Mid-latitude Cyclones

Chapter 12p

Part 2March 24, 2009

Surface air converges and obtains cyclonic spin after few hours due to coriolis force

2

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)

3

Upper level divergence moving over cyclonic circulation increases circulation (vorticity)- aircirculation increases circulation (vorticity) air

forced upward

4

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

5

6

Upper Level VorticityUpper Level Vorticity

7

Relative VorticityRelative Vorticity

8

9

• 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), )

10

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

12

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

13