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Outline for Lesson 20

Outline for Lesson 20. Chapter 9 Case study in an extratropical cyclone Case study in an extratropical anticyclone Chapter 10 Thunderstorms Air Mass Thunderstorms Severe Thunderstorms. 4/08/03. Cloud patterns typically associated with a mature middle-latitude cyclone.

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Outline for Lesson 20

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  1. Outline for Lesson 20 Chapter 9 Case study in an extratropical cyclone Case study in an extratropical anticyclone Chapter 10 Thunderstorms Air Mass Thunderstorms Severe Thunderstorms 4/08/03

  2. Cloud patterns typically associated with a mature middle-latitude cyclone

  3. 3-D view of atmospheric wave Idealized depiction of the support that convergence and divergence aloft provide to anticyclonic and cyclonic circulation at the surface.

  4. Weather Map Depicting the Norwegian Cyclone Model

  5. 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 waves turn the minutes to hours..."~Gordon Lightfoot

  6. Path of the November extratropical cyclone that sunk the Fitzgerald

  7. Winds generate waves; the size of the waves depends on • wind speed • duration of the winds • the fetch the distance of open water

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

  9. 45/37 70/64 Developing Stage The temperature contrast across 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-mb for 0700 EST November 9, 1975

  10. By 0700 AM on the 9th the cyclone has taken classic form at an early stage. Comma cloud - the comma head is made up of clouds and light precipitation 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 along a trailing cold fronts. Not observed in tropical cyclones. Satellite image on November 9, 1975

  11. Upper level support is key to surface cyclone development The combination of the two types of divergence (speedand diffluence) helps lead to sudden pressure drops over the upper Midwest. Therefore, the cyclone accelerates to the northeast and deepens rapidly. 300 mb winds and heights November 9, 1975

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

  13. 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 warm front is now in Canada. Surface weather and 500-mb for 0700 EST November 10, 1975

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

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

  16. 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.

  17. Mid Latitude Anticyclones

  18. Typical High Pressure Systems

  19. 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.

  20. Anticyclones High pressure systems inhibit vertical mixing of air and have light winds. Therefore in the summertime it is not uncommon for pollution to build over urban areas and areas “downwind” when strong anticyclones persist. Surface weather and 500-mb for 0700 EST July 16, 1999

  21. A dense layer of haze is evident from satellite imagery taken on the day of the accident. Satellite image of for 1720 EST July 16, 1999.

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

  23. Outline for lesson 21 Thunderstorms Air Mass Thunderstorms Severe Thunderstorms Lightning and Thunder 4/10/03

  24. Average number of days each year with thunderstorms. The humid, subtropical climate that dominates the southeastern United States receives much of its precipitation from thunderstorms.

  25. 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.

  26. Stages of Development of Air Mass Thunderstorms: • The stages of development of an air mass thunderstorm can be summarized as follows: • Cumulus Stage, in which the updrafts dominate throughout the the cloud and growth from a cumulus to a cumulonimbus cloud occurs. • The Mature Stage, the most intense phase, with heavy rain and possibly small hail, in which downdrafts are found side by side with updrafts. • The Dissipating stage, dominated by downdrafts and • entrainment, causing evaporation of the structure.

  27. Cumulus Stage

  28. Mature Stage

  29. Dissipating Stage

  30. 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.

  31. 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

  32. 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 become tilted. The tilt of the updrafts results in precipitation falling into the downdraft rather than the updraft.

  33. Diagram of a well developed cumulonimbus tower lower stratosphere updrafts downdrafts

  34. Severe Thunderstorms (cont.) Gust fronts can thought of as mini cold fronts; these “fronts” can provide the updraft for new thunderstorms. As warm air is lifted along the leading edge of the gust front a roll cloud is formed which can often be seen.

  35. Supercell Thunderstorms A supercell consists of a very powerful single cell. Supecells can extend as high as 20 km 65,000 ft and persist for several hours. They can be as wide as 20-50 km (12-30 mi). Very complex wind structure present which can lead to tornadoes.

  36. Supercell Thunderstorms 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. 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?

  37. 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.

  38. Drylines can often indicate squall line location A squall line with severe thunderstorms can also form along a boundary called a dryline. A dryline is a narrow zone where there is an abrupt change in moisture. Often forms in between cT and mT air masses

  39. Several tornadoes were associated with this squall line. Notice the sharp moisture gradient with the squall line.

  40. 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.

  41. 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 convective systems are self-propagating as gust fronts from existing cells feed the development of new thunderstorms; MCC’s can remain as long as the favorable conditions exist.

  42. Lightning and Thunder Lightning is the result of charge seperation (part of the cloud develops an excess of negative charge, whereas another part acquires a positive charge. The rapid vertical motion present in mature cumulonimbus clouds (most lightning is occurs during the mature stage) is key to charge separation.

  43. G.W. Richmann, in 1753,systematically studied lightning and proved that thunderclouds contain electrical charge. He was killed instantly when lightningstruck him The Storm of the Century March 13, 1993 viewed from space

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