Chapter 13Chapter 13Middle Latitude CyclonesMiddle Latitude Cyclones

This chapter discusses:This chapter discusses:

1.1. The location, vertical structure, and The location, vertical structure, and developmental stages of middle latitude developmental stages of middle latitude cyclonescyclones

2.2. How upper level convergent winds, abrupt How upper level convergent winds, abrupt topographic features, and planetary topographic features, and planetary longwaves may enhance cyclonic longwaves may enhance cyclonic development at the surfacedevelopment at the surface

Polar Front Theory (Norwegian)Polar Front Theory (Norwegian)

Figure 13.1A-Cylconic Figure 13.1A-Cylconic ShearShear

Figure 13.1B-Frontal WaveFigure 13.1B-Frontal Wave

Figure 13.1C-Open WaveFigure 13.1C-Open Wave

Figure 13.1D-Figure 13.1D-Mature CycloneMature Cyclone

Mid-latitude Cyclone Life Cycle• Begins with a stationary front with warm and cold winds

in opposite directions (e.g. wind shear).• A wavelike kink, such as a low pressure system, then

creates a frontal wave, or incipient cyclone.• As the storm develops into an open wave, a broad band

of precipitation forms ahead of the warm sector.• As cyclone matures, the cold air “catches up” with the

warm sector, and low becomes occluded• The mature cyclone is now at it’s strongest, and begins

to slowly weaken• Eventually, the low becomes cut-off and dissapates

Figure 13.1E-OcclusionFigure 13.1E-Occlusion Figure 13.1F-DissapationFigure 13.1F-Dissapation

Triple Point

Wave Cyclone DevelopmentWave Cyclone Development

Figure 13.2Figure 13.2

A series, or family, of A series, or family, of cyclones, at various cyclones, at various stages of development, stages of development, may extend across may extend across North America.North America.

Energy originates from Energy originates from kinetic sources such as kinetic sources such as rising warm air, sinking rising warm air, sinking cold air, and converging cold air, and converging air, as well as latent heat air, as well as latent heat of condensation.of condensation.

Cyclone & Anticyclone PathsCyclone & Anticyclone Paths

• Many well-known paths for low and high pressure systems extend Many well-known paths for low and high pressure systems extend across North Americaacross North America

• Cyclone development most common in a few specific areasCyclone development most common in a few specific areas• CyclogenesisCyclogenesis describes the strengthening of these storms into huge describes the strengthening of these storms into huge unstable waves.unstable waves.

Figure 13.3AFigure 13.3A Figure 13.3BFigure 13.3B

Generally SE

GenerallyNE

LeeSidelows

Convergence & DivergenceConvergence & DivergenceDeepening of cyclones Deepening of cyclones through explosive through explosive cyclogenesis is prohibited cyclogenesis is prohibited when low pressure aloft is when low pressure aloft is directly above the surface directly above the surface low.low.

In this scenario, In this scenario, convergence at the convergence at the surface low builds up surface low builds up mass and fills in the low mass and fills in the low (converging air has (converging air has nowhere to go).nowhere to go).

The same stacking of high The same stacking of high pressure, with divergence pressure, with divergence at the surface, will at the surface, will weaken the anticyclone.weaken the anticyclone.

Figure 13.4Figure 13.4

Vertical Storm StructureVertical Storm Structure

Figure 13.5Figure 13.5

Divergence of air aloft occurs as contour Divergence of air aloft occurs as contour interval widen (the inverse happens interval widen (the inverse happens when height contours become more when height contours become more packed together)packed together)

Low pressure systems deepen and Low pressure systems deepen and intensify (cyclogenesis) when upper-intensify (cyclogenesis) when upper-level divergence is stronger than the level divergence is stronger than the surface convergencesurface convergence

This which requires a vertical This which requires a vertical offset of the surface low and low offset of the surface low and low pressure above it.pressure above it.

Upper Level WavesUpper Level WavesEarth's poles are encircled by 3 Earth's poles are encircled by 3 to 6 to 6 longwaveslongwaves, or Rosby waves, , or Rosby waves, directing upper level winds directing upper level winds around lows at the 500 mb around lows at the 500 mb surface.surface.

Small disturbances (shortwaves) Small disturbances (shortwaves) in these larger waves can in these larger waves can trigger stormstrigger storms

Cyclones move Cyclones move in the direction in the direction of the 500mb flowof the 500mb flow, at about , at about ½ the ½ the speedspeed

Figure 13.6Figure 13.6

Shortwave DisturbanceShortwave DisturbanceShortwave “kinks” within Shortwave “kinks” within the Rossby waves move the Rossby waves move faster, and propagate faster, and propagate downwind into the downwind into the Rossby troughs and cause Rossby troughs and cause them to deepen.them to deepen.

BarotropicBarotropic conditions, conditions, where isobars and where isobars and isotherms are parallel, isotherms are parallel, then degenerate into a then degenerate into a baroclinic baroclinic state where the state where the lines cross and cold or lines cross and cold or warm air is advected warm air is advected downwind.downwind.

Cyclone Development: Upper Cyclone Development: Upper WindsWinds

Figure 13.8A-BarotropicFigure 13.8A-Barotropic

Figure 13.8B-Barclinic InstabilityFigure 13.8B-Barclinic Instability

Figure 13.8C-BarotropicFigure 13.8C-Barotropic

Differential temperature advection intensifies the wave. + Existing surface frontal boundary upper level shortwave moving over the boundary--cyclogenesis occurs

Atmospheric conditions at the surface Atmospheric conditions at the surface and aloft affect cyclogenesis.and aloft affect cyclogenesis.

An upper level shortwave can trigger An upper level shortwave can trigger baroclinic instability.baroclinic instability.

-Convergent flow aloft helps -Convergent flow aloft helps strengthen surface high pressure, and strengthen surface high pressure, and supports cold air advection supports cold air advection

-Downwind of the upper level trof, -Downwind of the upper level trof, divergent flow aloft deepens the surface divergent flow aloft deepens the surface low, and warm advection aids rising air low, and warm advection aids rising air flowflow

Finding a Low Center Using Wind Observations

Jet StreakJet Streak

Divergence aloft is Divergence aloft is enhanced by the polar jet enhanced by the polar jet stream, where the jet stream, where the jet maximum, or maximum, or jet streakjet streak, , forms in the tightly forms in the tightly packed pressure packed pressure gradients.gradients.

Figure 13.9Figure 13.9

Strong C Strong D

Weak D Weak C

Jet Convergence & DivergenceJet Convergence & Divergence

-The polar jet forces air convergence aloft upstream of the -The polar jet forces air convergence aloft upstream of the deepening open wave cyclone, and then divergence deepening open wave cyclone, and then divergence downstream.downstream.-When these winds are gone, the cyclone degrades.-When these winds are gone, the cyclone degrades.

Figure 13.10AFigure 13.10A Figure 13.10BFigure 13.10B

Summary of Cyclone WeatherSummary of Cyclone Weather

Figure 13.11Figure 13.11

Upper and surface Upper and surface maps summarize maps summarize the role of the role of convergence and convergence and divergence aloft; divergence aloft; and the pattern of and the pattern of clouds, clouds, precipitation, and precipitation, and temperatures on temperatures on the ground.the ground.

Conveyor Belt ModelConveyor Belt ModelThis model describes This model describes rising and sinking air rising and sinking air along three conveyor along three conveyor belts, warm conveyor belts, warm conveyor belt rises with water belt rises with water vapor above the cold vapor above the cold conveyor belt which also conveyor belt which also rises and turns.rises and turns.

Finally the dry conveyor Finally the dry conveyor belt descends brining belt descends brining clearer weather behind clearer weather behind the storm.the storm.

Figure 13.12Figure 13.12

(Upper Level)

Comma Cloud Pattern

Comma CloudsComma Clouds

Figure 13.13Figure 13.13

Rising and turning Rising and turning moist air, illustrated in moist air, illustrated in the conveyor belt the conveyor belt model, condenses into model, condenses into a large comma-shaped a large comma-shaped cloud typical of the cloud typical of the open wave cyclone.open wave cyclone.

This March 1993 This March 1993 storm wreaked havoc storm wreaked havoc along the East Coastalong the East Coast..

Mar 1993 “Storm of the Mar 1993 “Storm of the Century”Century”

During the During the “storm of the “storm of the century”, the century”, the low pressure low pressure center reached center reached 980 mb, and 980 mb, and the storm the storm extended extended across several across several southeastern southeastern states.states.

Figure 13.14Figure 13.14

500mb Temperature Advection500mb Temperature Advection

Upper level winds flowed along a deep trough with steep Upper level winds flowed along a deep trough with steep baroclinic cold and warm air advectionbaroclinic cold and warm air advection ..

Figure 13.15Figure 13.15

Storm of Century PathStorm of Century PathLow pressure Low pressure values and values and location are location are charted with time charted with time to illustrate the to illustrate the storm track and storm track and intensity, moving intensity, moving from Texas to from Texas to Maine in 2 days.Maine in 2 days.

Figure 13.16Figure 13.16

Size of Polar LowsSize of Polar LowsCyclones that Cyclones that develop north of develop north of the polar front, the polar front, called polar lows, called polar lows, are smaller in size are smaller in size than mid-latitude than mid-latitude cyclones.cyclones.

They form during They form during the winter in the winter in arctic air, have arctic air, have (relatively) warm (relatively) warm central cores, central cores, strong winds, and strong winds, and can generate snow.can generate snow. Figure 13.17Figure 13.17

Vorticity & Cyclonic SpinVorticity & Cyclonic Spin

- Vorticity describes the spin of an air parcel, which is positive in - Vorticity describes the spin of an air parcel, which is positive in counterclockwise cyclonic flow.counterclockwise cyclonic flow.- Due to the conservation of angular momentum, vorticity increases - Due to the conservation of angular momentum, vorticity increases with a decrease in parcel radius (e.g. stretching due to divergence with a decrease in parcel radius (e.g. stretching due to divergence aloft) and increase in earth's latitude.aloft) and increase in earth's latitude.

Figure 13.18Figure 13.18

Figure 13.19Figure 13.19

Sources of VorticitySources of Vorticity

- Curvature of upper level isobars and wind, as well as strong - Curvature of upper level isobars and wind, as well as strong changes in wind speed (shear), generate the spin of changes in wind speed (shear), generate the spin of relative relative vorticity.vorticity.- Additional earth vorticity is generated by the earth's spin, and - Additional earth vorticity is generated by the earth's spin, and together they comprise absolute vorticity.together they comprise absolute vorticity.

Figure 13.20Figure 13.20

Figure 13.21Figure 13.21

Trough to Ridge Vorticity Trough to Ridge Vorticity ChangeChange

Figure 13.22Figure 13.22

Anticyclonic spin Anticyclonic spin around a ridge around a ridge reduces absolute reduces absolute vorticity, but the vorticity, but the convergence and convergence and cyclonic spin in the cyclonic spin in the trough enhance the trough enhance the relative and hence relative and hence absolute vorticity, absolute vorticity, as illustrated in this as illustrated in this upper level upper level diagramdiagram..

Vorticity & Vertical MotionVorticity & Vertical Motion

Figure 13.23Figure 13.23

The 500 mb map vorticity The 500 mb map vorticity maximum is a signal that to its east, maximum is a signal that to its east, air is diverging aloft.air is diverging aloft.

If there is also convergence below, If there is also convergence below, then an open wave cyclone will then an open wave cyclone will likely deepen.likely deepen.

Hence, 500 mb charts are useful to Hence, 500 mb charts are useful to find the “vort max” and predict find the “vort max” and predict potential storms.potential storms.

500mb Vorticity Chart

Imaging Shortwave CentersImaging Shortwave Centers

GOES West GOES West satellite infrared satellite infrared imagery of imagery of water vapor are water vapor are useful in useful in identifying identifying swirling swirling vorticities, seen vorticities, seen off Pacific off Pacific Northwest Northwest coast.coast.

Figure 13.24Figure 13.24

Vertical Storm ProfileVertical Storm Profile

Figure 13.25Figure 13.25

Surface, 500mb, and 200mb Surface, 500mb, and 200mb charts are used to illustrate charts are used to illustrate the structure of the the structure of the February 1983 open wave February 1983 open wave cyclone exploding over cyclone exploding over North Carolina.North Carolina.

The 500mb chart shows a The 500mb chart shows a shortwave dashed line shortwave dashed line moving into the longwave moving into the longwave trough and baroclinic cold trough and baroclinic cold air advection.air advection.

February 1983 VorticityFebruary 1983 VorticityLines of equal Lines of equal vorticity are plotted on vorticity are plotted on the 500 mb chart for the 500 mb chart for the February 1983 the February 1983 open wave cyclone open wave cyclone that buried the east that buried the east coast in snowfall.coast in snowfall.

Note that the vort max Note that the vort max is west of the storm is west of the storm center, strengthening center, strengthening the cyclogenesis.the cyclogenesis.

Figure 13.26Figure 13.26

Key Facts and Concepts• The polar front theory is a model of how an ideal

storm progresses through the stages of birth, maturity, and dissipation

• For a surface mid-latitude cyclone to develop or intensify, the upper-level low must be located to the west (or behind) the surface low

• Convergence is the piling up of air, divergence is the spreading out of air– Upper-level divergence above the developing surface

cyclone must be greater than surface convergence– Upper-level convergence above developing surface

anticyclone must be greater than surface divergence

• As the jet stream develops into a looping wave pattern, it

produces areas of convergence and divergence• The jet stream tends to direct anticyclones to the southeast and

cyclones to the NE• When upper-level flow is disturbed by a shortwave, horizontal

and vertical air motions begin to enhance the formation of a surface storm– Rising warm air and sinking cold air provide energy for storm

formation• As air flows over a mountain range, wind flow causes storms to

form or intensify• Polar lows are storms that develop over water in polar regions

behind the main polar front (on the arctic front)• Vorticity is a measure of the spin in a column of air

– An area of positive relative vorticity at 500mb can initiate surface mid latitude cyclogenesis