EAS 106 LABORATORYATMOSPHERIC SOUNDINGS AND WEATHER

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EAS 106 LABORATORY ATMOSPHERIC SOUNDINGS AND
WEATHER This package contains 10 slides of
text Excel Spreadsheet (E106_LAB11) 7 Slides of
Exercises Introduction to Atmospheric
Soundings A sounding is a vertical profile of
temperature, humidity (via dew point) and wind in
the atmosphere. It helps to tell the weather in
the air directly above and is particularly useful
for telling the vertical extent of clouds, the
type of precipitation and the likelihood of
thunderstorms or air pollution. How Soundings
are Obtained Every 12 hours at several hundred
stations around the world, instrumented balloons
are released into the atmosphere. These balloons
measure temperature, pressure, and humidity as
they ascend through the atmosphere, and radio the
readings to the ground-based weather station. The
wind above the ground is determined by tracking
the balloon by a radar as it rises. The height of
the balloon is determined from the pressure and
the temperature mathematically by the hydrostatic
equation, which equates changes of pressure to
the weight of a column of air. The soundings on
thee Excel Sheet were taken at 0000 UTC on 01
December 2006 and come from Jackson Mississippi,
Springfield, Illinois, Topeka, Kansas, and North
Platte, Nebraska. Web sources http//weather.uwyo
.edu/upperair/sounding.html
http//vortex.plymouth.edu/u-make.html Units
for the Weather Variables Temperature, T, for
soundings is expressed in Centigrade. Dew Point
Temperature, Td, is expressed in Centigrade and
gives a measure of Humidity. Pressure, p, is
expressed in millibars (mb) or hectoPascals. 1 mb
100 Pa (Pascals) Height, h, is expressed in
meters.
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The dew point temperature is the temperature at
which the air becomes saturated with water vapor.
Dew and clouds first appear when the air is
cooled to the dew point. Further cooling can
produce precipitation. Td is an indirect measure
of the amount of water vapor in the air. For
example, when Td 0?C, each kilogram of air at a
pressure of 1000 millibars (mb) is holding 3.84
grams of water vapor. In rough terms, The amount
of water vapor that the air can hold doubles for
each 10?C increase of T. The amount of water
vapor the air actually holds doubles for each
10?C increase of Td. Thus, when Td 10?C, 1 kg
of air at 1000 mb is holding 7.76 g (nearly
double 3.84), while when Td 20?C, 1 kg of air
at 1000 mb pressure is holding 14.95 g (nearly 4
times 3.84).
Plotting a Sounding Soundings are profiles of
both T and Td. They are often plotted on a graph
known as an adiabatic chart. For most adiabatic
charts, the x-axis of this chart is proportional
to T (or Td) while the y axis is related to
pressure or height. T and Td are plotted in the
same manner as you would plot any graph. For each
data point of T make a large dot and then connect
the dots with a solid line. For each data point
of Td, make a small x and connect xs with a
dashed line.
Data for Sounding z(m) T Td 0 10
3 1000 15 4 2000 10 10 4000 -9 -9 6000
-23 -40
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Clouds and Precipitation Clouds and precipitation
occur when the air is saturated, or when T ? Td.
In the sounding to the right, the cloud layer
occurs from about 2000 to 4000 m. Many clouds are
too thin or have bases that are too high to
produce precipitation at the ground. In order to
produce precipitation a cloud base should be
within about 3000 m of the ground and should in
general be at least 2000 m thick. The thicker the
cloud and the lower its base, the more likely it
will produce precipitation. Clouds often occur
even where the sounding suggests that T is up to
about 5?C higher than Td! This is due to slow
instrument response time, or layers in which
there are breaks in the clouds. As mentioned,
this is common with thunderstorms. Temperature
Inversions An inversion is a region in which
temperature increases with height. In the
sounding above, the inversion occurs from 0 m to
1000 m. Inversions are most common just above the
ground on clear nights and early mornings.
Pronounced inversions also occur in polar lands
during winter. Inversions also occur at elevated
surfaces of fronts. Inversions are associated
with pollution episodes because they act like
lids, suppressing vertical motions in the
atmosphere. Ironically, weak inversions often
occur several hours before severe thunderstorms,
where they serve the purpose of intensifying the
thunderstorms by delaying them until the rest of
the atmosphere becomes extremely unstable, and by
restricting the area over which they form.
Instability will be discussed below. Thunderstorm
s Thunderstorms, which rise to heights of 8 17
km are exceptional because they pump warm, humid
air to great heights and form in the midst of
soundings that are colder and may be dry at most
altitudes. Therefore, thunderstorms do not match
the sounding of the surrounding atmosphere. Few
soundings are actually taken within
thunderstorms, because they are so narrow,
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so short-lived, and because their weather is so
severe they shred the balloons. But the
thunderstorm sounding can be estimated from the
sounding of the surrounding atmosphere. Soundings
for Classical Weather Situations Very often it
is possible to tell the type of weather to expect
from the sounding. Soundings for different types
of weather conditions are described and graphed
below. 1. Fair weather, with possible fog and
pollution. Here the air near the ground is almost
saturated (T - Td ? 0) so that fog is likely,
particularly when the air cools at night. The air
at 800 m is also cooler (and denser) than the air
at 1300 m. This inversion will trap any pollution
near the ground if the winds are too light to
blow it away. By contrast, the air aloft is very
warm and dry, (T - Td 10) so that no clouds can
form in it and no humid but cooler and denser air
from the ground can rise into it. Soundings like
this are very common along the coast of
California, where fog is frequent, and they also
resemble the soundings that occurred during some
of the worst pollution episodes in London. In
these cases, and on most clear nights, the
coldest air is right at ground level and the
inversion typically extends several hundred
meters above ground. 2. Unstable air with a
possible thunderstorm. Here air near the ground
is humid and much warmer than air above.
Temperature decreases rapidly with height (i. e.,
the sounding line slopes sharply to the left).
For these soundings, thunderstorms are possible
whenever the sounding temperature decreases more
rapidly with height than the cooling rate for
rising saturated air (discussed below). This
situation is called unstable. 3. Warm front
above with snow. Here, a mass of humid, tropical
air ascends a dome of cold air at the surface.
Clouds and precipitation usually form in the warm
air above the frontal surface (i. e. the
inversion). The type of precipitation reaching
the ground is determined by the temperature
structure of the sounding. Because temperatures
remain below 0?C at all heights in sounding 3,
snow is
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most likely. If temperatures near the ground are
well above 0?C, rain will occur. You should be
able to draw a sounding with a frontal inversion
that produces rain.
4. Warm front above with freezing precipitation.
This is the same general situation as sounding
3, but now temperatures near the ground are
below 0?C, while temperatures in some layer aloft
are above 0?C. Snow will form near the top of
this sounding, melt or partly melt as it falls
through the zone with T 0?C and then, as it
falls through the cold air near the ground,
either refreeze to ice pellets in mid-air or
freeze on contact with the ground or any other
object as freezing rain. _________________________
__ The red dotted line in panel 2 indicates T
inside the thunderstorm or the adiabatic cooling
rate of rising, saturated air. It shows that a
rising parcel (balloon) of saturated air will be
buoyant because it is warmer than the surrounding
air.
T of surrounding atmosphere
T in thunderstorm
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Cross Sections Soundings provide a
one-dimensional view of the vertical structure of
the atmosphere above one point on earth. Cross
sections provide a two-dimensional view of the
atmosphere along a line on earth. Cross sections
are "slices through the pie" that reveal the
structure of fronts and the associated forms of
precipitation. This is done by contouring the
cross section for temperature.
Cross sections show the sequence of precipitation
form. With warm fronts there is a sequence of
rain, freezing rain, sleet (ice pellets), and
snow as you move from the front to the cold air.
However, freezing rain and sleet only occur near
sharp fronts with large temperature contrasts (as
in the diagram below to the left), because only
then is there band where a refreezing zone lies
below a melting zone. This does not occur with a
weak front (as in the diagram below to the
right). Even when freezing rain and ice pellets
do occur, they generally form in a narrower zone
than either rain or snow.
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Process Lapse Rates When air rises it expands and
its temperature falls. The lapse rate or the rate
of cooling for rising air depends on whether or
not the air is saturated. The cooling rate is
slower for saturated air because the latent heat
released during condensation heat offsets the
cooling rate. The amount of condensation is
greater for warm air than for cold air, but we
will here ignore the differences, and use the
rules below. These rules are valid whether the
air rises vertically in a thunderstorm, or
obliquely over a front.
Rising Unsaturated Air (T Td) T cools
10?C per km (upward) Td cools 2?C per
km Rising Saturated Air (T Td) T and Td
both cool (about) 6?C per km Sinking Air T
warms 10?C per km (downward) Td warms 2?C
per km (downward) Because rain and snow fall
out of the clouds, little water is left to
reevaporate once air begins to descend. The
descending air quickly becomes unsaturated and
thereafter T and Td increase at the rates for
unsaturated air. On the next page we use these
rates to find the temperature of air above a
cross section with a front and thereby can infer
the form of precipitation.
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Cross Sections, Process Lapse Rates and
Precipitation Form
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Stability Thunderstorms form when a rising bubble
or parcel of air, which is indicated by the
process lapse rate, is warmer and hence lighter
than the souding of the surrounding atmosphere.
This condition is known as unstable. On the other
hand, vertical motions are suppressed when the
rising parcel is colder and denser than the
surrounding atmosphere. Therefore, Unstable
Atmosphere Sounding cools faster than rising
parcel. Stable Atmosphere Sounding cools slower
than rising parcel. Atmospheric stability is
determined by the following steps 1. Draw the
sounding. 2. Lift a parcel of air from ground
level using the process cooling rate for
unsaturated air until the point of saturation and
the process cooling rate for saturated air
thereafter. 3. Compare temperatures of sounding
and lifted parcel. The atmosphere is unstable if
the lifted parcel is warmer then the sounding and
stable if the lifted parcel is cooler than the
sounding.
The diagram to the right contains both a stable
sounding and an unstable sounding and compares
them to the temperature of a rising
parcel. Sometimes, the lifted parcel will be
cooler than the sounding for the first few
hundred meters of lifting but warmer at some
greater heights. This means that the atmosphere
will need a push to get it going. Such an
atmosphere is potentially unstable, and the
instability will be realized only if the air near
the ground is heated further, or if some
atmospheric process forces the entire atmosphere
to rise. Examples of forced rising include
upslope flow against mountains or over frontal
surfaces.
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Temperature Inversions and Stability
Implications for Air Pollution and
Thunderstorms Inversions represent extremely
stable situations. Cold air below is far too
dense to rise, while light air above is far to
light to sink. Inversions are associated with air
pollution outbreaks precisely because vertical
motions are so suppressed that pollutants cannot
be dispersed. Inversions are also associated with
fronts, because warm air masses slide over cold
air masses that hug the ground because of their
greater density. Breaking Inversions Inversions
can be broken by heating the air at the base of
the inversion. This happens almost every clear
day near the ground. Overnight the ground cools
and an inversion forms in the lowest few hundred
meters of the atmosphere. Once the sun comes up
it heats the ground, which in turn heats the air
near the ground rapidly, until it becomes warmer
than the air above and thereby destroys the
inversion. Pollutants are dispersed and cumulus
clouds or even thunderstorms may pop up.
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Exercises
1. In Excel, graph the four soundings (0000 UTC
on 01 December 2006 and come from Jackson
Mississippi, Springfield, Illinois, Topeka,
Kansas, and North Platte, Nebraska). using height
as the y axis and plot T as a black line and Td
as a red line on the x axis. Then copy the graphs
to this slide. 2. Mark all cloud layers and
inversions. 3. In a text box, tell what form of
precipitation (if any) each is associated with,
and explain.
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On this and the next slide, fill out the missing
T and Td in the rising air above the frontal
surface and tell the form of precipitation at the
ground.
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Color Contour the Cross Section Below and tell
the Precipitation Form
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Air at sea level has T 24C, Td 8C. Lift the
air to 6000 m allowing it to cool at the
appropriate adiabatic rates (Show process lines).
Find the condensation level and T and Td at 6000
m.
Condensation Level _______m T(6000)
_____ Td(6000) _____
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Exercise For the sounding shown below lift air
from the surface to the top of the chart using
the appropriate adiabatic process and showing the
lines. On the next slide tell the condensation
level and if this sounding could support a
thunderstorm (i. e., if it is unstable). Briefly
justify your answer.
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Properties of 04 May 1999 Sounding