Wednesday, Apr. 13, 2016

Noora Noor "Forget What I Said" (3:18), Iyeoka "Simply Falling" (3:57), Danger Mouse, Daniele Luppi, & Norah Jones "Black" (3:41)


Spinning motions in cases where the PGF is stronger than the Coriolis force
The situations we have been looking at so far are representative of large "country size" storm systems.  There are smaller scale 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. 





Large scale winds upper level winds blowing around Low pressure.  You  must take into account both PGF and CF forces. Winds only spin in a CCW direction around L in the northern hemisphere and change direction in the southern hemisphere.
 A net inward force is need to keep winds spinning in a circular path.  The inward pointing PGF provides the needed net inward force in this case and winds can spin in either direction around the L in either hemisphere.



Water draining from a sink or toilet - direction of spin
This is what happens when water drains from a sink or toilet.  The PGF is present, there is no CF.  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 (located between the Canadian provinces of New Brunswick and Nova Scotia).  It is apparently the largest whirlpool in the Western Hemisphere (source).  The Bay of Fundy also has some of the highest tides in the world.


The Great Toilet Flushing Experiment
You may have heard that water draining from a sink or flushed toilet spins in a different direction in the southern hemisphere than it does here in the northern hemisphere.  As mentioned above there are situations where the pressure gradient force is much stronger than the Coriolis force.  In these situations clockwise or counterclockwise spin should be equally likely.  For the past few years we've been conducting and experiment in ATMO 170 to see if this is indeed the case.  Students would go out, note the direction of spin after flushing a toilet, and report back to me.  Because the Coriolis force does not play a role, we should expect to see roughly equal numbers of reports of clockwise (CW) and counterclockwise (CCW) spin.  Here's the summary of results after the Fall 2015 version of the experiment.

It isn't possible to find spinning winds around high pressure when the CF is not present?




The CF plays an important role here, it is the force that provides the net inward force needed to keep the winds blowing in a circular trajectory.
 With just the PGF there's nothing to provide a net inward force.  Circular winds around centers of high pressure is not possible when there is no CF.


What if just the Coriolis force were present?
The following figure is on the back of the class handout.  Which of these would be possible if just the CF were present?



When you think you have the answer, click here.



Thunderstorms introduction

Severe thunderstorms in Texas Sunday night with baseball size hail (report from the Lubbock National Weather Service Forecast Office)




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). 

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 1st and 5th images in the gallery show the base of a supercell thunderstorms photographed in Texas with wall clouds.  There are additional images further down in the gallery.

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 buildup to an air mass thunderstorm
The following somewhat tedious material is intended to prepare you to better appreciate a time lapse video movie of a thunderstorm developing over the Catalina mountains.  The newest 1S1P/Optional Assignment makes uses of a couple of the numbers below (the rates of cooling of rising parcels of unsaturated and saturated air).


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.  A dew point temperature of -5 C was used in this example.  It could be warmer or colder than that. 

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.  Saturated air 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, it doesn't need Mother Nature's help anymore.  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 rising air will quickly become colder and denser than the surrounding air if it travels into the stratosphere).

Here's a time lapse video showing a day's worth of work leading eventually to the development of a thunderstorm over the Catalina mountains north of Tucson (Firefox seems to have trouble sometimes downloading the file, you may need to use another browser).


Air mass thunderstorm life cycle
The events leading up to the initiation of a summer air mass thunderstorm are summarized in the figure below (p. 151 in the ClassNotes).    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 an air mass thunderstorm gets above the level of free convection it 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 denser).  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, 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 at the very beginning of this sections on thunderstorms.