1. NATS 101 Lecture 17 Curved 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 Initial Velocity Final Velocity Acceleration directed toward center of circle Initial Velocity Final Velocity Circular Path Circle Center Centripetal (center seeking) acceleration is required for curved flow, i.e. to change the direction of the velocity vector!

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

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

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

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

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

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

10. Gedzelman, p249 Vertical Motion 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 Coriolis Force Geostrophic Wind 1004 mb 1008 mb 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. Friction

13. Force of Friction 2 Pressure Gradient Force Coriolis Force Wind 1004 mb 1008 mb Because PGF becomes larger than CF, air parcel will turn toward lower pressure. Friction Turns Wind Toward Lower Pressure. Friction

14. Force of Friction 3 PGF CF Wind 1004 mb 1008 mb 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. Fr

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

16. Force of Friction 5 1004 mb 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. SFC ~1 km 0.6 km 0.3 km

17. Gedzelman, p250 Ekman Spiral Speed and direction change with height. Wind direction turns clockwise with height in the NH. Wind speeds increase with height. Wind gets close to geostrophic/gradient wind balance

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

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

20. 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

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

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