Thursday Nov. 15, 2007
Today was the first of the 1S1P Assignment #3 due dates. You
could turn in one or two reports today. Next Tuesday is the
second due date. You can only turn in one report next week, not
two. Remember you can only turn in a total of 2 reports for this
assignment. You can't turn in two reports today and another
report next week because that would be a grand total of 3 reports.
The Experiment #4 reports were also collected today. Those will
be graded and returned by next Tuesday so that you have time to revise
your report before the end of the semester.
This class will meet just a few more times before the end of the
semester. Here's a warning that was
delivered at the beginning of class and also a matter of common courtesy.
We will be
spending the next three class periods in Chapter 10 in the
textbook. This will lead up to Quiz #4. The first section
in Chapter 10 covers thunderstorms.
Some general information on different types of
thunderstorms. We will mostly be concerned with ordinary
single-cell thunderstorms
(also referred to as air mass thunderstorms). We'll watch a short
video next week that shows a computer simulation of the complex air
motions inside a supercell thunderstorm.
Before
looking at how air mass thunderstorms development we need to review
some
material.
Rising air always expands and cools. It cools at
different rates depending on whether the air is saturated (RH=100%) or
unsaturated. Saturated air cools more slowly with increasing
altitude than unsaturated air. This is because as the air is
rising, expanding, and cooling; condensation of
water vapor inside the rising parcel releases latent heat energy inside
the parcel. This latent heat offsets some of the
cooling due to expansion.
As air is lifted from the ground up to some level in the
atmosphere, the temperature of the air inside the lifted parcel may be
different from the air outside (we assume energy doesn't flow into or
out of the rising parcel). If the rising air that ends up warmer
than the surroundings the parcel will, after being lifted and released,
begin to rise on its own. If the parcel ends up colder
than the surroundings the parcel will sink.
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. Temperature could
decrease more quickly than shown here or less rapidly.
Temperature in the atmosphere can even increase with increasing
altitude
(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). The
rising air will expand and cool as it is
rising. Unsaturated (RH<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.
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, if released, 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 repeats what we just
discussed: it takes some effort and often a good part of
the
day before a thunderstorm forms. The air must be lifted to or
above the level of free convection. The level of free convection
can change from one day to the next.
An ordinary single cell
thunderstorm goes through a 3-stage life cycle. In
the first stage, the cumulus 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 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.
We have talked about the shelf cloud and gust
front (think dust front) features in the top picture
before. The dust storms that thunderstorm winds stir up can cause
a
sudden drop in visibility and are a serious risk to automobile traffic
on the interstate highway.
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, 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 (see Fig. 10.15 in the text).
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
microburst winds that can reach 100 MPH.
The demonstration was followed with a short time lapse
video showing a
microburst that occurred over the Santa Catalina mountains. Cold
air and rain suddenly fell out of a thunderstorm sank to the ground and
then spread out sideways. The surface winds could well have been
strong enough to blow down a tree or two.
The following figure wasn't shown or
discussed in class.
The winds are increasing in speed with increasing
altitude in the figure above. This is vertical wind shear
(changing wind direction
with altitude is also wind shear).
The thunderstorm will move to the right more
rapidly than the air in the thunderstorm updraft which originates at
the ground. 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.
Remember that an ordinary air mass thunderstorm will begin to dissipate
when the downdraft grows horizontally and cuts off the updraft.
In a severe storm the updraft is continually
moving to the right and staying out of the downdraft's way.
Severe thunderstorms can get bigger, stronger, and last longer than
ordinary air mass thunderstorms. The strong updraft winds can
keep hailstones in the cloud longer which will allow them to grow
larger.
We will find that sometimes the tilted updraft will begin to
rotate. A thunderstorm with a rotating updraft is capable of
producing tornadoes.