Download Ch12Mid-LatCyclones

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
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
A series of wave cyclones (a “family” of cyclones) forming along the polar front.
Where do mid-latitude cyclones tend to
form?
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Cyclogenesis – cyclone development
Lee side low’s
Nor’easters
Hatteras low
Alberta Clipper
Explosive cyclogenesis, bomb
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.
(a) Typical paths of winter midlatitude cyclones. The lows are
named after the region where
they form.
(b) (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
If lows and highs aloft were always directly above lows and highs at the surface, the
surface systems would quickly dissipate.
An idealized vertical
structure of
a middle-latitude
cyclone and
anticyclone.
Convergence and divergence
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).
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
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
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).
(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.
Summary of clouds, weather, vertical motions, and upper-air support associated with a
developing midlatitude cyclone.
The conveyor belt model of a developing mid-latitude cyclone.
The warm conveyor belt (in orange)
rises along the warm front, causing clouds and precipitation to
cover a vast area.
The cold conveyor belt (in blue) slowly rises
as it carries cold, moist air westward ahead of the warm front
but under the rising warm air. The cold conveyor belt lifts
rapidly and wraps counterclockwise around the center of the
surface low.
The dry conveyor belt (in yellow) brings very dry,
cold air downward from the upper troposphere.
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.
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.
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.
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.
Air flow aloft at an altitude above 10,000 m (33,000 ft) on March 13, 1993. Notice that a
jet streak (orange shade) swings over northern Florida. The letters DIV represent an
area of strong divergence that formed above the surface low.
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
causing cyclogenesis and upward air
movement
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
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.
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.
Areas of cyclonic (positive) relative vorticity and anticy-clonic (negative) relative vorticity
can form in a region of strong horizontal wind-speed shear that can occur near a jet
stream.
Notice that parcels of air on either side of the jet stream have opposite directions of spin.
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.
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.
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
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.
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
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.