Chapter 9 Winds: Small scale and local systems

1. Chapter 9 Winds: Small scale and local systems

2. Scales of motion • Smallest - microscale (few meters or less) • Middle - Mesoscale (few to about 100 km) • Large - Synoptic scale (hundreds to thousands of km) • Global - Planetary scale (tens of thousands of km) • -sometimes synoptic and planetary are combined • -macroscale

3. Scales of Motion Winds and whirling eddies evident at the microscale and mesocale are no longer observed at the synopitic scale. Macroscale winds that act at the planetary level include the Jet Stream.

4. Friction and turbulence in boundary layer • friction of fluid flow is called viscosity • slowing of air due to random motion of gas molecules is called molecular viscosity • if molecular viscosity were the only type of friction acting on moving then the friction would disappear quickly above the surface • friction produced by turbulent eddies is called eddy viscosity • eddies created by air blowing over/around/into obstructions • eddies created by these obstructions are called mechanical turbulence • turbulence that is caused by surface heating and instability is called thermal turbulence

5. Turbulence & Eddies Surface obstructions slow surface winds, which due to viscosity creates eddies of whirling winds in a much thicker layer. The vertical depth of these eddies and gusting winds thickens with surface wind velocity.

6. Scales of Eddies Whirling eddies of wind are generated at various scales and orientations. Here are horizontal whirls leeward of an obstructing mountain and vertical rotors, or roll eddies, that can extend for several kilometers.

7. Turbulent eddies • Form aloft as well as near the surface • Turbulence aloft can occur suddenly and unexpectedly • Occur where winds changes direction and/or speed abruptly • This is called wind shear • - shearing produces eddies along the mixing zone • - if it occurs in cloud-free air it is called clear-air turbulence Shear boundary Air Flow Small shear Increasing shear Boundary deforms Waves appear Turbulent eddies break

8. Friction and Mixing • friction of air flow is a result of the exchange of air molecules moving at different speeds • this exchange is quite small compared to turbulent motions • frictional effect of the surface on moving air depends mainly on mechanical and thermal turbulent mixing • the depth of mixing depends on three factors • - surface heating • - strong winds • - rough/hilly landscape

9. Wind Profiles Changes in air temperature causing warm air to rise and cool air to sink can also generate horizontal winds. Rising warm air creates a surface low and upper level high. Sinking cool air creates a surface high and upper level low.

10. Thermal Circulations Solar heating and radiational cooling of earth's surface generates cold-core thermal highs and warm-core thermal lows. Winds can circulate between these two systems.

11. Cold & Warm Cores Land heats more quickly than water, creating land-water temperature differences along a coastline. During the day the land's warm-core thermal low draws a sea breeze, while at night, the warmer sea draws a land breeze.

12. Sea & Land Breezes Opposing breezes may converge on an isthmus of land, and this rising moist unstable air will trigger thunderstorms. The leading edge of land-water breezes can bring rapid changes in humidity or pollutants.

13. Sea Breeze

14. Sea Breeze

15. Water, Friction, & Wind Speed Seasonal reversal of monsoon winds in southern Asia is explained by continental temperature shifts. Summer monsoon depressions of low pressure and rains are enhanced by insolation, latent heat of condensation, and jet stream patterns.

16. Seasonal Wind Changes Monsoon like winds develop in July across the southwestern U.S. region. As the continental interior heats and rises, humid Gulf air sweeps in creating instability and thunderstorms.

17. Mountain and Valley Breezes Solar heating of hill slopes creates air that is warmer and less dense than air at the same elevation over the valley, and generates rising valley breezes. Nocturnal radiational cooling of the mountain slope creates relatively denser air that sinks as a mountain breeze, or gravity wind.

18. Valley & Mountain Breezes Daily warming of mountain slopes that trigger valley breezes of moist air may result in cumulus clouds above mountain summits.

19. Katabatic Wind Mountain snow cover creates a thin layer of high pressure cold air that rushes into lower valleys. Elevated plateaus with snow cover may foster development of a thin layer of high pressure cold air. Pressure gradient winds are triggered due to lower pressure above the adjacent valley, pushing cold air into the lower valley.

20. Chinook Winds Westerly winds that condense and precipitate their moisture when ascending the Rockies, and then compressionally warm and dry when descending, are described as chinook winds.

21. Santa Ana Winds Easterly winds that descend from southern California's elevated desert plateau are compressionally heated and lowered in humidity. Many fires have been triggered by winds of this type.

22. Desert Winds Strong desert winds can lift dust particles in the air, creating sandstorms, or haboobs.

23. Dust Devils On a hot, dry day solar radiation generates predominantly sensible heat and rising thermals of air.