In this homework we will analyze the temperature and moisture profile of the atmosphere and how itrelates to the intensity of thunderstorms. An atmospheric profile is simply a way of displaying how any weather parameter (such as temperature and moisture) changes with height.We will be plotting the weather data on a Stüve diagram. A Stüve diagram is a type of graph that has height on the vertical axis and temperature across the horizontal axis. Since air pressure ALWAYS decreases with height, we can use air pressure as a height measurement (using millibars) on the left side of the chart as well as the standard method of measuring altitude (using kilometers above sea level) on the right side of the chart.
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CEE 1331 Homework #3: Atmospheric Soundings and Severe Weather
In this homework we will analyze the temperature and moisture profile of the atmosphere and how it
relates to the intensity of thunderstorms. An atmospheric profile is simply a way of displaying how any
weather parameter (such as temperature and moisture) changes with height.
We will be plotting the weather data on a Stüve diagram. A Stüve diagram is a type of graph that has
height on the vertical axis and temperature across the horizontal axis. Since air pressure ALWAYS
decreases with height, we can use air pressure as a height measurement (using millibars) on the left side
of the chart as well as the standard method of measuring altitude (using kilometers above sea level) on
the right side of the chart.
Note that at an altitude of 0 km (which is sea level), the air pressure is indicated to be near 1000 mb. At
an altitude of approximately 2 km above sea level, the air pressure drops to around 800 mb. Likewise,
when you get up to around 8 km above sea level, the air pressure drops to around 350 mb. Keep in
mind that these must be understood as averages because the values do change from day to day!
As we go higher into the atmosphere the air pressure will decrease until it eventually reaches NEAR
zero millibars at the edge of outer space. In this homework we will not be going quite that high. We will
only be going up to the level where the air pressure drops to 100 mb (which is approximately 16
kilometers or about 10 miles above sea level). You can see this by looking at the 100 mb level at the top
left side of the Stüve chart and how it corresponds to 16 km on the top right of the chart.
When you look at the Stüve graph you will note that in addition to the standard vertical and horizontal
lines denoting height and temperature, respectively, we also have diagonal lines and curves. The
diagonal lines are called DRY ADIABATS. The dashed curves are call WET ADIABATS. You can think of
these as the paths that air bubbles take as they rise into the atmosphere. And just like their names
suggest, a DRY air bubble (i.e., one that is not saturated and thus whose relative humidity is less than
100%) will follow the DRY ADIABATS as it rises into the sky. Similarly, a SATURATED air bubble (i.e., one
whose relative humidity is 100%) will follow the curved WET ADIABATS as it rises.
As you follow these curves, you will see how the temperature of rising air bubbles change with height.
Let’s do an example. Let’s say you have an air bubble located at 1000 mb, and the temperature of the
air bubble is 38°C. You would plot this air bubble by finding the 1000 mb horizontal line, and mark
where it intersects the 38°C vertical temperature line. Of course, not every single possible temperature
is going to have a vertical line. But you can see the vertical lines for 30°C and for 40°C. Therefore, you
can ESTIMATE where the 38°C line would be and make your plot.
OK, now let’s assume for this example that our air bubble at 1000 mb and 38°C is NOT saturated. In
other words, its relative humidity is less than 100%. We are going to lift this air bubble up to the 800
mb level. Since it is not saturated, we will follow the DRY ADIABAT. In this example, it just so happens
that we have a DRY ADIABAT already on the chart that passes through our point of 38°C and 1000 mb.
This is NOT the case for every possible scenario. If you do not have a visible dry adiabat passing
through your point of interest, simply use the DRY ADIABAT that would pass through your point by
drawing one for yourself that is parallel to the DRY ADIABAT on either side of your point!
But for now, we can just use the visible DRY ADIABAT that happens to conveniently pass through our
example point. Now lift your bubble to the 800 mb level following the DRY ADIABAT and stop.
What is the temperature of your air bubble now that it is at 800 mb? By dropping a vertical line down to
the temperature axis, you can see that your air bubble has cooled to about 19°C.
As long as the air bubble stays dry, that is to say, as long as its relative humidity is less than 100%, you
would follow the DRY ADIABAT to see what the temperature of your air bubble would be at any level in
the atmosphere.
But we know by now that an air bubble that is rising will probably eventually cool to the point to where
it can no longer “hold” all of the water vapor that may be in it. Once it reaches that point, the relative
humidity will hit 100%, and condensation will begin. In other words, a cloud will form.
But where will this happen? It happens at the level in the atmosphere called the LIFTING
CONDENSATION LEVEL (LCL). This is where the base of a cloud is produced by our rising air bubble.
So how do we find this LCL? First you need to know what the DEW POINT TEMPERATURE of the air
bubble is at the surface where it starts rising. Remember, the dew point temperature is just another
way of measuring how much water vapor is in the air. The higher the dew point, the greater amount of
water vapor (i.e., moisture) is in the air.
So, let’s say the dew point temperature of our surface air bubble at 1000 mb is 24°C. To find the
bubble’s approximate* LCL (i.e., where cloud base will form), you simply follow the dashed-curve WET
ADIABAT that passes through the dew point temperature up to where it crosses the DRY ADIABAT line
that passes through the temperature of the air bubble.
When you plot 24°C at 1000 mb, you will quickly note that there is no WET ADIABAT that conveniently
passes through that point. BUT…you will also note that there are WET ADIABATS on either side of your
plotted dew point temperature. So, you simply make your own WET ADIABAT that parallels both of
these, and then you follow it!
Therefore, when you lift your “dry” air bubble on its DRY ADIABAT, and intersect that path with the WET
ADIABAT that passes through your air bubble’s surface dew point temperature, you have found the
approximate level in the atmosphere where it will saturate (i.e., RH=100%) and become a cloud. You
have found its LCL. In our example above, the air bubble saturates at the 775 mb level. This is where
the base of the cloud will be! And you can see this corresponds to a little more than 2 km above sea
level.
Any further lifting of our air bubble must now follow the WET ADIABAT because it will remain saturated
for the rest of its journey into the sky.
At any step of the way, we can compare the rising air bubble’s temperature to its surroundings. Thus,
we can determine if the air bubble is warmer than its surroundings, or colder than its surroundings, or
the same temperature as its surroundings.
If the air bubble is warmer than its surroundings, then it will continue to rise on its own much like a hot
air balloon rising into the relatively cool air around it. In this scenario, we say the atmosphere at this
level is UNSTABLE relative to the air bubble.
If, however, the air bubble is colder than its surroundings, then it will want to fall back to where it came
from since it would be more dense than its surrounding environment. In this case, we would say the
atmosphere at this level is STABLE relative to the air bubble.
If the air bubble finds itself at the same temperature as its surrounding environment, then it would tend
to stop rising because it would be equally as dense as its surroundings. We would say that the
atmosphere at this level is NEUTRAL relative to the air bubble.
RISING AIR BUBBLES and THUNDERSTORM POTENTIAL
Thunderstorms form whenever the atmosphere is mostly unstable. This allows air bubbles to rise
through great depths in the atmosphere. And the warmer these bubbles are compared to their
surrounding environment, the faster they will rise and the stronger a thunderstorm will be.
In this part of the homework, we are going to lift an air bubble in a specific environment and then
determine whether thunderstorms are possible, and if they are, just how strong they will be.
First, we need to define the atmospheric environment in which air bubbles will be rising. Plot the
following MORNING temperatures at each pressure level on your Stüve diagram. You may skip plotting
the dew point temperatures, except for the first one, for right now. We will get back to this parameter
later in the homework.
Pressure Level (mb)
Temperature (°C)
Dew Point Temperature (°C)
1000
850
700
22
20
2
16
19
-25
500
400
300
200
150
100
-20
-27
-40
-50
-50
-35
-45
-55
-65
-75
-80
-65
Once you have plotted your temperature points on the Stüve (called an atmospheric sounding), connect
your points with straight line segments. (Do NOT include your dew point temperature with one of the
straight line segments). You will then be able to see how the temperature is changing with height from
the surface (1000 mb) up to 100 mb (approximately 16 kilometers above sea level).
Now you are ready to lift surface air bubbles into this environment. Using the procedure discussed
earlier in this homework,
1)
2)
3)
4)
5)
Find the pressure level where the LCL is located.
What happens to the air bubble at the LCL?
What is the relative humidity of the air bubble at its LCL?
What is the air bubble’s temperature at the LCL?
Is the air bubble at the LCL (warmer, cooler) than its environment?
_______________ mb
_______________
_______________ %
_______________ °C
_______________
Let us now assume that we can lift our bubble even higher than its LCL. As mentioned earlier, any
further lifting must be done on a WET ADIABAT since our air bubble is now saturated (i.e., RH= 100%)
and will remain saturated for the rest of its trip into the atmosphere.
Lift the air bubble until it crosses the environmental temperature line. When it crosses this line, any
further lifting will result in the air bubble being warmer than its environment. This means once it
crosses this line, it will freely rise on its own. This level is called the LEVEL OF FREE CONVECTION (LFC).
6) At what pressure level is the LFC for this air bubble?
7) What is the bubble’s relative humidity at the LFC?
_______________ mb
_______________ %
Continue lifting the air bubble (although now it is doing it on its own because it needs no help). Lift it
until once again it equals the environmental temperature. This is the point where the bubble will
ultimately stop rising because any further lifting will result in the air bubble being colder than its
environment. This level is called the EQUILIBRIUM LEVEL (EQL).
8) At what pressure level is the EQL?
9) What is the relative humidity of the air bubble at the EQL?
_______________ mb
_______________ %
The EQL is often the height to which a cloud will grow. Using the altitude scale in kilometers on the
right-hand side of the Stüve diagram,
10) To what height – in kilometers – will the cloud reach?
_______________ km
The amount of energy available to thunderstorms growing in any environment is given by what is called
the POSITIVE area on the Stüve diagram. Specifically, the POSITIVE area on the graph is defined as the
area TO THE RIGHT of the environmental temperature line and TO THE LEFT of the air bubble’s path
between the LFC and the EQL.
This area is known as Convective Available Potential Energy, or CAPE. The larger the CAPE area the
more intense a storm can be.
In the figure below, the heavy black line is the atmospheric temperature profile. The bubble’s path
starts with the DRY ADIABAT at temperature T and switches to a WET ADIABAT after the LCL is reached.
It then rises along the WET ADIABAT, crossing the environmental temperature line at the LFC. It
continues on the WET ADIABAT until it crosses the environmental temperature line again at the EQL.
The green shaded area between the LFC and the EQL is an example of CAPE.
Note that there is also a portion of the bubble’s path – shaded in red – that is to the LEFT of the
environmental temperature line between the SURFACE and the LFC. This is called the NEGATIVE area
and is known as Convective Inhibition, or CINH. This is the amount of energy required to FORCE an air
bubble to rise.
The reason the bubble must be forced to rise is because as soon as it rises from the surface, it becomes
colder than its surrounding environment, and thus it wants to return from where it came. Therefore, the
atmosphere is INHIBITING the air bubble from rising. But if the bubble is sufficiently forced (i.e., such as
along a frontal boundary or by being pushed up a mountain), it is possible to get an air bubble to rise to
its LCL and then eventually on to its LFC.
11) Shade in the CINH and the CAPE areas on your Stüve chart using a “cool” color (i.e., green or
blue) for the CAPE and a “warm” color (i.e., yellow, orange, or red) for CINH.
So far, we have seen that the atmosphere – as depicted by our MORNING temperature profile – does
have some energy available (CAPE) for thunderstorm development IF AND ONLY IF enough forcing can
be provided to overcome the CINH and allow air bubbles to reach their LFC. Once there, the air bubbles
will continue to rise on their own – being warmer than their surroundings – until they reach their EQL.
Now let us jump ahead into the AFTERNOON and heat up the lower levels of the atmosphere and see
how this changes the potential for thunderstorms and their intensity. Keep in mind that the mid- and
upper-levels of the atmosphere do not change very much during the day, but near the surface the
temperature can heat up dramatically.
Let’s say the surface winds throughout the day are out of the SE bringing in warm, more humid air from
the Gulf of Mexico. And therefore, by late afternoon, the surface temperature rises to 30°C, and the
surface dew point rises to 22°C reflecting the rich, Gulf moisture flowing into the area. Let’s assume
conditions aloft do not change. Plot this afternoon temperature profile on a second Stüve chart.
Now, lift an air bubble with a temperature of 30°C and a dewpoint of 22°C, starting once again at 1000
mb.
12) What is the LCL of this afternoon rising air bubble?
13) What is the air bubble’s temperature at this LCL?
14) Is the air bubble at the LCL (warmer, cooler) than its environment?
_______________ mb
_______________ °C
_______________
Continue lifting your air bubble until it first equals the temperature of the environment in order to find
its LFC. Shade in the CINH area with a “warm” color (yellow, orange, or red).
15) At what level does the air bubble reach its LFC?
_______________ mb
16) Is the amount of afternoon CINH (less, greater) than in the morning? _______________
17) What does this amount of CINH tell you about the difficulty in producing thunderstorms in the
afternoon compared to the difficulty you found in the morning?
_________________________________________________________________________________
Continue lifting your air bubble until it reaches its EQL.
18) At what pressure level does the air bubble reach its EQL?
_______________ mb
19) To what height – in kilometers – will the cloud reach?
_______________ km
20) Shade in the afternoon CAPE area on your Stüve chart using a “cool” color (i.e., green or blue).
21) How does the amount of CAPE in the afternoon compare to the amount of CAPE you found in
the morning? _________________________________________________________________
_____________________________________________________________________________
22) What does this tell you about the intensity of afternoon thunderstorms compared to the
intensity of thunderstorms that might form in the morning?
_____________________________________________________________________________
*Using the WET ADIABAT that passes through the surface dew point temperature and finding its
intersection with the DRY ADIABAT that passes through the surface temperature APPROXIMATES the
location of the LCL. Ideally, one would use MIXING RATIO lines to determine the EXACT value for the
LCL. But in the interest of saving time for this lab session and reducing the complexity of the Stüve
chart, this approximation is introduced.
…
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