Download Figure 12.16 A Developing Mid

Mid-latitude Cyclones
Chapter 12
Polar Front Theory
• Polar front is a semi-continuous boundary
separating cold, polar air from more
moderate mid-latitude air
• Mid-latitude cyclone (wave cyclone) forms
and moves along polar front in wavelike
manner
• Frontal wave, warm sector, mature cyclone,
triple point, secondary low, family of
cyclones
Figure 12.1
The idealized life cycle of a mid-latitude cyclone (a through f) in the Northern
Hemisphere based on the polar front theory. As the life cycle progresses, the system
moves eastward in a dynamic fashion. The small arrow next to each L shows the
direction of storm movement.
Midlatitude Cyclones
Figure 12.2
A series of wave cyclones (a ―family‖ of cyclones) forming along the polar front.
Where do mid-latitude cyclones tend to
form?
•
•
•
•
•
•
Cyclogenesis – cyclone development
Lee side low’s
Nor’easters
Hatteras low
Alberta Clipper
Explosive cyclogenesis, bomb
Figure 12.4
As westerly winds blow over a mountain range, the airflow is deflected in such a way
that a trough forms on the downwind (leeward) side of the mountain. Troughs and
developing cyclonic storms that form in this manner are called lee-side
lows.
Figure 12.5
(a) Typical paths of winter mid-latitude cyclones. The lows are named after the region
where they form. (b) Typical paths of winter anticyclones.
• Topic: Northeasters
– Mid-latitude cyclones
that develop or intensify
off the eastern seaboard
of North America then
move NE along coast
Vertical Structure of Deep Dynamic
Lows
• Dynamic low = intensify with height
• When 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
Figure 12.6
If lows and highs aloft were always directly above lows and highs at the surface, the
surface systems would quickly dissipate.

Convergence and divergence
Figure 2
The formation of convergence
(CON) and divergence (DIV) of air
with a constant wind speed
(indicated by flags) in the upper
troposphere. Circles represent air
parcels that are moving parallel to
the contour lines on a constant
pressure chart.
Below the area of convergence the
air is sinking, and we find the
surface high (H).
Below the area of divergence the air
is rising, and we find the surface low
(L).
Figure 3
As the faster-flowing air in the ridge moves toward the slower-flowing air in the trough,
the air piles up and converges. As the slower-moving air in the trough moves toward the
faster-flowing air in the ridge, the air spreads apart and diverges.
Upper Level Waves and Mid-latitude
Cyclones
• Longwaves – planetary or Rossby waves
• Shortwave disturbances
Figure 12.8
A 500-mb map of the Northern Hemisphere from a polar perspective shows five
longwaves encircling the globe. Note that the wavelength of wave number 1 is as great
as the width of the United States. Solid lines are contours. Dashed lines show the
position of longwave troughs.
(a) Upper-air chart showing a longwave with three shortwaves (heavy dashed lines)
embedded in the flow.
(b) Twenty-four hours later the shortwaves have moved rapidly around the longwave.
Notice that the shortwaves labeled 1 and 3 tend to deepen the longwave trough, while
shortwave 2 has weakened as it moves into a ridge. Notice also that as the longwave
deepens in diagram (b), its length actually shortens. Dashed lines are isotherms in °C.
Solid lines are contours. Blue arrows indicate cold advection and red arrows, warm
advection.

Barotropic vs. baroclinic
 Barotropic – contours parallel to isotherms
 winds blow parallel to isotherms
 Baroclinic - isotherms cross contours
 So, Temperature advection occurs
 Causing - Cold and warm air advection
The Necessary Ingredients for
Development of Mid-latitude Storm
• Baroclinic instability
• Upper-Air Support: the overall effect of
differential temperature advection is to
amplify the upper level wave
• Upper air low may break away from main
flow and become a cut-off low
Figure 12.10
- formation of a mid-latitude cyclone during baroclinic instability.
(a) A longwave trough at 500 mb lies parallel to and directly above the surface stationary
front.
(b) A shortwave (not shown) disturbs the flow aloft, initiating temperature advection (blue
arrow, cold advection; red arrow, warm advection). The upper trough intensifies and
provides the necessary vertical motions (as shown by vertical arrows) for the
development of the surface cyclone.
c) The surface storm occludes, and without upper level divergence to compensate for
surface converging air, the storm system dissipates.
• Topic: Jet Streaks and Storms
– Entrance and exit regions associated with
divergence and convergence, right exit allows
divergence.
Figure 5
Changing air motions within a straight jet streak (shaded area) cause strong
convergence of air at point 1 (left entrance region) and strong divergence at
point 3 (left exit region).
Figure 12.11
(a) As the polar jet stream and its area of maximum winds (the jet streak, or core)
swings over a developing mid-latitude cyclone, an area of divergence (D) draws
warm surface air upward, and an area of convergence (C) allows cold air to sink.
The jet stream removes air above the surface storm, which causes surface
pressures to drop and the storm to intensify.
(b) When the surface storm moves northeastward and occludes, it no longer has the
upper-level support of diverging air, and the surface storm gradually dies out.
Figure 12.12
Summary of clouds, weather, vertical motions, and upper-air support associated with a
developing midlatitude cyclone.

Conveyor Belt Model: air constantly glides through
storm
 warm, cold, and dry conveyor belts
Figure 12.14
Visible satellite image of a mature mid-latitude cyclone with the three conveyor belts
superimposed on the storm. As in Fig. 12.13, the warm conveyor belt is in orange, the
cold conveyor belt is in blue, and the dry conveyor belt (forming the dry slot) is in yellow.
Figure 12.16 A Developing Mid-Latitude Cyclone – Storm of March 1993
Surface weather map for 4 a.m (EST) on March 13, 1993. Lines on the map are
isobars. A reading of 96 is 996 mb and a reading of 00 is 1000 mb. (To obtain the
proper pressure in millibars, place a 9 before those readings between 80 and 96, and
place a 10 before those readings of 00 or higher.) Green shaded areas are receiving
precipitation. Heavy arrows represent surface winds. The orange arrow represents
warm, humid air; the light blue arrow, cold, moist air; and the dark blue arrow, cold,
arctic air.
Figure 12.15
A color-enhanced infrared satellite image that shows a developing mid-latitude cyclone
at 2 a.m. (EST) on March 13, 1993. The darkest shades represent clouds with the
coldest and highest tops. The dark cloud band moving through Florida represents a line
of severe thunderstorms. Notice that the cloud pattern is in the shape of a comma.
Figure 12.17
The 500-mb chart for 7 a.m. (EST) March 13, 1993. Solid lines are contours where 564
equals 5640 meters. Dashed lines are isotherms in °C. Wind entries in red show warm
advection. Those in blue show cold advection. Those in black indicate no appreciable
temperature advection is occurring.
Figure 12.18
The development of the
ferocious mid-latitude cyclonic
storm of March, 1993. A small
wave in the western Gulf of
Mexico intensifies into a deep
open-wave cyclone over
Florida. It moves northeastward
and becomes occluded over
Virginia where its central
pressure drops to 960 mb
(28.35 in.). As the occluded
storm continues its
northeastward movement, it
gradually fills and dissipates.
The number next to the storm is
its central pressure in millibars.
Arrows show direction of
movement. Time is Eastern
Standard Time.
Vorticity, Divergence and
Development of Cyclones
• Vorticity is a measure of the spin of small
air parcels
– Positive: cyclonic
– negative: anticyclonic
• Divergence aloft causes an increase in the
cyclonic vorticity of surface cyclones =
cyclogenesis and upward air movement
Figure 12.19
When upper-level divergence moves over an area of weak cyclonic circulation, the
cyclonic circulation increases (that is, it becomes more positive), and air is forced
upward.
• Earth’s vorticity always positive
• Relative vorticity due to curvature of wind flow
+ shear ( change of wind speed over horizontal
distance)
– trough: cyclonic
– ridge: anticyclonic
• Absolute vorticity = Earth v + relative v
• An increase in absolute vorticity is related to
upper level convergence
• A decrease in absolute vorticity is related to
upper level divergence
• Vorticity maxima/minima
Figure 12.20 Earth’s Vorticity
Due to the rotation of the earth, the rate of spin of observers about their vertical axes
increases from zero at the equator to a maximum at the poles.
Figure 12.21 Relative Vorticity
In a region where the contour lines curve, air moving through a ridge spins clockwise
and gains anticyclonic relative vorticity. In the trough, the air spins counterclockwise
and gains cyclonic relative vorticity.
Figure 12.22
Areas of cyclonic (positive) relative vorticity and anticyclonic (negative) relative vorticity
can form in a region of strong wind-speed shear. Notice that the pinwheel changes its
direction of spin when placed above and below the region of maximum winds.
Figure 12.23
The vorticity of an air parcel changes as we follow it through a wave. From position 1 to position
3, the parcel’s absolute vorticity increases with time. In this region (shaded blue), we normally
experience an area of upper-level converging air. As the air parcel moves from position 3 to
position 5, its absolute vorticity decreases with time. In this region (shaded green), we normally
experience an area of upper-level diverging air.
Figure 12.24
A region of high absolute
vorticity—a vorticity
maximum—on its downwind
(eastern) side has diverging
air aloft, converging surface
air, and ascending air
motions. On its upwind
(western) side, there is
converging air aloft, diverging
surface air, and descending
air motions.
Figure 12.25
This infrared water vapor image shows regions of maximum vorticity as cyclonic swirls
of moisture off the coast of Oregon and Washington and out over the Pacific. The
stretched-out band of clouds toward the bottom of the picture is the intertropical
convergence zone.
• Vorticity and Longwaves
– Longwaves develop in upper-levels due to the
conservation of absolute vorticity.
– Absolute V = Earth’s V + relative V = CONSTANT
• Putting It All Together
• Forecasters review 200mb, 500mb, and surface maps to
examine pressure, convergence, vorticity, and advection
Figure 12.26
The atmospheric conditions for
February 11, 1983, at 7 a.m., EST. The
bottom chart is the surface weather
map. The middle chart is the 500-mb
chart that shows contour lines (solid
lines) in meters above sea level,
isotherms (dashed lines) in °C, and the
position of a shortwave (heavy dashed
line). The upper chart is the 200-mb
chart that illustrates contours, winds,
and the position of the polar jet stream
(dark blue arrow). The letters DIV
represent an area of strong
divergence. The region shaded orange
represents the jet stream core—the jet
streak.
Figure 12.27
The 500-mb chart for February 11, 1983, at 7 a.m., EST. Solid lines are height
contours in meters above sea level. Dashed lines are lines of constant absolute
vorticity × 10−5/sec.
Polar Lows
• Storms that develop over water behind (poleward of)
main polar front.
– Comma cloud, eye
– Warm central core, strong winds, heavy showery
precipitation.
– Arctic front = baroclinic instability
Figure 12.28
An enhanced infrared satellite image
of an intense polar low situated over
the Norwegian Sea, north of the
Arctic Circle. Notice that convective
clouds swirl counterclockwise about
a clear area, or eye. Surprising
similarities exist between polar lows
and tropical hurricanes described in
Chapter 15.