chapter_09 - Wind: Small Scale and Local Systems Chapter 9...

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Unformatted text preview: Wind: Small Scale and Local Systems Chapter 9 Chapter 9 Scales of Motion Scales of Motion Friction in the Boundary Layer Friction in the Boundary Layer ► Friction decreases with height, so wind speed increases with height 10 meter wind is often double the speed of the surface wind The friction layer (planetary boundary layer) may extend up to 1000 meters Friction in the Boundary Layer Friction in the Boundary Layer ► Friction within fluid flow Molecular Viscosity – slowing of a fluid due to the random motion of molecules moving at different speeds Eddy Viscosity – refers to the internal friction generated as laminar flow becomes irregular and turbulent as it passes over irregularities on the surface Turbulence in the Boundary Layer Turbulence in the Boundary Layer ► Mechanical Turbulence – eddy motions created by obstructions Produces frictional drag on the air flow that is much larger than molecular viscosity Big whirls have little whirls That feed on their velocity; And little whirls have lesser whirls, and so on to viscosity. -Lewis Fry Richardson Eddies – Big and Small Eddies – Big and Small ► An eddy forms when wind encounters a solid object ► The size and shape of the eddy depend on: Size and shape of the object Speed of the wind Turbulence in the Boundary Layer Turbulence in the Boundary Layer ► Thermal Turbulence – eddy motions created by surface heating and instability as thermals rise and convection cells form Reaches a maximum during peak daytime heating Can produce convective clouds http://apollo.lsc.vsc.edu/classes/met130/notes/chapter9/therm_turb.html Mechanical & Thermal Turbulence Mechanical & Thermal Turbulence ► Mechanical and Thermal turbulence occur together in the atmosphere Wind speed and surface heating both increase the eddy size http://apollo.lsc.vsc.edu/classes/met130/notes/chapter9/therm_turb.html Mechanical & Thermal Turbulence Mechanical & Thermal Turbulence ► Mechanical and Thermal turbulence occur together in the atmosphere There is an exchange of fast moving air from the upper levels with slower moving air near the surface ►Wind gusts ►Surface wind speed increases Mechanical & Thermal Turbulence Mechanical & Thermal Turbulence ► The depth of mechanical and thermal turbulent mixing in the boundary layer depend on: Degree of surface heating Strength of wind speed Rough or hilly landscape Eddies and Mountain Barriers Eddies and Mountain Barriers ► Consider wind moving over a mountain range in a stable atmosphere Turbulent Eddies Aloft Turbulent Eddies Aloft ► Turbulent eddies form aloft where wind changes speed or direction (wind shear) Eddies form in the upper troposphere near the jet stream Can produce CAT Determining Wind Direction & Speed Determining Wind Direction & Speed ► Wind is characterized by direction, speed, and gustiness ► Prevailing winds are Wind direction is the direction the wind is coming from not to! described by the direction most often observed Impact climate Impact pollution Impact city planning/building Topography can change the prevailing wind Measuring Wind Direction & Speed Measuring Wind Direction & Speed Wind Energy: Big Business! • From milliseconds up to a few minutes, forecasts can be used for the turbine active control. Such type of forecasts are usually referred to as very short-term forecasts • for the following 48-72 hours, forecasts are needed for the power system management or energy trading. They may serve for deciding on the use of conventional power plants (unit commitment) and for the optimization of the scheduling of these plants (economic dispatch). Regarding the trading application, bids are usually required during the morning of day d for day d+1 from midnight to midnight. These forecasts are called short-term forecasts • for longer time scales (up to 5-7 days ahead), forecasts may be considered for planning the maintenance of wind farms, or conventional power plants or transmission lines. For the specific case of offshore wind farms maintenance costs may be prohibitive, and thus an optimal planning of maintenance operations is of particular importance Local Wind Systems Local Wind Systems ► Thermal Circulation – circulation due to changes in air temperature (warm air rises, cold air sinks) Produces thermal high and low pressure areas ►Cooling high pressure ►Warming low pressure H L Thermal Circulation Thermal Circulation Sea & Land Breezes Sea & Land Breezes ► Thermal circulation ► Due to uneven heating rates of land and water ► Sea Breeze Land heats rapidly during the day Thermal low develops over the land Thermal high develops over the cooler water Breeze is directed from H to L pressure or Sea to Land ► Reverse the process for Land Breeze Sea or Lake Breeze Convergence Sea or Lake Breeze Convergence ► The leading edge is the sea breeze front Boundary between cool, moist air and hot, sultry air Wind shift High RH Clouds Fog Thunderstorms Local Winds & Water Local Winds & Water ► Wind speeds up and changes direction as it crosses a water body ► Less friction ► Increased speed means more coriolis deflection ► On the other side, surface roughness slows the wind down A Pile­Up on the Downwind Side A Pile­Up on the Downwind Side Winds & Seiches Winds & Seiches ► High winds, especially due to a strong low pressure system, can push the water to the far shores of a lake ► When the wind dies down the water oscillates back and forth ► This oscillation is called a Seiche 3 ft water level change Last ~8 hours Stirs nutrients Stirs pollutants Reverse flow in rivers http://www.seagrant.umn.edu/superior/processes Seasonally Changing Winds ­ Monsoon ► Monsoon – the seasonal reversal of winds ► Typical in eastern and southern Asia ► Similar to a large scale Sea and Land Breeze http://en.wikipedia.org/wiki/Image:Cherrapunji.jpg Monsoon not confined to India North American Monsoon Mountain and Valley Breezes Mountain and Valley Breezes ► Valley Breeze – upslope flow due to daytime heating of the slopes ► Mountain Breeze – downslope flow due to cooling of the slopes at night Katabatic Winds Despite downsloping, air is so cold that compressional warming does not warm up the air sufficiently! Chinook Winds: “The Snow Eater” Chinook Winds: “The Snow Eater” Santa Ana Winds http://www.noaawatch.gov/2008/santa_ana.php Haboob: Dust Storm Haboob: Dust Storm Dust Devils http://www.youtube.com/watch?v=tOITKe-H6HE Summary Summary ► Circulations in the atmosphere can be classified by size and time duration. The boundary layer is the layer of frictional influence and may extend upward about 1000 m. Mechanical and Thermal turbulence work together to create eddies in the atmosphere, these eddies cause high wind gusts. The Sea and Land Breezes are local thermal circulations created by the uneven heating of land and water. The Monsoon is a seasonal reversal of winds, similar to the sea breeze, but on a larger scale. Height of the boundary layer depends on the instability, wind speed, and the surface roughness Microscale, mesoscale, synoptic scale, global scale ► ► ► ► Summary Summary ► The Mountain and Valley Breezes are local winds that blow down or upslope depending on heating or cooling of the slopes. ► A Katabatic wind is a cold, dry downslope wind ► A Chinook wind is a warm, dry wind that descends the eastern slope of the Rockies ► The Santa Ana winds is a warm, dry, and strong downslope wind that blows into Southern California ...
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This note was uploaded on 04/18/2011 for the course EAS 107 taught by Professor Pan,z during the Spring '08 term at Saint Louis.

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