Transcript Slide 1

Chapter 13
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 (Norwegian)
Figure 13.1C-Open Wave
Figure 13.1A-Cylconic
Shear
Figure 13.1B-Frontal Wave
Figure 13.1DMature Cyclone
Mid-latitude Cyclone Life Cycle
• Begins with a stationary front with warm and cold winds
in opposite directions (e.g. wind shear).
• A wavelike kink, such as a low pressure system, then
creates a frontal wave, or incipient cyclone.
• As the storm develops into an open wave, a broad band
of precipitation forms ahead of the warm sector.
• As cyclone matures, the cold air “catches up” with the
warm sector, and low becomes occluded
• The mature cyclone is now at it’s strongest, and begins
to slowly weaken
• Eventually, the low becomes cut-off and dissapates
Triple Point
Figure 13.1E-Occlusion
Figure 13.1F-Dissapation
Wave Cyclone Development
A series, or family, of
cyclones, at various
stages of development,
may extend across
North America.
Energy originates from
kinetic sources such as
rising warm air, sinking
cold air, and converging
air, as well as latent heat
of condensation.
Figure 13.2
Cyclone & Anticyclone Paths
Lee
Side
lows
Generally
NE
Generally SE
Figure 13.3A
Figure 13.3B
• Many well-known paths for low and high pressure systems extend
across North America
• Cyclone development most common in a few specific areas
• Cyclogenesis describes the strengthening of these storms into huge
unstable waves.
Convergence & Divergence
Deepening of cyclones
through explosive
cyclogenesis is prohibited
when low pressure aloft is
directly above the surface
low.
In this scenario, convergence
at the surface low builds up
mass and fills in the low
(converging air has nowhere
to go).
The same stacking of high
pressure, with divergence at
the surface, will weaken the
anticyclone.
Figure 13.4
Vertical Storm Structure
Divergence of air aloft occurs as contour
interval widen (the inverse happens
when height contours become more
packed together)
Low pressure systems deepen and
intensify (cyclogenesis) when upperlevel divergence is stronger than the
surface convergence
This which requires a vertical
offset of the surface low and low
pressure above it.
Figure 13.5
Upper Level Waves
Earth's poles are encircled by 3
to 6 longwaves, or Rosby waves,
directing upper level winds
around lows at the 500 mb
surface.
Small disturbances (shortwaves)
in these larger waves can
trigger storms
Cyclones move in the direction
of the 500mb flow, at about ½ the
speed
Figure 13.6
Shortwave Disturbance
Shortwave “kinks” within
the Rossby waves move
faster, and propagate
downwind into the
Rossby troughs and cause
them to deepen.
Barotropic conditions,
where isobars and
isotherms are parallel,
then degenerate into a
baroclinic state where the
lines cross and cold or
warm air is advected
downwind.
Cyclone Development: Upper Winds
Figure 13.8A-Barotropic
Figure 13.8C-Barotropic
Differential temperature
advection intensifies the wave.
+ Existing surface frontal
boundary upper level shortwave
moving over the boundary-cyclogenesis occurs
Figure 13.8B-Barclinic Instability
Atmospheric conditions at the surface and
aloft affect cyclogenesis.
An upper level shortwave can trigger baroclinic
instability.
-Convergent flow aloft helps strengthen
surface high pressure, and supports cold air
advection
-Downwind of the upper level trof,
divergent flow aloft deepens the surface low,
and warm advection aids rising air flow
Finding a Low Center Using Wind
Observations
Jet Streak
Divergence aloft is
enhanced by the polar jet
stream, where the jet
maximum, or jet streak,
forms in the tightly
packed pressure
gradients.
Strong C
Strong D
Weak D
Figure 13.9
Weak C
Jet Convergence & Divergence
Figure 13.10A
Figure 13.10B
-The polar jet forces air convergence aloft upstream of the
deepening open wave cyclone, and then divergence
downstream.
-When these winds are gone, the cyclone degrades.
Summary of Cyclone Weather
Upper and surface
maps summarize
the role of
convergence and
divergence aloft;
and the pattern of
clouds,
precipitation, and
temperatures on
the ground.
Figure 13.11
Conveyor Belt 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.
(Upper Level)
Finally the dry conveyor
belt descends brining
clearer weather behind
the storm.
Figure 13.12
Comma Cloud Pattern
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.
Figure 13.13
Mar 1993 “Storm of the Century”
Figure 13.14
During the
“storm of the
century”, the
low pressure
center reached
980 mb, and
the storm
extended
across several
southeastern
states.
500mb Temperature Advection
Figure 13.15
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.
Figure 13.16
Size of Polar Lows
Cyclones that
develop north of
the polar front,
called polar lows,
are smaller in size
than mid-latitude
cyclones.
They form during
the winter in arctic
air, have
(relatively) warm
central cores,
strong winds, and
can generate snow.
Figure 13.17
Vorticity & Cyclonic Spin
Figure 13.18
Figure 13.19
- 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) and increase in earth's latitude.
Sources of Vorticity
Figure 13.21
Figure 13.20
- Curvature of upper level isobars and wind, as well as strong
changes in wind speed (shear), generate the spin of relative
vorticity.
- Additional earth vorticity is generated by the earth's spin, and
together they comprise absolute vorticity.
Trough to Ridge Vorticity Change
Figure 13.22
Anticyclonic spin
around a ridge
reduces absolute
vorticity, but the
convergence and
cyclonic spin in the
trough enhance the
relative and hence
absolute vorticity,
as illustrated in this
upper level
diagram.
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 to
find the “vort max” and predict
potential storms.
Figure 13.23
500mb Vorticity Chart
Imaging Shortwave Centers
GOES West
satellite infrared
imagery of
water vapor are
useful in
identifying
swirling
vorticities, seen
off Pacific
Northwest
coast.
Figure 13.24
Vertical Storm Profile
Surface, 500mb, and 200mb
charts are used to illustrate
the structure of the
February 1983 open wave
cyclone exploding over
North Carolina.
Figure 13.25
The 500mb 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.
Figure 13.26
Key Facts and Concepts
• The polar front theory is a model of how an ideal
storm progresses through the stages of birth,
maturity, and dissipation
• For a surface mid-latitude cyclone to develop or
intensify, the upper-level low must be located to the
west (or behind) the surface low
• Convergence is the piling up of air, divergence is
the spreading out of air
– Upper-level divergence above the developing surface
cyclone must be greater than surface convergence
– Upper-level convergence above developing surface
anticyclone must be greater than surface divergence
• As the jet stream develops into a looping wave pattern, it
produces areas of convergence and divergence
• The jet stream tends to direct anticyclones to the southeast and
cyclones to the NE
• When upper-level flow is disturbed by a shortwave, horizontal
and vertical air motions begin to enhance the formation of a
surface storm
– Rising warm air and sinking cold air provide energy for storm
formation
• As air flows over a mountain range, wind flow causes storms to
form or intensify
• Polar lows are storms that develop over water in polar regions
behind the main polar front (on the arctic front)
• Vorticity is a measure of the spin in a column of air
– An area of positive relative vorticity at 500mb can initiate
surface mid latitude cyclogenesis