Middle Latitude CyclonesMiddle Latitude Cyclones

This chapter discusses:This chapter discusses:

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

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

Polar Front Theory – Wave Cyclone CyclePolar Front Theory – Wave Cyclone CycleStage 1

Frontal WaveFrontal Wave

Overrunning Overrunning PrecipitationPrecipitation

Open WaveOpen WaveStage 3

Stage 2

Stage 1: One explanation for development of middle latitude cyclones begins with a stationary front with warm and cold winds in opposite directions (e.g. wind shear).

Stage 2: A wavelike kink, such as a low pressure system creates a frontal wave, or incipient cyclone under the right conditions.

Stage 3: As the storm develops into an open wave, a broad band of precipitation forms ahead of the warm sector.

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

•Stage 4: When occlusion occurs when cold air lies on both sides of the occluded front. Without an ample supply of energy of rising, warm, moist air, the system will dissipate.

•Stage 5: The end stage is where precipitation-free area is observed. In addition, only a weak cyclonic circulation is seen in the wind field.

Stage 4Stage 3a

Stage 5

Wave Cyclone DevelopmentWave Cyclone Development

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

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

•Baroclinic instability drives the middle latitude cyclones even further.Baroclinic instability drives the middle latitude cyclones even further.

Baroclinic instability arises from temperature advection. Baroclinic instability arises from temperature advection.

Where on the globe is there an overall strong thermal gradient and Where on the globe is there an overall strong thermal gradient and why?why?

Cyclone & Anticyclone PathsCyclone & Anticyclone Paths

•Many well known paths for low and high pressure systems extend across Many well known paths for low and high pressure systems extend across North America, and their interaction helps develop the open wave cyclone.North America, and their interaction helps develop the open wave cyclone.

•Cyclogenesis describes the strengthening or development of these storms Cyclogenesis describes the strengthening or development of these storms into huge unstable waves.into huge unstable waves.

•Cyclones developing on the eastern slopes of the Rockies are called lee-Cyclones developing on the eastern slopes of the Rockies are called lee-side lows and are sometimes associated with redevelopment of previous side lows and are sometimes associated with redevelopment of previous systems come onshore from the Pacific Ocean.systems come onshore from the Pacific Ocean.

4 main stages in Cyclogenesis

Initial Development of Wave Cyclone

• Jet streak upstream of short-wave trough (often diffluent)

• Sinking motion upstream of trough axis

• PVA aloft over incipient surface low

• Weak surface system downstream of short-wave trough

• Elongated area of clouds (baroclinic leaf) parallel to mid-level flow

• Near surface low center: low-level convergence, upper-level divergence, rising motion, and pressure falls

Rapid Development Stage

• 500 hPa trough amplifies (height falls west of trough axis and height rises downstream )

• Cyclonic circulation results in warm advection to east and cold advection to west of surface low

• Temperature advection yields dipole vertical motion field

• Comma cloud develops

• Surface pressures fall due to net warming of column (low-level WAA + tropopause undulation)

• Surface low moves east-northeast towards greatest pressure falls and max upward motion

Mature (Open Wave) Stage

• mid-tropospheric flow changes dramatically; trough becomes negatively tilted

• SW flow increases SW of surface low

• Absolute vorticity increases

• Jet core migrates to southwest of surface

• Expansion of clouds around warm front and west of surface low

• Narrower cloud band along and behind cold front

• Some western clouds are Sc by mixing

• Dry slot develops due to subsidence; part of dry conveyor belt and high IPV air entering system

Occluded (Maximum Intensity) Stage

• Center cut off from warm sector

• Cold air at mid-lower levels over cyclone

• 500 hPa vorticity maximum near surface low center

• “Stacked” system in vertical – advections are small

• Broad west-east cloud shield• Occlusion not completely due to cold front catching up to warm front – but to low moving into cold air due to dynamic processes

• Increasing ascent NW of low + cold advection yields thickness minimum which coincides with cyclone center

• Warm air in upper trop/lower stratosphere overlays cold mid-lower tropospheric air, allows pressures to fall

Lee-Side CycloneLee-Side Cyclone• As westerly winds flow over a mountain range, the airflow is deflected to the south initially.

• These air parcels after traversing over the mountain range will reemerge with a northeasterly path.

• This motion leads to a formation of a semi-persistent trough feature on the eastern plains of Colorado called a lee-side trough.

• If there is enough upper-level support, this setup can develop even further and creating a lee-side low.

•This concept is associated with the “conservation of potential vorticity.”

Convergence & DivergenceConvergence & Divergence

•Rapid intensification of cyclones is Rapid intensification of cyclones is prohibited when low pressure aloft is prohibited when low pressure aloft is directly above the surface low.directly above the surface low.

•In this scenario, the convergence at the In this scenario, the convergence at the surface low builds up air pressure and surface low builds up air pressure and fills in the low with excessive mass.fills in the low with excessive mass.

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

•In order for a surface cyclone to In order for a surface cyclone to strengthen, a high pressure must be over strengthen, a high pressure must be over it and vice versa for a surface high it and vice versa for a surface high pressure. pressure.

•This configuration will allow for mass This configuration will allow for mass to be transported through the to be transported through the atmosphere and promote a constant up atmosphere and promote a constant up or downward vertical motion.or downward vertical motion.

Idealized case for cyclogenesis

Not idealized case for cyclogenesis

Storm Vertical StructureStorm Vertical Structure

•Divergence of air aloft occurs as Divergence of air aloft occurs as height contours intervals widen.height contours intervals widen.

•Low pressure systems Low pressure systems deependeepen and and intensify (e.g. cyclogenesis) when intensify (e.g. cyclogenesis) when upper-level divergence is stronger upper-level divergence is stronger than the surface convergence, than the surface convergence, which requires a vertical which requires a vertical staggering of surface and upper staggering of surface and upper lows.lows.

•When more air converges at the When more air converges at the surface than it is removed at the surface than it is removed at the top, this signifies a storm is top, this signifies a storm is fillingfilling or dissipating and weakening as or dissipating and weakening as surface pressure rises. surface pressure rises.

Divergence and ConvergenceDivergence and Convergence

•Convergence or confluence of air aloft creates downward motion.

•Divergence or diffluence of air aloft generates upward motion.

DIV = DivergenceDIV = Divergence

CON = ConvergenceCON = Convergence

Upper Level WavesUpper Level Waves

•Earth's poles are Earth's poles are encircled by 3 to 7 encircled by 3 to 7 longwaves, or Rossby longwaves, or Rossby waves, directing upper waves, directing upper level winds around level winds around lows at the lows at the 500 mb500 mb surface.surface.

•Longwaves or Rossby Longwaves or Rossby waves are more waves are more trend trend maker than weather maker than weather maker.maker.

•Small disturbances in Small disturbances in these Rossby waves can these Rossby waves can trigger storms.trigger storms.

Shortwave DisturbanceShortwave Disturbance

•Shortwave troughs within the larger Rossby wave move faster, and propagate downwind Shortwave troughs within the larger Rossby wave move faster, and propagate downwind into the Rossby troughs and cause them to deepen.into the Rossby troughs and cause them to deepen.

•Barotropic conditions, where height contours and isotherms are parallel (no temperature Barotropic conditions, where height contours and isotherms are parallel (no temperature advection) dominate in Rossby waves.advection) dominate in Rossby waves.

•Within shortwaves, the height contours and isotherms are usually slight out of phase Within shortwaves, the height contours and isotherms are usually slight out of phase creating temperature advection.creating temperature advection.

•A shortwave can deepen or amplify a longwave collapse into a baroclinic state.A shortwave can deepen or amplify a longwave collapse into a baroclinic state.

General Characteristics: Long Waves Short Waves

Number of Waves 3-7 across hemisphere; typically 4-5

~ 15º-40º long. wide, move through long wave troughs

Amplitude Meridionally, on the order of several 1000 km

Meridionally, on the order of several hundred km, up to

1000 km

Wavelength ~50-120 longitude ~10-40 longitude

Diagnosed best at At 500 hPa and above At 500 hPa and below

Movement Generally eastward at 10-15 knots; but can remain quasi-stationary or even retrogress

Dominantly eastward with a northerly or southerly

component; faster than the long wave troughs

Energy Regime Barotropic or Equivalent Barotropic

Definitely baroclinic

1530 m

1500 m

10C 5C

Vg

Weak Baroclinicity - Weak Temperature Advection

Vg

Strong Baroclinicity - Strong Temperature Advection

1530 m

1500 m

0C 5C

Vg

No Baroclinicity - No Temperature Advection(Equivalent Barotropic Atmosphere)

1530 m

1500 m

10C

5C

L

L

L

L

Vertical Baroclinicity – Tilting of the System with Increasing Heights

Cold Cold

Cold

Cold

Cold

Cold

Cold Cold

z

x

Classic ETC Model – Open Wave

Lsfc

L500

Warm SectorWarm Sector

Classic ETC Model – Occlusion

LsfcL500

Warm SectorWarm Sector

Cyclone Development: Upper WindsCyclone Development: Upper Winds

A)A)

B)B)

C)C)

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

•An upper level shortwave can trigger baroclinic An upper level shortwave can trigger baroclinic instability, which converges flow aloft upstream, raises instability, which converges flow aloft upstream, raises high pressure, and supports cold air advection.high pressure, and supports cold air advection.

•Downwind, divergent flow aloft deepens the surface Downwind, divergent flow aloft deepens the surface low, and warm advection increases rising air flow.low, and warm advection increases rising air flow.

•Eventually the system occludes.Eventually the system occludes.

Jet Streak InfluenceJet Streak Influence

Divergence aloft is Divergence aloft is enhanced by the polar enhanced by the polar front and/or subtropical jet front and/or subtropical jet streams, where the jet streams, where the jet maximum, or jet streak, maximum, or jet streak, forms in the tightly packed forms in the tightly packed gradients.gradients.

Jet Streak Convergence & DivergenceJet Streak Convergence & Divergence

•Jet streaks can help to intensify a surface cyclone even farther.Jet streaks can help to intensify a surface cyclone even farther.

•The polar front or subtropical jet streak forces air convergence The polar front or subtropical jet streak forces air convergence aloft upstream of the deepening open wave cyclone, and then aloft upstream of the deepening open wave cyclone, and then divergence downstream where the surface cyclone is located.divergence downstream where the surface cyclone is located.

•When these wind maximums are gone, the cyclone degrades.When these wind maximums are gone, the cyclone degrades.

90 kt

70 kt

Movement of the Jet Streak

Right Front Region

Upward Motion

Right Exit Quadrant

Downward Motion

Left Exit Quadrant

Upward Motion

Left Front Region

Downward Motion

Summary of Cyclone WeatherSummary of Cyclone Weather

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

A mid-latitude A mid-latitude cyclone experiences cyclone experiences its maximum its maximum intensity when the intensity when the upper-level low or upper-level low or trough is juxtaposed trough is juxtaposed to the northwest of to the northwest of the surface low the surface low pressure.pressure.

The upper- and lower-level lows are tilted vertically to provide for the maximum upward vertical motion.

Conveyor Belt Conveyor Belt ModelModel

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

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

Old ModelOld Model

Old but Slightly Revised ModelOld but Slightly Revised Model

Ng (2005) Doctoral Dissertation

Revised View of Conveyor Belts Illustrating 3-D Deformation

Comma CloudsComma Clouds

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

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

3/93 Storm Size & Pressure3/93 Storm Size & Pressure

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

3/93 Storm Temperature Advection3/93 Storm Temperature Advection

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

Storm of Century PathStorm of Century Path

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

Vorticity & Cyclone SpinVorticity & Cyclone 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 Due to the conservation of angular momentum, vorticity increases with a decrease in parcel radius (e.g. stretching due to increases with a decrease in parcel radius (e.g. stretching due to divergence aloft) (local vorticity) and increase in earth's latitude divergence aloft) (local vorticity) and increase in earth's latitude (earth’s vorticity).(earth’s vorticity).

4 Types of Vorticity

• Relative (local)Relative (local)

• Earth’s (Coriolis parameter; Earth’s (Coriolis parameter; ff =2 =2sinsin))

• Absolute (local + earth’s)Absolute (local + earth’s)

• Potential (absolute + depth)Potential (absolute + depth)

Sources of VorticitySources of Vorticity

•Curvature of upper level heights and winds, as well as strong Curvature of upper level heights and winds, as well as strong changes in wind speed, or shear, generate the spin of relative changes in wind speed, or shear, generate the spin of relative vorticity.vorticity.

•Additional earth’s vorticity is generated by the earth's spin, and Additional earth’s vorticity is generated by the earth's spin, and together they comprise absolute vorticity.together they comprise absolute vorticity.

y

x

Curvature Vorticity

y

x

Shear Vorticity

y

x

as vorticity decreases, area

increases

Absolute Circulation = Constant = Absolute Vorticity x Area

as vorticity increases, area

decreases

DIV

CON

Trough to Ridge Vorticity ChangeTrough to Ridge Vorticity Change

Anticyclonic spin around a ridge reduces absolute vorticity, but the convergence Anticyclonic spin around a ridge reduces absolute vorticity, but the convergence and cyclonic spin in the trough enhance the relative vorticity and hence increases and cyclonic spin in the trough enhance the relative vorticity and hence increases absolute vorticity as wellabsolute vorticity as well

+

+ 2

Con+

Con

Div+

Div Div

PVAPVANVANVA

NVANVAPVAPVA

Maximum Maximum at at the base of the the base of the

trough:trough:

> 0; > 0; > 0 > 0

Minimum Minimum at at the crest of the the crest of the

ridge:ridge:

< 0; < 0; >> 0 >> 0

30N

35N

40N

45N

Vorticity & Vertical MotionVorticity & Vertical Motion

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

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

• Hence, 500 mb charts are useful in Hence, 500 mb charts are useful in analyzing “vorticity maximum” and analyzing “vorticity maximum” and predict potential storms in a quick, predict potential storms in a quick, crude way.crude way.

Vertical Storm ProfileVertical Storm Profile

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

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

February 1983 VorticityFebruary 1983 Vorticity

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

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

Imaging Vorticity CentersImaging Vorticity Centers

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

Potential Vorticity = Absolute Vorticity / Depth = Constant

PV PV 0: 0: D D < <

0 0 PV > 0: PV > 0: D D > >

0 0

PV > 0: PV > 0: D D > >

0 0

TROPOPAUSETROPOPAUSE

Maximum Maximum at at the windward the windward

side of the side of the mountain:mountain:

> 0; > 0; > 0 > 0

Minimum Minimum at at the top of the the top of the

mountain:mountain:

< 0; < 0; 0 0

Maximum Maximum at at the leeward the leeward side of the side of the mountain:mountain: > 0; > 0; > 0 > 0

acquiring anticyclonic

curvature

acquiring cyclonic curvature

Polar LowsPolar Lows

•Cyclones that develop above Cyclones that develop above the polar front, called polar the polar front, called polar lows, are smaller in size than lows, are smaller in size than mid-latitude cyclones.mid-latitude cyclones.

•They form during the winter, They form during the winter, have warm central cores, have warm central cores, strong winds, and generate strong winds, and generate snow.snow.

•Size ~ 1000 to 500 kmSize ~ 1000 to 500 km

•Eye-like clear structure Eye-like clear structure similar to a hurricane with the similar to a hurricane with the most intense winds and most intense winds and precipitation near the eye core. precipitation near the eye core.