Atmospheric Forces Wind Relationships

Atmospheric Forces Wind Relationships

• Pressure ForcesPressure Forces

• Apparent ForcesApparent Forces

• FrictionFriction

• Gradient windsGradient winds

• Super and Subgradient Winds Super and Subgradient Winds

• Geostrophic WindsGeostrophic Winds

• Confluence and DifluenceConfluence and Difluence

Forces of NatureForces of NaturePressure Gradient (PG) - Pressure Gradient (PG) -

Pressure Gradient Pressure Gradient ForcePressure Gradient Pressure Gradient Force Measured on the surface of the Earth.Measured on the surface of the Earth.

2. If the gradient force is2. If the gradient force ishigher, what happens tohigher, what happens tothe wind?the wind?

NaturalCoordinateSystem

Pressure Gradient Force (PGF) - A force acting on a parcel of air, Pressure Gradient Force (PGF) - A force acting on a parcel of air, resulting from the difference in pressure on either side ofresulting from the difference in pressure on either side ofthe parcel.the parcel.

The rate of pressure change withThe rate of pressure change with distance. Always measured perpendicular to the isobars, ordistance. Always measured perpendicular to the isobars, or PG=PG=p p / / n (Change in pressure over distance)n (Change in pressure over distance)

1. What force initiates1. What force initiatesthe wind?the wind?

Pressure Gradient ForcePressure Gradient Force

It IncreasesIt Increases

Forces of NatureForces of NatureHow do we find highs and lows in the upper atmosphere?How do we find highs and lows in the upper atmosphere?

Contour GradientContour Gradient 500mb Surface (Avg 18 kft)500mb Surface (Avg 18 kft)

Use a constant pressure surface chartUse a constant pressure surface chart

Forces of NatureForces of NatureContour Gradient (CG) -Contour Gradient (CG) -

Differences in virtual temperatureDifferences in virtual temperatureand quantity of mass over a givenand quantity of mass over a givenpoint in the atmosphere results in point in the atmosphere results in differences in thickness between differences in thickness between layers. These variations in height layers. These variations in height result in the CG.result in the CG.

Contour Gradient Contour Gradient ForceContour Gradient Contour Gradient Force 500mb Surface (Avg 18 kft)500mb Surface (Avg 18 kft)

The rate of height change (The rate of height change (z) with z) with distance (distance (n) on a constant pressure surface.n) on a constant pressure surface.

CG=CG=z z / / n (Change in height over distance)n (Change in height over distance)

Contour Gradient Force (CGF) - The force that represents PGFContour Gradient Force (CGF) - The force that represents PGFon a constant pressure chart.on a constant pressure chart.

500mb

500mbA

B

Forces of NatureForces of NatureCoriolis Force (CoF) - An apparent force exerted on movingCoriolis Force (CoF) - An apparent force exerted on movingobjects caused by the rotation of the Earth and it’s coordinateobjects caused by the rotation of the Earth and it’s coordinatesystem. system.

• The closer you get to the poles,The closer you get to the poles, the stronger the CoF, if the wind the stronger the CoF, if the wind speed remains the same. speed remains the same.

• Zero at the Equator and maximum at the poles Zero at the Equator and maximum at the poles

• Velocity dependent. With constant latitude, as a parcel’s Velocity dependent. With constant latitude, as a parcel’s velocity increases, then CoF increases.velocity increases, then CoF increases.

• CoF acts 90 degrees to the wind direction (toward the right of CoF acts 90 degrees to the wind direction (toward the right of motion in the northern hemisphere) motion in the northern hemisphere)

Forces of NatureForces of Nature

Strength of force is dependent onStrength of force is dependent onthe velocity of the parcel, and the the velocity of the parcel, and the rate of curvature:rate of curvature:

Constant curvature Constant curvature Constant VelocityConstant Velocity as v CeF as v CeF as r Cef as r Cef

What are the relationships of What are the relationships of velocity and curvature on the velocity and curvature on the

strength of CeF?strength of CeF?

Centrifugal Force is always directedCentrifugal Force is always directedoutward from the axis of rotation andoutward from the axis of rotation andperpendicular to rotation.perpendicular to rotation.

CeF helps weaken the gradient CeF helps weaken the gradient in a High and tighten the gradientin a High and tighten the gradientin a Low.in a Low.

Centrifugal Force (CeF) - The apparent force that deflects Centrifugal Force (CeF) - The apparent force that deflects particles and therefore winds away from the center of rotation.particles and therefore winds away from the center of rotation.

?? ??

Forces of NatureForces of NatureFrictional Force (Fr) - The force that opposes motion.Frictional Force (Fr) - The force that opposes motion.

• Acts opposite to the direction of motion. Acts opposite to the direction of motion.

Wind without the effects of friction,Wind without the effects of friction,and PGF/CGF and CoF in balance, isand PGF/CGF and CoF in balance, iscalled a … ?called a … ?

• No direct effect on PGF or CGF, but does affect CoF and CeF. No direct effect on PGF or CGF, but does affect CoF and CeF.

• Increasing Fr results in decreasing wind velocity. Increasing Fr results in decreasing wind velocity.

• Fr causes both CoF and CeF to decrease, therefore… Fr causes both CoF and CeF to decrease, therefore…

•Winds tend to back Winds tend to back towards the stronger towards the stronger force (PGF or CGF).force (PGF or CGF).

Geostrophic WindGeostrophic Wind

Forces of NatureForces of Nature

In Conclusion…. Pop Quiz!In Conclusion…. Pop Quiz!

Force Effect Wind Speed… Effect Wind Direction…Force Effect Wind Speed… Effect Wind Direction…

PGF/CGFPGF/CGF CeFCeF CoFCoF FrFr

Yes (intiates wind)Yes (intiates wind) Yes (high to low)Yes (high to low)

NoNo

NoNo

Yes (slows)Yes (slows) Yes (backs)Yes (backs)

Yes (90 deg to RT of flow)Yes (90 deg to RT of flow)

Yes (curved flow)Yes (curved flow)

Wind RelationshipsWind Relationships

• Can there ever truly be a Geostrophic wind in our atmosphere?Can there ever truly be a Geostrophic wind in our atmosphere?

Straight Line Flow (No Friction)Straight Line Flow (No Friction)

Geostrophic Wind: The wind that would result if there were a Geostrophic Wind: The wind that would result if there were a balance between the Coriolis Force and Pressure Gradient Force.balance between the Coriolis Force and Pressure Gradient Force.

• Assumes: Straight line flow - no curvature.Assumes: Straight line flow - no curvature.

• Assumes: No friction. Most often possible above the Earth’s surface.Assumes: No friction. Most often possible above the Earth’s surface.

• Forces are in balance, so wind speed is constant.Forces are in balance, so wind speed is constant.

Maybe? How about very short periods of time?Maybe? How about very short periods of time?

Ageostrophic flow (friction added-straight line)Ageostrophic flow (friction added-straight line)

Wind RelationshipsWind RelationshipsGradient Winds - In Curved Flow with no change in gradient -Gradient Winds - In Curved Flow with no change in gradient -

H

CeF

CGF

CoF

Va

Max Speed is limited dueMax Speed is limited dueto finite CoF.to finite CoF.

* Va will be stronger than the expected * Va will be stronger than the expected geostrophic wind at the same latitude andgeostrophic wind at the same latitude andPGF or CGF (supergeostrophic). PGF or CGF (supergeostrophic).

** Remember - with latitude constant, an ** Remember - with latitude constant, an increase in CoF will increase wind speed.increase in CoF will increase wind speed.

Anticyclonic Gradient Wind: Anticyclonic Gradient Wind: Coriolis force must balance both Pressure/ContourCoriolis force must balance both Pressure/ContourGradient Force and Centrifugal Force.Gradient Force and Centrifugal Force.

Wind RelationshipsWind RelationshipsGradient Winds - In Curved Flow with no change in gradient -Gradient Winds - In Curved Flow with no change in gradient -

L

CeF

CoF

CGF

Vc

Max speed is unlimited.Max speed is unlimited.

** Vc will be weaker than the expected ** Vc will be weaker than the expected geostrophic wind at the same latitude and geostrophic wind at the same latitude and PGF or CGF (subgeostrophic).PGF or CGF (subgeostrophic).

Cyclonic Gradient Wind: Cyclonic Gradient Wind: Coriolis Force and Centrifugal Force act Coriolis Force and Centrifugal Force act together to balance Pressure Gradient Force.together to balance Pressure Gradient Force.

Wind RelationshipsWind RelationshipsGradient Level WindsGradient Level Winds

What is the Gradient Level?What is the Gradient Level?

The top of the boundary layer, or the height where friction,The top of the boundary layer, or the height where friction, wind shear, and/or surface stability end.wind shear, and/or surface stability end.

Sfc Heating NormalSfc Heating Normal Sfc Cooling Sfc Cooling

Gradient LevelGradient Level

• Terrain - Rougher the terrain, the greater the mixing.Terrain - Rougher the terrain, the greater the mixing.

• Stability - The less stableStability - The less stable the atmosphere, the morethe atmosphere, the more mixing (stronger upwardmixing (stronger upward vertical motion)vertical motion)

• Wind - Greater wind means more mechanical mixingWind - Greater wind means more mechanical mixing

Wind RelationshipsWind Relationships

Confluence and DifluenceConfluence and Difluence

What is Confluence?What is Confluence?

The directional merging of wind flow.The directional merging of wind flow.

What is Difluence?What is Difluence? The directional spreading apart of wind flow.The directional spreading apart of wind flow.

Wind RelationshipsWind RelationshipsSupergradient and Subgradient WindsSupergradient and Subgradient Winds

Supergradient Winds - A wind adjusting to the rapid decrease Supergradient Winds - A wind adjusting to the rapid decrease in Contour Gradient Force downstream.in Contour Gradient Force downstream.

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CGFCGF

CGFCGF

CGFCGF

CoFCoF

CoFCoF

CoFCoF

VgVg

VgVg

VgVg

552552

552552

546546

540540

546546

Winds turn “Uphill”Winds turn “Uphill”towards higher valuestowards higher values

Wind RelationshipsWind RelationshipsSupergradient and Subgradient WindsSupergradient and Subgradient Winds

Subgradient Winds - A wind adjusting to a rapid increase in Subgradient Winds - A wind adjusting to a rapid increase in Contour gradient force downstream.Contour gradient force downstream.

CGFCGF

CoFCoF

VgVg

CGFCGF

CoFCoF

VgVg

CGFCGF

CoFCoF

VgVg

558558

546546

564564

558558

546546

552552

564564

552552

Winds turn “downhill”Winds turn “downhill”towards lower valuestowards lower values