How do the blizzards from?

1
How do the blizzards from?
2
Review of last lecture

• Tropical climate
• Mean state The two basic regions of SST? Which
  region has stronger rainfall? What is the Walker
  circulation?
• Mean state Two types of ocean upwelling,
  ocean-atmosphere feedback
• El Nino and La Nina Which region has warm SST
  anomaly during El Nino? 4-year period.
• Land-sea contrasts seasonal monsoon
• Extratropical climate
• Mean state westerly winds, polar vortex
• What is the primary way El Nino affect
  extratropics? (PNA)
• The oscillations associated with
  strengthening/weakening of polar vortex AO, AAO

3
Video Weather fronts

• http//www.youtube.com/watch?vtkK4_F0VKhM

4
Air masses

• An airmass is a large (usually thousands of km
  across) volume of air that has horizontally
  uniform properties of temperature and moisture.
• Airmasses acquire their properties from spending
  days to weeks over the same part of the Earth.
• Polar airmasses are colder than tropical
  airmasses
• Maritime airmasses are wetter than
  "continental" airmass
• Other specific airmass types include "arctic",
  "equatorial", and monsoon

5
Bergeron classification of air masses

• 3 letters e.g. mTk, cPw
• 1st letter for moisture properties c -
  continental, m - maritime
• 2nd letter for thermal characteristics T -
  tropical, P -polar, A - Artitic/Antarctic, M -
  monsoon, E - equatorial, S -superior air(dry air
  formed by significant downward motion in the
  atmosphere)
• 3rd letter for stability k/w - air colder/warmer
  than ground

6
Source regions
The areas where air masses form are called
source regions.
7
Fronts

• A weather front is a boundary separating two air
  masses
• Types cold front, warm front, stationary front,
  occluded front, dry line, squall line

8
Cold Fronts

• A cold front is a mass of cold air advancing
  towards warm air.
• Typically associated with heavy precipitation,
  rain or snow, combined with rapid temperature
  drops.
• Since friction decreases with height, winds move
  faster at higher altitude. Then the surface of
  cold front becomes more steeper through time,
  leading to a narrow belt of precipitation.
• Moving speed up to 30mph

9
Satellite and radar images of cold fronts(narrow
belt of clouds/precipitation)
10
Warm Fronts

• Warm fronts are warm air moving towards cold air.
• Friction decreases with height, so winds move
  faster at higher altitude. This causes the
  surface of the front to become less steep through
  time. Then clouds will be spread to a wider
  region.
• Shallow stratus clouds dominate and bring light
  precipitation. Frontal fogs may occur as rain
  evaporates in the colder air near the surface.
• Moving speed about 12 mph

11
Satellite and radar images of warm fronts(wide
region of clouds/precipitation)
12
Stationary Fronts

• Stationary fronts do not move. They do not
  advance. They are two unlike air masses side by
  side.
• They may slowly migrate and warmer air is
  displaced above colder.

From Environment Canada
13
Video Science of a Blizzard

• http//www.history.com/videos/science-of-a-blizzar
  dscience-of-a-blizzard

14
What is a mid-latitude cyclone?

• The mid-latitude cyclone is a synoptic scale low
  pressure system that has cyclonic
  (counter-clockwise in northern hemisphere) flow
  that is found in the middle latitudes (30N-55N,
  30S-55S).
• It has a larger size than a tropical cyclone

15
Midlatitude cyclones often form near the fronts
and jet streams

• Jet streams are caused by steep temperature
  gradients between cold and warm air masses
• Polar front - marks area of contact, steep
  pressure gradient ? polar jet stream
• Low latitudes ? subtropical jet stream
• Stronger in winter, affect daily weather patterns
  

16
How does a mid-latitude cyclone form?
In mid-latitude there is a boundary between
northern cold air and southern warm air
In the boundary a initial cyclone can advect warm
air northward and cold air southward
If the upper level low is to the west of surface
low, the cyclone will amplify and precipitation
will form.
Mature stage. Cold air begins to catch up with
warm air (occluded).
Cold air cools down the cyclone. Dissipation.
17
Why do some frontal waves develop into huge
cyclonic storms, but others dont?

• Complex challenge to forecasting
• Atmospheric conditions at the surface and aloft
  affect cyclogenesis.
• Key is to look at the upper level winds
  (longwaves, shortwaves).

18
Longwave disturbances (Rossby waves)
Earth's poles are encircled by 3 to 6 longwaves,
or Rosby waves, directing upper level winds
around lows at the 500 mb surface. Small
disturbances in these waves can trigger storms.
19
Shortwave Disturbances
Shortwave ripples within the Rossby waves move
faster, and propagate downwind into the Rossby
troughs and cause them to deepen. Barotropic
conditions, where isobars and isotherms are
parallel, then degenerate into a baroclinic state
where the lines cross and cold or warm air is
advected downwind.
20
Regions of cyclogenesis and typical tracks

• Gulf of Mexico, east coast
• Alberta Clipper from eastern side of Canadian
  Rockies
• Colorado Low from eastern slope of American
  Rockies
• Lee-side lows, lee cyclogenesis

21
Summary

1. Definition of airmasses. Bergeron classification
   of air masses (3 letters).
2. Fronts 6 types (cold, warm, stationary,
   occluded, dry line, squall line)
3. Cold front (narrow, fast, heavy precipitation),
   Warm front (wide, slow, light precipitation)
4. The developmental stages and vertical structure
   of middle latitude cyclones (boundary between
   northern cold air and southern warm air, upper
   level low to the west of surface low)
5. How upper level longwaves and shortwaves may
   enhance cyclonic development at the surface
   (upper level low to the west of surface low)
6. The three regions of cyclogenesis and typical
   tracks