Ch 7 – Scales of Ch 7 – Scales of Atmospheric CirculationsAtmospheric Circulations

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• IntroductionIntroduction– Weather is not a random occurrence.Weather is not a random occurrence.– Every weather event is the result of the Every weather event is the result of the

development of some sort of development of some sort of atmospheric circulation.atmospheric circulation.

– In this context, circulation means a In this context, circulation means a more or less organized movement of airmore or less organized movement of air

– The word eddy is often used in the same The word eddy is often used in the same sense (Lester, 2006).sense (Lester, 2006).

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• IntroductionIntroduction– The motion in a given circulation or The motion in a given circulation or

eddy may be vertical, horizontal, or eddy may be vertical, horizontal, or both.both.

– A very important characteristic of the A very important characteristic of the atmosphere is that circulations occur atmosphere is that circulations occur with many different dimensions, ranging with many different dimensions, ranging from organized motions on the scale of from organized motions on the scale of the entire earth to turbulent eddies as the entire earth to turbulent eddies as small as your hand (Lester, 2006).small as your hand (Lester, 2006).

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• IntroductionIntroduction– In this chapter, we formally introduce the In this chapter, we formally introduce the

concept of scales of atmospheric concept of scales of atmospheric circulations to help you organize your study circulations to help you organize your study of various atmospheric weather of various atmospheric weather phenomena.phenomena.

– We then apply this idea to the examination We then apply this idea to the examination of two important circulations of very of two important circulations of very different sizes: the general circulation and different sizes: the general circulation and the monsoon circulation (Lester, 2006).the monsoon circulation (Lester, 2006).

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• IntroductionIntroduction– When you finish this chapter, your When you finish this chapter, your

knowledge of these two macroscale knowledge of these two macroscale circulations will provide you with circulations will provide you with important background for understanding important background for understanding global climate and for your subsequent global climate and for your subsequent study of smaller scale circulations and study of smaller scale circulations and their related weather (Lester, 2006).their related weather (Lester, 2006).

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• Section A – Scales of CirculationsSection A – Scales of Circulations• Section B – The Largest Scale CirculationsSection B – The Largest Scale Circulations

– The General CirculationThe General Circulation– The Monsoon CirculationThe Monsoon Circulation

• Section C – The Global Circulation SystemSection C – The Global Circulation System– The Global Circulation AloftThe Global Circulation Aloft

• Section D – The Global Circulation & Section D – The Global Circulation & ClimatologyClimatology

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations• Section A:Section A: Scales of CirculationScales of Circulation- refer to the sizes and - refer to the sizes and

lifetimes of individual circulationslifetimes of individual circulations– MacroscaleMacroscale – greater circulations; have the longest – greater circulations; have the longest

lifetimes; refers to horizontal dimensions of 1,000 to lifetimes; refers to horizontal dimensions of 1,000 to 10,000 nautical miles; general circulation, monsoon 10,000 nautical miles; general circulation, monsoon circulation, jet stream circulation, jet stream

– MesoscaleMesoscale – refers to horizontal dimensions of 1 to 1,000 – refers to horizontal dimensions of 1 to 1,000 nautical miles; occluded cyclone, hurricane, front, nautical miles; occluded cyclone, hurricane, front, land/sea breeze, lee wave, thunderstorm, downburstland/sea breeze, lee wave, thunderstorm, downburst

– MicroscaleMicroscale – smaller circulations; have the shortest – smaller circulations; have the shortest lifetimes; tornado, dust devil, thermal, turbulencelifetimes; tornado, dust devil, thermal, turbulence

– Embedded circulationsEmbedded circulations – at any one time, several – at any one time, several circulations may be present, with smaller ones embedded circulations may be present, with smaller ones embedded in, and often driven by larger scale circulationsin, and often driven by larger scale circulations

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• Section B: The largest Scale Section B: The largest Scale CirculationsCirculations– The General CirculationThe General Circulation – the general – the general

circulation refers to the wind system circulation refers to the wind system that extends over the entire globethat extends over the entire globe

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

– Circulation CellCirculation Cell – each of the vertical circulation – each of the vertical circulation systems is called a circulation cell or simply a cellsystems is called a circulation cell or simply a cell

•Hadley CellHadley Cell – at the surface between the equator – at the surface between the equator and 30 degrees north we again find the and 30 degrees north we again find the northeasterliesnortheasterlies

– This cell is called the Hadley Cell for an 18th This cell is called the Hadley Cell for an 18th century scientist who first proposed a model of century scientist who first proposed a model of the general circulationthe general circulation

•Polar CellPolar Cell – in the highest latitudes a polar cell – in the highest latitudes a polar cell has developedhas developed

– It is defined by air rising near 60 degrees north It is defined by air rising near 60 degrees north and sinking over the poleand sinking over the pole

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

•Ferrel CellFerrel Cell – In the latitude belt between 30 – In the latitude belt between 30 degrees north and 60 degrees north the faster degrees north and 60 degrees north the faster rotation and strong north-south temperature rotation and strong north-south temperature gradient in midlatitudes favors the development gradient in midlatitudes favors the development of smaller scale eddies in that regionof smaller scale eddies in that region

– Their influence on the general circulation is to Their influence on the general circulation is to cause the average surface winds to be cause the average surface winds to be southwesterly in this latitude belt, and to southwesterly in this latitude belt, and to remain westerly up through at least the remain westerly up through at least the tropopausetropopause

– These average winds define a midlatitude These average winds define a midlatitude circulation cell called the Ferrel Cellcirculation cell called the Ferrel Cell

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• Trade windsTrade winds – the three cell circulation generates – the three cell circulation generates some important and well known features in the some important and well known features in the surface wind patternsurface wind pattern– These include the steady northeasterly trade These include the steady northeasterly trade

winds between the equator and 30 degrees winds between the equator and 30 degrees northnorth

• Prevailing westerliesPrevailing westerlies - the three cell circulation - the three cell circulation generates some important and well known generates some important and well known features in the surface wind patternfeatures in the surface wind pattern– These include the prevailing westerlies between These include the prevailing westerlies between

30 degrees north and 60 degrees north.30 degrees north and 60 degrees north.

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• The Monsoon CirculationThe Monsoon Circulation - the three cell - the three cell circulation generates some important and well circulation generates some important and well known features in the surface wind patternknown features in the surface wind pattern– These include the polar easterlies north of 60 These include the polar easterlies north of 60

degrees north.degrees north.

• Intertropical Convergence Zone (ITCZ)Intertropical Convergence Zone (ITCZ) – The low – The low pressure area near the equator is called the pressure area near the equator is called the “Doldrums.”“Doldrums.”– Because of the convergence of trade winds from Because of the convergence of trade winds from

both hemispheres into that area, it is also known both hemispheres into that area, it is also known as the Intertropical Convergence Zone (ITCZ).as the Intertropical Convergence Zone (ITCZ).

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations• Horse latitudesHorse latitudes – air sinks in a region of diverging – air sinks in a region of diverging

surface winds that correspond with a subtropical high surface winds that correspond with a subtropical high pressure near 30 degrees northpressure near 30 degrees north– This part of the general circulation is known as the This part of the general circulation is known as the

horse latitudeshorse latitudes– Cloud formation is suppressed and precipitation is Cloud formation is suppressed and precipitation is

typically low in these areas.typically low in these areas.• Polar FrontPolar Front – Near 60 degrees north, pressures are – Near 60 degrees north, pressures are

low and surface winds converge, bringing warm low and surface winds converge, bringing warm airmasses from tropical regions into contact with cold airmasses from tropical regions into contact with cold airmasses from polar regionsairmasses from polar regions– The line separating the airmasses at this location is The line separating the airmasses at this location is

called the Polar Frontcalled the Polar Front– It is another region of cloudiness and precipitation.It is another region of cloudiness and precipitation.

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations• Monsoon circulationMonsoon circulation – The monsoon is a – The monsoon is a

macroscale wind pattern that undergoes a macroscale wind pattern that undergoes a seasonal reversal in directionseasonal reversal in direction– The low-level winds of the “wet” monsoon of The low-level winds of the “wet” monsoon of

summer flow from the ocean to the continent. summer flow from the ocean to the continent. – The “dry” monsoon flow is in the opposite The “dry” monsoon flow is in the opposite

direction (the continent to the ocean)direction (the continent to the ocean)– A rough measure of the scale of a monsoon is A rough measure of the scale of a monsoon is

5,000 n.m. or about the size of a continent.5,000 n.m. or about the size of a continent.

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• Section C: The Global Circulation SystemSection C: The Global Circulation System– The Global Circulation SystemThe Global Circulation System – Most global – Most global

climatological wind charts are based on a climatological wind charts are based on a monthly or seasonal average of the world-wide monthly or seasonal average of the world-wide windswinds

•This averaging process eliminates This averaging process eliminates circulations with smaller time scales leaving circulations with smaller time scales leaving what is called the global circulation systemwhat is called the global circulation system

• It is a combination of the general and It is a combination of the general and monsoon circulations.monsoon circulations.

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• Long wavesLong waves – These are the average 500 mb heights – These are the average 500 mb heights and wind directions for January and Julyand wind directions for January and July– The January chart also shows positions of three The January chart also shows positions of three

long-wave troughs along the east coasts of North long-wave troughs along the east coasts of North America and Asia, and over Eastern EuropeAmerica and Asia, and over Eastern Europe

– The three waves in figure 7-8 are examples of the The three waves in figure 7-8 are examples of the largest scale wave distributions that occur in the largest scale wave distributions that occur in the atmosphereatmosphere

– Appropriately, these are called long wavesAppropriately, these are called long waves– They tend to move much more slowly than the They tend to move much more slowly than the

wind. wind.

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• Jet StreamJet Stream – Very important upper-air features – Very important upper-air features known as jet streams are often embedded in the known as jet streams are often embedded in the zone of strong westerlieszone of strong westerlies– A jet stream is a narrow band of high-speed A jet stream is a narrow band of high-speed

winds that reaches its greatest speed near the winds that reaches its greatest speed near the tropoause (24, 000 to 50, 000 feet MSL)tropoause (24, 000 to 50, 000 feet MSL)

– Typical jet stream speeds range between 60 Typical jet stream speeds range between 60 knots and about 240 knotsknots and about 240 knots

– Jet streams are typically thousands of miles Jet streams are typically thousands of miles long, hundreds of miles wide, and a few miles long, hundreds of miles wide, and a few miles thick.thick.

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• The polar front jet streamThe polar front jet stream – On the – On the average, two jet streams are found in the average, two jet streams are found in the westerlies, the polar front jet stream is one westerlies, the polar front jet stream is one of themof them– As the name implies, the polar front jet stream As the name implies, the polar front jet stream

is found near the latitude of the polar frontis found near the latitude of the polar front– Similar to the behavior of the polar front, it is Similar to the behavior of the polar front, it is

stronger and farther south in the winter and stronger and farther south in the winter and weaker and further north in summer.weaker and further north in summer.

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• The subtropical jet streamThe subtropical jet stream - On the - On the average, two jet streams are found in the average, two jet streams are found in the westerlies, the subtropical jet stream is one of westerlies, the subtropical jet stream is one of themthem– The subtropical jet stream has no related surface The subtropical jet stream has no related surface

frontal structure and shows much less fluctuation frontal structure and shows much less fluctuation in positionin position

– It is typically found near 25 degrees north to 30 It is typically found near 25 degrees north to 30 degrees north latitude near North Americadegrees north latitude near North America

– The subtropical jet stream reaches its greatest The subtropical jet stream reaches its greatest strength in the wintertime and generally strength in the wintertime and generally disappears in summer.disappears in summer.

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• ****THE POLAR FRONT JET STREAM WEAKENS ****THE POLAR FRONT JET STREAM WEAKENS

AND MOVES POLEWARD IN THE SUMMER****AND MOVES POLEWARD IN THE SUMMER****

Ch 7 – Scales of Atmospheric Ch 7 – Scales of Atmospheric CirculationsCirculations

• Section D: Global Circulation and Section D: Global Circulation and ClimatologyClimatology– ClimatologyClimatology – the study of the average – the study of the average

conditions of the atmosphereconditions of the atmosphere

SummarySummary

• This chapter can be viewed as a transition This chapter can be viewed as a transition between aviation weather basics and between aviation weather basics and some useful applications of those basics.some useful applications of those basics.

• The concept of “scales of motion” has The concept of “scales of motion” has been introduced as a learning and been introduced as a learning and organizing device.organizing device.

• The observed state of the atmosphere is The observed state of the atmosphere is usually due to the effects of one or more usually due to the effects of one or more individual circulations (Lester, 2006). individual circulations (Lester, 2006).

SummarySummary

• By separating the variety of atmospheric By separating the variety of atmospheric disturbances according to their space and disturbances according to their space and time scales, they become easier to time scales, they become easier to understand.understand.

• The interpretation of current and predicted The interpretation of current and predicted weather also is much easier when you have weather also is much easier when you have an appreciation of the types and scales of an appreciation of the types and scales of the disturbances involved.the disturbances involved.

• The scale approach has been applied in this The scale approach has been applied in this chapter to describe the causes and chapter to describe the causes and characteristics of the general circulation characteristics of the general circulation and the monsoon (Lester, 2006).and the monsoon (Lester, 2006).

SummarySummary

• In combination, these largest In combination, these largest circulation systems account for the circulation systems account for the average global winds, and help us average global winds, and help us explain many of the characteristics explain many of the characteristics of global climatology (Lester, 2006).of global climatology (Lester, 2006).