1 / 37

Atmospheric Forces and Wind Dynamics

Learn about the Coriolis force, geostrophic wind, and the role of centripetal force in atmospheric motion. Discover how pressure gradients and friction influence wind direction and speed around high and low pressure systems.

mantone
Download Presentation

Atmospheric Forces and Wind Dynamics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Announcements Midterm exam #1 will be given back in class Friday. First homework assignment due next Monday.

  2. The Coriolis force: • Causes the wind to blow from high to low pressure • Causes the wind to blow perpendicular to lines of constant pressure • Causes apparent deflection of a ball in a rotating reference frame, like a merry-go-round. • All of the above

  3. Survey question: If you flush a toilet in the southern hemisphere: • The water will swirl counterclockwise. • The water will swirl clockwise. • The water can swirl either direction or may not even swirl at all. • I don’t know.

  4. Summary of Lecture 15 Newton’s first law of motion: an object will remain at rest and an object in motion will maintain a constant velocity if the net force is zero. Newton’s second law of motion: F = ma. Change acceleration by a change in speed or direction. The simplified equation of horizontal atmospheric motion has four force terms: pressure gradient force, Coriolis force, centripetal force, and friction. The pressure gradient force is due to the difference in pressure over a distance. The Coriolis force is an apparent force due to the rotation of the Earth, and depends on speed (of the wind) and latitude. It causes deflection from the reference point of an observer in a rotating frame. Coriolis force deflects the wind to the right or left depending on which hemisphere. Geostrophic wind occurs when the pressure gradient force balances the Coriolis force and the wind is parallel to the isobars. A good approximation for upper level winds.

  5. NATS 101 Section 4: Lecture 16 Why does the wind blow? Part II

  6. Last time we talked about two of the force terms in the simplified equation for horizontal air motion Geostrophic Balance: PRESSURE GRADIENT = CORIOLIS

  7. Simplified equation of horizontal atmospheric motion FOCUS ON LAST TWO THIS TIME… (1) (2) (3) (4)

  8. The centripetal force and friction force are typically much smaller, but they are very important for two reasons: Cause mass divergence and convergence Can be relatively large in special cases that are meteorologically important (i.e. cool)

  9. MASS DIVERGENCE MASS CONVERGENCE AIR RISING ABOVE AIR SINKING ABOVE INITIAL WIND FASTER WIND INITIAL WIND SLOWER WIND AIR SINKING BELOW AIR RISING BELOW MASS LOST MASS GAINED

  10. To begin a discussion of centripetal force, let’s address the popular belief about how water goes down the drain…

  11. Popular belief: The way the toilet flushes or the sink drains depends on which hemisphere you’re in. Bart vs. Australia Simpson’s episode: Bart calls an Australian boy to see if his toilet really does flush clockwise…We’ll see what the surprising answer is later.

  12. Centripetal Force = Arises from a change in wind direction with a constant speed (v) due to the curvature of the flow around a radius (r) Centripetal acceleration (a) (towards the center of circle) Center of circle -V1 V2 Final velocity a V2 V1 Initial velocity The centripetal acceleration is always directed toward the center of the axis of rotation. Note to be physically correct, the expression should have a negative sign, so +V2/r is actually the centrifugal acceleration.

  13. Centripetal Force CENTRIFUGAL FORCE You experience acceleration without a change in speed, for example, on a tilt-a-whirl carnival ride. The force is directed toward the center of the wheel. An equal an opposite (fictitious) centrifugal force is exerted by the inertia of your body on the wheel—so you stay put and don’t fall off even when upside down. CENTRIPETAL FORCE

  14. CENTRIPETAL ACCELERATION NEEDED ACCOUNT FOR THE CURVATURE OF THE FLOW WINDS IN GEOSTROPIC BALANCE

  15. Flow around curved height iso-lines Assume PGF constant size along entire channel Height 1 L Height 2 H Centripetal acceleration (towards low pressure) Centripetal acceleration (towards high pressure) When wind curves, it must have an centripetal acceleration towards the axis of rotation, so it is NOT geostrophic.

  16. Gradient Balance: Curved Flow PGF WIND AROUND LOW PRESSURE Centripetal + PGF = Coriolis WIND PGF WIND Height 1 PGF Cent. Height 2 Coriolis Cent. Coriolis WIND WIND AROUND HIGH PRESSURE Centripetal + Coriolis = PGF Coriolis

  17. The effect of curvature has curious—and counter intuitive--implication for winds around high and low pressure, if the pressure gradient is constant

  18. Changes in wind speed around highs and lows due to gradient balance WIND AROUND HIGH PRESSURE PGF = Centripetal + Coriolis Effectively INCREASES the pressure gradient force, Wind speeds up. WIND AROUND LOW PRESSURE Centripetal + PGF = Coriolis OR, better to think… PGF = Coriolis – Centripetal Effectively REDUCES the pressure gradient force Wind slows down.

  19. PGF WIND AROUND LOW PRESSURE Centripetal + PGF = Coriolis FASTEST WIND Height 1 PGF Cent. Height 2 Coriolis Cent. SLOWEST WIND WIND AROUND HIGH PRESSURE Centripetal + Coriolis = PGF Coriolis SLOWEST WIND AT THE BASE OF A TROUGH FASTEST WIND AT THE TOP OF THE RIDGE

  20. Because of the effect of centripetal force, winds increase to the east of trough and decrease to the east of a ridge. PGF FASTEST WIND Height 1 PGF Cent. WIND INCREASES Height 2 Coriolis WIND DECREASES Cent. SLOWEST WIND Coriolis THERE MUST BE COMPENSATING VERTICAL MOTION DUE TO CHANGES IN WIND SPEED AHEAD OF THE TROUGH AN RIDGE.

  21. MASS DIVERGENCE AND COVERGENCE AT UPPER LEVELS (DUE TO CURVATURE OF THE FLOW) MASS DIVERGENCE MASS CONVERGENCE Stratosphere (acts as a lid) Stratosphere (acts as a lid) INITIAL WIND FASTER WIND INITIAL WIND SLOWER WIND AIR RISING AIR SINKING AHEAD OF A TROUGH AHEAD OF A RIDGE

  22. Relationship between upper level troughs and ridges and vertical motion PGF FASTEST WIND Height 1 PGF WIND INCREASES MASS DIVERGENCE Cent. Height 2 Coriolis Cent. WIND DECREASES MASS CONVERGENCE SLOWEST WIND RISING MOTION AHEAD OF TROUGH SINKING MOTION AHEAD OF RIDGE Coriolis

  23. Relationship between upper level troughs and ridges and vertical motion UPPER LEVEL ~300 mb SURFACE Surface High Surface Low RISING MOTION MAY BE CONDITIONALLY UNSTABLE (if clouds form and air is saturated) SINKING MOTION TYPICALLY STABLE

  24. RIDGE SINKING MOTION RISING MOTION SINKING MOTION TROUGH

  25. SURFACE UPPER LEVEL RISING MOTION TROUGH SURFACE LOW (in Colorado) IS LOCATED DIRECTLY AHEAD OF TROUGH AT 300-MB, BECAUSE AIR IS RISING AHEAD OF THE TROUGH

  26. Gradient balance and flow around lows and highs (Northern Hemisphere) Cent. force Cent. force Counterclockwise flow around lows Clockwise flow Around highs

  27. Flow around low pressure NORTHERN HEMISPHERE SOUTHERN HEMISPHERE Counterclockwise flow Clockwise flow (because Coriolis force reverses with respect to wind direction)

  28. There is another force balance possibility if the Coriolis force is very small or zero, so it’s negligible. In that case, the pressure gradient force would balance the centripetal force.

  29. Cyclostrophic Balance PGF + centripetal force = 0 OR PGF = Centrifugal force L Pressure gradient balances the centrifugal force. Occurs where flow is on a small enough scale where the Coriolis force becomes negligible. Pressure gradient force Centrifugal force Important in (the really cool) meteorological phenomena that have really strong winds and tight pressure gradients!

  30. Examples of Cyclostrophic Flow TORNADOES HURRICANES And the flushing toilet, too!!

  31. The Great Mystery of the Flushing Toilet Solved! Centrifugal force PGF To Bart and Lisa: “A swirling, flushing toilet is in cyclostrophic balance, so the way it flushes depends more on the shape of the drain—and nothing to do with whether you’re in Australia or not!”

  32. One last force to consider… Friction

  33. Effect of Friction Force (at the surface) Friction acts to slow the wind at the surface The slower wind decreases the magnitude of the Coriolis force. Weaker Coriolis force no longer balances the pressure gradient force. Wind crosses the isobars, more toward the pressure gradient.

  34. Surface friction and flow around surface highs and lows Air curves outward away from surface high pressure Mass divergence and sinking motion. Air curves inward toward surface low pressure. Mass convergence and rising motion

  35. Zoom-in on surface low in Colorado from earlier.

  36. Summary of Force Balances:Why the wind blows

  37. Reading Assignment and Review Questions Reading: Chapter 9 Chapter 8 Questions Questions for Review (8th ed.): 15,16,17,18,21,22 (9th ed.): 16,17,18,19,22,23 Questions for Thought: 9,10,13,14,15

More Related