Download 3 November

Ch. 12 Study materials
Questions for review: 1-6, 8-13, 15-19
Questions for thought: 2
Wednesday (Mid-latitude cyclones)
Chapter 12 (313-329)
Friday (Mid-latitude cyclones)
Chapter 12 (313-335)
Monday (Thunderstorms)
Chapter 14 (371-394)
Low pressure systems
Always …
Cyclones: winds spiral into low
• CCW in northern hemisphere
Single air mass
Tropical
• Warm core: warm air rises,
forming low
Polar front
2-3 air masses, 2 fronts
• Cold air pushed by warm
• Warm air pushed by cold
Polar front theory
Polar front theory
Genesis & movement of cyclones
Develops as a weak disturbance
Strengthening of system
• Wave develops
Occlusion over east coast, western
North Atlantic
Cyclone is cut off, leaving stationary
front
Typical life cycle
Cyclogenesis E of Japan
Guided by upper level winds to central
CA
E to Rocky Mountains
Intensifies coming down mountains
Strong storm moves to NE USA
Complete occlusion over N Atlantic
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What you see on the ground
Warm front approaches
Deepening cloud cover
Increasing light precipitation
Warm front passes
Warmer, sunny conditions for ~1 day
Winds shift from S to SW
Cold front
Strong, fast-moving band of clouds
Heavy precipitation
Then, cold and clear
Polar front—step 1
Winds
Polar front—step 2
Winds
Polar front—step 3
Winds
Polar front—step 4
Winds
Polar front—step 5
Winds
Where do mid-latitude cyclones tend to
form?
Lee-side lows
Hatteras low
Alberta Clipper
Nor’easters
Lee-side low
Where do mid-latitude cyclones tend to
form?
Topic:
Northeasters
Mid-latitude
cyclones that
develop or
intensify off the
eastern
seaboard of
North America
then move NE
along coast
Vertical structure
Vertical Structure of Deep Dynamic Lows
Dynamic low = intensify with height
When upper-level divergence is
stronger than surface convergence
(more air is taken out of the top than
the bottom) surface pressure drops and
low intensifies
Decay cycle
Self-sustaining
Fig. 2, p. 320
Vertical Structure of Deep Dynamic Lows
Topic: Convergence and
divergence
Convergence between
ridge and trough,
divergence between
trough and ridge
Convergence and divergence
Fig. 3, p. 320
Upper Level Waves
Long (Rossby) waves and short waves
Planetary scale pattern vs. subcontinental
scale
Upper level waves
Baroclinic (vs. barotropic)
Winds cross (vs. parallel) temperature
contours
Cold and warm air advection
Baroclinic instability
The Necessary Ingredients for Development
of Mid-latitude Storm
Baroclinic instability
Upper-Air Support:
Differential temperature advection
amplifies the upper level wave;
Cut-off low leads to dissipation
Role of the polar jet stream:
Removes air (through divergence) from
surface cyclone
Supplies air (through convergence) to
surface anti-cyclone
Baroclinic instability
Fig. 12-10, p. 323
The Necessary Ingredients for Development
of Mid-latitude Storm
Topic: Jet Streaks and Storms
Entrance and exit regions associated with
divergence and convergence, right exit
allows divergence.
Fig. 5, p. 324
Fig. 6, p. 324
Role of the polar jet stream
Polar jet swings over a developing mid-latitude cyclone,
An area of divergence (D) draws warm surface air upward
An area of convergence (C) allows cold air to sink
The jet stream removes air above the surface storm, which causes
surface pressures to drop and the storm to intensify.
When the surface storm moves northeastward and occludes, it no
longer has the upper-level support of diverging air, and the surface
storm gradually dies out.
Fig. 12-11, p. 325
Vorticity, Divergence and Development
of Cyclones
Vorticity is a measure of the spin of
small air parcels
Positive: cyclonic
Negative: anticyclonic
Divergence aloft causes an increase in
the cyclonic vorticity of surface
cyclones = cyclogenesis and upward air
movement
Vorticity on a Spinning Planet
Earth’s vorticity always positive (north)
Larger near poles
Vorticity on a Spinning Planet
Relative vorticity: curvature + shear
Trough: cyclonic, ridge: anticyclonic
Changes in vorticity through a Rossby wave
Vorticity on a Spinning Planet
Absolute vorticity
Earth + relative
Always positive in the north
• Lowest near equator and where relative vorticity is
negative
Increase is related to upper level convergence
Decrease is related to upper level divergence
Vorticity maxima/minima
Maximum positive/negative relative vorticity is
at sharpest turn
Therefore absolute maxima/minima are at
sharpest turns