NATS Lecture 13 Curved Flow and Friction Local winds

Supplemental References for Today’s Lecture Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN ) Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN )

PGF PGF CF CF

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!

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

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

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

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

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

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

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

Convergence Divergence Divergence 500mb WV Animation (Java applet)

Sub-geostrophic Super-geostrophic

Convergence Divergenc e Divergence Convergence

Convergence Divergence Convergence Divergence

Now Add Friction near the surface… This changes the force balance

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 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

Force of Friction 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

Force of Friction 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

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

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

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

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

Atmospheric Scales of Motion Ahrens, Fig 7.1

Examples of Different Scales Danielson et al

Review: Thermally Direct Circulation Heat WarmColdRisingSinking DIV CON Heat

Sea Breeze Development (Courtesy of Mohan Ramamurthy, WW2010) 12 34

56 7 RisingSinking DIV CON Heat

Mature Sea Breeze isobars  Coriolis Turning Ahrens, Fig 7.4 PGF 2 km 

Coriolis Impact on Sea Breeze Danielson et al, Fig 11.15

Sea Breeze versus Land Breeze (Courtesy of Mohan Ramamurthy, WW2010) Stronger Temperature contrast during PM than during AM Sea breezes are stronger than land breezes PM AM LAX Airport 4 PM upper 7 AM lower

Sea Breeze Regular feature of many coastal areas California, Florida, Gulf Coast Occurs along large lakes-Great Lakes Typically strongest during Spring-Summer Can penetrate inland 50 km or more Temperatures can drop ~10 o C Nose of cool air can trigger thunderstorms Florida Satellite Loop

Lake Breeze Ahrens, Older Ed.

Mountain-Valley Breeze Ahrens, Older Ed. Sun warms slopes Density decreases Air rises IR cools slopes Density increases Air drains Mountain-Valley circulation important to Tucson Convection over Catalinas during PM summer. SE drainage flows during early AM all year.

Mountain-Valley Breeze Ahrens, Older Ed. Sun warms slopes Density decreases Air rises IR cools slopes Density increases Air drains RisingSinking DIVCON Heat CON DIV Mountain-Valley circulation important to Tucson Convection over Catalinas during PM summer. SE drainage flows during early AM all year.

Phoenix-Tucson Diurnal Winds 5 PM 5 AM PM heating AM cooling 5 PM5 AM PM heating AM cooling PM AMTUS PM AMPHX

Assignment for Next Lecture Reading - Ahrens pg Reading - Ahrens pg Problems - 7.3, 7.4, 7.5Problems - 7.3, 7.4, 7.5