Mid-latitude Cyclones

Chapter 12p

Part 1March 19, 2009

Polar Front TheoryPolar Front Theory• Polar front theory – a model explaining thePolar front theory a model explaining the

life cycle of extratropical (mid-latitude) cyclonic stormscyclonic storms

• Polar front theory developed in early 1900’s by group of Norwegian meteorologist describes, formation, growth, and decay of , , g , ymid-latitude cyclonesOriginal theory basically correct but• Original theory basically correct, but modified to take into account new information learned

Polar Front TheoryPolar Front Theory

P l f t i i ti b d• Polar front is a semi-continuous boundary separating cold, polar air from more moderate mid-latitude air

• Mid-latitude cyclone (wave cyclone) forms y ( y )and moves along polar front in wavelike mannera e

• Frontal wave, warm sector, mature cyclone triple point secondary low familycyclone, triple point, secondary low, family of cyclones

Idealized Life Cycle of CycloneIdealized Life Cycle of Cyclone

Series of Wave Cyclonesy

Two developing mid latitude cyclonesTwo developing mid-latitude cyclones

Where do mid latitude cyclones form?Where do mid-latitude cyclones form?• Lee-side lows (cyclogenesis)Lee side lows (cyclogenesis)

– Eastern side of Southern RockiesN ’ t• Nor’easters– Along Atlantic coastg

• Hatteras lowCape Hatteras NC (warm Gulf Stream water)– Cape Hatteras, NC (warm Gulf Stream water)

• Alberta Clipper– Eastern side of Canadian Rockies

• Explosive cyclogenesis bomb• Explosive cyclogenesis, bomb

Cyclones

Typical Paths ofTypical Paths of winter Mid-

latitude Systemsy

AnticyclonesAnticyclones

Lee side Trough DevelopmentLee-side Trough DevelopmentAir on downwind side curves cyclonically and adds to the development of a cyclonic storm

Lee-side low developed

NortheastersNortheasters

• Also called a “Nor’easter”

• Mid-latitude cyclones that develop orthat develop or intensify off the

feastern seaboard of North America then move NE along coast

Vertical Structure of Deep Dynamic Lows

• Dynamic low = intensify with height• When upper-level divergence is strongerWhen upper level divergence is stronger

than surface convergence (more air is taken out of the top than the bottom)taken out of the top than the bottom) surface pressure drops and low formation

• Convergence and divergence– Convergence between ridge and troughConvergence between ridge and trough,

divergence between trough and ridge

Idealized Vertical

Structure ofStructure of Mid-latitude

Systems

Upper Level Waves and Mid-latitude Cyclones

• Longwaves and shortwaves• Barotropic vs baroclinicBarotropic vs. baroclinic• Cold and warm air advection

5 Longwaves5 Longwaves

Longwave and 3 ShortwavesLongwave and 3 Shortwaves

The Necessary Ingredients for Development of Mid-latitude Storm

• Baroclinic instability• Upper-Air Support: the overall effect ofUpper Air Support: the overall effect of

differential temperature advection is to amplify the upper level wave; cut off lowamplify the upper level wave; cut-off low

• Role of the Jet Stream: the polar jet stream removes air from surface cyclone and supplies air to surface anti-cycloneand supplies air to surface anti cyclone

The Necessary Ingredients for Development of Mid-latitude Storm

• Jet Streaks and Storms– Entrance and exit regions associated with g

divergence and convergence, right exit allows divergence.g

• Conveyor Belt Model: air constantly glides through storm; warm cold and drythrough storm; warm, cold, and dry conveyor belts

• March Storm of 1993

Conveyor Belt Model of Developing Mid-latitude Cyclone

Surface weather during deep troughg p g

500mb chart of deep trough500mb chart of deep trough