Tue., Nov. 18, 2014

3 maybe 4 selections from the following before class today: Calexico "Mi Vida y El Circo", "Inspiracion", "Two Silver Trees";  Leila Lopez "Sea and The Mountainside", "Pick Your Prize";  Norah Jones "Out on the Road"

All of the Scientific Paper, Book, and Experiment reports turned in last week have been graded and were returned today.  Revised reports (if you decide to do one, they aren't required) are due by Tuesday Dec. 2, though if you could get them in before Thanksgiving that would be appreciated.  Keep an eye out for a list of students that don't seem to have done a report yet.  It's getting late in the semester but there is still time to get that work done.  It's about half of your overall writing grade.

The 1S1P reports on Fog in Tucson have been graded (nearly everyone received full credit) and were returned today.  The 1S1P Atmospheric Stability worksheet and the Optional Assignment handed out last Thursday were collected today.  There are a couple of additional 1S1P topics that are due on Thursday this week.

Note there is now a list of people that have earned 45 1S1P pts (the maximum number allowed).  The list will be updated as reports are graded.

You'll also find mention of a short easy Optional Assignment embedded in the notes.

Here's a quick review of what we covered in class last Thursday followed by a common misconception involving the Coriolis force

Upper level winds review
Winds spin counterclockwise around L pressure in the northern hemisphere then switch direction and spin clockwise around L pressure in the southern hemisphere.   I think by just remembering a couple of things you can figure this out rather than just trying to memorize it.


Remember 1st that stationary air will start moving toward low pressure.  The dots in the figure above show this initial movtion.  Then the wind will turn to the right or left depending on the hemisphere.  This is the effect of the Coriolis force, it turns wind to the right in the northern hemisphere and to the left in the southern hemisphere.    The northern hemisphere winds are shown at left in the figure above, the southern hemisphere winds are shown at right.  The inward pointing force is always stronger than the outward force so that there is a net inward pointing force.

The same approach can be used with H pressure.  The initial motion is again toward L pressure which is on the outside of the picture.  The winds move away from the center then turn to the right or left depending on the hemisphere. 



The left figure above shows winds blowing around H pressure in the northern hemisphere.

You might have heard that water spins in a different direction when it drains from a sink or a toilet bowl in the southern hemisphere than it does in the northern hemisphere.  You might also have heard that this is due to the Coriolis force or the Coriolis effect. 

The Coriolis force does, as we have seen, cause winds to spin in opposite directions around large scale high and low pressure centers in the northern and southern hemisphere. 

Situations where the PGF is stronger than the Coriolis force
There are situations, though, where the PGF is much stronger than the CF and the CF can be ignored.  A tornado is an example.  Spinning water draining from a sink or toilet is another.  The PGF is much much stronger than the CF and the CF can be ignored. 


He we have clockwise and counterclockwise spinning motions around both H and L pressure.  The direction of the PGF is shown in all four cases (the PGF always point toward L pressure).  Remember you need an inward pointing force in order to keep something moving in a circular path.  The PGF can provide this needed force so either clockwise or counterclockwise motion is possible around L pressure. 

Spinning motion around H pressure isn't possible when just the PGF is present because there isn't any inward pointing force. 

Water draining from a sink or toilet - direction of spin
This is what happens when water drains from a sink or toilet.  The water can spin in either direction in either hemisphere.  It might not be obvious though what causes the inward pointing PGF in the case of spinning water.




If you look carefully at some spinning water you'll notice the surface has a "bowl" or "funnel" shape as sketched above.  The water at the edges is a little deeper.  That additional water has more weight and produces more pressure.  The water in the middle is shallower, doesn't weigh as much and the pressure is lower.  Thus there is a PGF pointing from the edges into the center of the vortex.


Here's a picture of the "Old Sow" whirlpool in the Bay of Fundy it is apparently the largest whirlpool in the Western Hemisphere (source). 

We can check out this idea that water can spin in either direction when draining from a sink or toilet with an Optional Experiment/Assignment.   Basically you'll need to find and flush a toilet and watch what direction the water spins.  Then report your observation back to me by midnight this 5 pm Friday, Nov. 21 (you'll need to send me an email).  Please let me know which class section you are in (8 am = Sect. 2, 9:30 am = Sect. 3).  I'll tabulate the results and let you know how things turn out next week.

What if just the Coriolis force were present?
The following figure was on the back of the class handout.

You'll find the answer at the end of today's notes.


Surface winds - H and L pressure, N and S hemispheres
Before we leave this topic and move onto thunderstorms, here's a little more information about surface winds. 



What information could you add to the figure above.  Does it show surface or upper level winds?  Are these winds blowing around H or L pressure.  These are pretty easy questions to answer. 

You need to remember that surface winds blow across the contours always in the direction of low pressure



Is this a northern or southern hemisphere situation?  This is also pretty easy to figure out also.


Imagine you're approaching the low pressure center and want to merge with the existing winds (kind of like approaching a traffic circle in your car and wanting to merge with the traffic that is already there).  Would you need to turn to the left or the right as you approach?  I think it's pretty clear you'd need to turn left.  So in addition to remembering that surface winds blow across the contours always toward low pressure if you remember that the Coriolis force acts to the left of the wind in the southern hemisphere and to the north of the wind in the northern hemisphere you can quickly figure out this is a southern hemisphere map.



Here are all the surface wind examples.



See if you can figure which of these are L pressure and which are H pressure?  Then determine whether each is a northern or southern hemisphere picture.


Thunderstorms

Here's a little bit of an introduction (found on p. 150 in the ClassNotes)



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).  They form in the middle of warm moist air, away from fronts.  Most summer thunderstorms in Tucson are this type.   An air mass thunderstorm has a vertical updraft.  A cell is just a term that means a single thunderstorm "unit" (a storm with an updraft and a downdraft).  See image #9 in the gallery mentioned below.

Tilted updrafts are found in severe and supercell thunderstorms.  As we shall see this allows those storms to get bigger, stronger, and last longer.  The tilted updraft will sometimes begin to rotate.  We'll see this produces an interesting cloud feature called a wall cloud and maybe tornadoes.  Supercell thunderstorms have a complex internal structure;  we'll watch a short video at some point that shows a computer simulation of the complex air motions inside a supercell thunderstorm.  In class I showed a gallery of storm images that were taken by Mike Olbinski.  The first image in the gallery shows the base of a supercell thunderstorm photographed in Texas.  Image #8 shows a somewhat smaller supercell that formed in Arizona.

We won't spend anytime discussing mesoscale convective systems except to say that they are a much larger storm system.  They can cover a large portion of a state.  They move slowly and often thunderstorm activity can persist for much of a day.  Occasionally in the summer in Tucson we'll have activity that lasts throughout the night.  This is often caused by an MCS.


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 meteorological 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 buoyant - that's called free convection).  The rising air will expand and cool as it is rising.  Unsaturated air (RH is less than 100%) 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. 

Early in the morning "Mother Nature" is only able to lift 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.  You can't see this because the air is clear, invisible.

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, the relative humidity is now 100%,  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 kilometers 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 has become neutrally buoyant, 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.

This was followed by a Time lapse video showing a day's worth of work leading eventually to the development of a thunderstorm. 

Thunderstorm life cycle
The events leading up to the initiation of a summer air mass thunderstorm are summarized 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 (the dotted line at middle left in the picture).  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.  I've tried to show this with colors below.  Cool colors below the level of free convection because the air in the lifted parcel is colder and denser than its surroundings.  Warm colors above the dotted line indicate parcel air that is warmer and less dense than the surroundings.  Once the parcel is lifted above the level of free convection it becomes buoyant; this is the moment at which the air mass thunderstorm begins. 




Once a thunderstorm develops it then goes through a 3-stage life cycle


In the first stage you would only find updrafts inside the cloud (that's all you need to know about this stage, you don't even need to remember the name of the stage).

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, hail, strongest winds, and most of the lightning occur.

Eventually the downdraft spreads horizontally throughout the inside of the cloud and begins to interfere with the updraft.  This marks the beginning of the end for this thunderstorm. 


The downdraft eventually fills the interior of the cloud.  In this dissipating stage you would only find weak downdrafts throughout 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.  Convergence between the prexisting and the thunderstorm downdraft winds creates rising air that can initiate a new thunderstorm.

Here's a sketch of 4 thunderstorm clouds and a question: what information could you add to each picture.



You should be able to say something about the first three.  The 4th cloud might be a bit of a puzzle.  You'll find the answer to the question at the end of today's notes.


Gust fronts and the dust storms they can produce
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 underneath 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, a photo not shown in class).


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".


There's lots of video on YouTube of an impressive dust storm a few summers ago.  Here's an example from Gilbert Arizona (July 5, 2011).  You can see day literally turn to night when the dust cloud is overhead (start about 1:20).  Here's another video of the same storm from a different location (South Mountain).  Finally a time lapse video of the July 5, 2011 storm taken by Mike Olbinski (click on the Massive Haboob Hits Pheonix link) and another time lapse video of a July 3, 2014 storm taken by him.   

Here's a video from a summer 2012 dust storm captured from the front window of a vehicle that drove through the storm.  Check the last minute or two of the video where visibility drops to near zero (about 5:00 minutes into the video).  Officials recommend that you drive off the highway under conditions like this, turn off your lights, and take your foot off the brake so that your brake lights are not on.  Otherwise someone might follow your lights thinking you're still on the highway and run into you from behind.

Here are answers to several questions embedded in today's notes.





The Coriolis force is perpendicular to the wind and to the right in the northern hemisphere, perpendicular and to the left in the southern hemisphere.  Spinning motions would be possible in Figs.  b and d work because the Coriolis force is pointed inward.  An inward force is needed to keep something moving in a circular path.


Surface winds blow across the contours always in the direction of low pressure.  So determining which of these is H and L pressure should be easy.  The figure below shows you how to go about determining whether figure belongs in the southern or northern hemisphere.
Rising air motions will be found with the two centers of L pressure.  The air sinks in the middle of centers of H pressure independent of hemisphere.



Here's the answer to another question embedded in today's notes:


The first 3 pictures shows the different stages in the lifetime of an air mass thunderstorm.  There's a tilted updraft in the 4th picture which is a characteristic of a severe thunderstorm.