Potential Vorticity

President’s Day http://www.atmos.washington.edu/academic/videos/PresidentsDayStorm.html

Positive PV Anomaly Near Trop

Negative PV Anomaly Near Trop

Surface +PV Anomaly

Piecewise PV Inversion

Stoelinga MWR. 124,5 1996: Overheads

Stoelinga 96 Intense case of western Atlantic baroclinic cyclogeneis: the Scamp storm Four approaches: Full physics mesoscale simulation Partitioned PV integration (temporally integrates accumulation of PV from various processes, e.g., explicit and parameterized LH) Piecewise inversion of particular PV anomalies to yield their direct contributions to the total circulation. Sensitivity experiments: No LH

Results Latent heat produces a large positive PV anomaly above surface warm/bent back fronts Explains about 70% of the non-divergent circulation a low levels Circulation associated with LH PV also enhanced vertical coupling between surface and upper level waves

Results Latent heating enhanced upper level divergence and developmet of downstream ridge. Cyclogenesis still occurred with LH-generated PV, but was much weaker (still had upper level PV) Friction was much smaller player.

Lee Troughing and PV

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

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

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

ATMS 316- Background 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  z becomes negative  air column drifts equatorward (Holton 2004, p. 98)

Conservation of potential vorticity (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  z becomes positive  air column drifts poleward

ATMS 316- Background 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)

(Ahrens 2005, p. 222)

Alps and Smaller Ranges More Complicated With All Kinds of Baroclinic Effects 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

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.

Wavebreaking

The End