NATS 101 Lecture 17 Curved Flow and Friction. Supplemental References for Today’s Lecture...
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Transcript of NATS 101 Lecture 17 Curved Flow and Friction. Supplemental References for Today’s Lecture...
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
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!
Flow Around Curved Contours
5700 m5640 m
Centripetal Acceleration is Required for Air Parcel to Curve
LL HHZer
Zer
oo
Assume PGF constant size along entire channel
Forces for Curved Flow
5700 m5640 m
Centripetal = CF + PGF
Centripetal << CF or PGF
Gradient Wind Balance
Wind
Wind
Geo
Win
dPGF
PGF
PGF
CF
CF
CF
Assume PGF constant size along entire channel
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
Win
d
Win
d Spe
ed
Incr
ease
s
Wind Speed
Decreases
Assume PGF constant size along entire channel
Gradient Wind Balance
Speeds and Areas: Increase downwind of trough Decrease downwind of ridge
Win
d Spe
ed
Incr
ease
s
Wind Speed
Decreases
Wind Speed
Decreases
5700 m5640 m
Area
Incr
ease
s
1
2
Assume PGF constant size along entire channel
Area
Decreases
Divergence and Convergence
Parcel Shapes: Stretch Downwind of Trough so Area Increases Compress Downwind of Ridge so Area Decreases
Area I
ncrea
ses
Diver
gence
Area Decreases
Convergence
Assume PGF constant size along entire channel
Divergence: Horizontal Area Increases with Time Convergence: Horizontal Area Decreases with Time
Divergence and Convergence
Diver
gence
Net M
ass L
oss
Convergence
Net Mass G
ain
Mass transport across channel
Large
Small
Assume PGF constant size along entire channel
Gedzelman, p249
Vertical Motion
Mass Conservation leads toMass 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
ConvergenceConvergence
DivergenceDivergenceDivergenceDivergence
500mb WV Animation (Java applet)
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
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
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
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).
MtnsWater
20o-30o
30o-40o
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
Gedzelman, p249
Flow at Surface LowsLows and HighsHighs
Spirals OutwardDivergence
Spirals InwardConvergence
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
Next Lecture
• Local Winds with Diurnal or Shorter Times
Land-Sea Breeze
Mountain-Valley Breeze• Charles Darwin was Right (maybe?)
Darwin “Natural Selection” Award• Stephen will lecture next week (and
maybe the following Monday)
I am on travel and APR duty