Transcript Atm-4 Winds [text KKC, pp.69-72]

Chapter 9 Winds, Temperature and
Precipitation
Objectives:
• Jet streams
• Sea breeze & land breeze
• Valley breeze, mountain breeze & chinooks
• Monsoons
• Wind power
• Global temp. distr.
• Hydrological cycle
• Precipitation
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Jet streams
• Jet streams: air
currents thousands of
km long, hundreds of
km wide, a few km thick
(centred near
tropopause).
• Max speed > 200 km/hr.
• Polar jet stream near
polar front, separating
cold air from mild air.
Jet stream turning south
=> cold air moves south.
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• So there is sinking air around 30 degree, which forms
divergence region on surface and convergence region
aloft. The convergence between cold air with warm air can
cause a great temperature gradient in this region, and
further causing a large gradient in pressure => speeding
the air flow => cause the jet.
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• Subtropical jet stream at ~30°
• Jet streams meander, polar jet may merge
with subtropical jet.
• Polar jet may also branch into 2.
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1998/3/10
Contours of wind speed (knots) at 300-mb level
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2 mechanisms for jet streams
• Where polar cell meets Ferrel cell, or Ferrel
cell meets Hadley, airs of different T meet
=> large T gradient => large p gradient
=> geostrophic winds.
•As air moves from low
to high lat., its circular
orbit shrinks.
=> orbiting speed incr.
(conservation of angular
momentum; e.g. spinning
skater moves arms
towards body).
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Sea breeze
• Daytime: land warms more than sea
=> rising air & low p on land. Air flows from
sea to land.
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Land breeze
• Night: Land cools more than sea.
=> Sinking air & high p over land. Air flows
from land to sea.
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Valley breeze & mountain breeze
• Daytime: At same elevation, air on mountain
slope heated more than air over valley
=> low p over mountain slope => air flows
upslope from valley (valley breeze).
• Night: Air on mountain slope cooled more
than air over valley => mountain breeze.
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Chinook wind
• Chinook: warm, dry wind on eastern slope of
Rockies.
• Western slope: condensation => release of latent
heat. Moisture lost from precip.
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• Descending wind on eastern slope =>
warming from compression
• Record of 27°C change in 2 minutes.
• Cities near the boundary between cold and
warm dry air can have large, rapid T
oscillations as boundary oscillates.
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Monsoons
• Winter: continents cool more than oc.
=> sinking air & high p over continent
• Summer: continents warm more than oc.
=> rising air & low p over continent
• Most prominent with the massive Asian land
mass.
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Temperature & Precipitation
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Mean air temp. at sea level (Jan.)
• Greater T range over land than over oc.
• Oc. currents affect land T (e.g. Gulf Stream
warms western Europe).
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Mean air temp. at sea level (July)
• Cool Californian Current => cools adjacent land
• Hottest regions at ~20°-30° (not at Eq.)
• High p, subsiding air, clear sky, low humidity =>
hot deserts.
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Annual T range
• Largest T range over land.
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Temp. records
•High T records:
•World: El Azizia, Libya (32°N) 58°C, in 1922
•Western Hem: Death Valley, CA(36°N) 57°C
•Canada: Midale, Sas.(49°N) 45°C
•Low T record:
•World: Vostok, Antarc. (78°S) -89°C, 1983
•N.Hem.: Verkhoyansk, Russia (67°N) -68°C
•N.America: Snag, Yukon (62°N) -63°C.
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Latitudinal
Variations
in Net Radiation
• tropic-to-tropic – energy surplus
• poles – energy deficits
• ~ 38o N/S – balance
• imbalance of net radiation at
surface 
Equator/Tropics vs. high latitudes
• drives global circulation
• agents: wind, ocean
currents,
weather systems
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• T decreases poleward
• larger T gradient in winter
• isotherms shift seasonally
• T over land > water in summer
• NH steeper T gradient
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Principal Controls on Temperature
1.
2.
3.
4.
5.
6.
Latitude
Altitude
Atmospheric Circulation
Land-Water Contrasts
Ocean Currents
Local Effects
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Marine v. Continental
Climates
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Ocean
Circulation
•
Primary driving force – wind (generated by pressure differences)
•
East coast of continents  northward moving currents
• Transfer energy poleward
West coast of continents  southerly currents
Energy transferred to atmosphere overlying oceans
• affects coastal areas
•
•
• Links atmosphere and ocean together
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Hydrological cycle
• Water has large latent heat of vaporization
(2260 kJ/kg at 100°C).
• Latent heat of fusion (335 kJ/kg at 0°C)
needed to convert ice to liquid water.
• H2O: 97% in oc., 3% on land, 0.001% in atm.
• H2O on land:
• 3/4 in polar ice sheets.
•Greenland ice sheet melted => 6m global sea
level (SL) rise
•Antarctic ice sheet melted => 60m SL rise
• Some in glaciers, ground water
• < 1% in lakes, rivers, soils.
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Meridional cells & precip.
• Northward shift of cells during summer, &
southward shift during winter => precip. changes.
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Mean annual precip.
• Driest regions near 30° and poles: high p,
subsiding air.
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