Transcript chapter 8 wind systems

CHAPTER 8
WIND SYSTEMS
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General refers to the average air flow, actual
winds will vary considerably
Average conditions help identify driving
forces
The basic cause of the general circulation is
unequal heating of the earth’s surface
◦ Warm air is transferred from the tropics to the
poles
◦ Cool air is transferred from the poles to the
tropics
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Single Cell Model
Assume
1. Uniform water surface
2. Sun always directly overhead the equator
3. Earth does not rotate
Result: huge thermally direct convection cell (Hadley)
Three Cell Model
Allow earth to spin = three cells (Hadley,
Ferrell, Polar)
Alternating belts of pressure starting with L at
equator
Alternating belts of wind with NE just north of
equator
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First, consider a nonrotating earth that’s
completely covered with ocean
The PGF is really the only force driving the
winds
http://veimages.gsfc.nasa.gov/643/itcz_goes11_lrg.jpg
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Adding some realism
◦ Semi-permanent high and lows
◦ Northern vs. Southern Hemisphere
◦ Major features (pressure systems, wind belts, ITCZ)
shift seasonally with the high sun
 Towards the warm pole
© Brooks Cole/Cengage Learning
Fig. 8.3, p. 212
© Brooks Cole/Cengage Learning
Fig. 8.3, p. 212
© Brooks Cole/Cengage Learning
Fig. 8.4, p. 213
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Average Wind Flow and Pressure Patterns
Aloft
◦ North-south temperature and pressure gradient at
high altitudes creates west-east winds, particularly
at mid to high latitudes.
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General Circulation and Precipitation Patterns
◦ Rain where air rises (low pressure)
◦ Less rain where air sinks (high pressure)
© Brooks Cole/Cengage Learning
Fig. 8.9, p. 216
© Brooks Cole/Cengage Learning
Fig. 8.9, p. 216
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http://daphne.palomar.edu/pdeen/Animation
s/23_WeatherPat.swf
© Brooks Cole/Cengage Learning
Fig. 8.5, p. 213
© Brooks Cole/Cengage Learning
Fig. 8.6, p. 214
© Brooks Cole/Cengage Learning
Fig. 8.7, p. 214
© Brooks Cole/Cengage Learning
Fig. 8.8, p. 215
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Where the circulation
cells meet, we
observe jet streams:
narrow regions of very
strong winds aloft
The polar jet usually
provides a good
estimate for the
dividing line between
warm and cold air
The subtropical jet
affects Texas weather
in the winter
The jet streams tend
to be wavy and aren’t
constant in time
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100-200 kt winds at 10-15 km, thousands of km long,
several 100 km wide and a few km thick (polar and
subtropical)
Established by steep temperature and pressure gradients
between circulation cells
◦ Gradients greatest at polar jet
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Jet Streaks are areas of stronger wind within the jet stream
Fig. 8.12, p. 218
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Arabic for “seasonal”
Winds that change drastically from season to
season
Have some similarities to land/sea breeze,
but on a much larger scale
Cooling over land in winter causes
sinking air, high pressure, dry conditions
Land heats up in summer (much more
than ocean), causes rising air, low
pressure, very wet conditions
Also a monsoon, though weaker, in the
southwestern United States
Douglas et al. (1993)
Fig. 8.15, p. 222
Fig. 8.16, p. 222
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Sea breezes and land breezes are mesoscale
circulations near coastlines (for example,
Texas Gulf coast)
Land and water heat and cool at different
rates during the day and at night (remember
heat capacity/specific heat?), causing
gradients in temperature and pressure
Sea breezes:
◦ Bring cooler air to coastal areas
◦ Bring more humid air as well
◦ Can cause thunderstorms inland from the coast
Land
Water
Land
Water
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At night, land cools faster than water---the
opposite processes take place
Offshore flow, sinking air over land, rising air
over water
Stepped Art
Fig. 9-25, p. 241
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Mountain and Valley Breeze
◦ On mountain slopes, warm air rises during the day
creating a valley breeze; during night nocturnal
drainage of cool air creating a mountain breeze;
gravity winds
◦ Associated with cumulus clouds in the afternoon
Fig. 8.19, p. 226
Fig. 8.20, p. 226
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Windy Afternoons
◦ Afternoon convection
◦ If the air begins to sink as part of a convective circulation, it
may pull some of the stronger winds aloft downward with it
◦ If this sinking air should reach the surface, it produces a
momentary gust of strong wind
◦ In addition, this exchange of air increases the average wind
speed at the surface
Katabatic (fall) winds
Cold wind rushes down elevated
slopes, usually 10 mi/hr or less but
can reach hurricane strength
Fig. 8.21, p. 227
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Chinook/Foehn Winds
◦ Dry warm descending on the leeward side of a
orographic barrier
◦ Eastern slope of Rockies (chinook), Europe (foehn),
Argentina (zonda)
◦ Snow-eater
Fig. 8.23, p. 228
Fig. 8.24, p. 229
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+49° F in seven minutes (Great Falls, MT)
Spearfish, SD: -4°F to 54, back to 11, up to 55!
Fig. 8.25, p. 229
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Santa Anna Winds
◦ Warm dry that blows from east or northeast
downslope into Southern California
◦ Very fast, desiccates vegetation, providing fuel for
fires
◦ Canyons can funnel and enhance
Fig. 8.26, p. 230
Fig. 8.27, p. 230
Fig. 8.28, p. 230
Fig. 8.29, p. 231
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Other extreme winds
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Texas or blue norther
Norte
Bora, Mistral (famous katabatic winds)
Blizzard, burga, purga
Duststorm, sandstorm
 Haboob, Shamal
◦ Dust devil, whirlwind
◦ Leste, levete, sirocco, khamsin, simoom
Fig. 8.30, p. 231
Fig. 8.31, p. 232
Fig. 8.32, p. 232
Fig. 8.33, p. 233
Fig. 8.34, p. 233
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Aircraft Turbulence
◦ CAT (clear air turbulence)
◦ Increasing wind speed shear
◦ Billow clouds