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.

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 pressure Dry air sinking Sunny weather Air spirals out Clockwise Low pressure Moist air rising Rainy weather Air spirals in Counter-clockwise

Tank demo Mid-latitude cyclones: high rotation rate (~ 5+ RPM) “Pole” Low lat.

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

Putting it altogether “Hadley” regime Subtropical high Intertropical convergence zone (ITCZ) “Eddy” regime Variable weather Westerly wind