Climate and Global Change
Why does the summer forecast
frequently predict afternoon showers?
Introduction
Have you ever wondered what atmospheric conditions are needed for a cloud to form? A cloud
is essentially air that has risen, cooled, and thereby reached saturation. Some air parcels rise and
form clouds, while others rise and don’t form clouds. So how can we determine which air parcels
will form clouds and which will not? To answer this question, we need to measure and record the
vertical profile, or sounding, of the atmospheric temperature and moisture content. This is
achieved by releasing rawinsondes balloons with instruments to record temperature, dewpoint
temperature and wind. These soundings are analyzed to determine the atmospheric stability.
Atmospheric stability is not only an
important parameter which determines
the occurrence and strength of
convection and afternoon showers, but
is also important when determining
vertical mixing of pollution, i.e.,
determining when to issue public
warnings concerning air pollution.
Adiabatic Diagrams
Atmospheric stability can be
determined by using special graph
paper called adiabatic diagrams, i.e., by
plotting temperature and dewpoint
versus pressure. As discussed in
lecture, one type of these diagrams is
the Skew-T Log-p diagram (see
diagram to the right).
Pressure on this type chart decreases from bottom to top along the vertical axis and temperature
increases from left to right along the horizontal axis. This is not your ordinary graph paper.
Temperature lines are not vertical, but slope upward slanting toward the right, hence the name
skew-T. Pressure lines are not equally spaced along the vertical axis, but because of the nonlinear
variation of atmospheric pressure with height, are spaced according to a logarithmic scale, hence
the name log-p. Looking at the diagram, you see several lines running horizontally and
diagonally, some straight and some curved. The small figure
to the right indicates what some of these lines represent. For
this exercise, we will concentrate on plotting and analyzing
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two variables, temperature and dewpoint temperature, versus pressure on skew-T diagrams.
Stability and the Skew-T Log-p Diagram
Now that we have a method of displaying the environmental conditions, i.e., how temperature
and dewpoint temperature vary with height, we can analyze the plot to determine the stability of
air parcels rising or sinking in this environment. From lecture, recall the dry and moist adiabatic
processes. The word adiabatic means no heat is added or subtracted during the lifting or
subsiding of a parcel of air. If a parcel of air is not saturated, it will cool (warm) about 10C / km
as it rises (sinks). This rate of temperature change with change of height is called the dry
adiabatic lapse rate. On the other hand, if an air parcel is saturated, it will cool (warm) about 5C
/ km as it rises (sinks) through the lower atmosphere. Recall that the moist adiabatic lapse rate is
not constant. It depends on the amount of latent heat being added to the parcel as it rises or
sinks. And the latent heat addition depends on vapor being condensed which is temperature
dependent. This is referred to as the moist adiabatic (only the latent heat of condensation or
evaporation is added or subtracted during the process, no other heating or cooling processes are
allowed) lapse rate. If for instance you have a dry (unsaturated) parcel that begins at 1000 mb and
is lifted, it will cool at the dry adiabatic (10C / km) rate until the parcel reaches a saturation
level, and then it will rise moist adiabatically cooling at about 5C / km thought the lower
troposphere.
Stability Rules
Recall that when a parcel is displaced (moved) from its original altitude to a new level, then
•
if the net force accelerates the parcel back toward its original position, the atmosphere is
considered “stable”.
•
if the net force accelerates the parcel away from its original position, the atmosphere is
considered “unstable”, and
•
if the net force is zero, the atmosphere is considered “neutral”.
Another way to discuss stability is to employ the ideas of we learned from the Buoyancy Lab. For
example, a parcel will rise as long as it is less dense (warmer) than its surrounding environment.
Conversely, a parcel will sink as long as it is denser (colder) than its surrounding environment.
There are three main characterizations of stability - absolute stability, conditional stability and
absolute instability (see figure below). In an absolutely stable environment, a rising air parcel
becomes denser than the surrounding environmental air as it rises, so a parcel in this environment
will not be able to continue to rise by buoyancy alone. In a conditionally unstable environment, a
parcel will continue to rise if it is saturated or will sink back to its original position if it is dry
(unsaturated). For an absolutely unstable atmosphere, regardless of saturation condition of the
parcel, a parcel will continue to rise if it is lifted. See the figure on the next page.
To determine stability from a skew-T, one needs to compare the moist and dry adiabatic
processes with the environmental sounding and apply the stability rules. If the environmental
lapse rate is less (tilted more toward the right) than the moist adiabatic lapse rate, the atmosphere
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is absolutely stable. If the environmental lapse rate is greater (tilted more toward the left) than the
dry adiabatic lapse rate, the atmosphere is absolutely unstable. If the environmental lapse rate is
greater than the moist adiabatic lapse rate and less than the dry adiabatic lapse rate, the
atmosphere is conditionally unstable.
400
Pr
es
su
re
(
m
b)
Dry Adiabat
500
Moist Adiabat
Conditional
Stability
600
Absolute
Stablility
700
800
850
Three
Observed
or
Measured
Lapse
Rates
Absolute
Instablility
1000
1050
Temperature (°C)
Exercise
The following sounding was taken on 11 November 2002 at 0000 UTC in Birmingham, AL. Plot
these data on the provided skew-T diagram. Using a pencil draw a dashed line connecting
dewpoint temperature data points and a solid line connecting temperature points.
Pressure
(mb)
986
850
800
762
700
650
585
500
400
300
200
Temp. (C)
25.6
15.8
12.2
12.6
7.6
2.4
-5.1
-9.9
-23.3
-39.3
-53.3
Dewpt.
Temp. (C)
20.7
14.0
10.1
-1.4
-9.4
-8.6
-19.1
-40.1
-43.3
-55.3
-70.3
Stable, Unstable or
Conditional Stability
XXXX
XXXX
XXXX
XXXX
XXXX
XXXX
XXXX
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Using the dry and moist adiabats on the skew-T chart, determine if a parcel is stable or unstable
for various layers of the atmosphere. An atmospheric layer is between two or more pressure
levels (e.g., 986-850 mb or 850-800 mb, etc.). Complete (the blank locations) the table above by
determining the stability of these layers.
Now suppose a dry air parcel begins to rise from 986 mb. Assume the parcel begins with the
same conditions as its surroundings. Trace the path this parcel would take as it rises to 200 mb.
Assume that the parcel reaches saturation at 910 mb, i.e., assume that the parcel first rises dry
adiabatic from 986 mb to 910 mb and then moist adiabatically to 200 mb. (The level that a parcel
will become saturated can be determined from a Skew-T diagram, but we will not have you learn
how this is done in this class.) With the above assumption, is the parcel Stable or Unstable
(Circle your answer).
At what level is the environmental temperature below 0ºC, i.e., below freezing?
Why might this level be critical for airplanes?
What kinds of clouds would you expect in this environment, tall thunderstorm-type clouds (i.e.,
thunderstorms or cumulonimbus clouds) or smaller “fair weather” clouds (cumulus)? Why?
Do you recall why this particular sounding might be significant; in other words, what weather
event happened around this time in northern Alabama?
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