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

Review of last lecture

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

2.2. Surface weather analysis: Station model, wind speed Surface weather analysis: Station model, wind speed code, present weathercode, present weather

3.3. Fronts: 6 types. Fronts: 6 types. 4.4. What is a cold front? Steep, narrow, fastWhat is a cold front? Steep, narrow, fast5.5. What is a warm front? Less steep, wide, slowWhat is a warm front? Less steep, wide, slow6.6. What is an occluded front? Two typesWhat is an occluded front? Two types

Winter Storms and Blizzards (Mid-latitude Winter Storms and Blizzards (Mid-latitude cyclones)cyclones) U.S. Annual U.S. Annual

mean fatalities 57, annual mean loss $329 millionmean fatalities 57, annual mean loss $329 million

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

bring abrupt changes in bring abrupt changes in wind, temperature, and wind, temperature, and sky conditionssky conditions

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

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

Low latitudes Low latitudes subtropical jet streamsubtropical jet stream

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

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

Conceptual modelsConceptual models

Norwegian cyclone model. Developed by Bjerknes in Norwegian cyclone model. Developed by Bjerknes in 1919, this model has formed the basis of our 1919, this model has formed the basis of our understanding of the life cycle of extratropical cyclones, understanding of the life cycle of extratropical cyclones, an amazing achievement given that this conceptual an amazing achievement given that this conceptual model was developed during a time period of highly model was developed during a time period of highly limited upper air data.limited upper air data.

Conveyer belt model. Developed by Browning (1973) Conveyer belt model. Developed by Browning (1973) and Carlson (1980), this model emphasizes the sources and Carlson (1980), this model emphasizes the sources of the incoming airstreams for an extratropical cyclone.of the incoming airstreams for an extratropical cyclone.

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.

Structure of Mature CyclonesStructure of Mature Cyclones• Well-developed fronts circulating about a deep

low pressure center describe a mature cyclone.• The system is capable of creating snow, sleet,

freezing rain, and/or hail. Chance of precipitation increases toward the storm center.

• Heavy precipitation (thunderstorms, squall lines) stems from cumulus development in association with the highly unstable conditions in the warm sector ahead of the cold front.

• Lighter precipitation is associated with stratus clouds of the warm front.

• Winds, spiraling counterclockwise toward the low, change accordingly as the system, and its associated fronts, moves over particular regions.

Conveyer belt modelConveyer belt model

Emphasizes the Emphasizes the sources of the sources of the incoming incoming airstreamsairstreams

Example of Mature CyclonesExample of Mature Cyclones

Video: The worst storm yet 2/1/2011

http://www.youtube.com/watch?v=Vnfwr1y016Q

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

Climate Change and Midlatitude CyclonesClimate Change and Midlatitude Cyclones

• Midlatitude cyclones are associated with the polar front and polar jet stream.

• It’s important to consider if climate change will cause a shift in the average tracks of midlatitude cyclones.

• Recent studies have shown that the average tracks for midlatitude cyclones has shifted towards the poles.

• The IPCC believes that continued warming and climate change will continue to move the tracks of the midlatitude cyclones towards the poles. This will have serious climate consequences to the Earth.

• Moreover, global warming weakens the polar vortex, causing more cold air intrusion into midlatitudes

Anticyclones, heat waves and droughtsAnticyclones, heat waves and droughts

• Anticyclones are associated with clear skies and calm conditions.

• If anticyclones linger over a region, it can lead to heat waves and/or droughts.

• Anticyclones are responsible for the Santa Ana wind conditions over the west coast, US.

SummarySummary1.1. The developmental stages and vertical structure of The developmental stages and vertical structure of

middle latitude cyclones (middle latitude cyclones (boundary between boundary between northern cold air and southern warm air,northern cold air and southern warm air, upper level upper level low to the west of surface lowlow to the west of surface low))

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

3.3. The three regions of cyclogenesis and typical tracksThe three regions of cyclogenesis and typical tracks4.4. Climate change: Climate change: move the tracks of the midlatitude

cyclones towards the poles5.5. Anticyclones, heat waves and droughtsAnticyclones, heat waves and droughts