Download Pressure Gradient Force

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Atmospheric Forces
Wind Relationships
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• Pressure Forces
• Apparent Forces
• Friction
• Confluence and Difluence
• Gradient winds
• Super and Subgradient Winds
• Geostrophic Winds
Atmospheric Forces
Wind Relationships
Forces of Nature
Pressure Gradient (PG) -The rate of pressure change with
distance. Always measured perpendicular to the isobars, or
PG=Dp / Dn (Change in pressure over distance)
Pressure Gradient Force (PGF) - A force acting on a parcel of air,
resulting from the difference in pressure on either side of
the parcel.
1. What force initiates
the wind?
Natural
Coordinate
System
Pressure Gradient Force
2. If the gradient force is
higher, what happens to
the wind?
It Increases
Pressure Gradient
Pressure Gradient Force
Measured on the surface of the Earth.
Forces of Nature
How do we find highs and lows in the upper atmosphere?
Use a constant pressure surface chart
Contour Gradient
500mb Surface (Avg 18 kft)
Forces of Nature
Contour Gradient (CG)The
- rate of height change (Dz) with
distance (Dn) on a constant pressure surface.
CG=Dz / Dn (Change in height over distance)
Contour Gradient Force (CGF) - The force that represents PGF
on a constant pressure chart.
Differences in virtual temperature
and quantity of mass over a given
point in the atmosphere results in
differences in thickness between
layers. These variations in height
result in the CG.
B
A
Contour Gradient
Contour Gradient Force
500mb Surface (Avg 18 kft)
500mb
500mb
Forces of Nature
Coriolis Force (CoF) - An apparent force exerted on moving
objects caused by the rotation of the Earth and it’s coordinate
system.
• Zero at the Equator and maximum at the poles
• Velocity dependent. With constant latitude, as a parcel’s
velocity increases, then CoF increases.
• CoF acts 90 degrees to the wind direction (toward the right of
motion in the northern hemisphere)
• The closer you get to the poles,
the stronger the CoF, if the wind
speed remains the same.
Forces of Nature
Centrifugal Force (CeF) - The apparent force that deflects
particles and therefore winds away from the center of rotation.
What are the relationships of
velocity and curvature on the
strength of CeF?
Strength of force is dependent on
the velocity of the parcel, and the
rate of curvature:
Centrifugal Force is always directed
outward from the axis of rotation and
perpendicular to rotation.
CeF helps weaken the gradient
in a High and tighten the gradient
in a Low.
Constant curvature
as v
CeF ?
Constant Velocity
as r
Cef
?
Forces of Nature
Frictional Force (Fr) - The force that opposes motion.
• Acts opposite to the direction of motion.
• No direct effect on PGF or CGF, but does affect CoF and CeF.
• Increasing Fr results in decreasing wind velocity.
• Fr causes both CoF and CeF to decrease, therefore…
•Winds tend to back
towards the stronger
force (PGF or CGF).
Wind without the effects of friction,
and PGF/CGF and CoF in balance, is
called a … ?
Geostrophic Wind
Forces of Nature
In Conclusion…. Pop Quiz!
Force
Effect Wind Speed…
Effect Wind Direction…
PGF/CGF
Yes (intiates wind)
Yes (high to low)
CeF
No
Yes (curved flow)
CoF
No
Yes (90 deg to RT of flow)
Fr
Yes (slows)
Yes (backs)
Wind Relationships
Geostrophic Wind: The wind that would result if there were a
balance between the Coriolis Force and Pressure Gradient Force.
• Assumes: Straight line flow - no curvature.
• Assumes: No friction. Most often possible above the Earth’s surface.
• Forces are in balance, so wind speed is constant.
• Can there ever truly be a Geostrophic wind in our atmosphere?
Maybe? How about very short periods of time?
Straight Line Flow (No Friction)
Ageostrophic flow (friction added-straight line)
Wind Relationships
Gradient Winds - In Curved Flow with no change in gradient -
Anticyclonic Gradient Wind:
Coriolis force must balance both Pressure/Contour
Gradient Force and Centrifugal Force.
Va
* Va will be stronger than the expected
geostrophic wind at the same latitude and
PGF or CGF (supergeostrophic).
** Remember - with latitude constant, an
increase in CoF will increase wind speed.
H
CoF
CGF
CeF
Max Speed is limited due
to finite CoF.
Wind Relationships
Gradient Winds - In Curved Flow with no change in gradient -
Cyclonic Gradient Wind:
Coriolis Force and Centrifugal Force act
together to balance Pressure Gradient Force.
L
** Vc will be weaker than the expected
geostrophic wind at the same latitude and
PGF or CGF (subgeostrophic).
CGF
CoF
Vc
CeF
Max speed is unlimited.
Wind Relationships
Gradient Level Winds
What is the Gradient Level?
The top of the boundary layer, or the height where friction,
wind shear, and/or surface stability end.
• Terrain - Rougher the terrain, the greater the mixing.
• Wind - Greater wind means more mechanical mixing
• Stability - The less stable
the atmosphere, the more
mixing (stronger upward
vertical motion)
Gradient Level
Sfc Heating
Normal
Sfc Cooling
Wind Relationships
Confluence and Difluence
What is Confluence?
The directional merging of wind flow.
What is Difluence?
The directional spreading apart of wind flow.
Wind Relationships
Supergradient and Subgradient Winds
Supergradient Winds - A wind adjusting to the rapid decrease
in Contour Gradient Force downstream.
540
Winds turn “Uphill”
towards higher values
CGF
Vg
540
CGF
Vg
546
CoF
CoF
552
CGF
Vg
552
CoF
546
Wind Relationships
Supergradient and Subgradient Winds
Subgradient Winds - A wind adjusting to a rapid increase in
Contour gradient force downstream.
546
CGF
552
546
Vg
552
CGF
CGF
Vg
CoF
558
Vg
558
CoF
564
CoF
564
Winds turn “downhill”
towards lower values