Mid-Latitude Cyclones
and Fronts
Lecture 12
AOS 101
Homework 4
COLDEST
TEMPS
GEOSTROPHIC BALANCE
Homework 4
L
FASTEST
WINDS
Rising Air
• Consider an air parcel rising
through the atmosphere
– The parcel expands as it rises
– The expansion, or work done
on the parcel causes the
temperature to decrease
• As the parcel rises, humidity
increases and reaches
100%, leading to the
formation of cloud droplets
by condensation
Rising Air
• If the cloud is sufficiently
deep or long lived,
precipitation develops.
• The upward motions
generating clouds and
precipitation can be
produced by:
– Convection in unstable air
– Convergence of air near a
cloud base
– Lifting of air by fronts
– Lifting over elevated
topography
Lifting by Convergence
• Convergence exists
when there is a
horizontal net inflow
into a region
• When air converges
along the surface, it is
forced to rise
Background on Cyclones
•A cyclone is: An area of low
pressure around which the
winds flow counter-clockwise
in the northern hemisphere,
and clockwise in the southern
hemisphere
• Hurricane (tropical cyclone)
• Mid-latitude cyclone
• Today, we’ll focus on midlatitude, or extra-tropical
cyclones, which have a life
cycle and frontal structures.
Hurricanes, which we
discussed earlier, have no
fronts.
http://www.wunderground.com/hurricane/history/iop4_sat.jpg
Background on Cyclones
Midlatitude cyclones are crucial in maintaining a
temperature equilibrium on our planet. This is
because in the northern hemisphere . . .
• . . . They advect warm air northward
• . . . And they advect cold air southward
• This helps maintain the radiative equilibrium on
our planet!
Background on Cyclones
• We already know that
friction near the surface
causes convergence into a
low pressure center and
that flow is
counterclockwise around
the low in the N.
Hemisphere.
• So we end up with the cold
air moving south and east
and the warm air moving
north and west
• Likewise, lifting by
convergence forces parcels
upward so we get clouds
and precipitation in the
vicinity of the low pressure
L
Background on Cyclones
The figure to the right
represents a typical
midlatitude cyclone:
• Cold, dry air is advected
eastward behind the cold
front
• Warm, moist air is
advected north behind
the warm front
• The fronts move in the
direction the “teeth” point
Background on Cyclones
• Definition - boundary, transition zone between
two different air masses
• The two air masses have different densities.
Frequently, they are characterized by different
temperatures and moisture contents
• Front has horizontal and vertical extent
• Frontal boundary/zone can be 1-100 km wide
• Types of synoptic-scale fronts:
–
–
–
–
warm fronts
cold fronts
stationary fronts
occluded fronts
Warm Front
• A transition zone where a warm air mass
replaces a cold air mass
• Drawn as a red line with red semi-circles
pointing in the direction of the front’s
movement
Warm Front
• Again, warm air is less dense than cold air.
• As the warm air moves north, it slides up the
gently sloping warm front.
• Because warm fronts have a less steep slope than
cold fronts, the precipitation associated with warm
fronts is more “stratiform” (less convective), but
generally covers a greater area.
Common Characteristics Associated with
Warm Fronts
Before Passing
While Passing
After Passing
Winds
South-southeast
Variable
South-southwest
Temperature
Cool-cold, slowly
warming
Steady rise
Warmer, then
steady
Pressure
Usually falling
Leveling off
Slight rise, followed
by fall
Clouds
Cirrus,
Cirrostratus,
Nimbostratus
Stratus-type
Clearing with
scattered
Stratocumulus
Precipitation
Light to moderate
rain, snow, sleet or
drizzle
Drizzle or none
Usually none,
sometimes light
rain in showers
Visibility
Poor
Poor, but
improving
Fair in haze
Dew Point
Steady rise
Steady
Rise, then steady
Cold Fronts
• A transition zone where a cold air mass
replaces a warm air mass
• Drawn as a blue line with blue triangles
pointing in the direction of the front’s
movement
Cold Fronts
•Cold air is more dense than warm air.
• As the dense, cold air moves into the warm air region, it forces
the warm air to rapidly rise just ahead of the cold front.
• This results in deeper clouds and precipitation than we saw with
a warm front. The clouds that form can be convective and can be
associated with more intense precipitation and thunderstorms
• Often, the precipitation along a cold front is a very narrow line of
thunderstorms
Common Characteristics Associated with
Cold Fronts
Before Passing
While Passing
After Passing
Winds
South-southwest
Gusty; shifting
West-northwest
Temperature
Warm
Sudden drop
Steadily dropping
Pressure
Falling steadily
Minimum, then
sharp rise
Rising steadily
Clouds
Increasing: Cirrus,
Cirrostratus,
Cumulonimbus
Cumulonimbus
Cumulus
Precipitation
Short periods of
showers
Heavy rains,
sometimes with
hail, thunder,
lightning
Showers, then
clearing
Visibility
Fair to poor in haze Poor, followed by
improving
Good, except in
showers
Dew Point
High; remains
steady
lowering
Sharp drop
Occluded Fronts
• A region where a
faster moving cold
front has caught up to
a slower moving warm
front.
• Generally occurs
near the end of the life
of a cyclone
• Drawn with a purple
line with alternating
semicircles and
triangles
Cold Occlusion
• The type most associated
with mid-latitude cyclones
• Cold front "lifts" the warm
front up and over the very
cold air
• Associated weather is
similar to a warm front as
the occluded front
approaches
• Once the front has passed,
the associated weather is
similar to a cold front
• Vertical structure is often
difficult to observe
http://apollo.lsc.vsc.edu/classes/met130/
notes/chapter11/index.html
Warm Occlusion
• Cold air behind cold
front is not dense
enough to lift cold air
ahead of warm front
• Cold front rides up and
over the warm front
• Upper-level cold front
reached station before
surface warm occlusion
http://apollo.lsc.vsc.edu/classes/met130/not
es/chapter11/index.html
Stationary Front
• Front is stalled
• No movement of the temperature gradient
• But, there is still convergence of winds,
and forcing for ascent (and often
precipitation) in the vicinity of a stationary
front.
• Drawn as alternating segments of red
semicircles and blue triangles, pointing in
opposite directions
How to Locate a Cyclone
1. Find the region of
lowest sea level
pressure
2. Find the center of the
cyclonic (counterclockwise) circulation
How to Locate a Cyclone
1. Find the region of
lowest sea level
pressure
L
2. Find the center of the
cyclonic (counterclockwise) circulation
How to Locate a Front
We know that we need to look for low pressure
and a boundary of cold and warm air.
To pinpoint the parts of our cyclone, look for
specifics in the observation maps
• Find the center of cyclonic rotation
• Find the large temperature gradients
• Identify regions of wind shifts
• Identify the type of temperature advection
• Look for kinks in the isobars
Polar Front Theory - Development and
Evolution of a Wave Cyclone
Also, referred to as Norwegian Cyclone Model
(NCM)
• The wave cyclone (often called a
frontal wave) develops along the
polar front
• When a large temperature gradient
exists across the polar front - the
atmosphere contains a large amount
of Available Potential Energy
NCM cont.
• Northward moving warm air
and southward moving cold
air are forced around each
other, forming a bend in the
temperature gradient (b). This
forms the warm front and the
cold front. Now with a
counter-clockwise spin, winds
converge at the newly formed
low pressure minimum at the
center of rotation.
NCM cont.
• (c)- A fully-developed cyclone is
seen 12-24 hours from its inception.
It consists of:
– a warm front moving to the northeast
– a cold front moving to the southeast
– region between warm and cold fronts
is the "warm sector"
– central low pressure (low, which is
deepening with time)
– wide-spread precip. ahead of the warm
front
– narrow band of precip. along the cold
front
– Wind speeds continue to get stronger
as the low deepens
– The production of clouds and precip.
also generates energy for the storm as
Latent Heat is released
http://apollo.lsc.vsc.edu/classes/met130/notes/chapte
r12/index.html
NCM cont.
(d) - As the cold front
moves swiftly eastward,
the systems starts to
occlude.
– Storm is most intense at
this stage
– have an occluded front
trailing out from the
surface low
– triple point/occlusion - is
where the cold, warm,
and occluded fronts all
intersect
http://apollo.lsc.vsc.edu/classes/met130/no
tes/chapter12/index.html
Final Stage
(e) - the warm sector
diminishes in size as the
systems further occludes.
– The storm has used most all of
its energy and dissipates
– cloud/precip production has
diminished
– The warm sector air has been
lifted upward
– The cold air is at the surface stable situation.
• The temperature
contrast which drove this
whole situation from the
surface perspective is no
longer near the center of
the wave of low pressure
http://apollo.lsc.vsc.edu/classes/met130/not
es/chapter12/index.html
Another view
Weather associated with a typical late fall to
early spring mid-latitude cyclone
Figure courtesy of Jon Martin
Precipitation Around a Cyclone and its Fronts
To the right is a major cyclone
that affected the central U.S. on
November 10, 1998.
Around the cold front, the
precipitation is more intense, but
there is less areal coverage.
North of the warm front, the
precipitation distribution is more
“stratiform”: Widespread and
less intense.
http://weather.unisys.com
Precipitation Around a Cyclone and its Fronts
Again, in this radar and surface
pressure distribution from
December 1, 2006, the
precipitation along the cold front
is much more compact and
stronger.
North of the warm front, the
precipitation is much more
stratiform.
Also note the kink in the isobars
along the cold front!
• We have all seen cyclones on weather maps,
but how do we know if it will strengthen or
weaken? The key to cyclone development is
in the upper level flow
So let’s look at Upper Levels…
Typical 500 mb height pattern
Similarly to lower levels, at upper levels of the atmosphere,
there is often a series of high pressures (high heights) and
low pressures (low heights)
Upper Levels
Ridge
Trough
Ridge
Why Do These Patterns Occur?
• The patterns of convergence and divergence
have to do with vorticity advection
• If there is positive vorticity advection,
divergence occurs
• If there is negative vorticity advection,
convergence occurs
• Let’s explain vorticity …
Vorticity
Vorticity is simply a
measure of how much the
air rotates on a horizontal
surface
Positive vorticity is a
counterclockwise (i.e.
cyclonic) rotation
Negative vorticity is a
clockwise (i.e. anticyclonic)
rotation
Therefore, troughs contain
positive vorticity, and
ridges contain negative
vorticity
Trough
Ridge
Let’s Revisit …
Vorticity < 0
Vorticity < 0
Vorticity > 0
Diagnosing Vorticity Advection
• To determine vorticity advection, first find the
locations of maximum (positive) vorticity and
minimum (negative) vorticity
• Then, determine what direction the wind is
moving
• Areas of negative vorticity advection (NVA)
will be just downstream of vorticity minima,
and areas of positive vorticity advection (PVA)
will be just downstream of vorticity maxima
Negative
vorticity
advection
Positive
vorticity
advection
Vorticity Advection and Vertical Motion
* Positive vorticity advection (PVA) results
in divergence at the level of advection
* Negative vorticity advection (NVA)
results in convergence at the level of
advection
Vorticity Advection and Vertical Motion
Remember that convergence at upper levels is associated with
downward vertical motion (subsidence), and divergence at upper
levels is associated with upward vertical motion (ascent).
Then, we can make the important argument that . . .
Upper Tropospheric Flow and Convergence/Divergence
• Downstream of an upper tropospheric ridge, there is convergence,
resulting in subsidence (downward motion).
• Likewise, downstream of an upper tropospheric trough, there is
divergence, resulting in ascent (upward motion).
Upper Tropospheric Flow and Convergence/Divergence
• Downstream of an upper tropospheric ridge axis is a favored
location for a surface high pressure, and of course, downstream of
an upper tropospheric trough axis is a favored location for a surface
low pressure center.
Upper Tropospheric Flow and Convergence/Divergence
• Surface cyclones also move in the direction of the upper
tropospheric flow!
• The surface low pressure center in the diagram above will track to
the northeast along the upper level flow