Tuesday Nov. 17, 2009
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I couldn't choose between Madeleine Peyroux, Tracy Chapman, and Brandi Carlile for this morning's music so I ended up playing one song from each ("Love and Treachery", "Give Me One Reason", and "Looking Out").

The first part of the Quiz #4 Study Guide is now online.

The 1S1P Assignment #2 reports are due on Thursday this week.  A list of people that have reached or are very close to 45 pts (the maximum number of points you can earn on 1S1P reports) in now online.

Thanks for the many toilet flushing experiment emails.  I started out by trying to reply to each of them, but there were too many.  Here are the results from the first 100 responses that I collected:

Clockwise: 57
Counterclockwise: 43

There are a few people that still think toilets should all spin in one direction in the northern hemisphere and in another direction in the southern hemisphere. That isn't correct.  Toilets can spin in either direction in either hemisphere.  The direction of spin is determined by the design of the toilet itself.

And there were few emails like this:


I'll try to get take some new cat pictures (particularly of the newcomers that stop by for something to eat)  In the meantime, here are some pictures of a creature that stopped by my house last Spring.  Something I'd never seen before in midtown Tucson.

We started by reviewing some additional examples of thermal circulations or large scale circulations that resemble thermal circulations that were stuck into the online notes from last Thursday.

And here is some additional information concerning the 3-cell model of the earth's global scale circulation.

The 3-cell model predicts subtropical belts of high pressure near 30 latitude.  What we really find are large circular centers of high pressure.  In the northern hemisphere the Bermuda high is found off the east coast of the US, the Pacific high is positioned off the west coast.  High pressure centers are found east and west of South America in the southern hemisphere.


The winds that blow around these large scale high pressure centers create some of the major ocean currents of the world.  If you remember that high pressure is positioned off the east and west coast of the US, and that winds blow clockwise around high in the northern hemisphere, you can determine the directions of the ocean currents flowing off the east and west coasts of the US.  The Gulf Stream is a warm current that flows from south to north along the east coast, the California current flows from north to south along the west coast and is a cold current.  A cold current is also found along the west coast of South America; winds blow counterclockwise around high in the southern hemisphere. 

Circular low pressure centers, the Icelandic low (off the east coast near Iceland and Greenland in the picture below) and the Aleutian low (off the west coast near the southern tip of Alaska), are found near 60 N.




Tucson gets about 12 inches of rain in a normal year (we are at about half of normal this year).  About half of this comes during the "summer monsoon" season.  The word monsoon, again, refers to a seasonal change in wind direction.  During the summer subtropical high pressure (the Pacific high) moves north of its normal position near 30 N latitude.  Winds on the southhern side of the subtropical high have an easterly component.   Moist air originating in Mexico and the Gulf of Mexico blows into Arizona.  The sun heats the ground during the day, warm moist air in contact with the ground rises and produces convective thunderstorms.

The close proximity of the Pacific high, with its sinking air motions, is what gives California, Oregon, and Washington dry summers.

In the winter the subtropical high moves south of 30 N latitude.  Winds to the north of the high blow from the west.  Air originating over the Pacific Ocean loses much of its moisture as it crosses mountains in California (remember the rain shadow effect).  The air is pretty dry by the time it reaches Arizona.  Significant winter rains occur in Arizona when storms systems are able to draw moist subtropical air from the southwest Pacific ocean into Arizona.

Between now and the end of the semester we will be covering Thunderstorms, Tornadoes, Lightning, and Hurricanes.  We just got started on thunderstorms today.  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.  At the other end of the spectrum are supercell thunderstorms.  We'll watch a short video at some point that shows a computer simulation of the complex air motions inside a supercell thunderstorm.

The following somewhat tedious material was intended to prepare you to better appreciate a time lapse video movie of a thunderstorm developing over the Catalina mountains.  I don't expect you to remember all of the details given below.  The figures below are more carefully drawn versions of what was done in class.


Refer back and forth between the lettered points in the figure above and the commentary below.

The numbers in Column A show the temperature of the air in the atmosphere at various altitudes above the ground (note the altitude scale on the right edge of the figure).  On this particular day the air temperature was decreasing at a rate of 8 C per kilometer.  This rate of decrease is referred to as the environmental lapse rate (lapse rate just means rate of decrease with altitude).  Temperature could decrease more quickly than shown here or less rapidly.  Temperature in the atmosphere can even increase with increasing altitude (a temperature inversion).

At Point B, some of the surface air is put into an imaginary container, a parcel.  Then a meterological process of some kind lifts the air to 1 km altitude (in Arizona in the summer, sunlight heats the ground and air in contact with the ground, the warm air becomes bouyant - that's called free convection).  The rising air will expand and cool as it is rising.  Unsaturated (RH is less than 100%) air cools at a rate of 10 C per kilometer.  So the 15 C surface air will have a temperature of 5 C once it arrives at 1 km altitude. 

"Mother Nature" lifts the parcel to 1 km and "then lets go."  At Point C note that the air inside the parcel is slightly colder than the air outside (5 C inside versus 7 C outside).  The air inside the parcel will be denser than the air outside and the parcel will sink back to the ground. 

By 10:30 am the parcel is being lifted to 2 km as shown at Point D.  It is still cooling 10 C for every kilometer of altitude gain.  At 2 km, at Point E  the air has cooled to its dew point temperature and a cloud has formed.  Notice at Point F, the air in the parcel or in the cloud (-5 C) is still colder and denser than the surrounding air (-1 C), so the air will sink back to the ground and the cloud will disappear.  Still no thunderstorm at this point.


At noon, the air is lifted to 3 km.  Because the air became saturated at 2 km, it will cool at a different rate between  2 and 3 km altitude.  It cools at a rate of 6 C/km instead of 10 C/km.  The saturated air cools more slowly because release of latent heat during condensation offsets some of the cooling due to expansion.  The air that arrives at 3km, Point H, is again still colder than the surrounding air and will sink back down to the surface.

By 1:30 pm the air is getting high enough that it becomes neutrally bouyant, it has the same temperature and density as the air around it (-17 C inside and -17 C outside).  This is called the level of free convection, Point J in the figure.

If you can, somehow or another,  lift air above the level of free convection it will find itself warmer and less dense than the surrounding air as shown at Point K and will float upward to the top of the troposphere on its own.  This is really the beginning of a thunderstorm.  The thunderstorm will grow upward until it reaches very stable air at the bottom of the stratosphere.

The top portion of this figure summarizes what we just covered: 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. 

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 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 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.

Warm moist air lifted by the gust front can form a shelf cloud.

 

This is a picture of a dust cloud stirred up by thunderstorm gust front winds (taken near Winslow, Az).  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.

The following picture shows a shelf cloud.

The gust front is moving from left to right in this picture.  The shelf cloud is very close to the ground, so the warm air that was lifted by the gust front must have been very moist.  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 could 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 (I'll show the demo video in class on Thursday)


 

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.

 

 

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.

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."

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.