NATS 101 Lecture 13 Curved Flow and Friction Local Diurnal Winds
NATS 101 Lecture 17 Curved Flow and Friction
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Transcript of NATS 101 Lecture 17 Curved Flow and Friction
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NATS 101
Lecture 17Curved Flow and Friction
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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)
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Recall: Uniform Circular Motion Requires Acceleration/Force
Initial Velocity
Final Velocity
Acceleration directed toward center of circleInitial
Velocity
Final Velocity
Circular Path
Circle Center
Centripetal (center seeking) acceleration is required for Centripetal (center seeking) acceleration is required for curved flow, i.e. to change the direction of the velocity vector! curved flow, i.e. to change the direction of the velocity vector!
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Flow Around Curved Contours
5700 m5640 m
Required Centripetal Acceleration
LL HHZero
Zero
Assume PGF constant size along entire channel
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Forces for Curved Flow
5700 m5640 m
Centripetal = CF + PGF
Centripetal << CF or PGF
Gradient Wind Balance
Wind
Wind
Geo W
indPGF
PGF
PGF
CF
CF
CF
Assume PGF constant size along entire channel
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Gradient Wind Balance
5700 m5640 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 W
ind
Wind Sp
eed
Incr
eases
Wind Speed
Decreases
Assume PGF constant size along entire channel
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Gradient Wind Balance
Speeds and Areas: Increase downwind of trough Decrease downwind of ridge
Wind Sp
eed
Incr
eases
Wind Speed
Decreases
Wind Speed
Decreases
5700 m5640 m
Area
Incr
eases
1
2
Assume PGF constant size along entire channel
Area
Decreases
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Divergence and Convergence
Parcel Shapes: Stretch downwind of trough Compress downwind of ridge
Area I
ncrea
ses
Diverg
ence
Area Decreases
Convergence
Assume PGF constant size along entire channel
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Divergence and Convergence
Diverg
ence
Net M
ass L
oss
Convergence
Net Mass Gain
Mass transport across channel
Large
Small
Assume PGF constant size along entire channel
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Gedzelman, p249
Vertical Motion
Mass Conservation leads to Mass Conservation leads to Upward motion beneath regions of divergence Upward motion beneath regions of divergence Downward motion beneath regions of convergenceDownward motion beneath regions of convergence
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ConvergenceConvergenceDivergenceDivergenceDivergenceDivergence
500mb WV Animation (Java applet)
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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
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Force of Friction 2
Pressure Gradient Force
Coriolis Force
Wind1004 mb
1008 mb
Because PGF becomes larger than CF, air parcel will turn toward lower pressure.
Friction Turns Wind Toward Lower Pressure.
Friction
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Force of Friction 3
PGFCF
Wind1004 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
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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 20o-30o from geostrophic).Rough surfaces (mtns) show the most slowing and turning (typically 30o-40o from geostrophic).
Mtns Water20o-30o
30o-40o
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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 km0.6 km
0.3 km
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Gedzelman, p249
Flow at Surface LowsLows and HighsHighs
Spirals OutwardDivergence
Spirals InwardConvergence
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upward motiondownward motion
Ahrens, Fig 6.21
Friction Induced Vertical MotionFriction Induced Vertical Motion
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Summary • Curved Flow
Requires Centripetal AccelerationDifference between PGF and Coriolis ForceSpeed Changes => Convergence-Divergence
• Frictional ForceCauses Winds to Turn toward Low Pressure Important in the lowest 1 km above the Surface Leads to Convergence-Divergence
• Curvature and FrictionLeads to Vertical Motions
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Next Lecture • Local Winds with Diurnal or Shorter Times
Land-Sea BreezeMountain-Valley Breeze
• Charles Darwin was RightDarwin “Natural Selection” Award
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Assignment for Next LectureAssignment for Next Lecture • Reading - Reading - Ahrens pg Ahrens pg 167-181167-181• Problems - Problems - 7.3, 7.47.3, 7.4