Pertinent observations about tornadoes
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Tornadoes occur typically in two places.
Regarding their intensity and form, we have the following general rules...
- Many tornadoes occur on the wind shift line where cold outflow from a thunderstorm meets inflow of warm/moist air. These tornadoes are often weak, and some people refer to them as landspouts or as Type-A tornadoes.
- Severe tornadoes occur below a rotation in a thundercloud (mesocyclone). The most severe tornadoes are found below rotating parent clouds that are moving across the ambient wind generally to its right.
Other interesting observations are:
- Wind speeds in the strongest of tornadoes do not exceed 140m/s (280mph) most likely.
- Inflow speeds are as strong as 50m/s (100mph), and occur at times not axisymmetrically but in the form of a spiral jet.
- Vertical flow may reach velocities of 80m/s (160mph). This starts at 0m/s at the ground surface and reaches its maximum a few hundred meters above the ground.
- Vertical flow is not confined to the core, but occurs as spirals on and outside the visible funnel. In the Cheyenne tornado of 1979 I saw structures orbiting around the tornado near the cloud base which must have risen with a verical flow well outside the core.
- In the northern hemisphere most tornadoes rotate cyclonically. Only about 7% rotate anticyclonically, and that fraction may be decreasing in recent years.
- Strong vortex motion occurs well within the parent cloud many minutes before the occurence of the tornado as evidenced by the tornado vortex signature (TVS) on doppler radar. The radar signature thereafter builds upward and downward rapidly. Presumably this occurs through the dynamic pipe.
- Winds below the parent cloud may remain very moderate for an hour prior to a rapid onset of a severe tornado (this was certainly true of the tornado that struck Spencer, South Dakota). What is going on during this time period? Is the incipient tornado concentrating vorticity, or are all necessary elements in place and we are just awaiting the occurence of the initiating event? In the section on concentrating rotation I explain this initiating event as something that breaks symmetry below the thunderstorm and allows the tornado to develop.
- Tornadoes seem to follow the most intense jet stream north and south during the seasons. When severe tornadoes struck Edmonton, Alberta in 1993(?), it was while the polar jet had wandered very far to the north. The connection between jet stream and tornadoes might be direct or indirect. The theory of J.R. Eagleman, which I discuss in the section on concentrating rotation, is that a rotating thunderstorm terminating in the jet stream is something like an aspirator which produces strong suction. On the other hand, tornado generation might be more closely tied to the occurence of jet-steaks within the jet stream. These are segments of maximum wind speed within the jet stream. They are "a-geostrophic," or lack balance between coriolis force, pressure gradient, and centrifugal force. The result is that they initiate a secondary circulation in the atmosphere that is perpendicular to the jet stream. When a jet streak flows around the southern curve of a "trough" the secondary circulation causes a convergence of air on the surface to the north and intensifies any thunderstorms that develop. These thunderstorms are also more likely to generate strong tornadoes.
- Tornadoes use no more than 10% of the available circulation in the parent thunderstorm. Providing, of course, that they are actually using this circulation. What is meant here is that concentrating all of the rotation of a thunderstorm into a vortex the size of a typical tornado would lead to wind speeds easily 10 times higher than those ever observed in a tornado. That such concentration is never observed is not just a lucky circumstance. The mechanism that limits this concentration is very likely a balance that must be maintained between the circulation in the environment around a developing tornado and the strength of the updraft that concentrates it. The details are explained well by Roger Smith in New Scientist Magazine, 28 Feb, 1980, p.650-653. This balance probably explains why, in the very scary rotating thunderstorm that my brother observed, small funnels shot out like darts but never evolved into a tornado.
- Severe tornadoes associate with wind veer (a wind that turns clockwise with increasing height.) Not only is this potentially a source of rotation, but veer or backing separates the updraft in a thunderstorm from downdrafts induced by falling precipitation. This allows a thunderstorm to grow in intensity, and to exist for a long time. For a stupendous photograph of a thunderstorm in a strongly veering ambient wind, refer to Warner, C. 1976. Bull. Am. Met. Soc., 57, 780-787.
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