Mid-latitude Cyclones
Chapter
p 12
Part 1
March 19, 2009
Polar Front Theory
• Polar front theory – a model explaining the
life cycle of extratropical (mid-latitude)
cyclonic storms
• Polar front theory developed in early 1900’s
by group of Norwegian meteorologist
describes,, formation,, growth,
g
, and decayy of
mid-latitude cyclones
• Original theory basically correct
correct, but
modified to take into account new
information learned
Polar Front Theory
• P
Polar
l ffrontt is
i a semi-continuous
i
ti
boundary
b
d
separating cold, polar air from more
moderate mid-latitude air
• Mid-latitude cyclone
y
((wave cyclone)
y
) forms
and moves along polar front in wavelike
a e
manner
• Frontal wave, warm sector, mature
cyclone triple point
cyclone,
point, secondary low
low, family
of cyclones
Idealized Life Cycle of Cyclone
Series of Wave Cyclones
y
Two developing mid
mid-latitude
latitude cyclones
Where do mid
mid-latitude
latitude cyclones form?
• Lee
Lee-side
side lows (cyclogenesis)
– Eastern side of Southern Rockies
• Nor’easters
N ’
t
– Along
g Atlantic coast
• Hatteras low
– Cape Hatteras,
Hatteras NC (warm Gulf Stream water)
• Alberta Clipper
– Eastern side of Canadian Rockies
• Explosive cyclogenesis
cyclogenesis, bomb
Cyclones
Typical Paths of
winter Midlatitude
Systems
y
Anticyclones
Lee side Trough Development
Lee-side
Air on downwind side
curves cyclonically and
adds to the
development of a
cyclonic storm
Lee-side low
developed
Northeasters
• Also called a
“Nor’easter”
• Mid-latitude cyclones
that develop or
intensify off the
eastern seaboard off
North America then
move NE along coast
Vertical Structure of Deep Dynamic
Lows
• Dynamic low = intensify with height
• When upper-level
upper level divergence is stronger
than surface convergence (more air is
taken out of the top than the bottom)
surface pressure drops and low formation
• Convergence and divergence
– Convergence between ridge and trough
trough,
divergence between trough and ridge
Idealized
Vertical
Structure of
Mid-latitude
Systems
Upper Level Waves and Mid-latitude
Cyclones
• Longwaves and shortwaves
• Barotropic vs
vs. baroclinic
• Cold and warm air advection
5 Longwaves
Longwave and 3 Shortwaves
The Necessary Ingredients for
Development of Mid-latitude Storm
• Baroclinic instability
• Upper
Upper-Air
Air Support: the overall effect of
differential temperature advection is to
amplify the upper level wave; cut
cut-off
off low
• Role of the Jet Stream: the polar jet
stream removes air from surface cyclone
and supplies air to surface anti-cyclone
anti cyclone
The Necessary Ingredients for
Development of Mid-latitude Storm
• Jet Streaks and Storms
– Entrance and exit regions
g
associated with
divergence and convergence, right exit allows
g
divergence.
• Conveyor Belt Model: air constantly glides
through storm; warm
warm, cold
cold, and dry
conveyor belts
• March Storm of 1993
Conveyor Belt Model of Developing
Mid-latitude Cyclone
Surface weather during
g deep
p trough
g
500mb chart of deep trough