Tue., Nov. 20 notes

Tuesday Nov. 20, 2007

The revised Expt. #3 reports were collected today.

The Experiment #4 reports have been graded. Revised reports are due in two weeks on Tue., Dec. 4 (the last day of class). Please return the original report with your revised report.

Today is the 2nd 1S1P Assignment #3 due date.

Just in case you've missed it, you should read this warning concerning next week's quiz reviews.

The United States has more tornadoes in an average year than any other country in the world (about 1000 per year). The central US has just the right mix of meteorological conditions. In the spring, cold dry air from Canada collides with warm moist air from the Gulf of Mexico to form strong cold fronts and thunderstorms.

Tornadoes have been observed in every state in the US, but tornadoes are most frequent in the central plains, a region referred to as "Tornado Alley."

About 2/3rds of tornadoes are F0 or F1 tornadoes and have winds of about 100 MPH or less. A very strong inwardly directed pressure gradient force is needed to keep winds spinning in a circular path typically a few hundred meters in diameter. The pressure in the center of a tornado can be 100 mb less than the surroundings. The PGF is much stronger than the CF and the CF can be neglected. Tornadoes can spin clockwise or counterclockwise, though counterclockwise rotation is more common.

Tornadoes usually last only a few minutes and leave a path a few miles long. We will look at an exception below. Most tornadoes move from the SW toward the NE at a few tens of MPH. The most and the strongest tornadoes occur during the spring.

Tornadoes probably don't occur more frequently now than they did a century ago. There are just more people in more locations to observe and report them when they occur. The number of tornado deaths (currently about 80 people per year) has decreased significantly in the past 50 years or so. Meteorologists are better able to predict when and where severe weather is likely to occur and are better able to detect and warn of tornadoes and tornadic thunderstorms.

Most tornadoes last only a few minutes and leave a path a few miles long on the ground. There are of course exceptions. One is discussed below.

The path of the 1925 "Tri-State Tornado" is shown above. The tornado path (note the SW to NE orientation) was 219 miles long, the tornado last about 3.5 hours and killed 695 people. The tornado was traveling over 60 MPH over much of its path. It is the deadliest single tornado ever.

Tornadoes often occur in "outbreaks." The paths of 148 tornadoes during the April 3-4, 1974 "Jumbo Tornado Outbreak" are shown above. Note the first tornadoes were located in the upper left corner of the map. The tornadoes were produced by thunderstorms forming along a cold front. During this two day period the front moved from the NW part toward the SE part of the figure. Note that all the tornado paths have a SE toward NE orientation.

Before looking at the first of four segments of tornado video, here's an easy to remember version of the Fujita Scale used to rate tornado intensity. Because it is so hard to make measurements of tornado wind speeds, intensity estimates are usually based on an examination of the damage caused by the tornado.

The Fujita Scale normally runs from F0 to F5 but there have been a F6 few tornadoes where winds might have exceeded 300 MPH.

We looked at a portion of a video tape with several different tornadoes. The tornadoes, Fujita scale ratings, and comments are given in the table below:

54a	F3	Grand Island, NE	Mar. 13, 1990	tornado cloud is pretty thick and vertical
61f	F3	McConnell AFB KS	Apr. 26, 1991	this is about as close to a tornado as you're ever likely to get. Try to judge the diameter of the tornado cloud. What direction are the tornado winds spinning?
52	F5	Hesston KS	Mar. 13, 1990	Watch closely, you may see a tree or two uprooted by the tornado winds
51	F3	North Platte NE	Jun. 25, 1989	Trees uprooted and buildings lifted by the tornado winds
65	F1	Brainard MN	Jul. 5, 1991	It's a good thing this was only an F1 tornado
57	F2	Darlington IN	Jun. 1, 1990	Tornado cloud without much dust
62b	F2	Kansas Turnpike	Apr. 26, 1991	It's sometimes hard to run away from a tornado. Watch closely you'll see a van blown off the road and rolled by the tornado.
47	F2	Minneapolis MN	Jul. 18, 1986	Tornado cloud appears and disappears.

The 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. "Wedge tornadoes" actually appear wider than they are tall.

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 ). 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 second of the 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. At first the tornado was almost stationary but then began to move toward the NE at 10 MPH. This tornado was eventually given an F3 rating, a moderately strong tornado.

Sketches showing some of the characteristic features of supercell thunderstorms. Supercells are first of all much larger than ordinary air mass thunderstorms (a purple air mass T-storm is superimposed on the top figure for comparison). 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. 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 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, however.

A radar picture of a supercell thunderstorm will often have a characteristic hook shape (outlined in brown 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 show 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. The two tornado formation regions are shaded yellow in the figure.

Actual radar display (not shown in class) with 4 thunderstorms with hook echoes. The hook echo feature is not always easy to spot. The "Xenia cell" produced a large tornado.

The Fujita Scale is used to rate tornado strength or severity. We'll look at some of the kinds of damage tornadoes can do.

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

Here are some photographs of actual tornado damage.

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 the photograph above. 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 last video featured a tornado observed in Pampa, Texas and was shown at this point. At one point the tornado winds just above the ground were estimated at 250 MPH. Several vehicles (pickups and a van) were seen on the video being thrown from the tornado cloud at a height of about 100 feet at speeds of 80 to 90 MPH. Imagine something like that coming down in your backyard.