Download Dewey Notes 05 Coriolis

EOS 110 Earth System I
Motion on a Rotating Planet
EOS 110 Earth System I
The Coriolis Effect
Atmospheric winds and Ocean currents want to go “straight”
relative to the stars (inertial reference frame). However, the
solid Earth turns underneath them.
Because we are on a rotating planet, we “see” the non-inertial
effects of our rotating reference frame. When an object moves,
relative to the solid rotating Earth, it’s straight path relative to
the stars appears to any observer on the planet to be a curved
path. A smart observer (you), says: “A change in uniform
velocity is acceleration, and Newton stated that acceleration
occurs when a force acts upon an object”. We call this force the
Coriolis force, and the effect of changing direction: The
Coriolis
Effect. The
Coriolis effect
is a positive
(right)
maximum at
the North
Pole,
decreasing
with latitude
to zero at the
equator, and a
negative (left)
maximum at
the South
Pole.
Since we (observer) are standing on the Earth and rotating also,
the winds/currents appear to turn.
Coriolis Effect
The Coriolis force causes the turning and always pulls at right
angles to the direction of motion (does not change the speed).
In the: Northern Hemisphere
Southern Hemisphere
pulls to the right
pulls to the left
Cyclonic (low pressure) circulation
- in NH is anti-clockwise - in the SH is clockwise
Anti-cyclonic (high pressure) circulation
- in NH is clockwise
- in SH is anti-clockwise
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EOS 110 Earth System I
Geophysical Fluid Dynamics
EOS 110 Earth System I
Geostrophic Balance
Analysis of Circulation
1) Consider all forces acting on the fluid
2) Conserve mass
3) Balance the energy budget
A pressure gradient accelerates the flow, Coriolis turns it, and
eventually a force balance is reached.
The Coriolis force is always at right angles to the flow, and it’s
magnitude is
2Ω sinθ ×U = fU
Four Primary Forces
1) Buoyancy Forces: Hot/Moist air rises, cool/dry air sinks.
Warm/fresh water rises, cold/salty water sinks.
where Ω is the Earth’s rotation rate [7.292x10-7 radians/second]
and θ is the Latitude. f is the Coriolis parameter f= 2Ω sinθ ,
maximum at the poles.
2) Pressure gradient Forces: Air/water accelerates from high
pressure to low pressure. The pressure at any on location is a
result of the weight of fluid above. Pressure gradients are
due to density differences, and variations in thickness.
The pressure gradient force is always from high to low
pressure. At some point, the flow will turn until the Coriolis
force is opposite to the pressure gradient force
Pressure Gradient Force = Coriolis Force
3) Coriolis Forces: Acts at right angles on any object in
motion relative to the rotating planet
a force balance, no more acceleration!
NH:right
Geostrophic Balance: F = net force
SH:left
4) Friction: Turbulence and molecule friction always act to
slow down the flow. Strongest in boundary layers.
Together, these forces cause Atmospheric and Oceanic
Circulation:
Large Scale Wind/Weather Patterns
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EOS 110 Earth System I
EOS 110 Earth System I
Review Of Atmospheric Processes
Geostrophic Balance
Pressure Gradient Force
1) The Earth’s distance from the sun sets our energy input and
Coriolis Force
therefore our “climate”.
This balance results in circular motion around a high or low
pressure region.
2) Energy is partitioned and exchanged between the
atmosphere, ocean, and land, and geographically by
Low pressure “cyclones” spin-up, intensify and last a long
time.
radiation, convection/advection, and latent heat.
3) Warm and/or moist air masses are light and rise, cool and/or
High pressure “anti-cyclones” spin-down and dissipate.
dry air masses are heavy and sink.
4) Differential heating and cooling, and evaporation and
Northern Hemisphere
condensation introduce “mass” and pressure differences.
5) Weight differences and convergences/divergences (ridges
and troughs) cause pressure gradients.
6) Air and water are accelerated by pressure gradients, from
higher-pressure regions to lower-pressure regions.
7) Motion on a rotating planet results in the Coriolis effect,
things turn right in NH and left in SH.
8) Conservation of angular momentum (vorticity) causes “spinup” towards a low pressure (concentrates) and “spin-down”
away from a high pressure (dissipates).
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