Circulation in the atmosphere

Circulation in the atmosphere

Circulation in the Atmosphere

How does planetary rotation affect fluid motions?

• Additional forces in the rotating frame of reference– Centrifugal force Geoid– Coriolis force

Coriolis force

Exmaple: playing catch on a merry-go-round

• Straight path in inertial (non-rotating) frame• Deflection to the right in rotating frame

Coriolis force• In northern hemisphere, planetary rotation is

counter-clockwise.• A moving object is deflected to the right in

rotating frame of reference

Coriolis force

• What is the sense of planetary rotation in the southern hemisphere?

- Looking down onto the south pole, planetary rotation is clockwise

• Which direction would Coriolis force defect a moving particle in the southern hemisphere?

- A moving object is deflected to the left in the southern hemisphere

Latitudinal variation of Coriolis force

• Projecting the merry-go-round on the planet

• Parallel rotation axis at the pole

Maximum Coriolis deflection

• Perpendicular rotation axis at the equator

No Coriolis deflection

General circulation (non-rotating atmosphere)

Hadley cell

• Rising air at tropics

• Sinking air at poles

• Poleward flow in the upper atmosphere

• Equatorward flow in the lower atmosphere

Hadley cell confined in low latitudes

General circulation (rotating atmosphere)

• Rising air at tropics

• Sinking air at subtropics

• Equatorward flow deflected westwards (trade wind)

• Poleward flow deflected eastwards (westerly wind)

Tank demo

Hadley circulation: very low rotation rate(~ 1-2 RPM)

“Pole”

Low lat.

Low lat.

Tank demo: Hadley cell

Low-level trade wind

Upper-level westerly wind

Hadley cell dominates low-latitude circulation

What controls the middle-high latitude circulation?

Mid-latitude cyclones

• Weather system – Strong rotation effect generates turbulent

motions– High and low pressures– Fronts: separating warm tropical air and cold

polar airs

Synoptic scale circulation

High pressureDry air sinkingSunny weatherAir spirals outClockwise

Low pressureMoist air risingRainy weatherAir spirals inCounter-clockwise

Tank demo

Mid-latitude cyclones: high rotation rate(~ 5+ RPM)

“Pole”

Low lat.

Low lat.

Tank demo

Mid-latitude cyclones: high rotation rate(~ 5+ RPM)

Putting it altogether

“Hadley” regime

Subtropical high

Intertropical convergence zone (ITCZ)

“Eddy” regimeVariable weatherWesterly wind