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Home , Occluded front

Middle-Latitude - 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 • catches up with a (more later) The symbols on a map are in the • direction of the 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. Vilhelm Jacob The model connects the storms with the Bjerknes Bjerknes • dynamics of the polar front: the transition zone between the cold air in the polar cell and the + Halvor Solberg warmer air at middle-latitude (Ferrel cell). + Tor Bergeron The polar front is a region of air conversions at • the surface, upward motion, and divergence aloft. This results in low surface pressure. The life of a mid-latitude (wave) 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 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

Dying out E: Advanced occlusion

C: Open wave

Just forming B: Frontal wave

Cyclogenesis

Any development or strengthening • of a mid-latitude cyclone Some regions have greater Lee • propensity for cyclogenesis: ♦ Gulf of Mexico ♦ Eastern slopes of Rockies and Sierra Nevada •Lee-side lows ♦ Atlantic ocean east of Carolinas •Nor’easters 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)