Lecture 5: General Circulation of the AtmosphereLecture 5: General Circulation of the Atmosphere

Questions?Questions?

1.1. Why do winds Why do winds blow aroundblow around a a high or a low high or a low pressure pressure center?center?

2.2. Why does a Why does a low arealow area mean mean more more precipitationprecipitation? ?

Why does a Why does a highhigh mean a mean a drydry area? area?

EarthsClimate_Web_Chapter.pdf, p. 15-22

The Global Energy Budget: Driver of Atmospheric Motion

However, the tilt of theEarth means this balanceis not maintained for eachlatitude

A balance exists between the incoming solar andoutgoing longwave energy averaged over the globe and the year

DEFICITSURPLUS

• To compensate for this energy imbalance, winds in the atmosphere and currents in the oceans transport cold air and water toward the equator

• About 1/3 of this transport occurs from the evaporation of tropical waters and subsequent transport into high latitudes, where it condenses and releases latent heat

• About 1/3 occurs from the poleward transport of warm waters by ocean currents

• The remaining 1/3 occurs from middle latitude cyclones and anticyclones

Global Wind Patterns

WesterliesWesterlies

WesterliesWesterlies

Polar easterliesPolar easterlies

Polar easterliesPolar easterlies

Hadley CellHadley Cell

Hadley CellHadley Cell

Atmospheric PressureAtmospheric Pressure

The The pressurepressure at the at the base of this column base of this column of air results of air results from from thethe weight of the weight of the gasses abovegasses above..

The earth's The earth's atmosphere has a atmosphere has a greater density of greater density of gases at its base gases at its base due due to gravityto gravity..

Forces & MotionForces & Motion

Air responds Air responds similarly as water to similarly as water to this force, moving this force, moving from higher pressure from higher pressure to lower pressureto lower pressure..

Pressure forces are Pressure forces are only one influence on only one influence on the movement of the movement of atmospheric air.atmospheric air.

PGF vs. Cyclonic WindsPGF vs. Cyclonic WindsPressure Pressure gradient force gradient force ((PGFPGF) winds ) winds acting acting alonealone would head would head directly into directly into low pressurelow pressure..

Surface Surface observations of observations of winds, such as winds, such as the cyclonic the cyclonic flow around flow around this low, reveal this low, reveal that that PGF winds PGF winds are deflected by are deflected by other forces.other forces.

Playing catch on a merry-go-round

Blue, Yellow, Red,

Green

Clockwise

Straight-line, watch from

above

Curved to the left, watch by the green on

board

Coriolis ForceCoriolis Force

Counterclockwise

Earth's rotation transforms straight line Earth's rotation transforms straight line motion for an outside viewer in space motion for an outside viewer in space into curved motion for an viewer on into curved motion for an viewer on Earth.Earth.

The Coriolis force explains this apparent The Coriolis force explains this apparent curvature of winds to the right curvature of winds to the right due to due to rotationrotation..

Its magnitude increases with wind Its magnitude increases with wind velocityvelocity and earth's and earth's latitudelatitude..

Cyclones and Anticyclones

Three forces:

PGF

Coriolis

Frictional

Vertical Air MotionVertical Air Motion

Winds Winds convergingconverging into a low pressure center generate into a low pressure center generate upwardupward winds that remove the accumulating air molecules.winds that remove the accumulating air molecules.

These updrafts may These updrafts may cause cloud formationcause cloud formation..

Likewise, Likewise, divergingdiverging air molecules from a air molecules from a high pressurehigh pressure area are area are replenished by replenished by downwarddownward winds. winds.

Northern Hemisphere FlowNorthern Hemisphere Flow

Figure 9.27AFigure 9.27A

Winds blow Winds blow counterclockwise counterclockwise around around low pressurelow pressure systems in the systems in the Northern Northern HemisphereHemisphere, but , but clockwise around clockwise around lows in the lows in the Southern Southern Hemisphere.Hemisphere.

Coriolis Force and Nature’s Greatest Storms

Hurricane MovementHurricane MovementFormationFormation

1.1. Low-pressure areas Low-pressure areas (easterly waves)(easterly waves)

2.2. Warm, moisture, Warm, moisture, unstable areaunstable area

3.3. Ocean surf. temp. Ocean surf. temp. > > 26–27°C26–27°C

4.4. Low wind shearLow wind shear5.5. Sufficient Coriolis Sufficient Coriolis

force (5-10 degrees force (5-10 degrees away from the away from the equator)equator)

6.6. Strong upper-level Strong upper-level anticyclone.anticyclone.

Their movement is Their movement is guidedguided by winds by winds around around subtropical highsubtropical high pressure systems.pressure systems.

R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html

R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html

R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html

R. Anthes http://www.ucar.edu/governance/meetings/oct05/followup/meeting_summary.html

Can you answer these questions at the end of this lecture?

• What causes atmospheric surface pressure gradient and change?

• What atmospheric forces drive winds?

• Why is cyclonic flow associated with cloudy and rainy weather, whereas anticyclonic flow is associated with clear and cooler weather?