Upload
hyatt-chambers
View
24
Download
1
Tags:
Embed Size (px)
DESCRIPTION
Atmospheric. Circulation. Air Pressure Experiments Lessons from Paper Cup experiment: 1. Air pressure is present everywhere Air “tries to” move from an area of higher pressure to an area of low pressure Lesson from Pop Bottle experiment: 3. Warm air occupies more space than an - PowerPoint PPT Presentation
Citation preview
Air Pressure Experiments
Lessons from Paper Cup experiment:
1. Air pressure is present everywhere
2. Air “tries to” move from an area of higher pressure to an area of low pressure
Lesson from Pop Bottle experiment:
3. Warm air occupies more space than an equal number of molecules of cold air
Wind Power Generation in Southern Alberta
“Don’t try this at home”
The speed and direction of windis determined by three forces:
1. Pressure Gradient Force
2. Inertial Coriolis Force
3. Friction Force
Pressure Gradient Force
H L102.2 99.8
101.4 100.6
600 km
Pressure Gradient Force = 2.4 kPa / 600 km= 0.4 kPa / 100 km
Definition:
The difference inatmosphericpressure per unitdistance
PGF acts at rightangles to isobarsof equal pressure
Where is the PGF forecast to be strongest today ?
Regina orLethbridge?
Solution:Check the spacing of theisobars of equal surfacepressure
Source: http://weatheroffice.ec.gc.ca/data/model_forecast/592_100.gif
The Inertial Coriolis Force
Objects moving in an “absolute” straight linebetween two points on the Earth’s surface aredeflected:
To the RIGHT in the N hemisphereTo the LEFT in the S hemisphere
Why ?
The Earth rotates more quickly at the equator.
Visualizing the Coriolis Force
Source: NASA
The Friction Force
Surface roughness decreases wind speed
Reduces impact of Inertial Coriolis Force
Winds cross isobars, spiralling out ofANTICYCLONES (H), and into CYCLONES (L)
H L
Can you infer wind direction and relative speed from this map ?
weather.unisys.com
Sea level pressure:
AltitudeCorrection
Source: Ahrens (1994)
Weather symbols andwind barbs
Classic Low Pressure SystemIn Temperate Latitudes
SHARPCOLDFRONT
WARM,MOISTSOUTHERLYFLOW
NORTH-EASTWINDS
www.atmos.washington.edu
0600h GMTAPRIL 52003
Cold Front
Arctic high pressure drives cold arctic air behind low
Warm Front
•Not as steep a division as in a cold front•It takes longer to scour out surface air (warm air rises)
WARM, SOUTHWIND
COOLNW WIND
COLD FRONT
WARM FRONT
HURRICANE ISABEL
The weather pattern last September
Main Low and High Pressure Zones
1. Equatorial Low Pressure Trough
2. Subtropical High Pressure Cells
3. Subpolar Low Pressure Cells
4. Weak Polar High Pressure Cells
Atmospheric Circulation Overview
HADLEYCELL
FERRELCELL
POLARCELL
Equatorial low pressure trough (warm, wet)
ITCZ shifts with season
High solar angle
HeatingConvergence
Consistent daylength
Hadley Cells
1. Warm, moist air rises in equatorial lowCools, condenses, and causes heavy rain
2. Outward flow to subtropical high at high altitude
3. Air descends in subtropical highHeats, compresses and becomes very dry
4. The subtropical high provides the gradient for trade winds and westerlies
eg. Bermuda/Azores and Pacific/Hawaii highs
Strahler and Strahler (2002)
Ferrel Cells
Between subtropical highs and subpolar lows
Poleward transport of excess heat througheddies and migration of lows toward polar front
Strong low pressure develops in a belt aroundAntarctica, near the Aleutians and near Iceland
Lows strongest in winter (shift and diminish periodically, especially in the summer)
Why ? Water much warmer than land in winter leading to lower pressure over oceans
H L
Air tends to be unstable in low pressure (tendency to rise)Air tends to be stable in high pressure (tendency to fall)
(more on stability in next class)
WINTER SUMMER
Generalized Overview of Seasonal Surface Pressure
Average GlobalSurface Pressurein January andJuly
Can you explainthe monsoon seasonof the Indian sub-continent with thischart ?
Polar High Pressure Cells
Tendency for higher pressure near polesthan at the polar front
Anticyclonic flow develops
Weak and variable polar easterlies result(stronger in southern hemisphere)
In northern hemisphere winter, the polar front usually lies over Canada and Russia,(further south than in the summer)
Geostrophic Winds 500 mbar height map
Lower heightswhere air is cold
Airflow parallel to isobars inupper troposphere
Why ?
Combination of PGF and Coriolis force
Source: http://weatheroffice.ec.gc.ca/data/model_forecast/134_100.gif
Source: Ahrens (1994)
Effect of Air temperature on 500 mb heights
Upper Atmospheric Circulation
Jet StreamsA band of wind in the upper troposphere
150 – 500 km wide0.9-2.2 km thickSpeeds may exceed 300 km/h
Polar Jet Stream: Between Polar and Ferrel cells
Subtropical Jet Stream: Between Hadley and Ferrel Cells
Source: http://apollo.lsc.vsc.edu
Source: http://apollo.lsc.vsc.edu
Tropopauseheight
18 000 m
12 000 m
6 000 m
Discontinuity orstep in tropopause height
“Rivers” of strong wind where cold and warm meet
Jet Stream Cross Section
See: www.avsim.com/avwx/avsim_wxus_jetstream.html
Polar Jet Stream
Subtropical Jet Stream
Meanders from 30-70° N or S
Moves more poleward in summer
Influences (and is influenced by) storm paths
Meanders from 20-50° N or S
May occur simultaneously with Polar Jet in NA
Rossby Waves
The polar jet stream follows the Rossby Waves
Rossby Waves are undulations in the upper-airwesterlies extending from the middle to upper troposphere
Form along the polar front
Mechanism of poleward heat transport
(Strahler and Strahler, 2002)
Daytime
Night
Source: Ahrens, 2001
Mountain Valley Breezes
Source: http://apollo.lsc.vsc.edu
Daytime
The sun heats the hillslope, causing air to move up theslope
Night
Night radiation coolsthe slopes
Cooler, denser air moves downslope
Katabatic Winds
•Air cools on a plateau or sloping terrain, becomes more dense and descends
•Winds get faster and faster downslope
•Relatively warm water at base can further increase winds, which can be very strong as a result
•Can occur on large scale (eg. Greenland, Antarctica)
•Also referred to as gravity drainage winds
VANCOUVER LETHBRIDGE8°C 12°C
X X
CoolingAt MALR6°C/km
WarmingAt DALR10 °C/km
CoolingAt DALR10 °C/km
CoolingAt MALR6°C/km
WarmingAt DALR10 °C/km
More sensible heat
Chinook Winds
•Water piles up around equator due to trade winds
•Along western edge of oceans, water spills N and S along shorelines of continents (also downwelling)
•Upwelling occurs near east edge of oceans (west coasts)
Upwelling of cool waters
Result: The water column becomes unstable and mixes vertically in the north. This newly formed water is carried southward at great depths - North Atlantic Deep Water (NADW)
The Thermohaline Circulation
(1) Intensive cooling at the ocean surface in North Atlantic(2) Northward transport of salty surface water from lower latitudes (both increase the density).
Interannual climatic variability atthe global scale
Caused by changing atmospheric andoceanic circulation in the tropicalPacific Ocean
Top La Nina December 1998; Middle Normal December 1993; Bottom El Nino Dec 1997
See http://www.cdc.noaa.gov/map/clim/sst_olr/sst_anim.shtml