ch09 - Jon Ahlquist 10/18/2006 Chapter 9: Wind: Small-scale...

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Jon Ahlquist 10/18/2006 MET1010 Intro to the Atmosphere 1 Chapter 9: Wind: Scales of motion, turbulence Microscale sculpts the Earth Water waves excited by the wind Measuring surface and upper level winds Air flow over a lake Chinook Radar winds Monsoon and Santa Ana in SW United States Dust devils Wind power Scales of motion (pp. 250-251) ± Atmospheric disturbances exist in all sizes Table 9. 1, p. 223: Bigger things last longer Week or more Planetary “Long waves” Days - week 1000’s of km Days - week 100’s of km Tropical storms, hurricanes Hours 10’s of km to 100’s of km Land/sea breeze, chinook, Santa Ana Minutes to hours Meters to kilometers Dust devils, tornadoes, thunderstorms Sec to min cm to meters Small eddies Life Span Size What Names of Different Scales Fig. 9.1, p. 222 Microscale Mesoscale Synoptic scale (meters or less) (1-100 km) (1000’s km) Turbulence (pp. 223-226) ± Irregular, 3D swirls, imperfectly predictable ± Most weather and many ocean disturbances involve turbulence ± Mechanical turbulence due to flow over or around rough objects (mountains, building, etc.) p. 223-224 ± Thermal turbulence due to warm air rising and cold air sinking due to instability caused by surface heating and/or upper level cooling (infrared emission from cloud tops) p. 224 ± Clear Air Turbulence (CAT) due to wind shear,i.e., (See story, p. 221. Wear safety belt on plane! ) Microscale sculpts Earth’s surface (pp. 226-230) ± Wind can erode Earth’s surface (see picture on p. 190) We’ve already mentioned clay dust in the air serves as ice nuclei. ± Creates ripples in sand as well as sand dunes (fig. 9.8, p. 228) ± Wind shapes snow on ground, e.g., snow drifts ² Snow rollers: wind creates a roll of snow, just as a child might (fig. 9.9, p. 228) ± Wind can permanently bend trees (fig. 9.11) ± Windbreaks (shelterbelts): lines of trees to protect buildings or fields (fig. 9.12) Water waves (p. 230) ± Most water waves are due to wind ± Wave activity depends on ² wind speed ² length of time wind blows over water ² distance (“fetch”) wind blows over water ± Extreme example: 50 kt wind blowing for 3 days over 1600 miles or more of ocean can generate waves 50 feet high ± Big waves rare in Gulf of Mexico: all three factors (wind speed, time, & fetch) are limited. Exception: Hurricane in Gulf of Mexico can generate large waves.
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Jon Ahlquist 10/18/2006 MET1010 Intro to the Atmosphere 2 Measuring surface winds (pp. 233-234) “Surface” winds are measured at 10 meters (~30 feet) above the surface. The most common tools are
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ch09 - Jon Ahlquist 10/18/2006 Chapter 9: Wind: Small-scale...

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