Middle-Latitude Cyclones - I

RECAP: Types of Fronts

• Cold fronts: cold, dry stable air is replacing warm, moist unstable air. Moves fast, showers along the leading edge (squall line).

• Warm fonts: warm, moist unstable air is replacing cold dry stable air. Overrunning: warm air rides up and over the cold air, widespread cloudiness, light-to-moderate precipitation well ahead of the front

• Stationary fronts: essentially no movement, winds blow parallel to the front, in opposite directions on both sides

• Occluded fronts: when a cold front catches up with a warm front (more later)

• The symbols on a map are in the direction of the air mass motion.

Formation of an occluded front

Cold occlusion

Weather Map•Shown: surface-pressure systems, air masses, fronts,

isobars, winds and air flow (large arrows)

•Green-shaded area: precipitation

Weather Map•The example from Chapter 1: Fig. 1.14.

Sample weather maps

Polar Front Theory• A model of how mid-latitudes storms develop:

their birth, growth, and decay.

• The model connects the storms with the dynamics of the polar front: the transition zone between the cold air in the polar cell and the warmer air at middle-latitude (Ferrel cell).

• The polar front is a region of air conversions at the surface, upward motion, and divergence aloft. This results in low surface pressure.

VilhelmBjerknes

JacobBjerknes

+ Halvor Solberg+ Tor Bergeron

The life of a mid-latitude (wave) cyclone

The life of a mid-latitude (wave) cyclone

• A: the polar front is stationary: the winds are in opposite directions on the two sides of the front. This creates a cyclonic wind shear.

• B: A local perturbation: a region of low pressure appears somewhere along the front. The front then breaks in two fronts: ♦ warm (moving northward - why? Coriolis force) ♦ cold (moving southward - why? Coriolis force)♦ Central pressure: the junction of the two fronts

• A frontal wave is formed.♦ The winds aloft set the general direction of motion (black arrow)♦ The wave starts moving to the east and gradually becomes

• C: Open wave. The cold front moves faster than the warm front -> polar front bends. Warm sector between the two fronts. The central pressure keeps dropping: isobars now encircle it.

Weather patterns around a cyclonic wave

•South of the wave:♦ First a warm front

•Warmer air advancing•Wide band of precipitation.•Starts with snow first.•Then rain and drizzle.

♦ Cold front•Cold air advancing•Sharp drop of pressure•Strong precipitation at the front.•Then dry, cold, clear weather.

•North of the wave: some clouds associated with the low pressure center but no strong precipitation because there is no warm moist air around.

The life of a mid-latitude (wave) cyclone• D: Mature cyclone (initial occlusion). Cold front closes in on the warm

front. Most intense stage of the storm. Clouds cover a large area.

• E: Advanced occlusion. Triple point: where all three fronts come together. The center of the storm gradually dissipates:♦ Cold air on both sides of the occluded front♦ Warm sector far removed – the rising warm and moist air provides

energy for the storm (kinetic energy, latent heat of condensation)

• F: Cut-off cyclone plus a stationary front once again.

Family of cyclones

Just formingB: Frontal wave

C: Open wave

Dying outE: Advanced

occlusion

Cyclogenesis

•Any development or strengthening of a mid-latitude cyclone

•Some regions have greater propensity for cyclogenesis:♦ Gulf of Mexico♦ Eastern slopes of Rockies

and Sierra Nevada•Lee-side lows

♦ Atlantic ocean east of Carolinas•Nor’easters

Lee cyclogenesis

Famous nor’easters: the Great Blizzard of 2006

•Began on Feb 11 2006

•Developed an “eye”

•All-time record snowfall (27 in) in New York City.

Where do mid-latitude cyclones form?

•Typical paths of winter mid-latitude (anti)cyclones:♦ Lows: towards the east-northeast♦ Highs: towards the east-southeast

•Explosive cyclogenesis (bomb): when the central pressure drops very rapidly (more than 24 mb in 24 hours)