Published on October 5, 2007
Tornadoes: Tornadoes Tornadoes: Tornadoes A rapidly rotating column of air that is in contact with the ground. Made visible by condensation, dust, dirt, debris, cows. Actual tornadic circulation is usually larger than that indicated by the condensation funnel. Wall cloud: Wall cloud Tornadoes: Tornadoes The vast majority of tornadoes are weak. They typically have lifetimes less than 5 minutes. They typically have a path length of less than 1.5 km. They typically have diameters of less than 100 m. Tornadoes: Tornadoes The deadliest of all recorded tornadoes was the “Tri-State Tornado” of 1925 219 Mile path from Missouri through Illinois to Indiana. 629 Dead, 2000+ Injured 11 000+ Homeless May have actually been more than a single tornado -- Possibly part of a tornado family. Tornadoes: Tornadoes If conditions are favorable, large numbers of tornadoes or tornado families may occur over a short period of time. These are called tornado outbreaks. Examples of outbreaks: 3-4 April 1974 -- 148 tornadoes in 13 states 26 April 1991 -- 55 tornadoes in 6 states 4-10 May 2003 -- >400 tornadoes in many states Tornado Distribution: Tornado Distribution Tornadoes have occurred in all 50 states. Most occur in “Tornado Alley” -- A region of the United States from Texas to Nebraska. Tornado maxima move as spring progresses. Early maximum in the Gulf states (GA - LA) Maximum moves to TX, OK, KS in late spring. Slide8: Tornado Occurrence Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually, but the highest frequency is observed in tornado alley of the Central Plains. Nearly 75% of tornadoes form from March to July, and are more likely when warm humid air is overlain by cooler dryer air to cause strong vertical lift. Figure 15.30 Slide9: Dryline Thunderstorms Figure 15.16 Tornado Winds: Tornado Winds Early knowledge of the tornadic winds from: Visual observations Motion picture footage Damage analysis Today we have: Doppler Radar Mobile research radars (DOW, UMASS) Tornadogenesis: Tornadogenesis The process of forming a tornado. A tornado is a rapidly rotating column of air in contact with the ground. A visible condensation funnel is not necessary to have a tornado. There are at least 11 different hypotheses to explain the formation of the tornado. We shall discuss a few of them. Convergence: Convergence Consider your sink. Initially there is some weak rotation, but as the water converges toward the drain, the speed of the rotation increases. This is similar to the way an ice skater speeds up when their arms are pulled in. This is the same process that may contribute to the development of the rotating updraft. For a tornado, this occurs on a much smaller scale. Stretching: Stretching Air with spin about the vertical axis can be stretched by the updraft. This stretching can increase the spin rate. Turns Rapidly Turns Slowly Stretch Vertically Tornado Formation: Tornado Formation One Theory (there are many others) Tilting and stretching of horizontal vorticity Push the top of the basketball. The top will have a greater velocity than the bottom. The ball will begin to rotate. Tornado Formation: Tornado Formation Vertical wind shear can cause a “rolling” action to the wind. We can do this with winds as well: The result is a horizontally rotating column. This horizontal vorticity is very weak. Tornado Formation: Tornado Formation Inflow with horizontal vorticity Updraft The updraft tilts the inflow wind into the vertical. As it does, the horizontal vorticity becomes vertical vorticity. Tornado Formation: Tornado Formation The tilting of the horizontal vorticity into the vertical can create vertical vorticity. The rotation is still very weak. The wind speed is increased by “stretching” this vertical vorticity -- similar to the spin up of an ice skater. Slide18: Vorticity from Horizontal to Vertical Figure 15.36 Figure 15.37 Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a tornado. Slide19: Rear Flank Downdraft Supercell thunderstorm development may create an area where the updraft and counterclockwise swirl of upper winds converge into a rear flank downdraft. This downdraft can then interact with lower level inflow winds and spawn a tornado. Figure 15.40 Slide20: Tornado Wind Speed Figure 15.31 As the tornado moves along a path, the circular tornado winds blowing opposite the path of movement will have less speed. For example, if the storm rotational speed is 100 knots, and its path is 50 knots, it will have a maximum wind of 150 knots on its forward rotation side. Tornado damage: Tornado damage Waterspouts: Waterspouts Rotating column of air over a large body of water. Normally develops along weak convergence lines over tropical water. Tend to form in very unstable air. Wind speeds much less than a tornado: typically < 45 kts (but occasionally can be stronger). Slide23: Doppler on Wheels -- DOW Mobile radar © Joshua Wurman Slide24: Doppler on Wheels -- DOW Mobile radar with tornado © Joshua Wurman Radar image of tornado: Radar image of tornado Doppler on Wheels The DOW From: Science -- 21 June 1996 Twister: Twister Doppler on Wheels The DOW From: Science -- 21 June 1996 Slide27: What did this tornado look like? Visual tornado DOW in a ditch: DOW in a ditch Super summary: Super summary Supercells are maintained by the vertical profile of the wind. Supercells are responsible for much of the severe weather in the Midwest Supercells can produce the strongest tornadoes Not all tornadoes are associated with supercells.