In the
winter the subtropical high moves south of 30 N latitude (bottom part
of the figure above).
Winds to the north of the high blow from the west. Air
originating over the Pacific Ocean is cool (because it moves over the
cold California current) and cool air can't contain as much moisture as
warmer air. Also the air 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 more tropical
southwest
Pacific ocean into
Arizona.
Normally about half Tucson's yearly rainfall (about 12 inches per year)
comes during
the 2 or 3 month long "summer monsoon" season. The word monsoon
refers to a
seasonal change in wind direction (it is often used incorrectly
by
people in Tucson to refer to thunderstorms). 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.
We'll be
spending today and part of next Tuesday talking about
thunderstorms. We'll also cover tornadoes on Tuesday next week.
The
following somewhat repetitious material is intended to
prepare you to better appreciate a time lapse video movie
of a thunderstorm developing over the Catalina mountains. The Bonus 1S1P Assignment that is currently online
covers this same type of material.
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 it
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.
A time lapse video of some thunderstorm development was shown in
class. As in the
example above, it often takes most of a day's worth of work before a
thunderstorm is initiated.
What we have just covered is summarized more concisely 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. 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.
Air
mass
thunderstorms
then go through a 3-stage life cycle (that lasts
about an hour).
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 its
name).
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 begins to interfere with and eventually chokes off the
updraft. This marks the
beginning of the end for this thunderstorm.
The
downdraft eventually fills the interior of the cloud. In this is
the dissipating stage you would only find weak downodrafts
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.
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 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 (see
below).
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".
The following picture shows a shelf
cloud.
Warm
moist air if lifted by the cold air behind the gust front which is
moving from left
to right in this picture. The shelf cloud is very close to the
ground, so
the warm air must have been very
moist because 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 might 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.
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.
Here are a couple of videos from
YouTube. The first
video shows a microburst from some distance away. The second video was
taken in the heavy rain and strong winds under a thunderstorm in the
microburst.
We didn't have time to discuss the next figure which shows some of
the characteristic features found on severe thunderstorms.
None of the
discussion below was covered in class on Thursday, we'll review
it quickly next Tuesday.
Severe
storms are more likely to form when there is vertical wind shear.
Wind
shear (pt 1) is changing wind direction and/or wind speed with
distance. In
this case, the wind speed is increasing with increasing altitude, this
is vertical wind shear.
A thunderstorm that forms in this kind of an environment
will move 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." If you remember in air mass
thunderstorms, the downdraft gets in the way of the updraft and leads
to dissipation of the storm.
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.
A wall cloud can form a little bit
below the rest of the base of the thunderstorm. The figure above
tries to explain why that is true. Clouds normally form when air
rises, expands, and cools as shown above at left. The rising air
expands because it is moving into lower pressure surroundings at higher
altitude.
At right the air doesn't have to rise to as high an altitude to
experience the same amount of expansion and cooling. This is
because it is moving into the core of the rotating updraft where the
pressure is a little lower than normal for this altitude. Cloud
forms a little bit closer to the ground.
Here's a
pretty nice photograph of a wall cloud and what is probably a
relatively weak tornado (from the
University
Corporation for Atmospheric Research)
