Mon., Apr. 14 notes

Mon., April 14, 2008

I didn't get a chance to grade the Expt. #4 reports this weekend, SORRY. Too much around the house type work to be done. I hope to have them done by Wednesday.

A couple more of the 1S1P Topic links are now operative. The 1S1P reports are due by Friday this week.

The short Optional Assignment handed out in class last Friday was collected today. You'll get those back on Wednesday.

Tomorrow is April 15, the deadline to file your income taxes. Many of you probably don't get the chance to try to read through and understand the instructions and complete your own return. With that kind of experience in mind, here's a worksheet and instructions that will allow you to estimate your current grade in this class.

Something relatively unusual happened this past weekend (and has continued into the early part of this week)
It would probably be best if I just told you what it was, but that often goes in one ear and out the other.
I tried something different, went to the grocery store and bought the following card.

Have a close look at the cat, can you get any sense of what happened over the weekend?
No? Well the message (the message is a little different from the one shown in class) that was inside the card is shown above at right.
Now look back at the cat. Doesn't the cat look like he(she?) was a little hot and maybe thirsty?

Now that today's moment of weirdness is over we can get on with the rest of the class.
Today we are going to take a step backward and learn a little bit more about thunderstorms. We do this so that we can eventually learn what make tornadic thunderstorms different from more ordinary thunderstorms that don't make tornadoes.

We'll first look at air mass thunderstorms. These are the kinds of thunderstorms we see most of the time in Tucson.

The top portion of this figure repeats what we discussed a week or two ago (see the Wed., Apr. 02 online notes) : it takes some effort and often a good part of the day before a thunderstorm forms. The air must be lifted to or above the level of free convection. Once air is lifted above the level of free convection it finds itself warmer and less dense that the air around it and floats upward on its own. The is the moment at which the air mass thunderstorm begins.

The thunderstorm then goes through 3 stages.

In the first stage you would only find updrafts inside the cloud.

Once precipitation has formed and grown to a certain size, it will begin to fall and drag air downward with it. This is the beginning of the mature stage where you find both an updraft and a downdraft inside the cloud. The falling precipitation will also pull in dry air from outside the thunderstorm (this is called entrainment). Precipitation will mix with this drier air and evaporate. The evaporation will strengthen the downdraft (the evaporation cools the air and makes it more dense). The thunderstorm is strongest in the mature stage. This is when the heaviest rain, strongest winds, and most of the lightning occur.

Eventually the downdraft spreads throughout the inside of the cloud and interferes with or cuts off the updraft. This marks the beginning of the end for this thunderstorm.

In the dissipating stage you would only find weak downodrafts throughout the interior of the cloud.

Note how the winds from one thunderstorm can cause a region of convergence on one side of the original storm and can lead to the development of new storms. Preexisting winds refers to winds that were blowing before the thunderstorm formed.

The cold downdraft air spilling out of a thunderstorm hits the ground and begins to move outward from underneather the thunderstorm. The leading edge of this outward moving air is called a gust front. You can think of it as a dust front because that is what it often causes in the desert southwest.

This is a picture of the dust cloud stirred up by thunderstorm gust front winds. The gust front is moving from the right to the left. Visibility in the dust cloud can drop to near zero which makes this a serious hazard to automobile traffic. Dust storms like this are sometimes called haboobs.

A narrow intense downdraft is called a microburst. At the ground microburst winds will sometimes reach 100 MPH (over a limited area); most tornadoes have winds of 100 MPH or less. Microburst winds can damage homes (especially mobile homes that aren't tied to the ground), uproot trees, and seem to blow over a line of electric power poles at some point every summer in Tucson. Microbursts are a serious threat to aircraft especially when they are close to the ground during landing or takeoff (see Fig. 10.15 in the text). An inattentive pilot could quickly lose altitude and hit the ground.

Falling rain could warn of a (wet) microburst. In other cases, dangerous (dry) microburst winds might be invisible (the virga, evaporating rain, will cool the air, make the air more dense, and strengthen the downdraft winds).

A simple demonstration can give you an idea of what a microburst might look like.

A large plastic tank was filled with water, the water represents air in the atmosphere. Then a colored mixture of water and glycerin, which is a little denser than water, is poured into the tank. This represents the cold dense air in a thunderstorm downdraft. The colored liquid sinks to the bottom of the tank and then spreads out horizontally. In the atmosphere the cold downdraft air hits the ground and spreads out horizontally. These are the strong microburst winds that can reach 100 MPH.

The demonstration was followed with a short time lapse video showing a microburst that occurred over the Santa Catalina mountains. Cold air and rain suddenly fell out of a thunderstorm sank to the ground and then spread out sideways. The surface winds could well have been strong enough to blow down a tree or two.

The winds are increasing in speed with increasing altitude in the figure above. This is vertical wind shear (changing wind direction with altitude is also wind shear).

The thunderstorm will move to the right more rapidly than the air in the thunderstorm updraft which originates at the ground. Rising air that is situated at the front bottom edge of the thunderstorm will find itself at the back edge of the storm when it reaches the top of the cloud. This produces a tilted updraft.

Remember that an ordinary air mass thunderstorm will begin to dissipate when the downdraft grows horizontally and cuts off the updraft. In a severe storm the updraft is continually moving to the right and staying out of the downdraft's way. Severe thunderstorms can get bigger, stronger, and last longer than ordinary air mass thunderstorms. The strong updraft winds can keep hailstones in the cloud longer which will allow them to grow larger.

We will find that sometimes the tilted updraft will begin to rotate. A rotating updraft is called a mesocyclone. Low pressure in the core of the mesocyclone creates an inward pointing pressure difference (pressure gradient) force that keeps the updraft winds spinning in circular path (just as low pressure keeps winds spinning in a tornado). 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.

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.

The next two figures weren't shown in class.

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 with 4 thunderstorms with hook echoes. The hook echo feature is not always easy to spot. The "Xenia cell" produced a large 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.