Fri., Apr. 20 notes

Friday, Apr. 20, 2007

Tornado life cycle (don't worry about learning the names of the various stages). Tornadoes begin in and descend from a thunderstorm. You might see a funnel cloud dropping from the base of the thunderstorm. Spinning winds will probably be present between the cloud and ground before the tornado cloud becomes visible. The spinning winds can stir up dust at ground level. The spinning winds might also be strong enough at this point to produce some minor damage.

In Stage 2, moist air moves horizontally toward the low pressure in the core of the tornado. This sideways moving air will expand and cool just as rising air does. Once the air cools enough (to the dew point temperature) a cloud will form. The tornado is colored green above just to reinforce the fact that it is a true cloud and isn't just composed of dust (dust may mix with the cloud and turn the tornado brown)

Tornadoes can go from Stage 2 to Stage 3 (this is what the strongest tornadoes do) or directly from stage 2 to stage 5. Note a strong tornado is usually vertical and thick as shown in Stage 3.

The thunderstorm and the top of the tornado will move faster than the surface winds and the bottom of the tornado. This will tilt and stretch the tornado. The rope like appearance in Stage 5 is usually a sign of a weakening tornado.

Tornadic thunderstorms have rotating updrafts called mesocyclones (cyclone refers to winds spinning around low pressure, meso means medium size scale). Air moving into toward the low pressure core of the mesocyclone will expand and cool. The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud. The largest and strongest tornadoes will generally come from the wall cloud.

At this point we looked at the first of three video segments. The video showed the early stages in the life cycle of a tornado in Laverne, Oklahoma. The tornado was initially 200 to 300 yards wide but grew to about 1/4 of a mile in diameter. The tornado was initially almost stationary then began to move toward the NE at about 10 MPH. This tornado was eventually given an F3 rating (on the Fujita Scale), a moderately strong tornado.

A sketch showing some of the characteristic features of supercell thunderstorms. Supercells are first of all much larger than ordinary air mass thunderstorms. In an ordinary thunderstorm the updraft is unable to penetrate into the very stable air in the stratosphere. The upward moving air just flattens out and forms an anvil. In a supercell the rotating updraft (shown in orange above) is strong enough to penetrate into the stratosphere a little ways. This produces the overshooting top or dome feature above. A wall cloud and a tornado are shown at the bottom of the mesocyclone. The flanking line is a line of new cells trying to form alongside the supercell thunderstorm.

A little more detailed sketch is shown below

A photograph of a distant supercell thunderstorm was shown in the next video tape. A computer simulation of the air motions inside a supercell thunderstorm was also shown. Researchers understand the development of a supercell pretty well. The exact process that initiates tornado development is still unknown.

A radar picture of a supercell thunderstorm will often have a characteristic hook shape (outlined in black above). The hook is caused by spinning motions inside the thunderstorm The large orange shaded area is the thunderstorm updraft, the mesoscylone. Smaller regions of rising air are shown along a gust front.

Blue shaded areas shown where precipitation falls out of the cloud. The flanking line of new cells is forming along the gust front produced when cold downdraft air from the thunderstorm (purple arrows) collides with prexisting winds (green arrows). Weak tornadoes can sometimes form along the gust front. The largest and strongest tornadoes come from the mesocylone and wall cloud.

Before going onto the next video it would be worthwhile to learn more about the Fujita Scale that is used to rate tornado strength or severity. The Fujita Scale runs from F0 (weakest) to F5 (strongest, though there are a very few tornadoes with winds over 300 MPH that have been given an F6 rating).

The scale below is an "easy to remember" version (you can compare this with the scale in Table 10.2 (p. 281) in the textbook). About 2/3 rds of all tornadoes are F0 or F1 tornadoes and have spinning winds of 100 MPH or less. Only a few percent of tornadoes develop into F4 or F5 tornadoes, but those account for about 2/3 of all tornado deaths.

Simplified, Easy-to-Remember version of the Fujita Scale

winds < 100 MPH

F0
F1	roof damage, mobile home tipped over

microburst winds can cause this degree of damage

winds 100 to 200 MPH

F2	roof gone, outside walls still standing
F3	outside walls gone, inside walls intact

winds 200 to 300 MPH

F4	home destroyed, debris nearby
F5	home destroyed, debris carried away

Some representative examples of F1 through F5 level damage were shown in class

The buildings on the left suffered light roof damage. The barn roof at right was more heavily damaged.

More severe damage to what appears to be a well built house roof.

F1 tornado winds can tip over a mobile home if it is not tied down (the caption states that an F1 tornado could blow a moving car off a highway). F2 level winds (bottom photo) can roll and destroy the mobile home.

Trees, if not uprooted, can suffer serious damage from F1 or F2 tornado winds.

F2 level winds have completely removed the roof from this building. The building walls are still standing.

The roof is gone and the outer walls of this house were knocked down. This is characteristic of F3 level damage. In a house without a basement or storm cellar it would be best to seek shelter in an interior closet or bathroom.

All of the walls were knocked down in the top photo but the debris is left nearby. This is characteristic of F4 level damage. All of the sheet metal in the car body has been removed in the bottom photo and the car chasis has been bent around a tree. Note the tree has been stripped of all but the largest branches.

An F5 tornado completely destroyed the home in the photo above and removed most of the debris. Only bricks and a few pieces of lumber are left.

Several levels of damage are visible in this photograph. It was puzzling initially how some homes could be nearly destroyed while a home nearby or in between was left with only light damage. One possible explanation is shown below

Some big strong tornadoes may have smaller more intense "suction vortices" that spin around the center of the tornado. Tornado researchers have actually seen the scouring pattern shown at right in the figure above that the multiple vortices can leave behind.

The sketch above shows a tornado located SW of a neighborhood.

As the tornado sweeps through the neighborhood, the suction vortex will rotate around the core of the tornado.

The homes marked in red would be damaged severely. The others would receive less damage (remember, however that there would probably be multiple suction vortices in the tornado).

The following material wasn't coveredin class.
Air motions inside tornadoes are complex and difficult (dangerous) to study. Researchers resort to laboratory simulations and computer models.

Winds thought to be found in a weak tornado.
Friction probably causes the closed circulation near the bottom center of the tornado.

Winds in a somewhat stronger tornado.

Downward moving air is found in the core of this tornado.
This tornado would also have a larger diameter than the weaker tornadoes above.

Vortex breakdown has reached the ground.
This may lead to the formation of multiple vortices.