Atmospheric Motions

1
Atmospheric Motions Climate
Science Concepts Newtons Laws of
Motion Vertical Forces Pressure Gradient
Force Gravitational Force Friction
Force Buoyancy Latent Heat
Vertical Atmospheric Motion Hydrostatic
Balance Non-hydrostatic Balance Vertical
Stability Dry Adiabatic Motion Moist
Adiabatic Motion Skew-T Log-p
Diagram Stability Rules Changes in Stability
The Earth System (Kump, Kastin Crane) Chap. 4
(pp. 56-57)
2
Atmospheric Motions
What keeps this balloon in the air? Scientist
Benjamin Franklin witnessed brothers Montgolfiers
launch the first manned balloon flight on
November 21, 1783 in France, after he had
negotiated the end of the Revolutionary
War. The Montgolfiers believed the balloons
lift was caused by hot air and smoke, so plied
the fire with wet straw and wool. Ten days
later, Jacques Charles (Charles Law fame) flew a
silk balloon filled with hydrogen for two hours
while traveling 21 miles. Franklin helped
finance Charles flight.
Franklin Parody - If you want to fill your
balloons with an element ten times lighter than
inflammable air, you can find a great quantity of
it, and ready made, in the promises of lovers and
of courtiers.
Walter Isaacson, 2003 Benjamin Franklin - An
American Life, Simon and Schuster, NY, pp.
420-422.
3
Atmospheric Motions
Vertical Motion Vertical Forces and
Acceleration - Gravitational force
(GF) gt Generally points toward the center of
the Earth gt Depends on the mass of the object
or in our case the air parcel - Pressure
gradient force (PGF) gt As before, points
toward lower pressure, i.e., upward in the
vertical gt Depends on the mass of the
displaced fluid, in our case the mass of the
displaced environmental air - Friction force
(FF) gt Not very important because friction
depends on the object's speed and vertical
velocities are in general small
4
Atmospheric Motions
Vertical Motion (Cont) Hydrostatic
Balance - As a first approximation the PGF is
equal to and in the opposite direction to the
GF. Thus these two forces cancel and the net
force is zero. Therefore, the acceleration is
zero. This state is called hydrostatic
balance. Non-Hydrostatic Balance - In cases
with large imbalances between the PGF and GF, air
parcels are accelerated vertically either up
or down. - This acceleration is referred to as
buoyancy or Archimedean acceleration. - Warm
air accelerating upward and cold air accelerating
downward, i.e., convection, are examples of
non-hydrostatic balance.
5
Atmospheric Motions
Hydrostatic Balance
Non-Hydrostatic Balance
PGF
PGF
PGF
PGF Gravity
PGF ? Gravity
Gravity
Gravity
Gravity
Zero Acceleration
Downward Acceleration
Upward Acceleration
Surface
6
Atmospheric Motions
In a Thunderstorm
PGF
PGF
Downdraft PGFltlt Gravity
Updraft PGF gtgt Gravity
Gravity
Gravity
Surface
7
Stability
Vertical Stability Consequences of the Gas
Law revisited
T Changes p Changes
Gas Law
Boyles Law
Dry Adiabatic Ascent
V Constant
T Constant
T
T
T
V
V
V
400
7000
-156C
-53.6C
1.00
1.91
15C
2.47
6000
500
5000
600
Pressure (mb)
Altitude (m)
4000
-99
C
-24.2C
1.00
1.42
15C
1.65
700
3000
800
2000
-48
C
-4.6C
1.00
1.19
15C
1.28
1000
900
1000
0
15C
15C
1.00
1.00
15C
1.00
8
Stability
Vertical Stability (Cont) Important to
determine the occurrence and strength of
convection and afternoon showers. Also
important to determine the vertical mixing of
pollution. Adiabatic Diagrams Use to
determine atmospheric stability Plot of
temperature versus pressure - Several
types - Skew-T Log-p diagrams Compare
measured lapse rate with dry or moist adiabatic
parcel lapse rates - Adiabatic - No energy
(heat) added or subtracted
9
Stability
Simple Skew-T Log-p Diagram
400
0
-10
-20
-30
-40
-50
-60
10
500
600
Pressure (mb)
20
700
800
850
30
1000
1050
Temperature (C)
10
Stability
Skew-T Log-p Diagram with Dry Adiabats
400
0

-10

-20

-30

-40

-50

-60

340

310

300

290

280

330

320

270

10

500
260

600
Pressure (mb)
20

700
250

800
850
30

1000

1050

Temperature (C)
11
Stability
Stable Atmosphere
400
Observed or Measured Lapse Rate
Dry Adiabat
Parcel beginning at 1050 mb is lifted to 600 mb
dry adiabatically. Note the parcel is colder
than its environment, thus it is accelerated
back toward its original position. This
atmosphere is considered to be stable.
500
TpltTe
600
Pressure (mb)
700
800
850
TpTe
1000
1050
Temperature (C)
12
Stability
Unstable Atmosphere
400
Continues to accelerate upward until TpTe
Dry Adiabat
Parcel beginning at 1050 mb is lifted to 600
mb dry adiabatically. Note the parcel is warmer
than its environment, thus it is accelerated away
from its original position. This atmosphere is
considered to be unstable.
500
600
Pressure (mb)
TpgtTe
700
Observed or Measured Lapse Rate
800
850
1000
1050
Temperature (C)
13
Stability
Unstable Atmosphere
400
Observed or Measured Lapse Rate
Dry Adiabat
500
600
Pressure (mb)
700
800
850
1000
1050
Temperature (C)
14
Stability
Stability Rules When a parcel is displaced
(moved) from its original position to a new
position, gt If the net force accelerates the
parcel back toward its original position then
the atmosphere is considered stable
(TpltTe) gt If the net force accelerates the
parcel away from its original position then
the atmosphere is considered unstable
(TpgtTe) gt If the net force is zero then the
atmosphere is considered neutral (TpTe)
15
Vertical Stability Consequences of the Gas
Law revisited again
Stability
T Changes p Changes
Gas Law
Gas Law
Boyles Law
Dry Adiabatic Ascent
Moist Adiabatic Ascent
V Constant
T Constant
V
T
T
T
T
V
V
V
(Ms)
400
7000
2.14
-30.2C
-156C
-53.6C
1.00
1.91
15C
2.47
(0.76)
6000
500
5000
600
Pressure (mb)
Altitude (m)
4000
1.52
-7.8C
-99
C
-24.2C
1.00
1.42
15C
1.65
(3.42)
700
3000
2000
800
1.23
4.4
C
-48
C
-4.6C
1.00
1.19
15C
1.28
(6.64)
1000
900
1000
0
1.00
15C
15C
15C
1.00
1.00
15C
1.00
(10.70)
16
Stability
Skew-T Log-p Diagram with Moist Adiabats
400
0

-10

-20

-30

-40

-50

-60

32

28

16

12

8

20

24

300

310

290

280

330

320

270

340

10

500
260

600
20

Pressure (mb)
700
250

800
850
30

1000

1050

Temperature (C)
17
Stability
Stable Moist Atmosphere
400
Observed or Measured Lapse Rate
Moist Adiabat
A saturated parcel beginning at 1050 mb is lifted
to 600 mb moist adiabatically. Note the parcel
is colder than its environment, thus it
is accelerated back toward its original position.
This atmosphere is considered to be stable.
500
TpltTe accelerates downward
600
Pressure (mb)
700
800
850
TpTe
1000
1050
Temperature (C)
18
Stability
Unstable Moist Atmosphere
400
Observed or Measured Lapse Rate
A saturated parcel beginning at 1050 mb is lifted
to 600 mb moist adiabatically. Note the parcel
is warmer than its environment, thus it is
accelerated away from its original position.
This atmosphere is considered to be unstable.
Moist Adiabat
Continues to accelerate upward until TpTe
500
Pressure (mb)
600
700
TpgtTe
800
850
1000
1050
Temperature (C)
19
Stability
Unstable Moist Atmosphere
400
Moist Adiabat
Observed or Measured Lapse Rate
500
Pressure (mb)
600
700
Unstable
800
850
1000
1050
Temperature (C)
20
Stability
Combined Stability Regions
400
Moist Adiabat
Dry Adiabat
Three Observed or Measured Lapse Rates
Conditional Stability
500
Pressure (mb)
600
Absolute Stablility
700
800
850
Absolute Instablility
1000
1050
Temperature (C)
21
Skew-T Log-p Chart
Pressure (mb)
Temperature (C)
22
Stability
Conditional Stability Note In this region the
stability criterion depends on if the parcel is
saturated or not - If the parcel is
saturated, then the atmosphere is considered to
be Unstable and if the parcel is
unsaturated, then the atmosphere is considered
to be Stable Causes of Changes in
Stability Destabilizes - Solar
heating - Cold air advection over a warm
surface Stabilizes - Radiational
cooling - Warm air advection over a cold surface
As it usually does on the Colorado Plateau,
night defeated the storm. It drifted
northeastward, robbed of the solar power that fed
it, and exhausted its energy in the thin, cold
air over the Utah canyons and mountains of
northern New Mexico. Tony Hillerman, 1986
Skinwalkers, p. 261.
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Stability
http//earthobservatory.nasa.gov/Newsroom/ NewImag
es/images.php3?img_id17108
Stability Effects on Convective Clouds 21
November 2005 cloud streets over Hudson Bay
Cloud streets are parallel lines of cumulus
clouds that align with the wind Result of
thermals, or rising columns of warmed air
formed when the surface is a little warmer than
the air above Here a cold northwest wind (red
arrow) is blowing off the ice covered land over
the still warmer water of Hudson Bay This
destabilizes the air and creating convective
clouds through lifting the air to its
saturation level These clouds are then carried
by the steady wind forming lines of clouds
aligned along the direction of the wind
Wind Direction
24
Stability
Stability Effects on Convective Clouds Surface
solar heating destabilizes the air sometimes
allowing afternoon convective clouds and
showers to form 7 September 1999 Radar
Reflectivity
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Stability
Stability Effects on Convective
Clouds Lightning intensity Diurnal distrib
ution Beginning at midnight local time
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Stability
Stability Effects on Convective
Clouds Lightning intensity Diurnal
distribution Clock - 1200 2400 up
Minimum 700 am
Maximum 500 pm
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Stability
Stability Effects on Convective
Clouds Lightning intensity Blue
less Green more
Global Lightning vs Time
Latitudinal Lightning Distribution vs Time White
Line Annual Mean Latitudinal
Lightning Distribution
Month/Year
Longitudinal White Line Annual Mean Latitudinal
Lightning Distribution Lightning Distribution
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Stability
Stability Effects on Convective
Clouds Lightning Intensity June 1995- May
1999 distribution
29
Stability
Stability Effects on Convective
Clouds Lightning intensity Annual distribu
tion (Jan-Dec)
30
Stability
Stability Effects on Convective
Clouds Lightning intensity Note difference
between February and August
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Stability
Stability Effects of Air Pollution Fanning F
umigation Looping Coning Lofting Trapp
ing