Jon Ahlquist 10/18/2006MET1010 Intro to the Atmosphere 1Chapter 9: Wind:Small-scale & local systemsScales of motion, turbulenceMicroscale sculpts the EarthWater waves excited by the windMeasuring surface and upper level windsThermal circulations: sea breeze & monsoonAir flow over a lakeChinookRadar windsMonsoon and Santa Ana in SW United StatesDust devilsWind powerScales of motion (pp. 250-251) Atmospheric disturbances exist in all sizesTable 9. 1, p. 223: Bigger things last longerWeek or morePlanetary“Long waves”Days - week1000’s of kmH & L areas on TV weatherDays - week100’s of kmTropical storms, hurricanesHours10’s of km to 100’s of kmLand/sea breeze, chinook, Santa AnaMinutes to hoursMeters to kilometersDust devils, tornadoes, thunderstormsSec to mincm to metersSmall eddiesLife SpanSizeWhatNames of Different ScalesFig. 9.1, p. 222Microscale 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., change in wind speed &/or direction. p. 226(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 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 wind speed length of time wind blows over water distance (“fetch”) wind blows over water Extreme example: 50 kt wind blowing for 3 days over 1600 miles or more of ocean can generatewaves 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.Jon Ahlquist 10/18/2006MET1010 Intro to the Atmosphere 2Measuring surface winds (pp. 233-234)“Surface” winds are measured at 10 meters (~30 feet) above the surface. The most common tools are pictured below.Cup anemometer & wind vane Aerovane (speed & direction)Fig. 9.18, p. 233 Fig. 9.19, p. 234Measuring upper-level winds (p. 234)Techniques: Track radiosonde balloons or pilot balloons Track clouds in satellite pictures Get automated readings from equipment packagesinstalled on many commercial aircraft (ACARS) Profiler: special continuously operating Doppler radar that gives speed & direction of wind at various levels from near surface up to 10 miles (16 km).Profiler antennaat Haskell, OKProfilers: www.profiler.noaa.gov National Weather Service “National Profiler Network” has 35 profilers. (Map on next slide.) 32 in in middle third of US 3 in Alaska Measure winds from 500 m to 16 km (10 miles) every 250 m continuously in time (reporting hourly). Particularly helpful in monitoring thunderstorms. In operation since 1988, continuously since 1992 Other agencies operate profilers, too: Environmental Protection Agency (EPA), Federal Aviation Admin (FAA), Dept of Energy (DOE), Dept of Defense (DOD), etc.National Profiler Network over central 1/3 of US monitors thunderstorm windsThermal Circulations: Sea and Land Breezes (pp. 236-240) During daytime, land is warmer than water. Thunderstorms occur over land where air rises. Wind blows from sea (“sea breeze”) to land.occurs during daytimeWARMCOOLLand Breeze at Night (p. 237) Land cools off more at night, so now water is warmer than land. Thunderstorms occur over water where air rises. Wind blows from land (“land breeze”) to sea.occurs during nightWARMCOOLJon Ahlquist 10/18/2006MET1010 Intro to the Atmosphere 3Florida sea breezes often converge from W & E over central FL (p. 240) Sea breezes from 2 sides (fig. 9.24, p.239) Florida is not the only place to get sea breezes from two sides. This example is from Michigan.Air Flow Over A Lake (pp. 239-240) Clearer sky where wind is leaving shore Cloudier & more precipitation where wind comes on shore More friction (slower wind) over land, less friction (faster wind) over water causes: divergence and sinking where wind leaves shore convergence and rising where wind reaches shoreLess friction over waterSlower Faster wind SlowerAir sinks to Air “piles up”replace air near coastlinerapidly moving outFig. 9.26, p. 239Monsoon (pp. 240-242): Fig 9.27 Like sea/land breeze but much larger & seasonalWinter monsoon: land cool Summer monsoon: land hotCherrapunji rainfall records: wettest year (87 FEET, Apr-Oct 1865) and wettest month (30.5 feet, July 1861)Monsoon also in southwest US (p. 242) In summer, southwest US gets hot. “Thermal low” develops at surface, i.e., low pressure caused by same thermal process that causes low pressure over hot land in other contexts, like sea breeze.See fig. 9.22, p. 237. Air is pushed from higher pressure over the waters by Baja California toward the low pressure over the SW. Moist, warm air rises over SW US, creating rain, as seen in red in the enhanced satellite picture shown at right (fig. 9.29, p. 242).Wind flowChinook (pp. 244-245) Read these notes about the chinook before studying the text. The book mentions the two kinds of chinooks (with and without rain) but could do a better job distinguishing them. Chinook: warm, dry wind flowing down east side of Rockies. Named after Chinook Indian tribe in Pacific Northwest. Similar dry winds blowing down mountains exist elsewhere in the world and have their own regional names. Examples: two of the German names for chinook are “schneefresser” (“snow eater”) and “foehn,” also spelled föhn. Two kinds of chinooks: with and without rainJon Ahlquist 10/18/2006MET1010 Intro to the Atmosphere 4Chinook with rain (fig. 9.33, p. 244) Air flows up a
View Full Document
Unlocking...