Today in Weather History
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1909- Tornado outbreak in Arkansas, The town of Brinkley AR was struck by
a tornado which killed 49 persons and caused 600,000 dollars damage. The
tornado, which was two-thirds of a mile in width, destroyed 860 buildings.
Entire families were killed as houses were completely swept away by the
tornado. Tornadoes killed 64 persons and injured 671 others in Dallas and
Monroe counties during the Arkansas tornado outbreak.
1989 - While arctic cold gripped the northeastern U.S., unseasonably warm
weather prevailed across the southwestern states. Albany NY reported a
record low of 2 degrees below zero. Tucson AZ reported a record high of 90
degrees.
1990 – Long track supercell produced severe weather in east central Iowa
and west central Illinois. Supercell spawned a tornado south of Augusta IL
which traveled 42 miles to Marbleton. Golf ball size hail was reported at
Peoria IL and near Vermont IL.
• Launch of new GOES-P
geostationary satellite
• Satellite also carries an
X-ray imager to sense xray emissions from the
sun
• Joins GOES 11, 12, 13
Global Circulation
Chapter 7
ATMO 1300
Fall 2009
General Circulation of the
Atmosphere
• Represents the average wind flow around the
globe…
• Winds at any one place may vary substantially
from this average
• Underlying cause of the general wind pattern
around the globe is the unequal heating of the
Earth’s surface
• Remember: Energy surplus at the equator
compared to the poles!
• Circulation patterns develop in an attempt to
achieve balance
General Circulation of the
Atmosphere
• Flow of air across the globe is quite
complex
• Can be viewed in the form of conceptual
models
• Simplified examples of the flow of air
Fig. 7-11, p. 197
Single-Cell Model
• Assumes Earth’s surface is uniformly
covered by water
• Assumes the Sun is always directly over
the equator
• Assumes the Earth is not rotating
• So… the only force we have to consider is
pressure gradient (no rotation = no
Coriolis)
Single-Cell Model
Single-Cell Model
• Single-cell model
referred to as Hadley
cell
• Where might this
model be applied?
Hadley Cell
• Thermally driven convective cell.
• Energy surplus at the equator creates and area of
rising motion and thus low pressure at the
surface near the equator
• At the poles, the energy deficit creates sinking
motion and high pressure near the surface
• Pressure gradient force is directed southward
(northern hemisphere) from the pole toward the
equator (remember no Coriolis)
Hadley Cell
• At the upper-levels the flow of air is reversed
• Air flow from the equator toward the poles
• Through this manner, some of the excess of
energy in the tropics is transported as sensible
and latent heat to the regions where there is an
energy deficit (poles)
• Closed circuit of air
• Does not happen on Earth, Earth is rotating
• So what does the Earth’s rotation do???
Three-Cell Model
• Allow the Earth to rotate now…
• The simplified convective cell is now
broken down into 3 cells.
• Remember: Tropics still have an energy
surplus
• Broad trough still located at the equator
• Broad ridge located near the poles
Fig. 7-6, p. 193
Three-Cell Model
• Between 0˚ and 30˚ of latitude the convective
cells resemble our Hadley Cell (Single Cell
model)
• Over equatorial oceans, air is warm and pressure
gradient is relatively weak. Therefore surface
winds are generally light. Referred to as the
Doldrums
• Often warm air rises into deep towering
cumulus clouds, can be referred to as “Hot
Towers” due to the enormous amount of Latent
Heat that is released
Three-Cell Model
• The latent heat released within tropical
convection aids in strengthening the Hadley Cell
due to enhanced rising motion over the tropics
• As the rising air reaches the top of the
troposphere it spreads out poleward
• Coriolis now deflects the flow to the right
(northern hemisphere) or left (southern
hemisphere)
• The Coriolis effect now produces westerly winds
aloft in both hemispheres
Three-Cell Model
• As air moves poleward it cools and converge as it
approaches the middle latitudes
• The convergence leads to a piling up of air aloft
• This increases the pressure at the surface (more stuff
now above)
• Creates sub-tropical highs
• The converging air aloft leads to sinking motion beneath
the sub-tropical highs. The subsiding air warms and
produces generally clear skies and warm surface
temperatures
• This is what produces the worlds deserts
• Over the oceans, the center of the subtropical highs
produce weak winds, referred to as “Horse Latitudes”
Three-Cell Model
• From the Horse Latitudes some of the surface air flows
back toward the equator
• Coriolis deflects this air creating our low-level easterlies
or trade winds
• Air moves from the northeast in the northern
hemisphere and the southeast in the southern
hemisphere
• These are fairly steady surface winds and provided
sailing ships with an ocean route to the Americas. Hence
the title “Trade Winds”
• Near the equator the trade winds in each hemisphere
collide creating a large area of surface convergence,
Inter-Tropical Convergence Zone (ITCZ)
Typical Locations of the ITCZ
ITCZ varies little by the season, due to the required energy to change the temperature
of water
Fig. 7-20, p. 203
Three-Cell Model
• Back to 30˚ Latitude… not all the surface air moves
toward the equator, some move towards the poles
• This air is deflected by Coriolis creating middle latitude
westerly winds
• This flow is not consistent as migrating areas of surface
high and low pressure break up the flow pattern
• As the air travels poleward it encounters cold air moving
equatorward from the poles. The two airmasses (more
later) do not mix readily (think density differences)
• The boundary between these airmasses is referred to as
the Polar Front
• The upper-level Jet Stream resides in this area
D:\Media\Active_Figures\56_General_Circulation\G_56.html
Fig. 7-13, p. 199
Fig. 7-14, p. 199
Fig. 7-1, p. 189
General Circulation of the
Atmosphere
• What happens in the “Real World”?
• A few area of semi-permanent highs and lows
• In the eastern Atlantic we see the BermudaAzores high, drives tracks of tropical cyclones
• Counterpart in the Pacific, Pacific High
• Islandic Low, Aleutian Low occur where we
expect the polar front to lie.
• Siberian High
Fig. 7-12a, p. 197
Fig. 7-12a (1), p. 197
Fig. 7-12a (2), p. 197
General Circulation of the
Atmosphere
• Thermal lows lead to monsoon
circulations
• Leads to moist flow off the oceans onto
the adjacent land surfaces.
• Often responsible for large percentage of
precipitation in these regions
Fig. 7-21, p. 205
General Circulation and
Precipitation Patterns
• On the global scale… we expect abundant
rainfall where we see general rising motion.
• Thus over the tropics, there are areas of high
precipitation due to the ITCZ
• Also high precipitation between 40˚ and 55˚
latitude where mid-latitude storms and the polar
front force air upward
• Areas of low precipitation are found near 30˚
latitude in the vicinity of sub-tropical highs
Fig. 7-4a, p. 191
Fig. 7-4b, p. 191
Average Wind Flow and Pressure
Patterns Aloft
Average Wind Flow and Pressure
Patterns Aloft
• In both hemispheres, the air is warmer over the
low latitudes and colder over the high latitudes
• This creates a temperature gradient which leads
to a pressure gradient which causes our
generally westerly flow in the mid-latitudes
• Temperature gradients are steeper in winter than
summer. Consequently the winds aloft are
stronger during the winter than summer
• In the mid and high latitudes wind speeds will
continue to increase above 500 mb (no slowing
due to surface friction)
Average Wind Flow and Pressure
Patterns Aloft
• Geostrophic wind is related to the pressure
gradient but also inversely proportional to
density
• Density decreases with altitude in the
atmosphere, therefore a given pressure gradient
will result in stronger winds with altitude.
• We see concentrated bands of strong winds near
the top of the troposphere – The Jet Stream
Jet Stream
• High speed “rivers” of
wind
• Several thousand km
long but only a few km
deep
• Wind speeds can often
exceed 100 kts
• Polar Jet (near the polar
front)
• Sub-tropical Jet (over the
subtropical high)
Polar Jet
• In the winter the Polar Jet is typically
stronger and located further south
• In the summer the jet is weaker and
retreats well into the higher latitudes (less
temperature gradient, less pressure
gradient)
Sub-Tropical Jet
• Forms along the northern side of the
Hadley Cell circulation
• Warm air carried poleward by the Hadley
Cell produces sharp temperature contrasts
• Sharp temperature contrasts produce
sharp pressure gradients and therefore
stronger winds
Jet Streams
• Another mechanism can produce strong westerly flow aloft (other
than temperature gradients)
• Conservation of Angular Momentum
• Air on the earth moves in a circular pattern due to the Earth’s
rotation and therefore has angular momentum
• Angular momentum is dependant on: Mass, Speed, and distance
(radius) between the blob of air and the axis about which it rotates.
Angular Momentum = mVr
So if there are no external forces at work on the rotating system…
angular momentum is conserved or does not change
So if we decrease distance or radius… Speed must increase to
compensate.
Fig. 7-8, p. 194
Conservation of Angular
Momentum and Jet Streams
• Consider heated parcels or blobs of air rising
from the equator…
• The air parcels rise and reach the tropopause,
they spread out and head poleward
• The air as it moves poleward gets closer to the
axis of rotation of the earth (e.g. radius of the
earth gets smaller with increasing latitude
• Because angular momentum is conserved, the
speed of the parcels must increase (mass of the
air is unchanged)
Fig. 7-7, p. 194
Fig. 7-9, p. 195
General Circulation of the
Atmosphere
• We know that the winds in the upperatmosphere (troposphere) flow in a wave-like
pattern with troughs and ridges
• These features move cold air equatorward and
warm air poleward
• The northern hemisphere is typically encircled
by several of these waves at any given time
• These waves are called long-waves or Rossby
Waves (named for Carl Gustav Rossby)
General Circulation of the
Atmosphere
• Just like electromagnetic waves, waves in the
atmosphere have a wavelength, amplitude and
period
• Describing the movement of these waves is a key
component in weather forecasting (remember
the vertical motions associated with troughs and
ridges)
• Small amplitude waves result in a nearly zonal
flow (west to east flow pattern). The flow is
nearly parallel to lines of latitude
• In this regime cold air tends to remain poleward
General Circulation of the
Atmosphere
• Meridional flow
pattern means highly
amplified troughs
and ridges
• In this pattern, cold
air flows toward the
equator and warm air
flows poleward
General Circulation of the
Atmosphere
• Superimposed on the long-waves or
Rossby waves are smaller features called
short-waves
• These features travel quickly through the
Rossby waves
• Difficult to observe and track, adds to
uncertainty in weather forecasts
Fig. 7-18a, p. 202
Fig. 7-18b, p. 202
Other Types of Jet Streams
• Tropical Easterly Jet
• Stratospheric polar night jet
• Low-level Jet
Low-Level Jet
• Common over the Great
Plains of the US
• Can reach 50-60 kts only
a few hundred meters
above the surface
• Surface winds remain
light
• Atmosphere is effectively
“decoupled” during the
night from surface
friction effects
• Sloping terrain east of the
Rockies also plays a role
Atmosphere – Ocean Interactions
• As wind blows over ocean surfaces it
imparts a force on the water below
• The surface water begins to move along
with the prevailing wind
• The moving water gradually piles up
creating pressure differences, just like we
see in the atmosphere
• This leads to motions within the ocean
several hundred meters deep
Atmosphere – Ocean Interactions
• The general low-level
flow around the globe
starts the major ocean
currents
• Due to larger
frictional drag, ocean
currents move slower
than the wind above
• Typical speeds range
from a few km/day to
several km/hr
Atmosphere – Ocean Interactions
• Ocean currents don’t quite follow the wind flow
perfectly
• They typically show a more spiral-like
appearance, called gyres
• Water is deflected by Coriolis just like the
moving air
• Surface water typically moves at an angle
between 20˚ and 45˚to the direction of the wind
• Water will move in a circular pattern as wind
moves outward from a high pressure center
• Gulfstream
along the US
east coast
• Transports
warm water
northward
Loop Current – Gulf of Mexico
California Current
• Southward flowing
current along the
coast of California
• Brings cool water
along the coast
• Responsible for
moderating
temperatures along
the west coast
Upwelling