Atmospheric Systems and Storms

Spatial and Temporal Relationships of Atmospheric Systems

  • Direct relationship between the size and duration of atmospheric systems.

    • Microscale atmospheric systems last hours.

    • Synoptic atmospheric systems last weeks.

Storm Energy Sources

  • Storm systems derive energy from:

    • Solar heating of Earth’s surface.

    • Condensation of water vapor in the atmosphere and subsequent release of latent heat energy.

Tropical Cyclones (TCs)

  • Nature's Deadliest Storms

Formation

  • Forms over tropical warm waters (temperature > 80°F).

  • Characterized by a closed low-level atmospheric circulation, where surface winds move continuously in a circular motion.

  • Defined as a cyclonic low-pressure system occurring in the tropics with sustained winds of 119 \, km/h (74 \, mph) or greater.

    • Rotate counterclockwise in the Northern Hemisphere around a region of low pressure.

Nomenclature

  • Tropical cyclones are known by different names depending on the region:

    • Hurricane: North and Central America.

    • Typhoon: Southeast Asia.

    • Cyclone: Countries bordering the Indian Ocean.

    • Tropical Cyclone: Australia.

Structure of a TC

  • Eye:

    • The center of the storm with the lowest pressure.

    • Typically calm with clear skies.

  • Eye Wall:

    • A ring of towering thunderstorms encircling the eye.

    • Features the fastest winds and heaviest rainfall.

  • Rain Bands:

    • Arms of heavy precipitation extending outward from the eye wall.

  • Thermal Signature:

    • Warmer in the eye and eye wall; colder in rain bands.

TC Strength

  • TC strength depends on the amount of water vapor that condenses to liquid, determining the latent heat exchange.

  • Condensation releases latent heat into the storm, intensifying it.

Latent Heat Positive Feedback

  • TCs derive high wind speeds from a latent heat positive feedback loop:

    1. Deep Low Pressure:

    • A deep central low pressure draws surface air in from all directions.

    1. Sea Spray:

    • Rapid air inflow whips up huge, frothy ocean waves and sea spray.

    • This sea spray evaporates, increasing the moisture content of the air.

    1. Latent Heat:

    • Inflowing air reaches the eyewall and rises, cooling to the dew point.

    • Water vapor condenses, releasing latent heat.

    1. Unstable Air:

    • Latent heat increases the instability of the rising air, deepening the surface-level low pressure as air rises faster. This restarts the cycle.

Saffir-Simpson Scale

  • Used to describe five categories of hurricane intensity based on measured wind speeds.

Hurricane Wind Scale

  • Category 1: Winds from 74 to 95 \, mph – Minimal.

  • Category 2: Winds from 96 to 110 \, mph – Moderate.

  • Category 3: Winds from 111 to 129 \, mph – Extensive.

  • Category 4: Winds from 130 to 156 \, mph – Extreme.

  • Category 5: Winds 157 \, mph or higher – Catastrophic.

TC Geography

  • Major tracks and frequency of hurricanes and typhoons vary by region and time of year.

  • Hotspots:

    • Western Pacific: June-December (Typhoons).

    • Has the greatest number of tropical cyclones in the world.

    • Eastern Pacific: June-October (Hurricanes).

    • Has the second greatest number of tropical cyclones in the world.

    • Western Atlantic and Gulf of Mexico: August-October (Hurricanes).

    • Has the third greatest number of tropical cyclones in the world.

  • Indian Ocean:

    • Smaller and doesn't have much room for hurricanes to develop, but when they do they are the world's deadliest due to the large populations living at sea level near the coast.

Influences

  • Influence of the Pacific high: Typhoons curve north as they approach Southeast Asia.

  • Coriolis effect: There are no hurricanes on the equator due to the lack of Coriolis force.

  • Cold water: The oceans west of South America and Africa are too cold for hurricanes.

Restrictions

  • TCs are restricted to tropical oceans.

  • Do not occur within about 5° latitude of the equator due to lack of Coriolis force.

  • Subtropical highs steer the movement of TCs.

TC Hazards

  • High winds, coastal storm surge, extreme rainfall, flooding, power outage, tornadoes, disease, etc.

  • Coastal storm surge: the most dangerous aspect of a TC

    • Storm Surge: Rise in sea level caused by strong winds and low atmospheric pressure of a TC

Deadliest Tropical Cyclones Worldwide

Top 10

  • 10/10 in south Asia – particularly vulnerable because large populations live near coast & poverty makes evacuation difficult.

    • Storm surge height depends on the category:

  • Category 1: Storm surge height of 1-1.7 meters (3-5 feet).

  • Category 2: Storm surge height of 1.8-2.6 meters (6-8 feet).

  • Category 3: Storm surge height of 2.7-3.8 meters (9-12 feet).

  • Category 4: Storm surge height of 3.9-5.6 meters (13-18 feet).

  • Category 5: Storm surge height of 5.7 or higher meters (19 or higher feet).

Deadliest Hurricanes in the United States

Top 10

  • 6/10 struck the Gulf Coast

Stages of TC Growth

Development Stages

  1. Tropical disturbance: A mass of thunderstorms forms on a tropical wave.

  2. Tropical depression: Winds increase and closed rotation begins because of the Coriolis effect.

  3. Tropical storm: Winds increase to at least 62 \, km/h (39 \, mph) with strong rotation.

  4. Tropical cyclone: Winds become 119 \, km/h (74 \, mph) or faster. Strong rotation and well-defined rain bands develop.

Stage Wind Speeds:

  • Tropical disturbance: Light.

  • Tropical depression: Up to 61 \, km/h (38 \, mph).

  • Tropical storm: 62-118 \, km/h (39-73 \, mph).

  • Tropical cyclone: Over 118 \, km/h (73 \, mph).

Characteristics

  • Tropical disturbance: A mass of thunderstorms with no rotation.

  • Tropical depression: Closed rotation begins.

  • Tropical storm: Stronger rotation, heavy rainfall.

  • Tropical cyclone: Strong circulation, heavy rain. Identifiable eye, eyewall, and rain bands often evident.

Important Hurricane Formation Conditions

  1. Tropical disturbance ("seed")

    • Low pressure spawns showers and storms.

  2. Warm moist air (“fuel”)

    • Large & warm ocean (temperature > 80°F)

  3. Enough Coriolis effect (to sustain a low pressure center)

    • Due to the Earth’s rotation, different parts of the Earth (including air parcels) move at different speeds.

    • Coriolis effect is too weak (i.e., no spin) at the equator, so central low pressure is easily filled up.

  4. Weak vertical wind shear (to allow hurricanes grow vertically)

    • Vertical wind shear: Changes in wind speed and direction with altitude

Dissipation

  • TC dissipates over land and cold ocean water in high latitudes

Hurricane Vulnerability

  • Almost all coastal regions in the Gulf of Mexico and up the East Coast of North America have been struck by hurricanes.

  • Miami is the metro area with the highest risk for a hurricane disaster.

    • Hurricane return period: the number of years, on average, between major hurricanes (category 3 or higher)

Recent Noteworthy Hurricanes

Hurricane Harvey

  • Maximum winds: 209 \, km/h (130 \, mph)

  • Maximum category: 4

  • Damage: 70 billion, extensive flooding throughout Houston area

  • Fatalities: 83

  • Broke rainfall record for a single storm in the United States.
    Hurricane Irma

  • Maximum winds: 298 \, km/h (185 \, mph)

  • Maximum category: 5

  • Damage: 63 billion, extensive wind damage to Florida

  • Fatalities: 124

  • Broke record for longest sustained high wind speed: 37 hours of winds of 298 \, kp/h.
    Hurricane Maria

  • Maximum winds: 282 \, km/h (175 \, mph)

  • Maximum category: 5

  • Damage: 139 billion; costliest in Puerto Rican history; extensive wind damage to Puerto Rico.

  • Fatalities: nearly 3,000

  • Fifth costliest hurricane on record.

Climate Change and Hurricanes

  • Hurricanes have increased by 2.58 events since 1851!

  • US hurricane hazards are more frequent & costlier!

Factors

  • Warmer Atlantic sea surface temperature (SSTs)

  • Rising greenhouse gases are the major driver of the warming.

Midlatitude Cyclones (Extratropical Cyclones)

  • Large cyclonic (low-pressure) systems that occur at midlatitudes (30° to 70°).

  • Form as a result of temperature contrasts between cold and warm air masses during the cool season (fall to spring).

Key Areas of Cyclogenesis (Formation)

  • Alberta Clipper

  • Gulf of Alaska Low

  • Hatteras Low

  • Colorado Low

  • Gulf Low

Main Settings in Which Cyclones Form

  • Downwind of mountain ranges (e.g., Rocky Mts.).

  • Warm water is located downwind of cold land surfaces/water.

Anatomy of a Midlatitude Cyclone

  • Most midlatitude cyclones are composed of a warm front and a cold front.

Elements

  1. Cold sector

    • Polar air is moving southward into warmer air.

    • Low air temperatures and dew points in the cold sector.

  2. Cold front

    • Warm air rises over the approaching mass of cold air.

    • The lifted air cools adiabatically and condenses to form clouds. Precipitation is shown in gray.

    • Cold fronts are symbolized on weather maps with a blue line and triangles. Triangles point in the direction of cold air movement.

  3. Storm center

    • The lowest pressure occurs at the center of the midlatitude cyclone.

    • In this scenario, the central low pressure is less than 992 \, mb.

  4. Warm sector

    • Warm mT air is moving north from the Gulf of Mexico.

    • High air temperatures and dew points prevail in the warm sector.

  5. Warm front

    • The warm air rises over relatively cold air to the north.

    • As the air rises, it cools adiabatically, forming clouds.

    • Precipitation usually comes from nimbostratus clouds ahead of the warm front.

    • Warm fronts are symbolized on weather maps with a red line and half circles that point in the direction of the movement of warm air.

Fronts

Cold Front

  • Region where cold air advances on relatively warm air

  • Have a slope of about 1:100. This means that 100 \, km (or 100 \, mi) behind the front, the air mass surface is 1 \, km (or 1 \, mi) above Earth's surface.
    Weather progression

  • Before the front passes, warm weather with cirrus clouds. Then a cold-front squall line follows. The squall line is followed by cold, clear air as the cold front moves past. This sequence may occur in just a few hours. Warm Front

    • Region where warm air advances on and flows over cooler, heavier air
      Have about a 1:200 slope. This means that 200 km (or 200 mi) in advance of the front, the height of the air mass boundary is 1 km (or 1 mi) above Earth's surface. The steepness of the front is exaggerated here.
      Weather progression

  • Fronts typically move about 40 \, km/h (25 \, mph). As the front passes, the progression of clouds might be cirrus, cirrostratus, altostratus, then nimbostratus. After the front passes, the temperature rises as the warm air arrives. If the warm sector is humid, the dew point rises as well. This sequence may take a day to complete.

Effects of Midlatitude Cyclones on Weather

  • Midlatitude cyclones bring storms to midlatitude regions (e.g., U.S. and Canada) in fall through spring.

  • Warm fronts are usually associated with nimbostratus clouds that bring steady precipitation.

  • Cold fronts are usually associated with cumulonimbus clouds that bring short bursts of rainfall and potentially severe weather.

Life Cycle of a Midlatitude Cyclone

  • A midlatitude cyclone experiences three stages (growth, maturation, and dissipation) over about 1-2 weeks

  1. Stationary front

    • In a stationary front, cold polar air and warm subtropical air move parallel to one another.

    • A stationary front is symbolized with alternating red half circles and blue triangles.

  2. Stationary wave

    • A wave in the stationary front can develop from mountain ranges, surface temperature contrasts, or changes in the location and strength of upper-level Rossby waves.

    • For the storm to strengthen, there must be an upper-level trough creating divergence aloft

  3. Midlatitude cyclone

    • As the wave develops, warm air pushes north and creates a warm front.

    • Cold air pushes south and creates a cold front. Because the cold front moves faster, it overtakes the warm front.

    • This is the strongest stage of the storm.

  4. Occluded front

    • An occluded front (shown in purple triangles and half circles) forms as the cold front overtakes the warm front.

    • The occluded stage of a storm can result in brief but heavy precipitation.

  5. Dissipation

    • The cold front has overtaken the warm front and moves the warm air aloft.

    • At this point, there is no more horizontal temperature or pressure gradient. The air is stable, and the storm dissipates.

Upper-Level Support

  • Midlatitude cyclones must have upper-level support with a trough in the polar jet stream to persist.

  1. Rossby wave trough

    • Long, undulating Rossby waves form in the upper-level westerlies. Where a Rossby wave bends toward the equator, a trough forms.

  2. Upper-level divergence

    • As air flows into the trough, it piles up and slows down, much as cars slow down as they enter a turn on a road.

    • As air exits the trough, it speeds up and creates a vacuum that pulls air upward from the surface.

  3. Rising central air column

    • As air flows upward from the surface, it rotates counter- clockwise due to Coriolis effect, forming a cylinder of rotating air that is several hundred kilometers across.

  4. Surface-level low pressure

    • The rising air column deepens the surface-level low-pressure system, strengthening the cyclone. Both the upper-level trough and the cyclone at the surface move eastward together.

  5. Anticyclonic flow

    • Where a Rossby wave bends poleward, a ridge develops. Ridges are associated with surface-level anticyclonic systems. Often an anticyclone feeds cold air from the north, enhancing the cold front in the cyclone.

Average and Actual Storm Tracks

  • Storm tracks vary based on factors, such as season.

  • In wintertime, midlatitude cyclones move over the Great Lakes, the cold air behind them often creates lake-effect snow (heavy snowfall).

Thunderstorms

  • Thunderstorm: a cumulonimbus cloud that produces lightning and thunder

Scales

Temporal Scale (Duration)

  • Hours

Spatial scale (size)

  • 1 \, km (0.62 \, mi)

Systems by Size

  1. midlatitude

  2. Tropical

  3. Supercell thunderstorms

  4. Multicell thunderstorms

  5. Single-cell thunderstorms

Energy

  • Thunderstorms are a way for the atmosphere to release energy from:

    • Solar heating of Earth’s surface

    • Condensation of water vapor in the atmosphere and subsequent release of latent heat energy

Lifting Mechanisms:

Convective uplift

  • Warm air parcels become unstable and rise.
    Orographic uplift

  • A moving air parcel meets a mountain range, which forces it to rise.
    Frontal uplift

  • Where warm air and cold air masses meet, the less dense warm air flows over the cold air.
    Convergent uplift

  • Where surface winds converge, they rise.

Relation to Synoptic-Scale Systems

  • Most thunderstorms are embedded within larger synoptic-scale systems:

    • Tropical cyclones (also called hurricanes)

    • Midlatitude cyclones

    • The ITCZ

Air Masses

  • A large region of air that is uniform in temperature and humidity.

  • An air mass extends over thousands of kilometers

Characteristics

Humidity (1st letter)

  • c = continental (dry)

  • m = maritime (humid)
    Temperature (2nd letter)

  • A = Arctic (very cold)

  • P = polar (cold)

  • T = tropical (warm)

Air Mass Influence

  • All thunderstorms form within or on the boundaries of mT air masses.

    • Weak thunderstorms form within mT air masses.

    • More powerful severe thunderstorms form where mT air masses come into contact with cP air to the north.

Thunderstorm Frequency

  • Each day, about 40,000 thunderstorms occur globally

  • Most of them are caused by afternoon ground heating & subsequent convective uplift in the ITCZ.

North America

Thunderstorms form:

  1. along frontal systems in the early spring.

  2. due to orographic lifting along mountain ranges such as the southern Rocky Mountains

  3. convective uplift of warm and moist air masses that move north from the Gulf of Mexico (making the southeast, particularly Florida, produce the highest frequency of thunderstorms in the US)

  • The cold Pacific Ocean inhibits evaporation and atmospheric moisture. Dry air reduces the number of thunderstorms in the western United States.

  • Orographic lifting over the southern Rocky Mountains increases thunderstorm activity for central Colorado and northern New Mexico.

  • Dry air from Mexico reduces thunderstorm activity in the southwestern United States.

  • The warm Gulf of Mexico and the Atlantic Ocean readily evaporate and increase atmospheric moisture. This moisture increases thunderstorm frequency in the south-eastern United States.

Types of Thunderstorms

  • Three types of thunderstorms:

    • Single-cell

    • Multicell

    • Supercell thunderstorms

  • The two most important factors that determine thunderstorm type:

    • Atmospheric humidity

    • Wind shear (changes in wind speed and direction with altitude)

Thunderstorm Specifics
Single-Cell Thunderstorms

  • Relatively mild and short-lived (last about 1 hour)

  • Form within mT air masses where wind shear is weak

  • Develop in late afternoon

  • Typically experience a predictable sequence of growth, maturation, and dissipation phases

Stages:

  1. Cumulus stage

    • A cumulus cloud develops, cooling to its dew point with condensation releasing latent heat, warming the cloud's interior and causing it to become more unstable and grow vertically.

  2. Mature stage

    • Rain develops dragging air downward, forming downdrafts. The upper regions of the cloud are so cold that liquid droplets become glaciated and freeze into ice. At this stage, lightning and thunder, heavy rain, and hail are possible.

  3. Dissipating stage

    • Downdrafts block the updrafts feeding moisture into the storm. Once the storm is starved of moisture, condensation and release of latent heat cease, weakening the updrafts and the cloud quickly evaporates.

Multicell Thunderstorms

  • Form under conditions of moderate wind shear (wind speeds of 40–65 \, km/h or 23–40 \, mph).

  • Form along fronts rather than within air masses

  • Arranged in clusters (mesoscale convective systems) or in squall lines

    • A line of multicell thunderstorm cells that typically forms along a cold front on a midlatitude cyclone.

Characteristics

  • Multicell thunderstorms often produce severe weather.

    • Severe thunderstorm: produces either hail 2.54 \, cm (1 \, in) in diameter, a tornado, or wind gusts of 93 \, km/h (58 \, mph) or greater

Supercell Thunderstorms

  • Supercell thunderstorm (or rotating thunderstorm): a severe thunderstorm containing a rotating cylindrical updraft.

  • Form over land where there is humid air and strong wind shear

  • Form most often in Tornado Alley in the southern Great Plains

  • Produce almost all powerful tornadoes

Mechanics

  1. Wind shear

    • Wind shear creates a horizontal rolling cylinder of air near the ground.

  2. Updraft and thunderstorm

    • An updraft in a thunderstorm (red arrow) tilts the cylinder to vertical.

  3. Mesocyclone

    • The rotating air creates a mesocyclone. The mesocyclone itself is not a tornado, although it may cause a tornado to form.

    • Mesocyclone: rotating portion of a supercell thunderstorm

Thunderstorm Hazards: Lightning and Tornadoes

Lightning

  • Electrical discharge produced by thunderstorms

  • Acoustic shock wave produced when lightning rapidly heats and expands the air around it

    • Follows channels of least resistance (e.g., wet soil, tree roots)

    • Runs cloud-to-cloud, within a single cloud, cloud-to-ground

Lightning Capital

  • The world’s lightning capital: Lake Maracaibo in Venezuela.

    • Average of 260 days of thunderstorms each year

    • 9 hours thunderstorm activity per day

    • 2,500 flashes of lightning per day

  • Formed by combination of warm, humid water from Lake Maracaibo and cool, dry air from the surrounding mountains.

Causes

  • Negative and positive charges build up.

  • Two oppositely charged regions develop an electrical connection, and a bolt of lightning is discharged.

Safety

  • To determine distance of a lightning bolt, count the number of seconds until you hear thunder (i.e., lag time)

    • Imagine you count 15 seconds: how far away is the lightning? (Speed of sound: 343 meters per second)
      (343 * 15 = 5145). Divide by 1000 to get km. 5.14 \, km away

    • Rule of thumb: dividing lag time by 3 (5) to get distance in kilometers (miles)

  • 30/30 rule – Not safe outdoors if lightning within 10 \, km (6 \, mi) (30 seconds between lightning and thunder); Wait 30 minutes after the storm has passed to go back outside.

  • During a thunderstorm:

    • Always shelter indoors if possible.

    • If outdoors, do not be the highest object or stand under the highest trees.

    • Cars are safe.

Tornadoes

  • Violently rotating column of air that descends from a cumulonimbus cloud and touches the ground.

  • Tornadoes form in thunderstorms, hurricanes, and cold fronts

Details

  • Only about 25\% of supercell thunderstorms produce tornadoes, and meteorologists still don’t know exactly how supercells make tornadoes

  • If a tornado does form from a supercell, it usually descends from a wall cloud (a cylindrical cloud that protrudes from the base of the mesocyclone)

The Enhanced Fujita Scale (EF scale)

  • A system used to rank the strength of a tornado

    • Directly measuring their wind speed is impossible to do safely.

  • Tornado strength is estimated by damage done to the landscape

Tornado Geography

  • U.S. has the most frequent (1,200 per year) and strongest tornadoes in the world.

  • Florida has the most tornadoes among US states.

  • Great Plains is called the “Tornado Alley”– warm, humid (mT) air masses interact with dry, cold air masses (cP) from its west and north.

    • April to July – most active period for tornadoes

Characteristics

  • Time of year: April–July

  • Time of day: 4 to 6 \, p.m.

  • Diameter: 50 \, m (160 \, ft)

  • Forward movement speed: 48 \, km/h (30 \, mph)

  • Length of ground path: 3 \, km (2 \, mi)

  • Duration: 5 minutes

Dixie Alley

Center of strong tornadoes (EF2-EF5) activity appears to be shifting southeastward to “Dixie Alley”
More fatalities and damage in the Dixie Alley due to:

  • A larger number of strong tornadoes

  • More heavily populated

  • More mobile homes

  • More nighttime tornadoes

Preparedness
Warning System

  • Tornado Watch: conditions are favorable for tornadoes to occur. Be ready to move to a safe place. Lead time: hours

  • Tornado Warning: a tornado has been spotted or indicated on weather radar. Take action! Get to a safe place now. Lead time: minutes

Safety

  • During the year 1925 was the most deadly in U.S. history.

  • The year 2011 had 550 fatalties.

Advancement

  • Radar development.

  • The Union City tornado

  • NEXRAD. Between 1990 and 1997 the U.S. government installed a network of 159 Doppler radar stations across the United States.

  • Increasing lead-time.

Measures
Indoors

  • Retreat to a basement or an interior room or hallway. windows, which are easily shattered into dangerous glass shards.

  • Retreat to the lowest floor.

  • Heavy objects such as refrigerators, which can fall on a person.

  • Crouch in a bathtub.

  • Exterior walls, which can fail in high winds.

  • Cover yourself with a mattress or sleeping bag.

  • Mobile homes, even those with anchored foundations, are unsafe in a tornado.

Outdoors

  • Find low ground and lie face down with arms over head.

  • Cars. If caught in a car, park on the side of the road. Keep a seat belt on and put your head down below the windows.

  • Bridges. Wind speed increases under bridges.

Weather Forecasting and Analysis

  • Weather forecasting is a big data-driven effort.

Measurements

  • How is precipitation measured?

Weather forecast model

Models

  • Surface data, upper-level data and vertical exchange

Error Trends

  • Hurricane track forecasts have improved markedly.

    • These improvements are largely tied to improvements in large- scale forecasts.

  • Hurricane intensity forecasts have only recently improved.

    • Improvement in intensity forecast largely corresponds with commencement of Hurricane Forecast Improvement Project