Wind Circulations, Fronts, &
Mid-Latitude Cyclones
Geostrophic Winds
• Geostrophic Wind
– Wind flowing in a straight path, parallel to the isobars at a
constant speed.
– Above the influence of friction (the Boundary Layer) a
given parcel will accelerate toward the lower pressure
due to the PGF.
– Coriolis force will deflect this parcel to the right (NH)
– The parcel will continue to accelerate until the PGF
balances the Coriolis force and the parcel is said to be in
geostrophic balance.
• Assuming geostrophic flow can usually give us a good
approximation of the speed and direction of the flow in the
upper atmosphere.
– Speed from spacing of the isobars.
• Closer together the PGF increases—wind speed
increases and the Coriolis force will increase to
maintain geostrophic balance.
Geostrophic Wind
Upper Level Low Pressure System
• PGF is greater than
Cor. The difference
b/w the PGF and Cor
is “net force”
• Centripetal force (due
to curvature) keeps
the wind blowing in a
circular pattern.
Upper Level High Pressure System
• Cor. > PGF
• Net force is outward
• Winds around a low
pressure system are
usually much stronger
due to the increased
PGF resulting from the
gradient wind.
Surface Pressure Patterns & Wind Flow
Surface Winds
• Winds in the BL are strongly influenced by friction
• Friction reduces the wind speed, which in turn reduces
the Coriolis force
• This creates an imbalance b/w the Cor and PGF, so the
wind blows across the isobras toward lower pressure.
• The winds near the sfc are not geostrophic due to friction
• Winds near sfc blow into a low and out of a high
• A way to remembering and applying the sfc and aloft
balances we have discussed is through the Buys-Ballot’s
Law
– Stand with wind aloft to our backs, lower pressure will
be to our left and higher pressure to our right in the
NH
– Stand with our backs to the wind, then turn clockwise
30º, lower pressure will be to our left.
Surface Map Showing Isobars
Local Winds
• Differential Heating
– Land and Sea Breeze
– Mountain Valley Breezes
– Country Breezes
• Chinook (Foehn) Winds
• Katabatic (Fall) Winds
Land Sea Breeze
• Land heats more rapidly, air above
• Land cools more rapidly,
expands and becomes less dense
contracts air above.
and rises
• High Pressure develops over
• At about 1 km AGL the air diverges ,
land.
blows toward sea, sinks and return
to land at low levels.
Since the strongest temp gradients occur right along the ocean land
boundary, the strongest winds occur at this time right along the edge of the
beach and diminish inland.
T contrast at night < T contrast day—stronger sea breeze
Rising motion—clouds
Day—land
Night—ocean
At the peak of the sea breeze (late afternoon), the cool ocean air be >300 m
thick and extend inland 12 miles, often dropping temps as much as 9 ºF in
an hour.
Warm air rising and cold air sinking—Direct Thermal Circulation (DTC)
Valley Breeze
• Daytime
• Valley Breeze
– Mountain slopes oriented
toward the sun (N-S) heat
more rapidly than flat land or
slopes oriented away from
the sun.
– The air above the warmer
slopes warms and expands
upward, then diverges at
higher altitudes
Mountain Breeze
• Nighttime
• Mountain Breeze
– Mountain slopes cool
more quickly and the
cooler dense air flows
from the slopes into the
valleys.
General Circulation
• Zonal (E-W) and Meridional (N-S)
• In 1735, George Hadley described a
model call the single cell model in
which
– Earth’s sfc is uniformly covered
– The sun is always directly over the
equator. (no seasonal shift of wind)
– The Earth does not rotate (no Cor,
only PGF)
• Strong heating at the Equator causes air
to expand upward, diverge toward both
poles at upper-levels and sink back to
the sfc before returning to the equator.
This scenario is far too simplified to
describe the earth’s flow, but it does
describe the “Thermally Direct
Circulations” of the tropics
• DTC warm air rises and cool air sinks
• ITC cold air rises and warm air sinks
3 Cell model
• Hadley Cell
– Solar heating along equator creates a zone of low
pressure called the Intertropical Convergence Zone
(ITCZ).
• Zone of strong UVM favoring heavy rainshowers,
especially in the afternoon.
• Easy to identify on Satellite imagery
• Migrates N or S about 10 º through seasons
• Also called the Doldrums, b/c of the hot and humid
conditions.
• 20 -30 º lat, air begins to sink forming the
Subtropical Highs
• Descending air warms adiabatically (10ºC/km),
suppressing cloud formation and producing desert
conditions in the subtropics.
• Weak pressure gradients and light winds
• “Horse Latitudes”
– Strongest in the winter hemisphere, when the Temp
gradient is strongest
• Ferrel Cell
– Located b/w 30 and 60 º latitude
– An ITC results from the circulation of the
Hadley and Polar Cells
– Sfc air moves toward the sub-polar lows at ~ 60
º latitude air rises, returns to subtropics and
sinks
– This creates the Westerlies in the NH, the
would be southerly winds are deflected by Cor.
Causing winds to become westerly
• Polar Cell
– b/w 60 and 90 º (the poles)
– DTC since warm air rises
– Sfc air moves from the Polar highs to sub-polar
lows
– These are called the Polar Easterlies.
Midlatitude Cyclones
• Also known as Extratropical Cyclones (ETCs)
• Often form lee of the Rockies
• Describe a low pressure system
– Cyclonic flow … low pressure leads to sfc con
which leads to UVM which in turn leads to clouds
and precip
• This is the principle wx-maker in the
midlatitudes
• Covers 1000’s of square miles, and has a
lifespan of 3-10 days
Midlatitude Cyclones
• Cyclogenesis—the birth of a cyclone,
originates along a polar front.
• Polar Front Theory—Theory of how a cyclone
develops…from the Norwegians in the early
1900’s.
Before Storm
• System originates as
stationary front b/w cold,
polar air and warmer air
• The front represents a
trough of low pressure with
High pressure on either
side
• The cold air to the north
and warm air to the south
flow parallel to the front
but in opposite directions.
This type of flow sets up
cyclonic wind shear.
Wave Develops
• This is known as a frontal wave
or incipient cyclone
• Now we can identify a warm
front pushing northward and a
cold front pushing southward
• The region of lowest pressure is
called the central pressure and is
located at the junction of the
two fronts
• As the cold air displaces the
warm air upward ahead of the
cold front, and as overrunning
occurs ahead of the warm front,
a narrow band of precip. usually
occurs.
Cyclonic-circulation
• Steered by winds aloft, the
system typically moves E or NE
and gradually becomes a fully
developed open wave in 12-24 h
• Central pressure much lower,
several isobars fully encircle the
wave’s apex
• Creates a stronger cyclonic flow,
as the winds swirl
counterclockwise and inward
toward the low’s center.
• Precip wide band ahead of the
warm front and along a a narrow
band of the cold front.
• Warm air region is called the
warm sector (usually partly
cloudy, but could have scattered
showers if air is unstable)
Storm’s Energy Source
• As the air masses try to attain equilibrium, warm air
rises and cool air sinks…thereby changing potential
energy to kinetic energy.
• Condensation supplies energy to the system in the
form of latent heat.
• As the surface air converges toward the low’s
center, wind speed may increase, producing an
increase in kinetic energy.
• NOTE: Kinetic energy is the energy of motion.
Occlusion begins
• As the open wave continues
to progress eastward, its
central pressure continues
to decrease, and the winds
move more vigorously
• The cold front continually
inches closer to the warm
front, squeezing the warm
sector into a smaller area.
Occluded front developed
• At this time the storm is usually the
most intense, with clouds and
precip covering the largest area
• The point of occlusion, where the
cold, warm, and occluded fronts all
come together is called the triple
point.
• Notice: in this region, the cold and
warm front resembles the open
wave cyclone discussed previously.
It is at the triple point where a new
wave will occasionally form. This is
called “secondary cyclone”, and
may move eastward and intensify
into a cyclonic storm.
Cyclone dissipates
• Center of the intense storm
will now weaken, as cold air
is on both sides of the
occluded front (decreases
the temp. gradient)
• The warm sector is still
present, but is far removed
from the center of the
system
• Without the supply of
energy provided by the
rising warm, moist air, the
old system usually dies out
and gradually dissipates.
• Total lifespan can last from about 3 days to well
over a week
• When an ETC deepens rapidly (in excess of 24 mb
in 24 h), the term explosive cyclogenesis or bomb
is used.
• Frontal waves that develop into huge storms are
called unstable waves. These waves form
suddenly, grow in size then dissipate with the
entire process typically taking 3-10 days.
• Other waves called stable waves remain small and
never grow into giant weather producers.
• Why is it that some waves develop into huge storms
while other simply dissipate in a day or so?
– There are many surface conditions that influence the
formation of a wave, including mountain ranges and
land-ocean temperature contrasts. However, the real
key to the development of a wave cyclone lies in the
upper-level flow in the region of high level westerlies.
• So, before we can really answer the question, we
need to see how the winds aloft influence the
surface pressure systems.
Common Cyclone Tracks
Upper-level support for cyclogenesis
• Jet Stream—Relatively strong winds
concentrated within a narrow band in the
atmosphere.
• Jet Streak—A region of high wind speed that
moves through the axis of a jet stream
• Why do some waves develop into huge storms
and others die out quickly?
• Recall the definitions of convergence and
divergence:
– Convergence – The piling up of air (L)
– Divergence– The spreading out of air (H)
Convergence
• If we have a low
pressure at both the
surface and upper
levels
– Then we have CON at
both levels
– We are “piling up” air
throughout the
column.
L
L
Initia l
a lo ft L
s fc L
F ina l
• We are increasing the
pressure and thus
getting rid of our low
pressure.
• But we want to make
our pressure lower,
not higher! We want
cyclogenesis.
So how can we explain this?
• When we look at a
low pressure system
at multiple levels, we
see that the vertical
axis tilts backward
toward the cold air.
Upper Air Support for Mid-Lat Cyclone
• As long as upper-level DIV is greater than sfc
CON the low will continue to intensify b/c we
are taking out ore air at the top than is
brought in at the bottom.
• The reverse is true for a weakening low, as
well as a strengthening high.
• So to understand this lets examine how we
describe the upper-level flow.
Longwaves
• The equator receives more radiation than the
poles.
• The atmosphere redistributes this heat
through the waves
• There are usually 3 to 6 longwaves that
encircle the earth at any time.
• We measure the wave from trough to trough
(or ridge to ridge)
Zonal Flow
• A smaller number of waves present
• Flow is more east to west
• A “flatter flow”
Meridional Flow
• A larger number of waves present
• Flow is more north south
• This when we can get cold air outbreaks.
Shortwaves
• Shortwaves are smaller disturbances or ripples
imbedded in longwaves
• So shortwaves propagate through the
longwaves
• Longwaves move slowly, while shortwaves
more more rapidly.
Two atmospheric States
• Barotropic
• Baroclinic
Barotropic
• This is a state where the temperature contours
are parallel to the height lines
• In the upper levels the winds blow parallel to
the height lines.
• This means we are not moving air of different
temps around.
• This state does not commonly occur, except in
the tropics.
Baroclinic
• Isotherms cross the heights lines, we are
moving different temp to different areas.
– Warm Air Advection (WAA) warm to cold
– Cold Air Advection (CAA) cold to warm
• Advection means moving air horizontally
• Convection mean moving air vertically
• This state (Baroclinic) is usually present in the
atmosphere to varying degrees.
• Shortwaves disturb the flow and increase
regions of baroclinicity.
• This is known as an instability
– Baroclinic Instability
• Enhances the formation of Low pressure systems through
horizontal and vertical air motions
• Vertical air motions result b/c cold, heavy air sinks and
warm, light air rises
– This provides the energy for the developing cyclone, as potential
energy is transformed into Kinetic Energy.
• Eventually the system occludes as the supply of warm is
cut off by the cold front. System is dying out.
• Sometimes the surface will have an upper level low over it
as a storm dies out.
– This will result in rapid filling of the low