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.

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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.
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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.
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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?

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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.
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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.
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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.