1 / 46

NATS 101 Lecture 13 Curved Flow and Friction Local Diurnal Winds

NATS 101 Lecture 13 Curved Flow and Friction Local Diurnal Winds. Mullen’s 1 st Law of NATS 101 Clicker=Points. RUC surface analysis for 1800 UTC Mar 01, 2010 that shows winds blowing from high to low pressure.

byron-osborne
Download Presentation

NATS 101 Lecture 13 Curved Flow and Friction Local Diurnal Winds

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. NATS 101Lecture 13Curved Flow and FrictionLocal Diurnal Winds Mullen’s 1st Law of NATS 101 Clicker=Points RUC surface analysis for 1800 UTC Mar 01, 2010 that shows winds blowing from high to low pressure

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

  3. Geostrophic Wind and Upper Level Charts Winds at upper levels are pretty close to being geostrophic: Wind is parallel to isobars Wind strength dependents on how close together isobars are PRESSURE GRADIENT FORCE CORIOLIS FORCE GEOSTROPHIC WIND

  4. Simplified equation of horizontal atmospheric motion (1) (2) (3) (4) FOCUS ON LAST TWO TODAY… • GEOSTROHIC BALANCE LAST TIME…

  5. 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)

  6. 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

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

  8. 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.

  9. 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.

  10. 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

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

  12. Recall: Uniform Circular Motion Requires Acceleration/Force Circle Center Final Velocity Acceleration directed toward center of circle Circular Path Final Velocity Initial Velocity Initial Velocity Centripetal (center seeking) acceleration is required for curved flow, i.e. to change the direction of the velocity vector!

  13. 5640 m 5700 m Flow Around Curved Contours Assume PGF constant size along entire channel Height 1 Height 2 L H Zero Centripetal Acceleration is Required for Air Parcel to Curve

  14. 5640 m 5700 m Flow Around Curved Contours Assume PGF constant size along entire channel Height 1 Height 2 L H Zero Centripetal Acceleration How does atmosphere produce the necessary centripetal force?

  15. 5640 m 5700 m Forces for Curved Flow Assume PGF constant size along entire channel PGF Wind PGF Height 1 Geo Wind Height 2 PGF CF CF Wind Centripetal= PGF +CF Centripetal<< PGF orCF Gradient Wind Balance CF

  16. Simplified equation of atmospheric motion Gradient Wind Balance   (1) (2) (3) (4) GRADIENT WIND BALANCE…

  17. Wind Speed Increases 5640 m 5700 m Wind Speed Decreases Gradient Wind Balance: End Result Assume PGF constant size along entire channel Faster than Geo Wind Height 1 Geo Wind Height 2 Wind speeds are Slower at trough Faster at ridge Slower than Geo Wind Therefore, wind speeds Increase downwind of trough Decrease downwind of ridge

  18. Wind Speed Increases Area Increases Wind Speed Decreases Wind Speed Decreases Area Decreases Gradient Wind Balance Assume PGF constant size along entire channel Height 1 Height 2 2 1 1 2 Speeds and Areas: Increase downwind of trough Decrease downwind of ridge

  19. Area Increases Divergence Area Decreases Convergence Divergence and Convergence Assume PGF constant size along entire channel Height 1 Height 2 Parcel Shapes: Stretch Downwind of Trough so Area Increases Compress Downwind of Ridge so Area Decreases Divergence: Horizontal Area Increases with Time Convergence: Horizontal Area Decreases with Time

  20. Divergence Net Mass Loss Convergence Net Mass Gain Divergence and Convergence Assume PGF constant size along entire channel Height 1 Height 2 Large Mass transport across channel Small THERE MUST BE COMPENSATING VERTICAL MOTION DUE TO CHANGES IN WIND SPEED AHEAD OF THE TROUGH AND 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 DOWNWIND OF A TROUGH UPWIND OF A RIDGE UPWIND OF A TROUGH DOWNWIND OF A RIDGE

  22. Relationship between upper-level troughs-ridges and vertical motion Ridge Ridge Trough JET LEVEL ~300 mb SURFACE Surface High Surface Low Gedzelman, p249 RISING MOTION MAY BE CONDITIONALLY UNSTABLE (if clouds form and air is saturated) SINKING MOTION TYPICALLY STABLE (clear skies likely)

  23. Faster than geostrophic Ridge Slower than geostrophic Trough

  24. Divergence Divergence Convergence Trough Trough Convergence

  25. Convergence Divergence Divergence Convergence

  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 possible 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. Simplified equation of atmospheric motion Cyclostrophic Balance    (1) (2) (3) (4) CYCLOSTROPHIC BALANCE…

  30. 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 for understanding (really cool) meteorological phenomena that have extremely strong winds and tight pressure gradients!

  31. TORNADOES Examples of Cyclostrophic Flow HURRICANES And flushing toilets, too!!

  32. The Unsolved 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!”

  33. One last force to consider… Friction

  34. Friction Pressure Gradient Force 1004 mb Friction Geostrophic Wind 1008 mb Coriolis Force Frictional Force is directed opposite to velocity. It acts to slow down (decelerate) the wind. Once the wind speed becomes slower than the geostrophic value, geostrophic balance is destroyed because the Coriolis Force decreases.

  35. Friction Pressure Gradient Force 1004 mb Wind Friction 1008 mb Coriolis Force Because PGF becomes larger than CF, air parcel will turn toward lower pressure. Friction Turns Wind Toward Lower Pressure.

  36. Friction 1004 mb Wind CF PGF 1008 mb Fr Eventually, a balance among the PGF, Coriolis and Frictional Force is achieved. PGF + CF + Friction = 0 Net force is zero, so parcel does not accelerate.

  37. Friction 1004 mb 30o-50o Mtns Water 10o-30o 1008 mb The decrease in wind speed and deviation to lower pressure depends on surface roughness. Smooth surfaces (water) show the least slowing and turning (typically 10o-30o from geostrophic). Rough surfaces (mtns) show the most slowing and turning (typically 30o-50o from geostrophic).

  38. Friction SFC 0.3 km 1004 mb 0.6 km ~1 km 1008 mb Friction is important in the lowest km above surface. Its impact gradually decreases with height. By 1-2 km, the wind is close to geostrophic balance, gradient wind balance, or cyclostrophic.

  39. www.met.tamu.edu

  40. Flow in SurfaceLowsandHighs Gedzelman, p249 Spirals Inward Convergence Spirals Outward Divergence

  41. MASS DIVERGENCE AND CONVERGENCE AT SURFACE (DUE TO THE FORCE OF FRICTION) MASS DIVERGENCE MASS CONVERGENCE AIR RISING AIR SINKING H L INITIAL WIND FASTER WIND INITIAL WIND SLOWER WIND Ground is a solid barrier Ground is a solid barrier Flow into Lows Flow out of Highs

  42. Friction Induced Vertical Motion Ahrens, Fig 6.22 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

  43. Divergence Convergence Convergence Divergence Surface Convergence and Divergence

  44. Summary of Force BalancesWhy the Wind Blows

  45. Assignment for Next LectureLocal Winds, Monsoons • Reading -Ahrens 3rd: Pg 165-178 4th: Pg 167-181 5th: Pg 169-184 • Homework06 -D2L (Due Monday Mar. 22) 3rd-Pg 194: 7.3, 4, 5 4th-Pg 198: 7.3, 4, 5 5th-Pg 200: 7.3, 4, 5 Do Not Hand in 7.3

  46. Assignment for Next Lecture QUIZ 2 this Thursday D2L only Similar format as before 7 am -12 pm time period for the exam 70 minutes to finish test

More Related