Outline for Lesson 20

Chapter 9

Case study in an extratropical cycloneCase study in an extratropical anticyclone

Chapter 10

ThunderstormsAir Mass ThunderstormsSevere Thunderstorms

4/08/03

Cloud patterns typically associated with a mature middle-latitude cyclone

Idealized depiction of the support that convergence and divergence aloft provide to anticyclonic and cyclonic circulation at the surface.

3-D view of atmospheric wave

Weather Map Depicting the Norwegian Cyclone Model

November 10, 1974: The Edmund Fitzgerald: The 729-ft. ore boat sank in a storm in Lake Superior; called the worst Great Lakes Disaster in 17 years.29 casualties.

"...Does anyone know where the love God goes when the wavesturn the minutes to hours..."                                                                                    ~Gordon Lightfoot

Path of the November extratropical cyclone that sunk the Fitzgerald

Winds generate waves; the size of the waves depends on

•wind speed•duration of the winds•the fetch the distance of open water

Surface weather and 500-mb for 0700 EST November 8, 1975

A weak low forms because of the westerly winds flowing over the mountains. With very dry air over Northwest Texas no clouds are evident on the satellite image. However the circulation begins to draw in moist, warm air from the south and cool air from the north.

Cyclogenesis

The temperature contrastacross the front is now 20 °F

The short-wave at 500 mb is deepening to the west of thesurface low.

Upper level support is present.

The cyclone is maturing and growing in strength.

Surface weather and 500-mbfor 0700 EST November 9, 1975

Developing Stage

70/6445/37

Satellite image on November 9, 1975

By 0700 AM on the 9th

the cyclone has taken classicform at an early stage.

Comma cloud - the comma headis made up of clouds and lightprecipitation circulating counter-clockwise around the low’s center.

Dry slot - a feature typically seen in mature cyclones and never observed in the tropical cyclones.

Comma trail is cloudiness alonga trailing cold fronts. Not observedin tropical cyclones.

300 mb winds and heights November 9, 1975

The combination of the two types ofdivergence (speed anddiffluence) helps lead to sudden pressuredrops over the upperMidwest.

Therefore, the cycloneaccelerates to the northeast and deepensrapidly.

Upper level support is key to surface cyclone development

The relationship between upper level winds and surfaceconditions during the first 3 stages of the cyclone model.

Surface weather and 500-mbfor 0700 EST November 10, 1975

The Mature Cyclone

Over the previous 24 hours the cyclone has lost almost 1 mb per hr. Heavy wet snow is being blown by ~70mph winds.

15 tornadoes were spawned ahead of the cold front from Iowa to Tennessee. The warmfront is now in Canada.

Hourly sea pressures at four stations in the general path of theFitzgerald cyclone. The Fitzgerald sank in rising pressure northwestSault Ste. Marie, Michigan.

Hourly wind speeds at Sault Ste. Marie and Marquette, Michiganduring the approach and passage of the Fitzgerald cyclone.

Death of a Cyclone

The strong temperature gradient associated with the Fitzgerald cyclone are now in the Atlantic.

Another cyclone has formed over the great plains. However, unlike the previous cyclone it does not have a supply of warm moist air and never reaches the extreme state of the cyclone which sunk the Fitzgerald.

Mid Latitude Anticyclones

                                                                    

                                             

Typical High Pressure Systems

A large high pressure system during July of 1999 likely was the cause of the plane crash that killed John Fitzgerald Kennedy, Carolyn Bessette Kennedy and Lauren Bessette.

Surface weather and 500-mbfor 0700 EST July 16, 1999

High pressure systems inhibit vertical mixing of air and have light winds. Therefore in the summertime it is not uncommon for pollution tobuild over urban areas and areas “downwind” when strong anticyclones persist.

Anticyclones

Satellite image of for 1720 EST July 16, 1999.

A dense layer of haze is evidentfrom satellite imagery taken on theday of the accident.

A schematic explaining how the polluted anticyclone contributed tothe crash of John F. Kennedy Jr.’s plane. Temperature inversionswithin the anticyclone trapped and concentrated the pollutants in a narrow layer corresponding to the altitude at which the plane went outof control.

Outline for lesson 21

Thunderstorms

Air Mass Thunderstorms

Severe Thunderstorms

Lightning and Thunder

4/10/03

The humid, subtropical climate that dominates the southeasternUnited States receives much of its precipitation from thunderstorms.

Average number of days each year with thunderstorms.

Thunderstorms

A thunderstorm is a storm which generates lightning and thunder. There are two classes of thunderstorms:

• Air Mass Thunderstorms• Severe Thunderstorms

Air Mass thunderstorms form when warm, humid air rises in an unstable environment (recall an unstable environment is when a warm layer of air is below a cool layer of air). They often occur in the summertime from a mT air mass. Air mass thunderstorms are short lived and seldom produce strong winds or hail.

The stages of development of an air mass thunderstorm can be summarized as follows:

1. Cumulus Stage, in which the updrafts dominate throughout the the cloud and growth from a cumulus to a cumulonimbus cloud occurs.

2. The Mature Stage, the most intense phase, with heavy rain and possibly small hail, in which downdrafts are found side by side with updrafts.

3. The Dissipating stage, dominated by downdrafts and entrainment, causing evaporation of the structure.

Stages of Development of Air Mass Thunderstorms:

Cumulus Stage

Mature Stage

Dissipating Stage

Stages of development of a thunderstorm.

During the cumulus stage, strong updrafts act to build the storm. The mature stage is marked by heavy precipitation and cool downdrafts in part of the storm. Dissipating stage is when the warm updrafts disappear completely, precipitation becomes light and the cloud begins to evaporate.

Occurrence of Air Mass Thunderstorms

Air Mass thunderstorms are more common over mountainous regions than over the flat, plains areas. This is so because the heating of a mountain slope is more intense than that over flat ground (higher sun angle).

It is important to keep in mind that there are many different “trigger mechanisms” (some process which initiates the lifting of an air parcel such that it is warmer than the surrounding air and, thus, unstable) for thunderstorms depending on location.

Common triggers for thunderstorm development:differential heating, convergence, frontal wedging, strong upper level divergence

Severe Thunderstorms

About 10% of the 100,000 thunderstorms which occur over the US every year are classified as severe. A severe thunderstorm is one in which the winds exceed 58 mph, or produce hailstones with a diameters larger than 1.9 cm (0.75 inch).

Severe thunderstorms usually have strong vertical shear. Strong vertical wind shear (i.e. change in wind direction or speed between different heights) supports updrafts which maintain the supply of warm, moist air to the thunderstorm. In severe thunderstorms the alignment of updrafts and downdrafts becometilted. The tilt of the updrafts results in precipitation falling into the downdraft rather than the updraft.

Diagram of a well developed cumulonimbus tower

updraftsdowndrafts

lower stratosphere

Gust fronts can thought of as mini cold fronts; these “fronts” can provide the updraft for new thunderstorms.

Severe Thunderstorms (cont.)

As warm air is lifted along the leading edge of the gust front a roll cloud is formed which can often be seen.

Supercell Thunderstorms A supercell consists of a verypowerful single cell.Supecells can extendas high as 20 km65,000 ft and persistfor several hours.They can be as wideas 20-50 km (12-30mi).

Very complex windstructure present which can lead to tornadoes.

The most severe tornadoes almost always come from Supercells.Vertical winds sometimes have updrafts which rotate in a cyclonic motion, this is called a mesocyclone and is often where tornadoes are formed.

Supercell Thunderstorms

For supercells to form there needs to be copious amounts of moisture available.One way for this to occur is the presence of a strong “inversion layer”…..recall that inversions are typically associated with stable atmosphere……

How can a temperature inversion lead to explosive thunderstorms?

Squall Lines and Mesoscale Convective Complexes

A squall line is a relatively narrow band of thunderstorms that develop in the warm sector of a middle-latitude cyclone, usually 100-300 km (60-180 mi) in advance of a cold front.

The linear band of cumulonimbus clouds may extend 500 km or more and be comprised of many thunderstorms in various stages of development.

The lifetime of a squall line is on average ~10 hours or so but they have been known to last more than a day.

Cumulonimbus mammatus clouds are often associated with squall lines.

Drylines can often indicate squall line location

A squall linewith severethunderstormscan also formalong a boundary calleda dryline.

A dryline is a narrow zone where there is anabrupt change inmoisture. Oftenforms in betweencT and mT air masses

Several tornadoes wereassociated with this squallline. Notice the sharpmoisture gradient withthe squall line.

Mesoscale Convective Complexes (MCC)

A MCC consists of many individual thunderstorms organized into a large oval to circular cluster.

Most MCC’s cover an area of 100,000 square kilometers (39,000 square miles). One easy way to remember the maginitude of this area is to think of MCC’s as being about the size of state. These systems are slow moving (compared to squall lines or fast moving cold fronts) and often persist for 12 hours or more.

The Great Plains is a favored location for MCC formation.

Mesoscale Convective Complexes (MCC)MCC’s are complex systems. They tend to form in the afternoon as a large group of air mass thunderstorms begin to decay. With a steady flow of low-level warm, moist air the atmosphere becomes unstable and an MCC begins to develop.

Mesoscale convectivesystems are self-propagating as gust fronts from existingcells feed the development of new thunderstorms; MCC’scan remain as long as the favorable conditions exist.

Lightning and Thunder

Lightning is the result ofcharge seperation (part ofthe cloud develops anexcess of negative charge,whereas another part acquires a positive charge.

The rapid vertical motionpresent in mature cumulonimbus clouds (most lightning is occurs during the mature stage) is key to charge separation.

The Storm of the Century March 13, 1993 viewed from space

G.W. Richmann, in 1753,systematically studied lightning and proved that thunderclouds contain electrical charge. He was killed instantly when lightningstruck him