Transcript 10-Atmos2
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 General circulation (rotating atmosphere) Hadley cell confined in low latitudes • 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) Low lat. “Pole” Low lat. Tank demo: Hadley cell Upper-level westerly wind Low-level trade 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) Low lat. “Pole” Low lat. Tank demo Mid-latitude cyclones: high rotation rate (~ 5+ RPM) Putting it altogether “Eddy” regime Variable weather Westerly wind Subtropical high “Hadley” regime Intertropical convergence zone (ITCZ)