How do the blizzards from?How do the blizzards from?

Review of last lectureReview of last lecture

Tropical climate:Tropical climate: Mean state: The two basic regions of SST? Which region has Mean state: The two basic regions of SST? Which region has

stronger rainfall? What is the Walker circulation? stronger rainfall? What is the Walker circulation? Mean state: Two types of ocean upwelling, ocean-atmosphere Mean state: Two types of ocean upwelling, ocean-atmosphere

feedbackfeedback El Nino and La Nina: Which region has warm SST anomaly El Nino and La Nina: Which region has warm SST anomaly

during El Nino? 4-year period.during El Nino? 4-year period. Land-sea contrasts: seasonal monsoonLand-sea contrasts: seasonal monsoon

Extratropical climate:Extratropical climate: Mean state: westerly winds, polar vortexMean state: westerly winds, polar vortex What is the primary way El Nino affect extratropics? (PNA)What is the primary way El Nino affect extratropics? (PNA) The oscillations associated with strengthening/weakening of polar The oscillations associated with strengthening/weakening of polar

vortex: AO, AAOvortex: AO, AAO

Video: Weather fronts

http://www.youtube.com/watch?v=tkK4_F0VKhM

Air massesAir 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”

Bergeron classification of air massesBergeron classification of air masses

3 letters: e.g. mTk, cPw3 letters: e.g. mTk, cPw 1st letter for moisture properties: c - continental, m - maritime1st letter for moisture properties: c - continental, m - maritime 2nd letter for thermal characteristics: T - tropical, P -polar, A - 2nd letter for thermal characteristics: T - tropical, P -polar, A -

Artitic/Antarctic, M - monsoon, E - equatorial, S -superior airArtitic/Antarctic, M - monsoon, E - equatorial, S -superior air(dry air (dry air formed by significant downward motion in the atmosphere)formed by significant downward motion in the atmosphere)

3rd letter for stability: k/w - air colder/warmer than ground3rd letter for stability: k/w - air colder/warmer than ground

Source regions The areas where air masses form

are called source regions.

FrontsFronts A A weather frontweather front is a is a boundary separating two air massesboundary separating two air masses Types: cold front, warm front, stationary front, occluded Types: cold front, warm front, stationary front, occluded

front, dry line, squall linefront, dry line, squall line

Cold FrontsCold 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

Satellite and radar images of cold frontsSatellite and radar images of cold fronts(narrow belt of clouds/precipitation)(narrow belt of clouds/precipitation)

Warm FrontsWarm 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

Satellite and radar images of warm frontsSatellite and radar images of warm fronts(wide region of clouds/precipitation)(wide region of clouds/precipitation)

Stationary FrontsStationary 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

Video: Science of a BlizzardVideo: Science of a Blizzard

http://www.history.com/videos/science-of-a-blizzard#science-of-a-blizzard

What is a mid-latitude cyclone?What is a mid-latitude cyclone?

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

It has a larger size than a It has a larger size than a tropical cyclonetropical cyclone

Jet streams are caused by steep Jet streams are caused by steep temperature gradients between temperature gradients between cold and warm air massescold and warm air masses

Polar front - marks area of Polar front - marks area of contact, steep pressure gradient contact, steep pressure gradient polar jet stream polar jet stream

Low latitudes Low latitudes subtropical jet subtropical jet streamstream

Stronger in winter, affect daily Stronger in winter, affect daily weather patterns weather patterns

Midlatitude cyclones often form near the Midlatitude cyclones often form near the fronts and jet streamsfronts and jet streams

How does a mid-latitude cyclone form?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.

Why do some frontal waves develop Why do some frontal waves develop into huge cyclonic storms, but others into huge cyclonic storms, but others

dondon’’t?t?

Complex challenge to forecasting;Complex challenge to forecasting; Atmospheric conditions at the surface and aloft affect Atmospheric conditions at the surface and aloft affect

cyclogenesis.cyclogenesis. Key is to look at the upper level winds (longwaves, Key is to look at the upper level winds (longwaves,

shortwaves).shortwaves).

Longwave disturbances (Rossby waves)Longwave disturbances (Rossby waves)

Earth's poles are Earth's poles are encircled by 3 to 6 encircled by 3 to 6 longwaves, or longwaves, or Rosby Rosby waveswaves, directing , directing upper level winds upper level winds around lows at the around lows at the 500 mb surface.500 mb surface.

Small disturbances in Small disturbances in these waves can these waves can trigger storms.trigger storms.

Shortwave DisturbancesShortwave Disturbances

Shortwave Shortwave ripplesripples within the Rossby within the Rossby waves move faster, waves move faster, and propagate and propagate downwind into the downwind into the Rossby troughs and Rossby troughs and cause them to deepen.cause them to deepen.

BarotropicBarotropic conditions, conditions, where isobars and where isobars and isotherms are parallel, isotherms are parallel, then degenerate into a then degenerate into a baroclinicbaroclinic state where state where the lines cross and cold the lines cross and cold or warm air is or warm air is advectedadvected downwind.downwind.

Regions of cyclogenesis and typical tracksRegions of cyclogenesis and typical tracks

• Gulf of Mexico, east coastGulf of Mexico, east coast• Alberta Clipper from eastern side of Canadian RockiesAlberta Clipper from eastern side of Canadian Rockies• Colorado Low from eastern slope of American RockiesColorado Low from eastern slope of American Rockies

Lee-side lows, lee cyclogenesisLee-side lows, lee cyclogenesis

SummarySummary

1.1. Definition of airmasses. Bergeron classification of air Definition of airmasses. Bergeron classification of air masses (3 letters).masses (3 letters).

2.2. Fronts: 6 types (cold, warm, stationary, occluded, dry line, Fronts: 6 types (cold, warm, stationary, occluded, dry line, squall line) squall line)

3.3. Cold front (narrow, fast, heavy precipitation), Warm front Cold front (narrow, fast, heavy precipitation), Warm front (wide, slow, light precipitation)(wide, slow, light precipitation)

4.4. The developmental stages and vertical structure of middle The developmental stages and vertical structure of middle latitude cyclones (latitude cyclones (boundary between northern cold air boundary between northern cold air and southern warm air,and southern warm air, upper level low to the west of upper level low to the west of surface lowsurface low))

5.5. How upper level longwaves and shortwaves may How upper level longwaves and shortwaves may enhance cyclonic development at the surface (upper level enhance cyclonic development at the surface (upper level low to the west of surface low)low to the west of surface low)

6.6. The three regions of cyclogenesis and typical tracksThe three regions of cyclogenesis and typical tracks