Monday Apr. 19, 2010
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Two or three songs from Patty Griffin's Downtown Church CD ("Waiting For My Child", "Wade in the Water", and "The Strange Man").  She was at the Rialto Theater last Tuesday night.

1S1P Assignment #3 (the last assignment of the semester) has appeared online.  A second topic will be added soon.  Keep an eye on the "45 pts list" which will be updated after I process the reports which were turned in today.
We spent today's class on Thunderstorms.  Here's a brief introduction.


Thunderstorms come in different sizes and levels of severity.  We will mostly be concerned with ordinary single-cell thunderstorms (also referred to as air mass thunderstorms).  Most summer thunderstorms in Tucson are this type.   An air mass thunderstorm has a vertical updraft (The small sketches above weren't shown in class)

Tilted updrafts are found in severe and supercell thunderstorms.  As we shall see this allows those storms to get bigger, stronger, and last longer.  Supercell thunderstorms have a complex internal structure;  we'll watch a short video on Wednesday that shows a computer simulation of the complex air motions inside a supercell thunderstorm.

We reviewed the events covered in class on Friday that lead up to the formation of a thunderstorm.  This is summarized more concisely in the figure below. it takes some effort and often a good part of the day before a thunderstorm forms.  The air must be lifted to just 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. 

This was followed by a time lapse video tape of actual thunderstorm formation and growth.


Once a thunderstorm develops it 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 horizontally 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 picture below shows some of the features at the base of a thunderstorm.



 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 the gust front winds often stir up a lot of dust here in the desert southwest (see below).


The gust front in this picture (taken near Winslow, Az) 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".

The following picture shows a shelf cloud.
Warm moist air if lifted by the gust front which is moving from left to right in this picture.  The shelf cloud is very close to the ground, so the warm air must have been very moist because it didn't have to rise and cool much before it became saturated and a cloud formed. 


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.  An inattentive pilot encountering headwinds at Point 1 might cut back on the power.  Very quickly the plane would lose the headwinds (Point 2) and then encounter tailwinds (Point 3).  The plane might lose altitude so quickly that it would crash into the ground before corrective action could be taken.

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 winds that can reach 100 MPH.


Here's a picture of a wet microburst, a narrow intense thunderstorm downdraft and rain.  I'll show a short video with a microburst in class on Wednesday.

Next we looked at some of the conditions that can lead to severe thunderstorm formation and at some of the features that allow severe storms to last longer, grow bigger, and become stronger than ordinary air mass thunderstorms.

 

 

Severe storms are more likely to form when there is vertical wind shear.  Wind shear (pt 1) is changing wind direction or wind speed with distance.  In this case, the wind speed is increasing with increasing altitude, this is vertical wind shear.

The thunderstorm itself will move in this kind of an environmen, at an average of the speeds at the top and bottom of the cloud (pt. 2).  The thunderstorm will move to the right more rapidly than the air at the ground which is where the updraft begins.  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 (pt. 3).  The downdraft is situated at the back of the ground.  The updraft is continually moving to the right and staying away from the downdraft.  The updraft and downdraft coexist and do not "get in each others way."  If you remember in air mass thunderstorms, the downdraft gets in the way of the updraft and leads to dissipation of the storm.

I've added a few features to this picture that weren't discussed in class.  Sometimes the tilted updraft will begin to rotate.  A rotating updraft is called a mesocyclone (pt. 4).  Meso refers to medium size (thunderstorm size) and cyclone means winds spinning around low pressure.  Low pressure in the core of the mesocyclone creates an inward pointing pressure gradient force needed to keep the updraft winds spinning in circular path (low pressure also keeps winds spinning in a tornado).  The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud (pt. 5).  The largest and strongest tornadoes will generally come from the wall cloud.

Note (pt. 6) that a tilted updraft provides a way of keeping growing hailstones inside the cloud.  Hailstones get carried up toward the top of the cloud where they begin to fall.  But they then fall back into the strong core of the updraft and get carried back up toward the top of the cloud.