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Mid-latitude Cyclones
Chapter 12p
Part 2March 24, 2009
Surface air converges and obtains cyclonic spin after few hours due to coriolis force
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Vorticity, Divergence and Development of Cyclones
• Vorticity is a measure of the spin of small air parcelsp
• Positive vorticity = cyclonic motionN ti ti it ti l i ti• Negative vorticity = anticyclonic motion
• Divergence aloft causes and increase in e ge ce a o t causes a d c easethe cyclonic vorticity of surface cyclones = cyclogenesis and upward air movementcyclogenesis and upward air movement
• Relative Vorticity = ζ = (dv/dx-du/dy) = (Δv/Δx - Δu/Δy)
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Upper level divergence moving over cyclonic circulation increases circulation (vorticity)- aircirculation increases circulation (vorticity) air
forced upward
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Vorticity on a Spinning PlanetVorticity on a Spinning PlanetE th’ ti it l iti• Earth’s vorticity always positive
• Relative vorticity = curvature + sheart h l i• trough: cyclonic,
• ridge: anticyclonicAb l t ti it E th + l ti• Absolute vorticity = Earth + relative
• An increase in absolute vorticity is related to upper level convergenceupper level convergence
• A decrease in absolute vorticity is related to upper level divergenceupper level divergence
• Vorticity maxima = “vort max” and usually shown on map by “X”shown on map by X
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Upper Level VorticityUpper Level Vorticity
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Relative VorticityRelative Vorticity
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• Divergence at jet stream g j(300 mb) downstream of vorticity max at 500 mb
• Convergence at surface and rising air in area of i i ti itincreasing vorticity (positive vorticity advection or PVA)advection, or PVA)
• Convergence aloft, sinking air divergence atsinking air, divergence at surface in area of decreasing vorticitydecreasing vorticity (negative vorticity advection, or NVA), )
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ITCZ 11
Last thoughts on VorticityLast thoughts on Vorticity
• Vorticity and Longwaves– Longwaves develop in upper-levels due to the g p pp
conservation of absolute vorticity• Use of Vorticity in Forecasting• Use of Vorticity in Forecasting
– Forecasters review 200mb, 500mb, and f t isurface maps to examine pressure,
convergence, vorticity, and advection– They look for PVA and NVA – clouds and ppt– Helps locate shortwavesp
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Feb. 11, 1983 Nor’easter
Make note of:• Vertical tilt ofVertical tilt of
system500 b t h• 500 mb trough position
• Jet stream locationlocation
• Low over warm water
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