Transcript Coriolis Force

Chapter 10: Atmospheric
Dynamics
General Concept
Definition:
- Wind: air in motion relative to earth’s surface
Air moves in response to
difference in pressure. Thus,
pressure difference is a driving
source. But winds do not blow
directly from a higher pressure
region to a lower pressure
regions because of influence
from different forces.
Solid lines: isobar, arrows: winds
Force
• Newton’s Second Law of Motion:
F = ma
Force = mass x acceleration
• Imbalance of forces causes net motion
Forces We Will Consider
• Gravity
• Pressure Gradient Force
• Coriolis Force
• Centrifugal Force / Centripetal Acceleration
• Friction
1.
Gravitational Force
2. Pressure Gradient Force
• Gradient – the change in a quantity over a
distance
• Pressure gradient – the change in atmospheric
pressure over a distance
• Pressure gradient – the resultant net force due
to the change in atmospheric pressure over a
distance
Pressure Gradient Force on the
Weather Map
• H = High pressure (pressure
decreases in all directions from
center)
• L = Low pressure (pressure
increases in all directions from
center)
• The contour lines are called
isobars, lines of constant air
pressure
• Strength of resultant wind is
proportional to the isobar spacing
• Less spacing = stronger pressure
gradient = stronger winds
Pressure
Gradient
Force
(PGF)
gradient: high pressure  low pressure
• pressure differences exits due to unequal heating of
Earth’s surface
• spacing between isobars indicates intensity of gradient
• flow is perpendicular to isobars
• pressure
Low Pressure Center
• Center of lowest
pressure
• Pressure increases
outward from the low
center
• Also called a cyclone
High Pressure Center
• Center of highest
pressure
• Pressure decreases
outward from the low
center
• Also called an
anticyclone
Low Pressure Trough
• An elongated axis of
lower pressure
• Isobars are curved
but not closed as in a
low
1000
1004
1008
1012
High Pressure Ridge
• An elongated axis of
higher pressure
• Isobars are curved
but not closed as in a
high pressure center
1000
1004
1008
1012
Convergence
•
•
•
•
Convergence -- the net horizontal inflow of air into an area.
Results in upward motion
Convergence occurs in areas of low pressure (low pressure centers and
troughs)
Lows and troughs are areas of rising air
Divergence
• Divergence -- the net horizontal outflow of air from an area.
• Results in downward motion (subsidence)
• Divergence occurs in areas of high pressure (high pressure centers
and ridges)
• Highs and ridges are areas of sinking air (subsidence)
3.
Coriolis Force
• Due to the rotation of
the Earth
• Objects appear to be
deflected to the right
(following the motion)
in the Northern
Hemisphere
• Speed is unaffected,
only direction
Fig. 6-9, p. 165
• Coriolis effect seen on a rotating platform,
as 1 person throws a ball to another
person.
Coriolis force (CF)
- The Coriolis force causes the wind to deflect to the right of its
intended path in the Northern Hemisphere and to the left of its
intended path in the Southern Hemisphere. It acts at a right angle
to the wind.
- The Coriolis force is largest at the pole and zero at the equator
- The stronger the wind speed, the greater the deflection
- The Coriolis force changes only wind direction, not wind speed.
- We measure motion on the rotating Earth. Thus, we need to be
concerned with the Coriolis force
The Coriolis
Effect
• objects in the atmosphere are influenced by the Earth’s rotation
– Rotation of Earth is counter-clockwise
• results in an ‘apparent’ deflection (relative to surface)
• deflection to the right in the Northern Hemisphere
(left, S. Hemisphere)
• Greatest at the poles, 0 at the equator
• Increases with speed of moving object
• CE changes direction not speed
4.
Centrifugal Force / Centripetal
Acceleration
• Due to change in direction of motion.
• A centrifugal force is a force on an
object that tends to move it away from
a center of rotation and always results
from the inertia of the object.
roller coasters in parks.
• A centripetal force is a force on an object that tends to
move it toward a center of rotation.
5.
Friction
•
factor at Earth’s surface  slows wind
•
Loss of momentum during travel due to roughness of surface
•
varies with surface texture, wind speed, time of day/year and
atmospheric conditions
•
Important for air within ~1.5 km of the surface, the planetary
boundary layer
•
Because friction reduces wind speed it also reduces Coriolis
deflection
•
Friction above 1.5 km is negligible
– Above 1.5 km = the free atmosphere
Atmospheric Force Balances
• First, MUST have a pressure gradient force
(PGF) for the wind to blow.
• Otherwise, all other forces are irrelevant.
• Already discussed hydrostatic balance, a
balance between the vertical PGF and gravity.
There are many others that describe
atmospheric flow…
Geostrophic Balance
• Balance between PGF and Coriolis force
Fig. 6-15, p. 172
• Therefore, wind blows parallel to isobars, which is useful
to consider when looking at weather map.
• Buy-Ballot’s “law”: If you stand with your back to the wind
in the N.H, low pressure will be on your left and high
pressure on your right.
• In N. Hem., geostrophic wind blow to the right of PGF
(points from high to low P), In S. Hem., geostrophic wind
to left of PGF.
PGF
Coriolis
wind
N. Hem.
wind
S. Hem.
PGF
Coriolis
• Converging contours of const. pressure (isobars)
=> faster flow => incr. CF & PGF
Get geostrophic
wind pattern
from isobars
Geostrophic balance
• P diff. => pressure gradient force (PGF)
=> air parcel moves => Coriolis force
• Geostrophy = balance between PGF & Coriolis force .
Upper Atmosphere Winds
• upper air moving from areas of higher to areas of lower pressure
undergo Coriolis deflection
• air will eventually flow parallel to height contours as the pressure
gradient force balances with the Coriolis force
• this geostrophic flow (wind) may only occur in the free
atmosphere (no friction)
• stable flow with constant speed and direction
• Wind flows in a counterclockwise sense around a low or trough
• Wind flows in a clockwise sense around a high or ridge
Gradient Wind Balance
• Balance between PGF, Coriolis force, and
centrifugal force
• Examples: hurricanes
Supergeostrophic flow
(CF > PGF )
PGF + Ce = CF
Subgeostrophic flow
(CF < PGF)
PGF = CF + Ce
• Difference between PGF
& Coriolis (CF) is the
centripetal force needed
to keep parcel in orbit.
• Geostrophic flow too simplistic  PGF is rarely uniform, height
contours curve and vary in distance
• wind still flows parallel to contours HOWEVER continuously
changing direction (and experiencing acceleration)
• for parallel flow to occur pressure imbalance must exist between
the PGF and CE  Gradient Flow
• Two specific types of gradient flow:
– Supergeostrophic: High pressure systems, CE > PGF (to
enable wind to turn), air accelerates
– Subgeostrophic: Low pressure systems, PGF > CE, air
decelerates
• supergeostrophic and subgeostrophic conditions lead to airflow
parallel to curved height contours
Surface Winds
• Friction slows the wind
• Coriolis force (dependent on wind speed) is therefore
reduced
• Pressure gradient force now exceeds Coriolis force
• Wind flows across the isobars toward lower pressure
Near Surface Wind
• Ground friction
slows wind => CF
weakens.
• CF+friction balances
PGF.
• Surface wind tilted
toward low p region.
• At the surface, if we
stand with our backs
to the wind, then turn
clockwise about 30 °,
lower pressure will
be to our left. “BuysBallots law”
Friction
Comparison
Convergence & divergence
• Cyclone has convergence near ground but divergence at upper
level.
• Anticyclone: divergence near ground, convergence at upper level.
• Air converges into a low pressure center, leading into ascending
motion. This ascending air cools by adiabatic expansion and
possible development of clouds and precipitation.
• Air diverges at the center of high pressure. Then the air aloft
converges and slowly descend.
Winds: examples
Aloft
Northern
Hemisphere
Sfc
Aloft
Sfc
Southern
Hemisphere
Pressure Gradient Force + Coriolis Force
Geostrophic
Wind
Pressure Gradient + Coriolis + Friction Forces
Surface
Wind
Cyclones, Anticyclones, Troughs and Ridges
• High pressure areas (anticyclones)  clockwise airflow in the Northern
Hemisphere (opposite flow direction in S. Hemisphere)
– Characterized by descending air which warms creating clear skies
• Low pressure areas (cyclones)  counterclockwise airflow in N.
Hemisphere (opposite flow in S. Hemisphere)
– Air converges toward low pressure centers, cyclones are
characterized by ascending air which cools to form clouds and
possibly precipitation
• In the upper atmosphere, ridges correspond to surface anticyclones
while troughs correspond to surface cyclones