Transcript PV551Finaln.ppt

Potential Vorticity
Barotropic Instability
• Water Vapor Loop
President’s Day
• http://www.atmos.washington.edu/academic
/videos/PresidentsDayStorm.html
http://www.atmos.albany.edu/dae
s/atmclasses/atm305/climomaps.
html
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

  Ertel Potential Vorticity

P     f   g
p 

– conserved for adiabatic frictionless motion
• Ratio of absolute vorticity and depth of vortex
(Holton 2004, p. 96)
• Conservation of potential vorticity
P   z  f  / h  Const
Potential Vorticity
– for a homogeneous incompressible fluid
•  evaluated at constant height
(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)
(a) (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),  = 0
• (b) deflection of upper  surface
upstream of barrier  increases h
 absolute vorticity must increase
 air column turns cyclonically
(Holton 2004, p. 98)
ATMS 316- Background
(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
(Holton 2004, p. 98)
(a) (b)
• 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
(c)
(d)
(e)
ATMS 316- Background
(a) (b)
• 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)
(c)
(d)
(e)
(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.
The End