Jon Ahlquist 10 18 2006 Chapter 9 Wind Small scale local systems Scales of motion pp 250 251 Scales of motion turbulence Microscale sculpts the Earth Water waves excited by the wind Measuring surface and upper level winds Thermal circulations sea breeze monsoon Air flow over a lake Chinook Radar winds Monsoon and Santa Ana in SW United States Dust devils Wind power Names of Different Scales Fig 9 1 p 222 Synoptic scale 1000 s km 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 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 MET1010 Intro to the Atmosphere Size Life Span Small eddies cm to meters Sec to min Dust devils tornadoes thunderstorms Land sea breeze chinook Santa Ana Meters to kilometers 10 s of km to 100 s of km Minutes to hours Hours Tropical storms hurricanes 100 s of km Days week H L areas on TV weather 1000 s of km Days week Long waves Week or more Planetary 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 Water waves p 230 Most water waves are due to wind Wave activity depends on wind speed of time wind blows over water distance fetch wind blows over water length Snow What Turbulence pp 223 226 Microscale Mesoscale meters or less 1 100 km Atmospheric disturbances exist in all sizes Table 9 1 p 223 Bigger things last longer 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 1 Jon Ahlquist 10 18 2006 Measuring surface winds pp 233 234 Measuring upper level winds p 234 Surface winds are measured at 10 meters 30 feet above the surface The most common tools are pictured below Techniques Cup anemometer wind vane Fig 9 18 p 233 Aerovane speed direction Fig 9 19 p 234 Track radiosonde balloons or pilot balloons Track clouds in satellite pictures Get automated readings from equipment packages installed 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 antenna at Haskell OK Profilers www profiler noaa gov National Profiler Network over central 1 3 of US monitors thunderstorm winds National Weather Service National Profiler Network has 35 profilers Map on next slide 32 3 in in middle third of US 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 Thermal 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 WARM Land 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 COOL COOL occurs during daytime MET1010 Intro to the Atmosphere WARM occurs during night 2 Jon Ahlquist 10 18 2006 Florida sea breezes often converge from W E over central FL p 240 Sea breezes from 2 sides fig 9 24 p 239 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 Florida is not the only place to get sea breezes from two sides This example is from Michigan Monsoon pp 240 242 Fig 9 27 Like sea land breeze but much larger seasonal Winter monsoon land cool Summer monsoon land hot divergence and sinking where wind leaves shore convergence and rising where wind reaches shore Fig 9 26 p 239 Less friction over water Slower Faster wind Air sinks to replace air rapidly moving out Slower Air piles up near coastline 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 MET1010 Intro to the Atmosphere Cherrapunji rainfall records wettest year 87 FEET Apr Oct 1865 and wettest month 30 5 feet July 1861 Chinook pp 244 245 Wind flow 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 f hn Two kinds of chinooks with and without rain 3 Jon Ahlquist 10 18 2006 Chinook with rain fig 9 33 p 244 Air flows up a mountain The air expands as it moves up into lower pressure Expansion involves work which cools the air The cooled air reaches saturation and condensation forms releasing latent heat into air on uphill side Chinook with rain cont The cooling experienced by the air as it goes up the mountain is reversed as the air is compressed going down the mountain Net effect on the air after passing over the mountain air is fairly dry because moisture rained out on the upwind side air is warm
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