today in weather history 1909- tornado outbreak in arkansas, the town of brinkley ar was struck by a...

Today in Weather History

• 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 x-ray emissions from the sun

• Joins GOES 11, 12, 13

Global CirculationGlobal CirculationChapter 7Chapter 7

ATMO 1300

Fall 2009

General Circulation of the General Circulation of the AtmosphereAtmosphere

• 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


• 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 ModelSingle-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 referred to as Hadley cell

• Where might this model be applied?

Hadley CellHadley 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 CellHadley 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 ModelThree-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


• 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


• 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 ModelThree-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 ModelThree-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)

Fig. 7-20, p. 203

Typical Locations of the ITCZTypical Locations of the ITCZ

ITCZ varies little by the season, due to the required energy to change the temperatureof water

Three-Cell ModelThree-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

Fig. 7-13, p. 199D:\Media\Active_Figures\56_General_Circulation\G_56.html

Fig. 7-14, p. 199

Fig. 7-1, p. 189


• What happens in the “Real World”?• A few area of semi-permanent highs and

lows• In the eastern Atlantic we see the

Bermuda-Azores 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


• 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 General Circulation and Precipitation PatternsPrecipitation 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 Average Wind Flow and Pressure Patterns AloftPressure 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)


• 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 StreamJet 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 JetPolar 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 JetSub-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 StreamsJet 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 Conservation of Angular Momentum and Jet StreamsMomentum 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


• We know that the winds in the upper-atmosphere (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)


• 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


• Meridional flow pattern means highly amplified troughs and ridges

• In this pattern, cold air flows toward the equator and warm air flows poleward


• 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 Other Types of Jet StreamsStreams

• Tropical Easterly JetTropical Easterly Jet• Stratospheric polar night jetStratospheric polar night jet• Low-level JetLow-level Jet

Low-Level JetLow-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 Atmosphere – Ocean InteractionsInteractions

• 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


• 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


• 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 Loop Current – Gulf of MexicoMexico

California CurrentCalifornia Current

• Southward flowing current along the coast of California

• Brings cool water along the coast

• Responsible for moderating temperatures along the west coast

UpwellingUpwelling

today in weather history 1909- tornado outbreak in arkansas, the town of brinkley ar was struck by a...

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