Middle Latitude Cyclones
This chapter discusses:
1. The location, vertical structure, and developmental
stages of middle latitude cyclones
2. How upper level convergent winds, abrupt
topographic features, and planetary longwaves may
enhance cyclonic development at the surface
Polar Front Theory – Wave Cyclone Cycle
Stage 1
Stage 2
Frontal Wave
Stage 3
Open Wave
Overrunning
Precipitation
Stage 1: One explanation for development of
middle latitude cyclones begins with a stationary
front with warm and cold winds in opposite
directions (e.g. wind shear).
Stage 2: A wavelike kink, such as a low pressure
system creates a frontal wave, or incipient cyclone
under the right conditions.
Stage 3: As the storm develops into an open wave, a
broad band of precipitation forms ahead of the
warm sector.
Stage 4
Stage 3a
Stage 5
•Energy driving their development originates from kinetic
sources such as rising warm air, sinking cold air, and
converging air, as well as latent heat of condensation.
•Stage 4: When occlusion occurs when cold air lies on both
sides of the occluded front. Without an ample supply of
energy of rising, warm, moist air, the system will dissipate.
•Stage 5: The end stage is where precipitation-free area is
observed. In addition, only a weak cyclonic circulation is
seen in the wind field.
Wave Cyclone Development
•A series, or family, of cyclones, at various stages of development, may extend
across North America.
•Energy driving their development originates from kinetic sources such as rising
warm air, sinking cold air, and converging air, as well as latent heat of
condensation.
•Baroclinic instability drives the middle latitude cyclones even further.
Baroclinic instability arises from temperature advection.
Where on the globe is there an overall strong thermal gradient and
why?
Cyclone & Anticyclone Paths
•Many well known paths for low and high pressure systems extend across
North America, and their interaction helps develop the open wave cyclone.
•Cyclogenesis describes the strengthening or development of these storms
into huge unstable waves.
•Cyclones developing on the eastern slopes of the Rockies are called leeside lows and are sometimes associated with redevelopment of previous
systems come onshore from the Pacific Ocean.
4 main stages in Cyclogenesis
Initial Development of Wave
Cyclone
• Jet streak upstream of short-wave
trough (often diffluent)
• Sinking motion upstream of trough
axis
• PVA aloft over incipient surface
low
• Weak surface system downstream of
short-wave trough
• Elongated area of clouds (baroclinic
leaf) parallel to mid-level flow
• Near surface low center: low-level
convergence, upper-level divergence,
rising motion, and pressure falls
Rapid Development Stage
• 500 hPa trough amplifies (height
falls west of trough axis and height
rises downstream )
• Cyclonic circulation results in
warm advection to east and cold
advection to west of surface low
• Temperature advection yields
dipole vertical motion field
• Comma cloud develops
• Surface pressures fall due to net
warming of column (low-level
WAA + tropopause undulation)
• Surface low moves eastnortheast towards greatest
pressure falls and max upward
motion
Mature (Open Wave) Stage
• mid-tropospheric flow changes
dramatically; trough becomes
negatively tilted
• SW flow increases SW of surface low
• Absolute vorticity increases
• Jet core migrates to southwest of
surface
• Expansion of clouds around warm
front and west of surface low
• Narrower cloud band along and
behind cold front
• Some western clouds are Sc by
mixing
• Dry slot develops due to subsidence;
part of dry conveyor belt and high
IPV air entering system
Occluded (Maximum Intensity) Stage
• Center cut off from warm sector
• Cold air at mid-lower levels over
cyclone
• 500 hPa vorticity maximum near
surface low center
• “Stacked” system in vertical –
advections are small
• Broad west-east cloud shield
• Occlusion not completely due to cold
front catching up to warm front – but
to low moving into cold air due to
dynamic processes
• Increasing ascent NW of low + cold
advection yields thickness minimum
which coincides with cyclone center
• Warm air in upper trop/lower
stratosphere overlays cold mid-lower
tropospheric air, allows pressures to
fall
Lee-Side Cyclone
• As westerly winds flow over a
mountain range, the airflow is deflected
to the south initially.
• These air parcels after traversing over
the mountain range will reemerge with
a northeasterly path.
• This motion leads to a formation of a
semi-persistent trough feature on the
eastern plains of Colorado called a leeside trough.
• If there is enough upper-level support,
this setup can develop even further and
creating a lee-side low.
•This concept is associated with the
“conservation of potential vorticity.”
Convergence & Divergence
•Rapid intensification of cyclones is
prohibited when low pressure aloft is
directly above the surface low.
•In this scenario, the convergence at the
surface low builds up air pressure and
fills in the low with excessive mass.
•The same stacking of high pressure,
with divergence at the surface, will
weaken the anticyclone.
Not idealized case for cyclogenesis
•In order for a surface cyclone to
strengthen, a high pressure must be over
it and vice versa for a surface high
pressure.
•This configuration will allow for mass
to be transported through the
atmosphere and promote a constant up
or downward vertical motion.
Idealized case for cyclogenesis
Storm Vertical Structure
•Divergence of air aloft occurs as
height contours intervals widen.
•Low pressure systems deepen and
intensify (e.g. cyclogenesis) when
upper-level divergence is stronger
than the surface convergence,
which requires a vertical
staggering of surface and upper
lows.
•When more air converges at the
surface than it is removed at the
top, this signifies a storm is filling
or dissipating and weakening as
surface pressure rises.
Divergence and Convergence
DIV = Divergence
CON = Convergence
•Convergence or confluence of air aloft creates downward motion.
•Divergence or diffluence of air aloft generates upward motion.
Upper Level Waves
•Earth's poles are
encircled by 3 to 7
longwaves, or Rossby
waves, directing upper
level winds around
lows at the 500 mb
surface.
•Longwaves or Rossby
waves are more trend
maker than weather
maker.
•Small disturbances in
these Rossby waves can
trigger storms.
Shortwave Disturbance
•Shortwave troughs within the larger Rossby wave move faster, and propagate downwind
into the Rossby troughs and cause them to deepen.
•Barotropic conditions, where height contours and isotherms are parallel (no temperature
advection) dominate in Rossby waves.
•Within shortwaves, the height contours and isotherms are usually slight out of phase
creating temperature advection.
•A shortwave can deepen or amplify a longwave collapse into a baroclinic state.
General Characteristics:
Long Waves
Short Waves
Number of Waves
3-7 across hemisphere;
typically 4-5
~ 15º-40º long. wide, move
through long wave troughs
Amplitude
Meridionally, on the order of
several 1000 km
Meridionally, on the order of
several hundred km, up to
1000 km
Wavelength
~50-120 longitude
~10-40 longitude
Diagnosed best at
At 500 hPa and above
At 500 hPa and below
Movement
Generally eastward at 10-15
knots; but can remain quasistationary or even retrogress
Dominantly eastward with a
northerly or southerly
component; faster than the
long wave troughs
Energy Regime
Barotropic or Equivalent
Barotropic
Definitely baroclinic
Weak Baroclinicity - Weak Temperature Advection
5C
10C
1500 m
Vg
1530 m
Strong Baroclinicity - Strong Temperature Advection
5C
0C
1500 m
Vg
1530 m
No Baroclinicity - No Temperature Advection
(Equivalent Barotropic Atmosphere)
1500 m
5C
Vg
1530 m
10C
Vertical Baroclinicity – Tilting of the System with Increasing
Heights
L
L
Cold
Cold
Cold
Cold
Cold
Cold
z
x
L
Cold
L
Cold
Classic ETC Model – Open Wave
L500
Lsfc
Warm Sector
Classic ETC Model – Occlusion
LL500sfc
Warm Sector
Cyclone Development: Upper Winds
A)
C)
B)
•Atmospheric conditions at the surface and aloft affect
cyclogenesis.
•An upper level shortwave can trigger baroclinic
instability, which converges flow aloft upstream, raises
high pressure, and supports cold air advection.
•Downwind, divergent flow aloft deepens the surface
low, and warm advection increases rising air flow.
•Eventually the system occludes.
Jet Streak Influence
Divergence aloft is
enhanced by the polar
front and/or subtropical jet
streams, where the jet
maximum, or jet streak,
forms in the tightly packed
gradients.
Jet Streak Convergence & Divergence
•Jet streaks can help to intensify a surface cyclone even farther.
•The polar front or subtropical jet streak forces air convergence
aloft upstream of the deepening open wave cyclone, and then
divergence downstream where the surface cyclone is located.
•When these wind maximums are gone, the cyclone degrades.
Left Exit Quadrant
Left Front Region
Upward Motion
Downward Motion
90 kt
Movement of
the Jet Streak
70 kt
Right Front Region
Upward Motion
Right Exit Quadrant
Downward Motion
Summary of Cyclone Weather
Upper and surface
maps illustrate the
role of convergence
and divergence aloft,
and the pattern of
clouds,
precipitation, and
temperatures on the
ground.
The upper- and lower-level lows are tilted vertically to provide
for the maximum upward vertical motion.
A mid-latitude
cyclone experiences
its maximum
intensity when the
upper-level low or
trough is juxtaposed
to the northwest of
the surface low
pressure.
Old Model
Conveyor Belt
Model
Old but Slightly Revised Model
•This model describes rising and sinking air along three conveyor belts, warm conveyor belt
rises with water vapor above the cold conveyor belt which also rises and turns.
•Finally the dry conveyor belt descends brining clearer weather behind the storm.
Revised View of Conveyor Belts Illustrating 3-D Deformation
Ng (2005) Doctoral Dissertation
Comma Clouds
•Rising and turning moist air, illustrated in the conveyor belt model,
condenses into a large comma-shaped cloud typical of the open wave
cyclone.
•This March 1993 storm wreaked havoc along the East Coast.
3/93 Storm Size & Pressure
For the storm of the century, the low pressure center reached 980
mb, and the storm extended across several southeastern states.
3/93 Storm Temperature Advection
Upper level winds flowed along a deep trough with steep baroclinic
cold and warm air advection.
Storm of Century Path
Low
pressure
values and
location are
charted with
time to
illustrate the
storm track
and
intensity,
moving from
Texas to
Maine in 2
days.
Vorticity & Cyclone Spin
•Vorticity describes the spin of an air parcel, which is positive in
counterclockwise cyclonic flow.
•Due to the conservation of angular momentum, vorticity
increases with a decrease in parcel radius (e.g. stretching due to
divergence aloft) (local vorticity) and increase in earth's latitude
(earth’s vorticity).
4 Types of Vorticity
• Relative (local)
• Earth’s (Coriolis parameter; f =2sin)
• Absolute (local + earth’s)
• Potential (absolute + depth)
Sources of Vorticity
•Curvature of upper level heights and winds, as well as strong
changes in wind speed, or shear, generate the spin of relative
vorticity.
•Additional earth’s vorticity is generated by the earth's spin, and
together they comprise absolute vorticity.
Curvature Vorticity
+
y
x
Shear Vorticity
y
x
Absolute Circulation = Constant = Absolute Vorticity x Area
DIV
as vorticity
decreases, area
increases
as vorticity
increases, area
decreases
y
x
CON
Trough to Ridge Vorticity Change
Anticyclonic spin around a ridge reduces absolute vorticity, but the convergence
and cyclonic spin in the trough enhance the relative vorticity and hence increases
absolute vorticity as well

 + 
 + 2
Div+
Div
Div
Con
Con+
Maximum  at
the base of the
trough:
45N
 > 0;  > 0
PVA
NVA
40N
35N
Minimum  at
the crest of the
ridge:
 < 0;  >> 0
30N
NVA
PVA
Vorticity & Vertical Motion
•The 500 mb map vorticity maximum is
a signal that to its east, air is diverging
aloft.
• If there is also convergence below,
then an open wave cyclone will likely
deepen.
• Hence, 500 mb charts are useful in
analyzing “vorticity maximum” and
predict potential storms in a quick,
crude way.
Vertical Storm Profile
Surface, 500 mb, and 200 mb
charts are used to illustrate the
structure of the February 1983
open wave cyclone exploding
over North Carolina.
The 500 mb chart shows a
shortwave dashed line moving
into the longwave trough and
baroclinic cold air advection.
February 1983 Vorticity
•Lines of equal
vorticity are
plotted on the 500
mb chart for the
February 1983
open wave cyclone
that buried the
east coast in
snowfall.
•Note that the
“vort max” is west
of the storm
center,
strengthening the
cyclogenesis.
Imaging Vorticity Centers
GOES West satellite infrared imagery of water vapor are useful in
identifying swilling vorticities, seen off Pacific Northwest coast.
Potential Vorticity = Absolute Vorticity / Depth = Constant
TROPOPAUSE
PV > 0:
D
>0
PV  0:
D
<0
PV > 0:
D
>0
Maximum  at
the leeward
side of the
mountain:
 > 0;  > 0
Maximum  at
the windward
side of the
mountain:
 > 0;  > 0
Minimum  at
the top of the
mountain:
 < 0;   0
acquiring
anticyclonic
curvature
acquiring
cyclonic
curvature
Polar Lows
•Cyclones that develop above
the polar front, called polar
lows, are smaller in size than
mid-latitude cyclones.
•They form during the winter,
have warm central cores,
strong winds, and generate
snow.
•Size ~ 1000 to 500 km
•Eye-like clear structure
similar to a hurricane with the
most intense winds and
precipitation near the eye core.