Upload
dangkhanh
View
227
Download
8
Embed Size (px)
Citation preview
1
Lecture 16: Wind 2Introduction to Oceanography
Wind speed and direction about 1.5 km above sea levelBy Trent Schindler (NASA) using satellite data. Public Domain
https://svs.gsfc.nasa.gov/4571
Coriolis Effect Movies
Movie: University of Illinois (not sure if that’s the original source) http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/fw/crls.rxml
2
The Coriolis Effect on Earth
National Snow and Ice Data Center, free for educational use, http://nsidc.org/arcticmet/factors/winds.html
• Surface velocity increases from pole to
equator• Points on the equator
must move faster than points near the poles to go around once a day• Latitude velocity
differences lead to curving paths
– Example: Merry-go round
The Coriolis Effect• To an Earthbound observer (i.e., us): • Northern Hemisphere: Earth’s
rotation causes moving things to curve to their right
Moving things: Air masses, oceanic flows, missiles, anything with mass
• Southern Hemisphere: Earth’s rotation causes moving things to curve to their left
National Snow and Ice Data Center, free for educational use, http://nsidc.org/arcticmet/factors/winds.html
3
Questions ?
Southern Hemisphere: Cyclone Drena (1997) NASA, Public Domain, http://www.ngdc.noaa.gov/dmsp/hurricanes/1997/drena.vis.gif (now moved)
Northern Hemisphere: Hurricane Isabel (2003) NASA, Public Domain, http://visibleearth.nasa.gov/view_rec.php?id=5862
But wait – why do storms(including hurricanes and cyclones) go
backwards?
Atmospheric Circulation including Coriolis
Figure from NASA, Public Domain, http://sealevel.jpl.nasa.gov/overview/climate-climatic.html
4
Actual forecast of
surface winds
Pacific surface wind forecast-hindcast, National
Weather Service Environmental Modeling
Center/NOAA, Public Domain, GIF by E. Schauble
using EZGif
Atmospheric Circulation including Coriolis
• 3 convection cells in each hemisphere– Each cell: ~ 30o latitudinal width
• Vertical Motions– Rising Air: 0o and 60o Latitude– Sinking Air: 30o and 90o Latitude
• Horizontal Motions– Zonal winds flow nearly along latitude lines– Zonal winds within each cell band
• DUE TO DEFLECTIONS BY CORIOLIS!
5
Atmospheric Circulation including Coriolis
3 Cells per hemisphere:Polar
Active (updraft on hot side, downdraft on cold side)
FerrelPassive (downdraft on
hot side!)
HadleyActive
UCLA figure – background image unknown.
Atmospheric Circulation including Coriolis
• Latitudinal winds:– 0-30o:
Trade Winds
– 30-60o: Westerlies
– 60-90o: Polar Easterlies
Figure by Hastings, Wikimedia Commons, Creative Commons A S-A 1.0 Generic, http://en.wikipedia.org/wiki/File:AtmosphCirc2.png
6
Atmospheric Circulation including CoriolisCell Boundaries:
60o: Polar Front
30o: Horse Latitudes
0o: Doldrums
Vertical air movement(up at Polar Front and Doldrums, down at Horse Latitudes)
Doldrums
Horse Latitudes
Polar Front
Figure by Hastings, Wikimedia Commons, Creative Commons A S-A 1.0 Generic, http://en.wikipedia.org/wiki/File:AtmosphCirc2.png
Questions
Figure from NASA, Public Domain, http://sealevel.jpl.nasa.gov/overview/climate-climatic.html
7
Local Meteorology of Southern CaliforniaMarine layer against the Southern California mountainsPhoto by Dr. Jonathan Alan Nourse, CalPoly Pomona, http://geology.csupomona.edu/janourse/Storms,%20Floods,%20Landslides.htm
Mediterranean Climate• LA: Subtropical latitude, abutting ocean• Subsiding flow: sinking air
– Clear most of the year• Effects of coast:
– Higher humidity--- thermal buffer• Winter Storms
– Pole-equator temp difference larger in winter– Speeds up jet stream, big storms get pushed our
way
8
Sea Breeze
Jesús Gómez Fernández, Wikimedia Commons, Creative Commons A S-A 3.0, http://commons.wikimedia.org/wiki/File:Diagrama_de_formacion_de_la_brisa-breeze.png
Land warms fastest during the day. Air expands and rises
Ocean surface temperature changes
slowly. Air displaces less dense rising air on land.
Result – wind from sea towards land
Land Breeze
Adapted from Jesús Gómez Fernández, Wikimedia Commons, Creative Commons A S-A 3.0, http://commons.wikimedia.org/wiki/File:Diagrama_de_formacion_de_la_brisa-breeze.png
Land cools fastest at night. Air
contracts and sinks
Ocean surface temperature changes slowly. Air is pushed
away and up by cooler denser land air.
Result – wind from land towards sea
9
Marine Layer• Cold waters, warm air: thin cloud layer
on ocean surface– Subtropics: H pressure, regional
subsidence• Cloud layer flows onto land at night• Evaporates over land by day
LAND OCEANUCLA figure
UCLA Marine Layer
Time lapse -- Sept. 23, 2003(?), J. Aurnou, UCLA
10
Santa Ana Winds
• Canada: Chinook• Colorado: Downslope Winds• Germany: Foehn Winds• France: The Mistral• Southern CA: Santa Ana Winds
Santa Ana WindsWinter: Canadian cold air
pushes down into Southwestern USHigh pressure pushes dry desert air
downslope, to seaCompression of sinking air causes
heatingHeating lowers humidityWind Speeds: up to ~ 70mph ≈115
km/hrFunneling effect through canyonsFeeds dangerous brush firesWeaker in summer
Piotr Flatau, Wikimedia Commons, Public Domain, http://en.wikipedia.org/wiki/File:Santa_ana_wind1.jpg
Mojave Desert
San Gabriel/Bernardino Mtns.
Los AngelesHigh Plateau
Adapted from N. Short Remote Sensing Tutorial/NASA, Public Domain,
http://rst.gsfc.nasa.gov/Sect14/katabatic.jpg
11
Santa Ana WindsNASA image, Public Domain, http://photojournal.jpl.nasa.gov/catalog/PIA03892
~ 30 mph
Santa Ana Winds
Santa Ana Winds: dry & warm,
Encourage destructive fires
Fertilize ocean?Porter Ranch Fire, Oct. 14 2008, NASA image, Public Domain,
http://www.nasa.gov/mission_pages/fires/main/usa/califires_20081014.html
12
Santa Ana Winds
• Canada: Chinook• Colorado: Downslope Winds• Germany: Foehn Winds• France: The Mistral• Southern CA: Santa Ana Winds
Questions?
UCSD GOES-10/NASA, Public Domain, http://meteora.ucsd.edu/cap/images/junegloom_16jun2004.gif
13
Currents
The Gulf Stream, Winslow Homer (1899), Metropolitan Museum of Art (NY). Public Domain, https://commons.wikimedia.org/wiki/File:Winslow_Homer_004.jpg
Currents in the OceanWhat is a current?A current is a flow of material
MASS IS TRANSPORTED
Ben Franklin, 1769, Map of the Gulf Stream, Public domain.
Ocean currents transport water
Wind is a current of airMap by Ben Franklin, 1787
Portrait of Ben Franklin, 1785, by Duplessis
14
Currents in the Ocean• Two Types: Surface and Deep• Driving Forces
– Surface Currents: Wind-driven– Deep Currents: Density-driven
American Meteorological Society, http://oceanmotion.org/images/oc
ean-vertical-structure_clip_image002.jpg
Surface CurrentsCaused by: Wind Stress Pressure Gradients
Coriolis Effects Friction
Gene Paull, UT Brownsville, Public Domain(?),http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif
15
Wind-Driven Currents
• Wind accelerates ocean currents– Frictional Drag
Atmosphere
OceanSIDEVIEW
Drag
TOPVIEW
Wind
Wind
Current Current
Map View
Figures, UCLA
Pressure-Driven Currents
• Wind drives ocean currents• Currents run into continents and can’t continue
– UNLIKE ATMOSPHERIC FLOWS• Water piles up ---Pressure Gradients form
Atmosphere
OceanSIDEVIEW TOPVIEW
Wind
Wind
Current Current
Drag H H
L
L
Figures, UCLA