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NATS 101 Lecture 17 Curved Flow and Friction

NATS 101 Lecture 17 Curved Flow and Friction. Supplemental References for Today’s Lecture. Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere . 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6). Recall: Uniform Circular Motion Requires Acceleration/Force. Circle Center.

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NATS 101 Lecture 17 Curved Flow and Friction

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  1. NATS 101Lecture 17Curved Flow and Friction

  2. Supplemental References for Today’s Lecture Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6)

  3. 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!

  4. 5640 m 5700 m Flow Around Curved Contours Assume PGF constant size along entire channel L H Zero Required Centripetal Acceleration

  5. 5640 m 5700 m Forces for Curved Flow Assume PGF constant size along entire channel PGF Wind PGF Geo Wind PGF CF CF Wind Centripetal = CF + PGF Centripetal << CF or PGF Gradient Wind Balance CF

  6. Wind Speed Increases 5640 m 5700 m Wind Speed Decreases Gradient Wind Balance Assume PGF constant size along entire channel Faster than Geo Wind Geo Wind Wind speeds are Slowest at trough Fastest at ridge Therefore, wind speeds Increase downwind of trough Decrease downwind of ridge Slower than Geo Wind

  7. Area Increases Wind Speed Increases 5640 m 5700 m Wind Speed Decreases Wind Speed Decreases Area Decreases Gradient Wind Balance Assume PGF constant size along entire channel 2 1 Speeds and Areas: Increase downwind of trough Decrease downwind of ridge

  8. Area Increases Divergence Area Decreases Convergence Divergence and Convergence Assume PGF constant size along entire channel Parcel Shapes: Stretch downwind of trough Compress downwind of ridge

  9. Divergence Net Mass Loss Convergence Net Mass Gain Divergence and Convergence Assume PGF constant size along entire channel Large Mass transport across channel Small

  10. Vertical Motion Gedzelman, p249 Mass Conservation leads to Upward motion beneath regions of divergence Downward motion beneath regions of convergence

  11. Convergence Divergence Divergence 500mb WV Animation (Java applet)

  12. Force of Friction 1 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.

  13. Force of Friction 2 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.

  14. Force of Friction 3 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.

  15. Force of Friction 4 1004 mb 30o-40o Mtns Water 20o-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 20o-30o from geostrophic). Rough surfaces (mtns) show the most slowing and turning (typically 30o-40o from geostrophic).

  16. Force of Friction 5 SFC 0.3 km 1004 mb 0.6 km ~1 km 1008 mb Friction is important in the lowest km above sfc. Its impact gradually decreases with height. By 1-2 km, the wind is close to geostrophic or gradient wind balance.

  17. Flow at Surface Lows and Highs Gedzelman, p249 Spirals Inward Convergence Spirals Outward Divergence

  18. Friction Induced Vertical Motion downward motion upward motion Ahrens, Fig 6.21

  19. Summary • Curved Flow Requires Centripetal Acceleration Difference between PGF and Coriolis Force Speed Changes => Convergence-Divergence • Frictional Force Causes Winds to Turn toward Low Pressure Important in the lowest 1 km above the Surface Leads to Convergence-Divergence • Curvature and Friction Leads to Vertical Motions

  20. Next Lecture • Local Winds with Diurnal or Shorter Times Land-Sea Breeze Mountain-Valley Breeze • Charles Darwin was Right Darwin “Natural Selection” Award

  21. Assignment for Next Lecture • Reading - Ahrens pg 167-181 • Problems - 7.3, 7.4

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