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Atmospheric Forces b b Wind Relationships. Atmospheric Forces b b Wind Relationships Pressure Forces Pressure Forces Apparent Forces Apparent Forces Friction.

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Presentation on theme: "Atmospheric Forces b b Wind Relationships. Atmospheric Forces b b Wind Relationships Pressure Forces Pressure Forces Apparent Forces Apparent Forces Friction."— Presentation transcript:

1 Atmospheric Forces b b Wind Relationships

2 Atmospheric Forces b b Wind Relationships Pressure Forces Pressure Forces Apparent Forces Apparent Forces Friction Friction Gradient winds Gradient winds Super and Subgradient Winds Super and Subgradient Winds Geostrophic Winds Geostrophic Winds Confluence and Difluence Confluence and Difluence

3 Forces of Nature Pressure Gradient (PG) - Pressure Gradient Pressure Gradient Force Pressure Gradient Pressure Gradient Force Measured on the surface of the Earth. Measured on the surface of the Earth. 2. If the gradient force is higher, what happens to the wind? Natural Coordinate System Pressure Gradient Force (PGF) - A force acting on a parcel of air, resulting from the difference in pressure on either side of the parcel. The rate of pressure change with The rate of pressure change with distance. Always measured perpendicular to the isobars, or distance. Always measured perpendicular to the isobars, or PG= p / n (Change in pressure over distance) PG= p / n (Change in pressure over distance) 1. What force initiates the wind? Pressure Gradient Force It Increases

4 Forces of Nature How do we find highs and lows in the upper atmosphere? Contour Gradient Contour Gradient 500mb Surface (Avg 18 kft) 500mb Surface (Avg 18 kft) Use a constant pressure surface chart Use a constant pressure surface chart

5 Forces of Nature Contour Gradient (CG) - 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. Contour Gradient Contour Gradient Force Contour Gradient Contour Gradient Force 500mb Surface (Avg 18 kft) 500mb Surface (Avg 18 kft) The rate of height change ( z) with The rate of height change ( z) with distance ( n) on a constant pressure surface. CG= z / n (Change in height over distance) CG= z / n (Change in height over distance) Contour Gradient Force (CGF) - The force that represents PGF on a constant pressure chart. 500mb A B

6 Forces of Nature Coriolis Force (CoF) - An apparent force exerted on moving objects caused by the rotation of the Earth and its coordinate 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 parcels Velocity dependent. With constant latitude, as a parcels 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)

7 Forces of Nature Strength of force is dependent on the velocity of the parcel, and the rate of curvature: Constant curvature Constant Velocity as v CeF as r Cef as v CeF as r Cef What are the relationships of velocity and curvature on the strength of CeF? 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. Centrifugal Force (CeF) - The apparent force that deflects particles and therefore winds away from the center of rotation. ??

8 Forces of Nature 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, and PGF/CGF and CoF in balance, is 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 Wind

9 Forces of Nature In Conclusion…. Pop Quiz! Force Effect Wind Speed… Effect Wind Direction… PGF/CGF CeF CoF Fr Yes (intiates wind) Yes (high to low) No No Yes (slows) Yes (backs) Yes (90 deg to RT of flow) Yes (curved flow)

10 Wind 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) 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: Straight line flow - no curvature. Assumes: No friction. Most often possible above the Earths surface. Assumes: No friction. Most often possible above the Earths 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? Ageostrophic flow (friction added-straight line)

11 Wind Relationships Gradient Winds - In Curved Flow with no change in gradient - H CeF CGF CoF Va Max Speed is limited due to finite CoF. * 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. Anticyclonic Gradient Wind: Coriolis force must balance both Pressure/Contour Gradient Force and Centrifugal Force.

12 Wind Relationships Gradient Winds - In Curved Flow with no change in gradient - L CeF CoF CGF Vc Max speed is unlimited. ** Vc will be weaker than the expected geostrophic wind at the same latitude and PGF or CGF (subgeostrophic). Cyclonic Gradient Wind: Coriolis Force and Centrifugal Force act together to balance Pressure Gradient Force.

13 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. wind shear, and/or surface stability end. Sfc Heating Normal Sfc Cooling Gradient Level Terrain - Rougher the terrain, the greater the mixing. Terrain - Rougher the terrain, the greater the mixing. Stability - The less stable Stability - The less stable the atmosphere, the more the atmosphere, the more mixing (stronger upward mixing (stronger upward vertical motion) vertical motion) Wind - Greater wind means more mechanical mixing Wind - Greater wind means more mechanical mixing

14 Wind Relationships Confluence and Difluence What is Confluence? The directional merging of wind flow. What is Difluence? The directional spreading apart of wind flow.

15 Wind Relationships Supergradient and Subgradient Winds Supergradient Winds - A wind adjusting to the rapid decrease in Contour Gradient Force downstream. 540 CGF CGF CGF CoF CoF CoF Vg Vg Vg Winds turn Uphill towards higher values

16 Wind Relationships Supergradient and Subgradient Winds Subgradient Winds - A wind adjusting to a rapid increase in Contour gradient force downstream. CGF CoF Vg CGF CoF Vg CGF CoF Vg Winds turn downhill towards lower values

17 Jet Streams What is a Jet Stream? Relatively strong, quasi-horizontal winds concentrated within a narrow air current in the upper troposphere. A a narrow air current in the upper troposphere. A meandering, fast-moving river of air. meandering, fast-moving river of air. What is its cause? Large horizontal temperature contrasts produce strong contour/pressure gradients and strong winds.

18 Jet Streams Forms at the Tropopause - CGF at its maximum Forms at the Tropopause - CGF at its maximum H L Jet Core - region of relatively uniform wind speeds within the Jet Core - region of relatively uniform wind speeds within the interior of the Jet. interior of the Jet. Jet Axis - Axis of max wind speed at the core (remember sub/supergeostrophic). Jet Axis - Axis of max wind speed at the core (remember sub/supergeostrophic). Jet Max - Intermittent regions of strongest wind speeds. Very Jet Max - Intermittent regions of strongest wind speeds. Very important to the strengthening/weakening of meteorological important to the strengthening/weakening of meteorological phenomenon. phenomenon.

19 Jet Streams Polar Front Jet- Primary Cause: Primary Cause: Horizontal Temperature Contrast Height varies with the season, near 200 mb in summer, Height varies with the season, near 200 mb in summer, 300 mb in winter. Why? 300 mb in winter. Why? The atmosphere is thicker in the summer. Migrates north and south with the seasons. Migrates north and south with the seasons. Thermal contrast is strongest in the winter. Stronger in the winter than the summer. Why? Stronger in the winter than the summer. Why? Found on the 500 mb chart at approximately the 560 line Found on the 500 mb chart at approximately the 560 line (5560 meters). (5560 meters). Found near the -17 degree line on a 500 mb chart. Found near the -17 degree line on a 500 mb chart.

20 Jet Streams Subtropical Jet- Primary Cause: Primary Cause: Convergence aloft and Horizontal Temperature Contrast Height approximately 150 to 200 mb. Height approximately 150 to 200 mb. Location: between 25 and 30 Deg latitude, mean position Location: between 25 and 30 Deg latitude, mean position about 28 Deg. about 28 Deg. Stronger in the winter than the summer. Stronger in the winter than the summer. Confluence with the PFJ results in favorable conditions Confluence with the PFJ results in favorable conditions for severe turbulence. for severe turbulence. Diffluence of the two jets results in severe weather just Diffluence of the two jets results in severe weather just downstream of the jet max. Look for the wedge on a downstream of the jet max. Look for the wedge on a satellite picture (MCC) satellite picture (MCC) Conservation of Angular Momentum - causes maximums Conservation of Angular Momentum - causes maximums AM=m v r (mass constant, so r decreases, increasing v) AM=m v r (mass constant, so r decreases, increasing v)

21 Jet Streams Polar Front Jet and Subtropical Jet interaction - PFJ STJ


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