Download Introductory materials

Chapter 6: General Circulation
This chapter discusses:
1. Mean Sea Level Pressure and Winds
2. Zonal winds
3. Meridional Transport of Momentum, Heat and
Moisture
Thermal Circulations
Solar heating and radiational cooling of earth's surface generates
cold-core thermal highs and warm-core thermal lows.
Winds can circulate between these two systems.
Wind Profiles
Changes in air temperature causing warm air to rise and cool air to
sink can also generate horizontal winds.
Rising warm air creates a surface low and upper level high.
Sinking cool air creates a surface high and upper level low.
Forces & Motion
Pressure forces are
only one influence on
the movement of
atmospheric air.
Air responds
similarly as water to
this force, moving
from higher pressure
to lower pressure.
Centripetal, friction,
and apparent
Coriolis are other
forces, however,
determining winds.
Sea and Land Breezes
Land heats more quickly than water, creating land-water
temperature differences along a coastline.
During the day the land's warm-core thermal low draws a sea
breeze, while at night, the warmer sea draws a land breeze.
Variation in Height
Isobaric (constant pressure) surfaces rise and fall in elevation with
changes in air temperature and density.
A low 500 mb height indicates denser air below, and less
atmosphere and lower pressure above.
Contour lines indicate rates of pressure change.
Ridges & Troughs
Upper level areas with high pressure are named ridges, and areas
with low pressure are named troughs.
These elongated changes in the pressure map appear as undulating
waves.
Surface & 500 mb Maps
Surface maps chart
pressure contours,
highs and lows, and
wind direction.
Winds blow clockwise
around highs, called
anticyclones.
500 mb maps reveal
patterns that on
average are 5600 m
above the surface,
where westerly winds
rise and fall across
ridges and troughs.
Pressure Gradient Force
Change in pressure per change in distance determines the
magnitude of the pressure gradient force (PGF).
Greater pressure changes across shorter distances creates a
larger PGF to initiate movement of winds.
PGF vs. Cyclonic Winds
Pressure
gradient force
(PGF) winds
acting alone
would head
directly into low
pressure.
Surface
observations of
winds, such as
the cyclonic
flow around
this low, reveal
that PGF winds
are deflected by
other forces.
Apparent Coriolis Force
Earth's rotation transforms
straight line motion into curved
motion for an outside viewer.
The Coriolis force explains this
apparent curvature of winds to
the right due to rotation.
Its magnitude increases with wind
velocity and earth's latitude.
Actual & Observed Paths
Airplane
travel paths
have an
apparent
curvature, just
as Coriolis
forces affect
winds.
Again, the
deflection
between actual
and observed
paths is
greater near
the poles.
Geostrophic Wind
Winds have direction and magnitude, and can be depicted by
vectors.
Observed wind vectors are explained by balancing the pressure
gradient force and apparent Coriolis force.
These upper level geostrophic winds are parallel to pressure
contours.
Wind Speed & Pressure Contours
Just as a river speeds and slows when its banks narrow and expand,
geostrophic winds blowing within pressure contours speed as
contour intervals narrow, and slow as contour intervals widen.
Isobars & Wind Prediction
Upper level pressure
maps, or isobars,
enable prediction of
upper level wind
direction and speed.
Northern & Southern Hemisphere Flow
Winds blow
counterclockwise
around low pressure
systems in the
Northern
Hemisphere, but
clockwise around
lows in the Southern
Hemisphere.
Friction & Surface Winds
Surface objects
create frictional
resistance to wind
flow and slows the
wind, diminishing
the Coriolis force
and enhancing the
effect of pressure
gradient forces.
The result is
surface winds that
cross isobars,
blowing out from
highs, and in
toward lows.
Figure 9.29A
Single-Cell Circulation Model
The basis for
average air flow
around the earth
can be examined
using a nonrotating, non-tilted,
ocean covered
earth.
Heating is more
intense at the
equator, which
triggers Hadley
cells to redistribute
rising heat from
the tropical low to
the polar highs.
Three Cell Circulation Model
A rotating earth
breaks the single cell
into three cells.
The Hadley cell
extends to the
subtropics, the reverse
flow Ferrel cell extends
over the mid latitudes,
and the Polar cell
extends over the poles.
The Coriolis force
generates westerlies
and NE trade winds,
and the polar front
redistributes cold air.
Observed Winds in January
Observed average
global pressure
and winds have
increased
complexity due to
continents and the
tilted earth.
Differential
ocean-land
heating creates
areas of semipermanent high
and low pressure
that guide winds
and redistribute
heat.
Observed Winds in June
Global pressure
and wind
dynamics shift
as the Northern
Hemisphere tilts
toward the sun,
bringing the
inter-tropical
convergence
zone, the Pacific
high, and
blocking highs
in the southern
oceans
northward.
North American Winter Weather
Semipermanent
highs redirect
North
American
winds, such as
cold interior
northerly flow
from the
Canadian high.
The Polar front
develops a
wave like
pattern as air
flows around
lows.
Global Precipitation Patterns
Global low pressure
zones around the
equator and 60°
latitude generate
convergence at the
surface, rising air
and cloud
formation.
Zones of high
pressure at 30° and
the Poles experience
convergence aloft
with sinking, drying
air.
Coastal Summer Weather
The semi-permanent Pacific high blocks moist maritime winds and
rain from the California coast, while the Bermuda high pushes
moist tropical air and humidity over the eastern states.
Coastal Winter Weather
During winter
months, the
Pacific high
migrates
southward and
allows for
maritime winds
with moisture
and rains to
reach California.
On the east
coast,
precipitation is
rather even
throughout the
year.
Mean Sea Level Pressure (January)
http://www.atmo.arizona.edu/students/courselinks/spring03/atmo421/sealevelpressure.html
Mean Sea Level Pressure (July)
Mean Sea Level Pressure and Winds
Excerpt:
RIME OF THE ANCIENT MARINER
Samuel Taylor Coleridge
The fair breeze blew, the white foam flew,
The furrow followed free:
We were the first that ever burst
Into that silent sea
Down dropt the breeze, the sails dropped down,
Twas sad as sad could be;
And we did speak only to break
The silence of the sea!
All in a hot and copper sky
The bloody Sun, at noon,
Right up above the mast did stand,
No bigger than the moon.
Day after day, day after day,
We stuck, nor breath nor motion:
As idle as a painted ship
Upon a painted ocean.
Excerpt:
RIME OF THE ANCIENT MARINER
Samuel Taylor Coleridge
The fair breeze blew, the white foam flew,
The furrow followed free:
We were the first that ever burst
Into that silent sea
(The trades)
Down dropt the breeze, the sails dropped down,
Twas sad as sad could be;
And we did speak only to break
(The doldrums)
The silence of the sea!
All in a hot and copper sky
The bloody Sun, at noon,
Right up above the mast did stand,
No bigger than the moon.
Day after day, day after day,
We stuck, nor breath nor motion:
As idle as a painted ship
Upon a painted ocean.
(Vicinity of the ITCZ)
Meridional & Zonal Flow
Wind direction and speed are indicated by lines, barbs, and flags,
and appear as an archer's arrow.
Upper level winds that travel a north-south path are meridional,
and those traveling a west-east path are zonal.
Jet Stream
High velocity Polar and subtropical jet stream winds are located
to “break” the tropopause, and they oscillate along planetary
ridges and troughs.
Polar Jet Formation
Steep gradients
of temperature
change at the
Polar front
trigger steep
pressure
gradients, which
then forces
higher velocity
geostrophic
winds.
This is the trigger
for jet stream
flow.
Winds & Angular Momentum
Angular momentum
is the product of
mass, velocity, and
the radius of
curvature and it must
be conserved.
As northwardflowing air
experiences a smaller
radius, it increases in
velocity and
augments the jet
stream flow.
Zonal Mean Winds
Convergence & Divergence
Deepening of cyclones
into explosive
cyclogenesis is
prohibited when low
pressure aloft is directly
above the surface low.
In this scenario, the
convergence at the
surface low builds up
air pressure and fills in
the low.
The same stacking of
high pressure, with
divergence at the
surface, will weaken the
anticyclone.
Storm Vertical Structure
Divergence of air aloft occurs as
isobars intervals widen.
Low pressure systems deepen and
intensify (e.g. cyclogenesis) when
upper-level divergence is stronger
than the surface convergence,
which requires a vertical staggering
of surface and upper lows.
Summary of Cyclone Weather
Upper and
surface maps
illustrate the role
of convergence
and divergence
aloft, and the
pattern of
clouds,
precipitation,
and
temperatures on
the ground.
Meridional Momentum Transport (DJF 2001)
Meridional Momentum Transport (JJA 2001)
Meridional Heat Transport (DJF 2001)
Meridional Heat Transport (JJA 2001)
Meridional Moisture Transport (DJF 2001)
Meridional Moisture Transport (JJA 2001)
Asian Monsoon Circulation
African Monsoon Circulation
North American Monsoon Circulation