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Lecture Note Packet #3: Extratropical (Midlatitude) Cyclones Blizzards and Ice Storms

I. The What and Where of Extratropical Cyclones (ECs) A. What...............

1. Any surface low pressure system (of synoptic (~1000 km) scale) that develops outside the tropics, in what are referred to midlatitudes (~ 30 – 70° latitude)

2. Most organized snow and rain storms are associated with ECs but not all of these are extreme

a. We will discuss what happens in the atmosphere to make a EC extreme

3. Due to the fact that strong temperature contrasts in the atmosphere favor strong ECs, and frozen precipitation has greater impacts, blizzards and ice storms are what are generally considered Extreme ECs

B. Where.............. 1. It should be apparent from last class that ECs will tend to develop along and follow the jet stream (Fig. 5.1) a. This is referred to as the extratropical “storm track” (Fig. 5.2) 2. Extreme ECs affecting continental land masses occur predominantly in the Northern Hemisphere (NH) (Fig. 5.3)

a. This is because, as we have discussed before, the SH is predominantly ocean and almost exclusively ocean under where the jet stream is located

3. Extreme ECs almost always occur during the cool/cold season when temperature contrasts are the largest and, thus, the jet stream is the strongest (Fig. 5.4)

4. Different types of Extreme ECs tend to occur in different locations a. Blizzards tend to occur along the East Coast of North America and Asia, particularly along the East Coast of the U.S. b. Ice storms are also a predominantly North American event c. The propensity for these events to occur in North America,

particularly along the East Coast, is due to the clash of very cold arctic air with very warm ocean currents (Gulf of Mexico and Gulf Stream) that occurs over the central and eastern portion of the continent as well as the orientation of the jet stream curvature in this region (Fig. 5.5)

d. Extreme ECs also occur along the West Coast of continents, predominantly the U.S. and Europe 1. The predominant impact of these events is usually wind

II. EC Structure and Features

A. In order to understand how and why extreme events occur it is important to examine the basic structure of a generic EC B. The Polar Front 1. ECs in general, and extreme ECs in particular, form along jet

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streams which exist along the boundary between colder air, throughout the full thickness of the atmosphere (troposphere), to the north and warmer air, throughout the full thickness of the atmosphere, to the south

a. This boundary extends circumferentially around the globe at middle latitudes

b. This boundary is called the polar front (Fig. 5.6) 2. This polar front is not straight but “wavy”

a. W aves of varying amplitudes propagate eastward along this front

3. There is a manifestation of this polar front at the surface separating cold air to the north from warm air to the south a. ECs develop along this front at the surface (Fig. 5.7) 4. As these surface low pressure systems (ECs) develop (upper-level

divergence) and then propagate with the wave in the polar front the counterclockwise circulation around the low generates an additional smaller scale waviness in the surface polar front which contain smaller scale “fronts” extending eastward and westward from the surface low

a. As a result of this circulation around the surface low there will be a surface “warm front” extending eastward from the EC and a surface “cold front” extending westward from the EC (Fig. 5.8)

b. As the EC moves (generally east/northeastward), the surface warm front to its east also moves east/northeastward and the surface cold front moves east/southeastward (Fig. 5.9)

5. Different types of weather and precipitation occur in different locations relative to the low pressure center, particularly as location relates to these fronts so in the case of an Extreme EC what type of extreme weather event you experience, or whether you experience an extreme event at all, depends on where you are located relative to the low pressure center and, by extension, relative to the surface fronts

C. Surface Fronts (Fig. 5.10) 1. Represent separation between cold and warm “airmasses” (Fig. 5.11)

a. Density difference between cold and warm air results in “lifting” (upward motion) of air which causes clouds/precipitation b. Symbols point in direction of motion of the front

2. Cold Front (Figs. 5.12 and 5.13) a. Represented on a surface map by a blue line with triangles

pointing in the direction of motion of the front b. Usually extend southward from the surface low pressure center c. The counterclockwise circulation around the surface low

pressure center pushes cold dry air southeastward where it “lifts” warm moist air to the south of the “polar front”

1. This “lifting” occurs because cold dry air is much denser than warm moist air (Fig. 5.13)

a. This “lifting” adds to the ascent that is already

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occurring due to the vertical circulation associated with the EC

2. Recall that if warm moist air rises or is “lifted” it will cool, water vapor will condense and clouds and precipitation will occur

3. Because of the “vertical” (steep slope) of the boundary/front, precipitation along this front usually occurs in a narrow band

4. Because the precipitation is occurring in the warmer air it is usually rain 5. If the low pressure center is “deep” (very low

pressure) and/or the cold front is “strong” (steep temperature gradient across the boundary) and, most importantly, if the warm moist air is “unstable” enough severe thunderstorms and even tornadoes can form along and out in advance of the cold front

a. As we will subsequently learn, most tornadoes are associated with thunderstorms which form in response to one of these cold fronts or a “dry line” (more on dry lines shortly and when we talk about thunderstorms)

3. Warm Fronts (Figs. 5.14 and 5.15) a. Represented on a surface map by a red line with semi-circles pointing in the direction of motion of the front b. Usually extends eastward from the surface low pressure center c. These fronts have a much more gradual slope (Fig. 5.15) than

cold fronts as the counter-clockwise circulation around the surface low results in warm air to the south of the “polar front” riding up over the denser (heavier) cold air to the north

1. The slope is more gradual and the movement of the front is much slower than the cold front since it is more difficult for the “lighter” warm air to displace the “heavier” (denser) cold air

2. Because of the gradual slope to the front, associated precipitation tends to cover a larger area 3. Although the warm air can be unstable enough in spring

and summer to result in thunderstorms, in winter, this air is usually relatively stable so steady light to moderate precipitation results

4. With a winter EC, there is frequently a transition of precipitation types as one progresses north to south from the leading edge of the precipitation shield toward the surface warm front from snow to sleet (ice pellets) to freezing rain to rain (Fig. 5.16)

a. This variation in precipitation type is due to the

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thickness of the cold layer under the clouds that the precipitation falls through before it hits the ground 1. To the north of slow moving warm fronts

(or stationary fronts) is where ice storms occur

4. Stationary Fronts (Figs. 5.17 and 5.18) a. Represented on a surface map by alternating blue triangles and red semi-circles pointing in opposite directions b. These fronts form along the “polar front”, frequently not associated with a surface low pressure center c. As the name implies, they do not move d. Because of their stationary nature, these fronts can lead

to prolonged periods of precipitation and, therefore, can be associated with flooding or ice storms (depending on the nature of the precipitation)

5. Occluded Fronts (Fig. 5.19) a. Represented on a surface map by alternating purple triangles and semi-circles pointing in the direction of motion of the front b. Develops late in the EC life-cycle as the fast moving cold front catches up to the slow moving warm front c. This front forms a boundary between cold, usually drier air to the west and cold, usually moister air to the east d. Usually associated with light to moderate precipitation due to

similar densities between the two cool air masses at the front 1. Extreme weather is not usually associated with these

fronts 6. Dry Lines (Fig. 5.20)

a. Frequently associated with severe thunderstorms in tornado alley (southern U.S. Plains) b. Represented on a surface map by a brown line with bunched semi-circles pointing in the direction of motion of the front c. Located on the warm side of the “polar front” extending south from the surface low pressure center d. Warm, dry air descending from the Rocky Mountains and the

high Mexican Plateau forms a boundary with warm, moist air from the Gulf of Mexico region

e. Behaves like a cold front because dry air is denser (heavier) than moist air f. Frequently associated with severe thunderstorms because the

warm, moist air being lifted is very unstable and contains lots of moisture (water vapor) 1. More on this when we discuss thunderstorms

III. “Extreme” Extratropical Cyclones.......Blizzards A. Definition of a blizzard

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1. Varies from country to country 2. Bottom line: Dangerous snowstorm with reduced visibility due to

falling and/or blowing snow, strong winds and cold temperatures 3. Most people tend to think of blizzards as storms associated with very heavy snowfall 4. Usually associated with “extreme” EC (very deep surface low pressure)

B. Causes of blizzards 1. Coastal Storms (Figs. 5.21 and 5.22)

a. Most common location for blizzards that contain all the ingredients we mentioned....heavy snow, strong winds and cold temperatures

b. Caused by explosive development of an EC c. These storms occur along the east coast of continents but are the most frequent and most severe along the east coast of the U.S. (requently called “Noreasters” because of the associated strong winds from the northeast because of the counterclockwise circulation around the surface low and the track of the low just off the coast.....) 1. Why this location? (Fig. 5.23)

a. All the ingredients for formation of a deep surface low pressure center

1. Temperature contrast between cold continent and warm ocean current (Gulf Stream)

a. Strong jet stream and Jet streaks 1. Upper level divergence

2. High amplitude jet stream wave pattern with sharp curvature. Because of the Rocky Mountains, East Coast of U.S. is frequently located on the curve between trough and ridge (I’ll show this in class) a. Upper level divergence

b. Instability (Figs. 5.24 and 5.25) 1. Warm Gulf Stream enhances upward motion (ascent) 2. Warm at surface – cold aloft...instability

c. Moisture 1. Lots of water vapor from evaporation from warm ocean/gulf waters

d. Less friction over water 1. Less friction means less convergence of air into the surface low 2. This enables upper-level divergence to rapidly outpace lower-level convergence resulting in rapid evacuation of air

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molecules from the air column above the developing surface low 3. Thus......rapid deepening of the surface low pressure system (cyclone)

e. Thus...strong winds (deep low pressure/strong pressure gradient) and heavy snow (marked ascent and available water vapor) (Fig. 5.26)

f. Arctic air from northern Canada adds cold to the storm (this was the cause of the strong temperature contrast that leads to the deep surface low in the first place)

d. Blizzard conditions occur just to the north and northwest of the storm track (track that the surface low takes) (Fig. 5.27)

1. Combination of strong ascent (heavy precipitation), cold air and strong pressure gradient (wind) 2. Snow can fall at rates of 1 – 4 inches per hour 3. Can even have “thundersnow” if instability is great enough

2. Orographic lifting (Fig. 5.28) a. Blizzards in mountainous terrain, particularly mountain ranges

along western aspects of continents (North America has Rocky Mountains – Sierra Nevada and Cascades)

1. Lifting of very moist stream of air off the ocean by high mountain ranges a. Heavy snowfall

2. Frequently associated with very mature ECs (occluded) near the end of their life cycle

a. Strong winds due to combination of eastward translation of the surface low and west winds circulating counter-clockwise around the surface low (area of blizzard located south of low center) unimpeded by surface friction over the ocean (Fig. 5.29)

3. Sometimes preceded by cold air mass sinking south from higher latitudes (Canada)

a. Cold temperatures b. Upslope snowstorms (Figs. 5.30 and 5.31)

1. Due to terrain enhanced easterly/northeasterly flow in areas to the north and northwest of the low center 2. East slope of Rockies and western Great Plains 3. Usually enhanced by clockwise flow around cold Canadian high

c. Other orographic lifting 1. European Alps

a. Lifting of moisture laden air circulating off the Mediterranean Ocean

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1. Not usually associated with the strong winds or cold temperatures which frequently define blizzards

3. Great Plains blizzards (Fig. 5.32) a. Strong temperature contrast between very cold arctic air mass

and warm moist flow off the Gulf of Mexico creates strong jet stream/jet streaks

1. Deep surface low pressure develops and creates strong pressure gradient with arctic high to the north.....very strong winds

b. Low pressure development and winds enhanced by reduced friction over flat and featureless Plains

4. Lake-effect blizzards (Fig. 5.33) a. Persistent bands of heavy snow downwind of Great Lakes b. No surface low pressure system

1. Frequently follows the passage of an EC and/or cold front

c. Caused by cold air blowing across warmer water 1. Instability a. Cause of upward motion 2. Moisture a. Evaporation of relatively warm lake water 3. Surface convergence a. Friction low over water and higher over land 1. Wind slows when hits far shore of lake 4. Orographic lifting a. Additional lifting by hills on far side of lake

C. Impacts of Blizzards....note that many of these impacts are related to strong coastal U.S. blizzards (“Noreasters”)

1. Transportation a. Disruption of car travel b. Closing of airports c. It is not just the heavy snow, but the reduced visibility that

markedly hinders travel and creates travel safety hazards 2. Economic a. Loss of business b. Property damage

1. Roof collapses are common when snow is “heavy” (high liquid water content because temperature is very close to 32°F)

3. Coastal flooding a. If strong onshore winds combine with high tides 4. Power a. Wind b. If snow is “heavy”, this can bring down trees on power lines 5. Human health

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a. Hypothermia 1. People can be trapped in cars or outdoors 2. Older people or those in poor health may be isolated in

their homes with no heat due to the lack of power b. Traffic accidents c. Heart attacks 1. Snow shoveling 6. Ecological a. Tree damage b. Livestock deaths c. Wildlife starvation 7. Positive impacts a. Water supply

1. Many regions, particularly mountainous rely on snowmelt runoff

b. Societal 1. Many stories of how this type of tragedy will bring together members of the affected community

c. Psychological 1. A deep snow cover can brighten the day through reflection of light (seasonal affective disorder) 2. NO SCHOOL!!! 3. Snowstorms make many people happy (lots of snow lovers)

d. Aesthetic 1. Is there anything more beautiful than snow (don’t tell my wife I said that.....) 2. Considerable artwork makes use of the beauty of snow

e. Human health 1. Many regions depend on heavy snow for snow sports

which are frequently the only way that people will get out and exercise during a time of year when it is critical that they do so

f. Economic 1. Many regions, particularly mountainous ones, depend on

heavy snow for tourism, the main source of jobs and income for many in these regions

IV. Blizzard Historical Case: March 12-15, 1993 “Superstorm” (“Storm of the Century”) A. Overview 1. This storm is historic for the large area and population affected a. Affected 26 U.S. States, much of Eastern Canada and Cuba b. 50% of the U.S. population had direct impact 2. Storm manifestations (Fig. 5.34) a. Record low atmospheric (barometric) pressure (960 mb) 1. Equivalent to a Category 3 Hurricane (Katrina)

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b. Record heavy snowfall over a large swath of the Eastern U.S. and Canada (Fig. 5.35) 1. Unusual for many southern states 2. Somewhat unexpected in many areas for mid-March a. Was well forecast several days before the storm

3. Heaviness (large liquid water content) of the snow caused problems, particularly in the south

c. Strong winds 1. Hurricane force wind gusts of 80 – 100 mph (not sustained [speed averaged over 1 minute]wind) in many locations 2. If the central pressure (and the pressure gradient force)

was equivalent to a Category 3 hurricane, why were the winds not equivalent to a Cat 3 hurricane (sustained winds of 111 – 130 mph)?

d. Cold temperatures 1. Record low temperatures across the eastern U.S.

B. Physical Causes and Behavior 1. Strong atmospheric temperature contrast (Fig. 5.36)

a. The temperature gradient in middle latitudes is critical to the development of extreme MLCs because of the necessity of a strong jet stream (jet streaks) that result in divergence aloft and strong vertical motion associated with the development of a “deep” surface low pressure system (very low atmospheric pressure)

b. A strong temperature contrast develops in the NH in late winter/early spring as the region of maximum sunlight (and heating) makes its journey north. The subtropics are heating up but the high latitudes remain very cold.

2. Amplified wave pattern of the jet stream with sharp curvature (Figs. 5.36 and 5.37)

a. Strong curvature effect and jet streaks 1. Divergence aloft (top of the troposphere) with rapid

deepening of the surface low pressure center (Fig. 5.38) (very strong winds) and vigorous ascent (heavy precipitation)

2. In this case, as with most of these extreme MLCs, there is a double jet streak configuration with superimposition of the upper level divergence area of one jet streak (left exit region) on the divergence area (right entrance region) of the other (Figs. 4c and 5c are good examples)

b. Another reason these storms occur frequently in late winter/early spring.....

1. Recall that the waves in the jet stream, and the weather that is subsequently generated, is an attempt of the atmosphere to restore heat balance between the tropics

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and the poles. At this time of year, this imbalance is enhanced (thus the strong temperature contrast previously discussed. Therefore, the atmosphere is making vigorous attempts to drive warm air north and cold air south, thus the amplified wave pattern.

3. Instability (Fig. 5.39) a. Warm water and air at surface (gulf stream) and cold air aloft 1. This steep environmental lapse rate enhanced ascent

4. Moisture a. Abundant moisture from evaporation of warm Gulf of Mexico and Atlantic Ocean surface waters (large amount of water vapor

capacity in near surface air) 5. Latent heat release

a. There was massive release of latent heat over the surface low pressure center due to an outbreak of vigorous thunderstorms over the Gulf of Mexico early in the storm’s development

1. This created a level of intensification not anticipated in the forecast models (although generally the storm was well forecast)

2. Remember....that latent heat release expands the atmosphere (separates the lines of equal pressure) causing lower than normal pressure at the surface and higher than normal pressure at the tropopause

a. This enhances the vertical circulation causing a deepening of the surface low and more vigorous ascent

5. All five of these ingredients combined “perfectly” (?) to create this historic storm

C. Impacts 1. Transportation a. Ground

1. Because heavy snow is rare in the deep south they were unprepared for removal and this was a particular problem there

2. Thousands were isolated, particularly in the southern Appalachians

a. Hundreds of hikers on the Appalachian Trail had to be rescued

1. Many “through hikers” had already begun their trek....

b. Air 1. Largest disruption to air travel in North America ever

(hmmm.....not sure how this compares with the aftermath of 9/11)

2. Every major airport on the East Coast closed, some for as long as 3 days (Asheville, NC)

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2. Economic a. Fortunately, it occurred over the weekend. However, the long term effects, particularly in the south, did impact businesses b. Structural damage

1. Roof collapses due to the weight of the “heavy” (high liquid water content) snow, particularly in southern locations

2. Damage from high winds 3. Power

a. 3 million without power due to high winds and downed trees, many without power for a full week

4. Coastal flooding a. Homes fell into the sea in Long Island and North Carolina b. Storm surge along western FL coast and Cuba 5. Tornadoes a. Strong squall line associated with the cold front 6. Human health a. Death toll of over 300 (including Canada and Cuba) 1. Dozens lost at sea a. 33 crew members from sunken tanker 2. Hypothermia 3. Storm surge 4. Tornadoes 5. Shoveling snow (heart attacks) 6. Traffic accidents V. Ice Storms A. What is an ice “storm”

1. A prolonged period of freezing rain which results in accumulation of a coating of ice on surfaces on or near the ground

a. U.S. National Weather Service requires a 0.25 inch coating to define an ice storm. However, the most damaging ice storms have considerably greater accumulation of ice, sometimes several inches

2. Freezing rain a. Supercooled water

1. Water droplets (rain drops) will not automatically turn to ice when the temperature drops below 32°F 2. Contact of these drops with a surface makes it easier for

the liquid water molecules to realign in the necessary crystal lattice structure of ice

b. Freezing rain usually occurs in situations where a layer of warm air overrides a thin layer of cold dense air near earth’s surface (Fig. 5.16) 1. Snow falls through the warm layer and melts 2. These raindrops fall through the cold layer near the

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ground, become supercooled, and then freeze on contact with surfaces on and near the ground

c. This thin layer of warm air above the surface results in a layer where the temperature rises with height, instead of falling, called an inversion (Fig. 5.40)

B. Where do they occur and under what type of weather “scenario” or pattern? 1. As with many types of extreme weather in the NH, ice storms are most

prominent across North America, particularly the East Coast (Fig. 5.41) a. Necessary ingredients are a thin low layer of very dense cold air

that is usually provided by an arctic air mass (anticyclone) abutting warm moist air

1. As with ECs, this type of temperature and moisture gradient occurs almost exclusively over North America

2. Significant ice storms are almost always caused by: a. A warm front associated with a EC that advances very slowly

or becomes stationary due to a very strong and dense entrenched arctic anticyclone (Fig. 5.42B)

b. A stationary front (surface representation of the polar front) with multiple ECs moving along it and the entrenched cold anticyclone to the north (Fig. 5.42C)

c. Cold air trapped east of the Appalachians (cold air damming) or in high mountain valleys of the west with warmer, moist maritime air from the Atlantic or Pacific Oceans (Columbia River Valley) flowing over it aloft (Fig. 5.43)

C. Impacts....many of the same impacts as blizzards but worse due to the weight of the ice on trees, buildings and power lines

1. Power outages a. Weight of ice on power lines or downed trees b. This can be prolonged as damaged power lines are fixed 2. Transportation a. Car and airline travel come to a stand still b. The reduced friction can make car travel impossible or

extremely dangerous c. Icing of planes and runways closes airports

3. Economic a. Lost business due to shutdown b. Structural damage caused by the weight of the ice 4. Human health a. Hypothermia b. Traffic accidents 5. Ecological a. Tree destruction b. Livestock death 1. Collapse of barn roofs c. Wildlife starvation 6. Positive Impacts

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a. Aesthetic? VI. Ice Storm Historical Case: 1998 Eastern North America Ice Storm A. Overview

1. Probably the most devastating ice storm in recent history 2. Prolonged period of freezing rain (5 days) resulted in up to 4 inches of

ice across southern Quebec and Ontario and some portions of northern New York and New England (Fig. 5.44)

3. 5 million people without electricity, some for as long as one month a. No light, heat and even water in mid-winter b. 1000 electrical power pylons collapsed in southern Quebec 4. 45 deaths

B. Physical Causes and Behavior (Multiple Figs. from NWS Burlington article and Weather Map project website)

1. Strong, cold surface arctic anticyclone over eastern Canada a. The clockwise circulation around this surface feature funneled

very cold dense air near the surface into southern Quebec and northern New England via northeasterly and easterly winds out of the Canadian Maritime region (Fig. 5.45)

b. This effect was particularly prominent in the St. Lawrence Valley due to its geographical orientation and the fact that cold air is denser and will collect in valleys and low elevations (Fig. 5.46)

2. Stationary Front (Fig 5.47) a. Formed the boundary between the cold arctic anticyclone to the

north and a stronger than normal “Bermuda High” over the Atlantic Ocean (this is part of the semi-permanent subtropical high pressure region which is stronger in summer than winter)

1. Clockwise flow around this surface high brought warm, moist air toward the stationary front from the Gulf Stream on southwesterly winds

2. This warm moist air was intensified by flow of warmth and moisture off the Pacific aided by formation of a strong subtropical jet stream that is typical of El Niño conditions (Fig. 5.48)

a. The 97 – 98 El Niño was the strongest in the past 50 years

3. This warm, moist air rode up and over the shallow layer of cold surface air just north of the front

a. This vertical temperature structure (inversion) resulted in freezing rain (Fig. 5.49)

4. This moisture laden air was lifted by the front and by large-scale lifting provided by upper level divergence that resulted in two extratropical cyclones which rode along the stationary front

a. This combination of moisture and lift over a

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prolonged period of time (5 days) resulted in large amounts of precipitation

3. Extratropical Cyclones (Fig. 5.50) a. Two ECs passed over the region over the course of the five days of the storm b. Both were “weak” (996 mb central pressure for the stronger of the two compared with 960 mb for the “Superstorm”) c. However, they were strong enough to provide additional lift/ascent to that being provided by the front

4. Bottom line.....1) Large amounts of moisture provided by the subtropical jet stream (El Niño) and the “Bermuda High”; 2) Ascent provided by the stationary front and the vertical circulation associated with the midlatitude cyclones; 3) Low level cold air provided by the strong surface arctic anticyclone to the north of the front; 4) Layer of warm air above the cold air provided by southwesterly flow to the south of the front

5. As with the “Superstorm” all ingredients have to come together “perfectly” in the atmosphere to have an extreme event

C. Impacts 1. Power (Fig. 5.51)

a. This was by far the most profound impact, particularly in southern Quebec b. Towards the end of the event, on Jan. 10, half the population of

Quebec province and 17% of the population of Canada was without power

c. It took several days to restore power to Montreal and some rural areas did not have power for a month, particularly those areas south of Montreal d. The lack of power did not just mean no light, it also meant no

heat and no water as temperatures dropped below zero after the storm passed

e. Even Montreal lost its water supply at the height of the storm after pumping stations lost power f. Whereas with blizzards the loss of electricity is due to the

combination of snow and wind blowing down power lines and knocking trees down on power lines, wind was not a significant factor in this storm, it was the shear weight of the ice that caused electricity poles and pylons to collapse

g. Cold temperatures after the storm passed delayed repair as the ice did not melt

2. Transportation a. Road and air travel came to a standstill b. Tunnels and bridges into Montreal closed due to safety and ice

weight concerns, essentially isolating the city c. Hazardous road conditions for several days due to the persistent

cold temperatures and downed power lines

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3. Human Health a. 45 deaths 1. Carbon monoxide poisoning a. Use of alternative fuels to heat 1. Fires as well b. Trapped in automobiles 2. Trauma a. Car accidents b. Falling ice!

1. Large chunks of ice were falling from buildings in Montreal for days after the storm

3. Hypothermia 4. Snow and ice removal 4. Economic a. Loss of business 1. 20% of Canada’s employees couldn’t make it to work b. Industry 1. Major losses from maple sugar industry a. Tree loss 2. Farm and dairy a. Collapsed barn roofs, livestock deaths 3. Lumber industry 4. Tourist and ski industry in northern New England c. Structural 1. Falling ice and roof collapse d. Loss of food from failing freezers 5. Ecological a. Livestock and wildlife deaths b. Tree loss 1. Millions of trees were destroyed 2. In northern New England and New York 70% of the

trees affected 3. Damage to the aesthetic beauty of this region

6. Positive impact? a. Crime down 57% in Quebec during and shortly after the storm

1. Probably multiple factors, like no one could move, but.....communities come together during crisis?

Impact of Global Warming

I. Atmospheric Factors Contributing to Extreme ECs

A. In order to ascertain whether these events are likely to change in frequency and severity with global warming we first must assess what atmospheric conditions contribute to the formation of extreme ECs and then evaluate the likelihood that those conditions will be more or less likely to occur in the future and

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where they might be more likely 1. Climate models, which are predicting future atmospheric conditions, are

very coarse resolution and can only ascertain processes of greater scale than weather events. Therefore, cannot directly predict whether the frequency and intensity of these events will increase with global warming. Any change in this regard must be inferred from the predicted change in the larger scale atmospheric properties and circulations.

B. Meridional (north-south) temperature contrast 1. As we have learned, the most important feature which contributes to an

extreme EC (“deep” surface low pressure – blizzard type) is the strength and configuration of the jet stream

a. A very strong jet stream and jet streaks are necessary to create the force imbalances necessary to generate enough upper-level divergence to create deep low pressure at the surface (wind) and a vigorous vertical circulation (strong vertical motion....clouds and precipitation)

1. Strong jet streams are created by a strong pressure gradient (pressure gradient force generates wind) which is created by a strong north/south temperature gradient in middle latitudes

b. Sharp jet stream curvature 1. A highly amplified wave pattern results in upper-level

divergence between the trough and ridge (between counterclockwise and clockwise curvature) that, as in a., results in a deep surface low-pressure and a vigorous vertical circulation

2. This wave amplification is created by the atmosphere’s attempt to balance the strong meridional temperature contrast (between equator and poles) by moving warm air north and cold air south

c. Bottom line....The upper-level conditions important to extreme EC development are based upon the existence of a strong meridional temperature gradient

2. Ice storm type of extreme EC a. Although usually associated with ECs, they are not necessarily as deep as the blizzard type b. However, the critical component is the low-level meridional temperature gradient associated with the warm/stationary front

1. In particular, a strong, cold arctic surface anticyclone is almost a necessity

3. Bottom line.....A strong meridional (north/south) temperature gradient is critical for development of all types of extreme ECs

C. Instability 1. Recall.....this is a measure of how quickly the environmental (not parcel) temperature decreases with altitude (environmental lapse rate)

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a. The steeper this lapse rate, or the relative difference in the temperature in the vertical direction between surface and tropopause, the more likely that rising air parcels will remain warmer than the environment and continue to rise....and the more vigorous the upward motion

2. This factor plays a secondary role compared with the meridional (horizontal) temperature gradient in developing extreme ECs (blizzard type)

a. Will not develop an extreme EC on its own....need the primary process of divergence aloft related to the jet stream (caused by the meridional temperature gradient)

b. Tends to enhance the intensification of the storm c. This is particularly true along the east coast of continents (U.S.

coastal storms) when the very cold pool of air at high-levels which originated at arctic latitudes moves over the warm ocean current (i.e. Gulf Stream)

D. Latent Heat 1. As with instability, latent heat release plays a secondary role by

enhancing the intensification process (this was certainly the case with the 1993 Blizzard)

2. Heavy precipitation, particularly with thunderstorms, can markedly enhance the vertical circulation by enhancing divergence aloft, warming the rising parcels, and deepening the surface low pressure system

3. Will not develop an extreme EC on its own....need the primary process of divergence aloft related to the jet stream (caused by the meridional temperature gradient)

E. Bottom line....a strong meridional (north/south) temperature gradient is the primary atmospheric component necessary for extreme EC development with instability and latent heat playing secondary roles

II. What do the climate models indicate are likely to be the effect of global warming on these atmospheric factors? A. Meridional temperature gradient (Fig. 5.52)

1. The models exhibit a very clear signal that the latitudinal asymmetry of global warming.....proportionally greater warming at high latitudes.....that has been occurring over the past several decades should continue, and even intensify, over the next century

2. The result of this asymmetric warming is a weakening of the meridional temperature gradient 3. As a result, one would expect the jet stream to be weaker, with a less amplified wave pattern (Fig. 5.53) 4. The climate models suggest that this change in the general circulation in midlatitudes is likely to occur 5. This projected change should also make the formation of strong, cold arctic anticyclones that are so critical for most ice storms less likely 6. The strong surface temperature contrast that is important for ice storm

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formation is also projected to decrease with global warming as the warming will be proportionally greater over continents than oceans (Fig. 5.52)

7. Although climate models cannot specifically forecast the number and intensity of specific ECs, these projected changes indicates that, if any change occurs, it would more likely be a decrease in number and intensity

B. Instability 1. The projected asymmetric warming between continents (particularly

high latitude continents) and oceans would be expected to reduce the instability associated with extreme ECs, particularly coastal storms

C. Latent Heat 1. With a greater amount of water vapor in the warmer atmosphere of the

future one might expect an increased amount of precipitation associated with these storms and thus a greater latent heat effect....

a. However, thunderstorms are the usual culprit here and it is not clear what effect global warming will have on their frequency or severity??....

III. Will global warming change the geographical locations affected by extreme ECs?

A. Climate models suggest that the midlatitude jet streams should not only weaken but also shift poleward as the coldest arctic air shrinks in areal coverage and the subtropical high pressure systems strengthen and expand northward (Fig. 5.54)

1. This change would likely lead to a northward shift in the tracks of ECs

IV. What does the observational evidence over the past century of global warming suggest about the trend in extreme ECs? A. In a single word......this evidence is inconclusive B. The biggest problem is how to quantify and qualify what an extreme EC is 1. Extreme ECs have always occurred, not something recent 2. Different studies utilize different parameters 3. Some suggest an increase in frequency and intensity, others a decrease

and still others no significant change a. Some suggest an increase in some locations and a decrease in others

C. One trend in the data that does appear to be consistent in numerous observational studies is that the midlatitude storm track has shifted poleward over the past century, particularly in recent decades

V. What conclusions can we draw from all this?

A. The storm track for extreme ECs has shifted poleward and is likely to continue this poleward shift in the future B. Evidence for likely change in the frequency and intensity of extreme ECs is

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not conclusive. However, our knowledge of the physical processes responsible for the development of extreme ECs and the future change in these processes suggested from climate model results suggest that if there is a change with global warming in the future, a decrease in frequency and intensity is most likely

1. There is certainly no evidence to suggest a marked increase in frequency and intensity of these storms in the future

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Fig. 5.1

Fig. 5.2

Fig. 5.3

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Fig. 5.4

Fig. 5.5

Fig. 5.6

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Fig. 5.7

Fig. 5.8

Fig. 5.9

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Fig. 5.10

Fig. 5.11

Fig. 5.12 Fig. 5.13

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Fig. 5.14 Fig. 5.15

Fig. 5.16

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Fig. 5.17 Fig. 5.18

Fig. 5.19

Fig. 5.20

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Fig. 5.21 Fig. 5.22

Fig. 5.23

Fig. 5.24 Fig. 5.25

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Fig. 5.26 Fig. 5.27

Fig. 5.28

Fig. 5.29

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Fig. 5.30 Fig. 5.31

Fig. 5.32

Fig. 5.33

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Fig. 5.34 Fig. 5.35

Fig. 5.36 Fig. 5.37

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Fig. 5.38

Fig. 5.39

Fig. 5.40

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Fig. 5.41

Fig. 5.42

Fig. 5.43

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Fig. 5.44

Fig. 5.45

Fig. 5.46

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Fig. 5.47

Fig. 5.48

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Fig. 5.49

Fig. 5.50

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Fig. 5.51

Fig. 5.52

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Fig. 5.53

Fig. 5.54