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“GRADIENT FLOW” (con’t.)
See Moran and Morgan (1997):
 Figure
9.14
Cyclonic Flow
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 Figure
9.13
Anticyclonic
Flow
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G. GRADIENT FLOW WITH
FRICTION
 Resultant
flow with Friction
FCentripetal = FPG,H + FCor + FFriction
(A vector
summation).
 Applicability
to the Atmosphere
 Situation
 Resultant
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Diagrams
114
H. RELATIONSHIPS BETWEEN
HORIZONTAL & VERTICAL MOTIONS
 Dilemma
 Convergence
/ Divergence
 Principle of Mass Continuity
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Features in a Surface Low
(Convergence & Ascent)
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Features in a Surface High
(Sinking & Divergence)
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VERTICAL PRESSURE GRADIENTS Dependency on density (temperature)
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Recall
VERTICAL PRESSURE GRADIENTS Dependency on density (temperature)
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