Wind And Global Circulations and Jet Streams

Font Size: Larger /Smaller

BBCC Aviation Meteorology

CHAPTER 7

WIND

  • Wind causality
  • Unequal heating of the earth’s surface causes temperature imbalances
  • Differences in temperature cause differences in pressure
  • Since the universe is all about equilibrium, those differences in pressure cause the flow
  • Flow is always from high pressure to low pressure
  • The bigger the pressure differential, the faster the winds
  • F=ma (Newton’s second law)
  • There are 4 main forces behind wind flow
  • Pressure gradient force
  • Coriolis force
  • Centripetal force
  • Friction

 CONVECTION

  • A really good illustration of the temp/pressure/wind example is the convection model
  • Warm air rises
  • Cold air sinks
  • With convection, warm air rises cools then sinks
  • The wind sets up an advection process whereby the cool air is blown along the ground until it is warmed then it rises again and repeats the process.

 PRESSURE GRADIENT

  • Pressure gradient = difference in pressure/distance
  • A common condition is when a high or a low pressure system sets up a flow pattern
  • This force is at 90° to the isobars
  • The closer the isobars, the stronger the pressure gradient force and the stronger the wind
  • In fact wind speed is directly proportional to PGF

 CORIOLIS FORCE

  • This force describes the apparent force due to the rotation of the earth
  • Really not a force by the way, more of an effect
  • All free moving objects such as ocean currents, old artillery projectiles, air molecules and aircraft seem to deflect from a straight line path because the earth rotates under them.
  • This deflection is to the right when viewed in the direction of travel in the northern hemisphere

 CORIOLIS FORCE

  • Flow would normally be 90º to isobars except for Coriolis Force
  • Causes a deflection of winds to the right in the Northern Hemisphere
  • To the left in the Southern Hemisphere
  • The deflection turns the winds parallel to the isobars at altitude
  • When viewing wx maps the flow will be roughly parallel to the isobars
  • Near the ground, the deflection depends on surface friction

 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 the earth’s rotation under the winds.
  • The earth rotates at about 15° longitude an hour.
  • Thus if say a missile were airborne for an hour flying from the north pole toward the equator, it would appear to deflect toward the southwest.

 Actual and Observed Paths

  • Latitude effects Coriolis by increasing the angular deviation the higher the latitude.
  • Since the earth is a sphere, a straight line will only be congruent with the equator.
  • Anywhere north of the equator will have an ever greater degree of divergence from the starting line of latitude.
  • Since the 60° circle is smaller than the 30° circle, a tangent line drawn to it will represent a greater angle.
  • Therefore greater Coriolis angle due to latitude would be present the higher the latitude.

 CORIOLIS and friction

  • http://www.youtube.com/watch?NR=1&feature=endscreen&v=__SlJtnpCD8
  • Surface friction slows the wind allowing the pressure gradient force to over power Coriolis
  • Over water 10º to the isobars
  • Surface friction is less so it only causes a small decrease in Coriolis causing the flow to be mostly parallel to the isobars
  • Over land 45º to the isobars
  • Surface friction is much greater decreasing Coriolis allowing PGF to turn the flow at a greater angle to the isobars
  • The magnitude varies with the speed of the wind and the latitude
  • As wind speed increases Coriolis increases
  • As latitude nears the poles, Coriolis increases
  • Under a no wind, no course correction scenario your airplane (a Cub @ about 60-70mph) will move 1,500 feet to the right for every 100 miles
  • https://www.youtube.com/watch?v=i2mec3vgeaI

 Geostrophic winds

  • Geo means earth
  • Strophic means turning
  • As pressure gradient starts to move the air, Coriolis starts working
  • Eventually as wind speed reaches its max Coriolis and PGF equalize and the wind blows straight along the isobars at altitude
  • This is useful for determining wind speed and direction for which there is no means for direct measurement

 Meridional and 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.
  • Zonal flow can enhance a flights groundspeed from west to east in the US.
  • Since the gradient force over the northwest is larger in winter, this can lead to greater meridional wind force.

 Curved winds (the low)

  • The flow around a Low is counterclockwise or cyclonic
  • At point 1 PGF is moving air towards the center of the low
  • Coriolis force deflects and at point 2 the air is parallel to the isobars
  • If the wind were geostrophic at point 3 it would continue northward along straight line isobars, but that’s not the case because the isobars are curved
  • Since it is moving parallel to curved isobars along a curved path the wind is known as a Gradient wind
  • This is where centripetal force comes into play
  • Centripetal force acts 90° to the Gradient wind keeping it on a curved path

 Curved winds (the low)

  • Acceleration occurs when there is both a change in speed or direction (or both)
  • Since the wind is changing direction it has an acceleration force
  • Newton’s law explains that if an object is accelerating there must be a force acting on it (F=ma)
  • In the low’s case there is an imbalance between a greater PGF and Coriolis
  • The resulting imbalance produces centripetal force acting toward the center of the low
  • Centripetal force is effected by velocity and the radius of the path

 Curved winds (the low)

  • When winds are light centripetal force is relatively weak
  • In the case of hurricanes and tornadoes this force is very strong and plays a large roll in the destructive forces

 Curved winds (the high)

  • The flow around a High is clockwise or anticyclonic
  • The same principle applies to the high except the greater Coriolis force is producing the imbalance
  • Centripetal force still pulls inward as per Newton’s law which causes the Gradient wind to result

 Curved winds (the high)

  • Normally we associate bad weather with the low since the air is being pulled inward, upward and forced aloft
  • Normally we associate good weather with the high since the flow is outward, downward and causing descending air
  • However the picture at the right shows some clouds associated with this high

Chapter 8

Global Circulations and Jet Streams

  • GLOBAL CIRCULATION PATTERNS
  • Weather books love to show the single-cell model
  • This is an over simplification of the heat engine at work
  • Heat rises at the equator, cools at the poles, sets up an advective wind whereby it flows back to the equator heats and rises then repeats the process

THE THREE CELL MODEL

  • But we have an earth that spins, land masses, a tilting axis and ocean currents so that results in a three cell model
  • The three cells have names
  • The Hadley cell
  • The Ferrel cell
  • The Polar cell

GLOBAL CIRCULATION PATTERNS

  • This 3 cell model sets up some semi-permanent highs and lows
  • At the equator we have a semi-permanent low
  • Horizontal pressure gradients are weak here so winds are light
  • This is referred to as the Doldrums
  • Convective “Hot Towers” form here often
  • Where the northeast trade winds converge with the southwest trade winds is known as the Intertropical Convergence Zone or ITCZ
  • At 30°we have a semi-permanent high
  • This belt is referred to as the Subtropical High
  • Sometimes referred to as the Horse Latitudes
  • Winds are light here so sailing ships were stuck here for days or weeks forcing the sailors to either eat their horses or toss them overboard
  • This is where we find large deserts like the Sahara
  • This is also where the trade winds originate or Westerlies
  • At 60°we again have a semi-permanent low
  • This is where warmer air meets the Polar Front producing the Subpolar Low
  • North of the polar front we have the Polar Easterlies
  • Remember from our discussion on lows and highs
  • Descending air is characteristic of a high pressure area
  • Rising air is a characteristic of a low pressure area

GLOBAL CIRCULATION PATTERNS

  • Remember Coriolis and Pressure Gradient Force result in the curved flow of the
  • Polar Easterlies
  • Westerlies
  • NE trade winds
  • SE trade winds
  • 10 years of weather in 9 minutes
  • https://www.youtube.com/watch?v=XD0_LVWasuA

DISCOVERY OF THE JET STREAM

  • As World War II was approaching its conclusion, the United States introduced the first high-altitude bomber plane called the B-29. It could fly at altitudes well above 20,000 feet.
  • When the B-29s were being put into service from a Pacific island base, two air force meteorologists were assigned the task of producing a wind forecast for aircraft operations at such altitudes.

DISCOVERY OF THE JET STREAM

  • To make their prediction, the meteorologists used primarily surface observations and what is known in meteorology as the “thermal wind” relationship.
  • In plain language, this relationship implies “that if you stand with your back to the wind, and the air is colder to the left and warmer to the right, the wind will get stronger on your back as you ascend in the atmosphere.”
  • Using this relationship, the meteorologists then predicted a 168-knot wind from the west. Their commanding officer could not believe the estimate. However, on the next day, the B-29 pilots reported wind speeds of 170 knots from the west!
  • The jet stream was discovered.

JET STREAM FACTS

  • Narrow river of wind
  • 50-150 knots
  • 100-400 miles wide
  • 1 mile thick
  • Subtropical jet 33,000 to 43,000
  • Polar Front jet 25,000 to 35,000

JET STREAM FACTS

  • Stronger in winter than summer
  • Farther south in winter than summer
  • Occurs at breaks in the tropopause
  • Has to be 50 kts or greater to be classified as a Jet Stream

THE JET STREAM

  • The subtropical jet will form between breaks between the Hadley and Ferrel cells
  • The polar jet will form between the Ferrel and Polar cells

Meridional and 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.
  • Zonal flow can enhance a flights groundspeed from west to east in the US.
  • Since the gradient force over the northwest is larger in winter, this can lead to greater meridional wind force.

THE JET STREAM

  • The jet blows west to east and occasionally shifting north and south
  • Jet streams follow the boundaries between hot and cold air
  • Since the temp gradient is stronger in the winter, the jet is also stronger

CAUSE OF THE JET STREAM

  • Great contrast of temperatures in adjacent air frontal zones
  • Rapid change in temp causes rapid change in pressure
  • Steep pressure gradient intensifies wind speed and causes the Jet
  • Temp difference is greatest in winter between polar and tropical air masses
  • This causes the jet to be more intense in winter than summer

THE JET STREAM

  • Why does the jet blow west to east?
  • The earth’s rotation is responsible for developing the 3 primary cells
  • Recall the difference in rotational velocity along different points of latitude causes this
  • As air moves north it keeps it eastward flow
  • This is know as conservation of angular momentum
  • The Earth below the air, however, moves slower as that air travels toward the poles.
  • The result is that the air moves faster and faster in an easterly direction (relative to the Earth’s surface below) the farther it moves from the equator.
  • The farther the air moves northward the slower the earth’s rotation
  • This causes a shift in the speed of eastward movement of the air causing faster velocity
  • Because of this the jet moves from west to east in both the northern and southern hemispheres

CONSERVATION OF ANGULAR MOMENTUM

  • As we move northward air moves closer to axis of rotation of the earth
  • Air must move faster east which looks like a wind out of the west to an observer on the ground
  • The best example is an ice skater

CAUSE OF THE JET STREAM

THE JET STREAM

  • At 30 and 60 degrees latitude, temp changes are the greatest
  • As the temp difference increases so does the wind speed

THE JET STREAM

  • The actual appearance of the jet is a complex interaction between many variables:
  • Location of high an low pressure systems
  • Warm and cold air masses
  • Seasonal changes
  • They meander around the earth dipping and rising in altitude and latitude
  • They can split, form eddies, disappear and then reappear elsewhere
  • The jet will tend to be located where the strongest surface temp contrast exists

JET STREAKS

  • Jet streams form along the upper air boundaries of large masses of warm and cold air.
  • Often, the jet stream contains “jet streaks” of wind speeds faster than the surrounding regions.
  • These jet streaks can play a very important role in precipitation and storm formation.
  • Depending on the strength and position of a jet streak, a weak area of low pressure can quickly become a raging monster.
  • Highest velocities in jet streaks have hit 260 kts

JET STREAM FACTS

  • The curvature in the jet stream and upper-air flow can have significant implications for surface weather.
  • Curvature’s effects are the strongest in a highly amplified, meridional flow pattern with large troughs and ridges.
  • Meridional flow is north – south
  • Zonal flow is east – west
  • As the graphic shows, the anticyclonic curvature in the western USA ridge induces a clockwise circulation while the cyclonic curvature in the east trough tends to favor a counter-clockwise circulation.

JET STREAM FORECAST

  • Meteorologists use the jet position to forecast weather
  • The polar front jet is a critical piece when generating a forecast
  • Changes in it indicate changes in weather
  • Typically north of the polar jet is relatively cold air exists, warmer to the south
  • The polar jet can promote development of storms
  • Storms are most likely to develop under a jet streak
  • The polar jet generally steers storms in the direction of its travel
  • There are usually 2 jet streaks located somewhere over the US at any given time

JET STREAM CIRRUS

  • Rising air on the equatorial side can cause cirriform clouds
  • Descending air on the polar side causes them to dissipate

JET STREAM TURBULENCE

  • The more serious turbulence is found on the polar side of the Jet
  • Rate of decrease of wind speed is greater on polar side
  • Usually associated with an upper level trough
  • The bend caused by the trough is where pressure gradients change over the smallest distance increasing the chance for turbulence (CAT)

CLEAR  AIR TURBULENCE

  • Cold air colliding with warm air causes shear and presto CAT
  • The worst turbulence is found
  • In winter when temp differential is greatest
  • Where greatest wind velocity is found
  • On the polar side where there is combo of strong shear, curvature in the flow and cold air advection
  • Make no mistake, it is found on the equatorial side as well
  • Located most of the time in an upper trough on the polar side of the jet
  • Cat is found in sharply curved contours of strong lows, troughs and ridges
  • Turbulence is greatest

ODDS OF HITTING FORCAST TURBULENCE

  • Light  10-15% of the time
  • Moderate  2-3% of the time
  • Severe to Extreme 2/10ths of 1% of the time or 1 in 500 chance

AVOIDING CAT

  • Usually climbing or descending will get you out of it
  • Use FSS and/or Flight Watch on 122.00 find out pireps
  • Remember stress is cumulative (Turb + Manev = total stress)
  • Maintain constant attitude, slow to Va, accept variations

CAT IS STRONG AT TIMES

  • Apr 4, 1981 DC 10 at 37,000 over central Illinois
  • Dropped 2,000 feet
  • 21 out of 154 injured including
  • Fractured hip
  • Jabbed nose with a fork then landed in the seat in front of him on it

CAT IS STRONG AT TIMES

  • Apr 4, 1981 DC 10 at 37,000 over central Illinois
  • Dropped 2,000 feet
  • 21 out of 154 injured including
  • Fractured hip
  • Jabbed nose with a fork then landed in the seat in front of him on it
Print Friendly