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Unformatted text preview: Wind: Small-Scale & Local Systems Wind: This chapter discusses: 1. Different spatial scales of wind movement and Different its impact on earth's surface its 1. Specific winds named for their geographic Specific locality locality 1. Measurement and recording of winds Scales of Motion Scales Figure 10.1A Figure 10.1B Figure 10.1C Winds and whirling eddies evident at the microscale and Winds mesocale are no longer observed at the synopitic scale. mesocale Macroscale winds that act at the planetary level include Macroscale the Jet Stream. the Turbulence & Eddies Turbulence
Surface Surface obstructions slow surface winds, which due to viscosity creates eddies of whirling winds in a much thicker layer. thicker The vertical The depth of these eddies and gusting winds thickens with surface wind velocity. velocity. Figure 10.2A Planetary Boundary Layer Planetary
The layer of air The influenced by surface friction is called the planetary boundary layer (PBL). layer The mixing depth of the The PBL can increase as: PBL a) air becomes more a) unstable (e.g. heats) b) terrain roughens c) wind speeds increase c)
Figure 10.3 Scales of Eddies Scales Figure 10.5 Figure 10.4 Whirling eddies of wind are generated at various scales and Whirling orientations. orientations. Here are horizontal whirls leeward of an obstructing mountain Here and vertical rotors, or roll eddies, that can extend for several kilometers. kilometers. Wind Forces & Concern Wind Figure 10.6 Reverse flows of wind, triggered by strong eddies, can upset ground Reverse traffic as well as aircraft. Clear air turbulence (CAT) refers to invisible wind shear hazards aloft. invisible Wind and Exposed Soil Wind Figure 10.7 Microscale winds can bounce and skip sand grains, expose desert Microscale pavement, erode ventifact patterns into rocks, and shape sand dunes with ripples revealing wind direction. with Wind & Snow Surfaces Wind Figure 10.8 Figure 10.9 Turbulent eddies of wind can trigger rolling of moist snow to Turbulent create cylindrical snow rollers. create Snow fences are used to accumulate snow so that it later melts to Snow replenish local groundwater supplies. replenish Wind & Vegetation Wind Figure 10.11 Figure 10.10 Wind speeds increase with elevation above earth's frictional Wind surface, where stronger winds sculpt and denude the branches of trees. trees. Shelterbelts may protect downwind property, but may also create Shelterbelts unwanted turbulent eddies. unwanted Wind & Waves Wind Figure 10.12 Waves of water are generated by surface friction and eddies that Waves force winds and water upward and downward. force Wave height increases with wind speed, wind duration, and fetch Wave (e.g. distance) of open water. (e.g. Wind Direction & Origin Wind Figure 10.13 Wind direction is typically given as the origin from which it blows, Wind such as "westerlies" coming out of the west. such Counter-examples use the adverbs 'on' and 'up' to indicate Counter-examples direction, such as 'onshore wind', and "upslope wind" for wind moving uphill. moving Charting Prevailing Winds Charting Figure 10.15 Figure 10.14 Winds shift in direction, noted by degrees or compass direction, Winds and knowledge of prevailing wind directions are useful in planning. planning. A wind rose of likely directions might be used to orient runways wind and smoky power plants. and Wind Instruments Wind Figure 10.17 Figure 10.18 Figure Wind vanes point into the wind, or toward its origin, while revolutions Wind of a cup anemometers record wind distance and speed. Aerovanes can measure both speed and direction. Pilot balloons observed with theodolites, infrared radiometers, and lidar are other wind detection tools. tools. Doppler radar detects changes in wind speed within large volumes of Doppler atmosphere.The vertical profile may extend 16km, and can help identify developing storms and tornadoes. Wind Profiles Wind Figure 10.19A Figure 10.19B Changes in air temperature causing warm air to rise and cool air to Changes sink can also generate horizontal winds. sink Rising warm air creates a surface low and upper level high. Sinking cool air creates a surface high and upper level low. Thermal Circulations Thermal Figure 10.20 Solar heating and radiational cooling of earth's surface generates Solar cold-core thermal highs and warm-core thermal lows. cold-core Winds can circulate between these two systems. Cold & Warm Cores Cold Figure 10.21A Land heats more quickly than water, creating land-water Land temperature differences along a coastline. temperature During the day the land's warm-core thermal low draws a sea During breeze, while at night, the warmer sea draws a land breeze. breeze, Sea & Land Breezes Sea
Opposing Opposing breezes may converge on an isthmus of land, and this rising moist unstable air will trigger thunderstorms. thunderstorms. The leading edge The of land-water breezes can bring rapid changes in humidity or pollutants. pollutants. Figure 10.22 Breeze Convergence & Fronts Breeze Figure 10.23 Water is smoother than the land surface, permitting increases in Water wind velocity. wind These increased speeds mean 1) a greater Coriolis force and These deflection, and 2) divergence and sinking of air at the upwind water surface with convergence at the downwind end. surface Water, Friction, & Wind Speed Water, Figure 10.24 Figure 10.25A Seasonal reversal of monsoon winds in southern Asia is explained Seasonal by continental temperature shifts. by Summer monsoon depressions of low pressure and rains are Summer enhanced by insolation, latent heat of condensation, and jet stream patterns. El Nino Southern Oscillation is also known to affect monsoon intensity. affect Seasonal Wind Changes Seasonal
Monsoon like winds Monsoon develop in July across the southwestern U.S. region. region. As the continental As interior heats and rises, humid Gulf air sweeps in creating instability and thunderstorms. and Figure 10.26 Southwest Monsoon Southwest Figure 10.27 Solar heating of hill slopes creates air that is warmer and less dense Solar than air at the same elevation over the valley, and generates rising valley breezes. valley Nocturnal radiational cooling of the mountain slope creates Nocturnal relatively denser air that sinks as a mountain breeze, or gravity wind. wind. Valley & Mountain Breezes Valley Figure 10.28 Daily warming of mountain slopes that trigger valley breezes of Daily moist air may result in cumulus clouds above mountain summits. summits. Katabatic Wind Katabatic Mountain snow cover creates a thin layer of high pressure cold air that Mountain rushes into lower valleys. rushes Elevated plateaus with snow cover may foster development of a thin layer of Elevated high pressure cold air. high Pressure gradient winds are triggered due to lower pressure above the Pressure adjacent valley, pushing cold air into the lower valley. adjacent Figure 10.29 Chinook Winds Chinook Figure 10.30 Westerly winds that condense and precipitate their moisture when Westerly ascending the Rockies, and then compressionally warm and dry when descending, are described as chinook winds. when Chinook Wall Cloud Chinook Figure 10.31 Chinook wall clouds are an indicator of their presence. Santa Ana Winds Santa Figure 10.32 Easterly winds that descend from southern California's elevated Easterly desert plateau are compressionally heated and lowered in humidity. desert Many fires have been triggered by winds of this type. Haboob Winds Haboob Figure 10.33 Desert winds of dust and sand may form from cold downdrafts Desert along the leading edge of a thunderstorm. along These winds are found in the African Sudan and Arizona Sonora These deserts. Dust Devils Dust Figure 10.34 On a hot, dry day solar radiation generates predominantly sensible On heat and rising thermals of air. heat Dust Devils Dust Cyclonic winds then entrain this Cyclonic rising air and associated loose material to create a rising column of dust and debris. column Figure 10.35 ...
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- Spring '09