Potential Vorticity

Barotropic Instability Water Vapor Loop

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Positive PV Anomaly Near Trop

Negative PV Anomaly Near Trop

Surface +PV Anomaly

Piecewise PV Inversion

Stoelinga 1996: Overheads

Lee Troughing and PV

Conservation of potential vorticity –conserved for adiabatic frictionless motion Ratio of absolute vorticity and depth of vortex (Holton 2004, p. 96) Ertel Potential Vorticity

Conservation of potential vorticity –for a homogeneous incompressible fluid  evaluated at constant height (Holton 2004, p. 96) Potential Vorticity

Conservation of potential vorticity –When the depth of the vortex changes following motion, its absolute vorticity must change to maintain conservation of potential vorticity (Holton 2004, p. 98)

Conservation of potential vorticity –For westerly flow impinging on an infinitely long mountain range… (a) upstream, zonal flow is uniform (  u/  y = 0, v=0),  = 0 (b) deflection of upper  surface upstream of barrier  increases h  absolute vorticity must increase  air column turns cyclonically (Holton 2004, p. 98) (a) (b)(c)(d) (e)

Conservation of potential vorticity –For westerly flow impinging on an infinitely long mountain range… poleward drift in (b) also causes increase in f (c) as column crosses mountain, h decreases  absolute vorticity must decrease   becomes negative  air column drifts equatorward ATMS 316- Background (Holton 2004, p. 98) (a) (b)(c)(d) (e)

Conservation of potential vorticity –For westerly flow impinging on an infinitely long mountain range… equatorward drift in (c) also causes decrease in f (d) as column crosses mountain, h increases  absolute vorticity must increase   becomes positive  air column drifts poleward (a) (b)(c)(d) (e)

Conservation of potential vorticity –For westerly flow impinging on an infinitely long mountain range… (e) alternating series of ridges and troughs downstream of mountain range cyclonic flow pattern immediately to the east of the mountains (lee side trough) ATMS 316- Background (a) (b)(c)(d) (e)

(Ahrens 2005, p. 222)

Lee cyclogenesis –Preferred regions of cyclogenesis Alps –Narrow mountain range –Theory that applies to Alps lee cyclogenesis is modifed from that used to describe lee cyclogenesis of the Rockies –Ageostrophic effects dominate and the modification of baroclinic instability by the Alps is more difficult to analyze Alps and Smaller Ranges More Complicated With All Kinds of Baroclinic Effects

Tropopause +PV anomalies often apparent in water vapor imagery

Trop Pressure

Terminology: PV Streamer A PV-streamer is an elongated band of potential vorticity, generally in the upper troposphere. It is mesoscale in width and synoptic scale in length. In the upper troposphere, they are associated with stratospheric–tropospheric mass exchange, particularly in the area where the tropopause folds.

The End