A new Optional Assignment
was handed out in class. It is due in one week, on Wed., Apr. 22.
We'll start out with a little
review. What information
could you add to the figure below given just the direction of the upper
level winds and knowing that the upper level winds are part of a
thermal circulation.
(1) The first thing to do would be to complete the circulation
loop. Then if you remember that "warm air rises" you can identify
the warm and cold parts of the picture that cause the thermal
circulation to develop in the first place. (3) The beach is
warmer than the ocean during the daytime. (4) The surface wind is
blowing from the ocean toward the land and is called a "sea breeze"
(the name tells you the direction the wind is coming from). The
surface wind is likely to be moist since it is coming from the
sea. Moist rising air could form the clouds shown at (5).
Here are some additional examples
of thermal circulations or large scale circulations that resemble
thermal circulations.
Cities are often warmer than the
surrounding
countryside,
especially at night. This is referred to as the urban heat island
effect. This difference in temperature can create a
"country breeze."
The Asian monsoon (monsoon refers to a seasonal change
in the direction of the prevailing winds) is a large scale circulation
pattern and is much more complex than a simple thermal
circulation. However you can
use the thermal circulation concept to get a general understanding of
what to expect at different times of the year.
In the summer India and SE Asia
become warmer than the
oceans
nearby. Surface low pressure forms over the land, moist winds
blow from the ocean onshore, and very large amounts of rain can
follow. The view above at left is from above, the view at right
is from the side.
The winds change directions in the
winter when the
land becomes colder
than the ocean.
You can
also use the thermal circulation to understand some of the basic
features of the El Nino phenomenon (you find a discussion of the El
Nino on pps 135-139 in the photocopied Classnotes).
First here is what conditions look like in the tropical Pacific Ocean
in non-El Nino years (top and side views again)
Cold ocean currents
along the west coasts of N. America and S.
American normally converge at the equator and begin to flow westward
(see top view above). As the water travels westward it
warms. Some of the warmest sea surface waters on the earth are
normally found
the western Tropical Pacific. A temperature gradient becomes
established between the W. and E. ends of the tropical Pacific. The
crossectional view above shows the normal temperature and circulation
pattern found in the equatorial Pacific Ocean. You would
find surface high pressure in the east and low pressure in the
west. Note that the wind circulation pattern is the same as the
simple thermal circulation we studied above.
Every few years El Nino conditions occur and the cold
currents don't
make it to the
Equator. Warm water is carried from the western Pacific to the
eastern Pacific
Now surface high
pressure is found in the west and surface low
pressure and rising air is found in the E. Pacific (the reversal in the
surface pressure pattern is referred to as the southern
oscillation). Indonesia and Australia often experience drought
conditions during El Nino events. In the desert SW we expect
slightly wetter than normal conditions (perhaps 20% wetter than
normal). Wetter conditions are also found in California and in
the SE US.
And
here is some additional information concerning the 3-cell model of the
earth's global scale circulation.
The 3-cell model predicts subtropical belts of
high
pressure near
30
latitude. What we really find are large circular centers of high
pressure. In the northern hemisphere the Bermuda high is found
off the east coast of the US, the Pacific
high is positioned
off the
west coast. High pressure centers are found east and west of
South America in the southern hemisphere.
The winds that blow around these
large scale high pressure
centers create some of the major ocean currents of the world. If
you
remember that high pressure is positioned off the east and west coast
of the US, and that winds blow clockwise around high in the northern
hemisphere, you can determine the directions of the ocean currents
flowing off the east and west coasts of the US. The Gulf Stream
is a warm current that flows from south to north along the east coast,
the California current flows from north to south along the west coast
and is a cold current. A cold current is also found along the
west coast of South America; winds blow counterclockwise around
high in
the southern hemisphere. These currents are shown in the picture above (not shown in class).
Circular low pressure centers, the Icelandic low (off the east coast
near Iceland and Greenland in the picture below) and
the Aleutian low (off the west coast near the southern tip of Alaska),
are found near 60 N.
Tucson gets about 12 inches of rain
in a normal year (we are well below
normal this year). About half of this comes during
the "summer monsoon" season. The word monsoon, again, refers to a
seasonal change in wind direction. 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.
In the winter the subtropical high moves south of 30 N latitude.
Winds to the north of the high blow from the west. Air
originating over the Pacific Ocean 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 southwest into
Arizona.
Between
now and the next quiz we will be covering Thunderstorms,
Tornadoes, Lightning, and maybe Hurricanes.
We just got started on thunderstorms. Here's a brief introduction.
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). Most summer
thunderstorms in Tucson are this type. At the other end of the
spectrum are supercell thunderstorms. We'll watch a short
video at some point that shows a computer simulation of the complex air
motions inside a supercell thunderstorm.
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 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 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.
