EAR 203
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EAR 203 EARTH SYSTEM SCIENCE
SYRACUSE UNIVERSITY
LAB 4D – ATMOSPHERIC CIRCULATION
PART 1 – UNDER PRESSURE
Atmospheric pressure is a maximum at the surface and decreases with increasing altitude through the atmosphere. The
relationship is not a linear, but rather an exponential, and is referred to as the barometric law. The law is based on the ideal gas
law (remember PV=MRT, where P= pressure, M= number of moles of gas, R= the idea gas constant and T= Temperature) so the
derivation would begin here:
Equation 1.
where ρ = density.
If you want to consider the change in pressure (dP, or delta pressure) for a given height you must introduce elevation and, of
course, the acceleration due to gravity:
Equation 2.
This expression reads, the change in pressure is equal to the density times gravity for a given change in height (dz).
After some substitutions and an integration you would arrive here:
Equation 3.
where Po = a sea level pressure of 1000 mb.
We can simplify this even further by making Mg/RT in the exponent equal to H and placing it in the denominator, making the
exponent -z/H. H is called the scale height and is defined as the height at which the atmosphere is 1/3 less dense. The value is
place dependent, but here we will use a generic value of 8400 m for H:
P = P0e(-z/H)
If you choose, you may make a plot of pressure with height in the atmosphere (using the above equation). Then, using the top
of the graph as a second X-axis for temperature, plot the temperature vs. altitude for the first 100 km of the atmosphere.
OR – you may use the information in Figure 1. NOTE: 1 hPa=1000 mbar
1.
Are the curves in the two plots related? If so, how? If not, why not?
2.
What controls the temperature distribution in the atmosphere?
Lab 4D – Atmospheric Circulation
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Figure 1.
Lab 4D – Atmospheric Circulation
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PART 2 – CIRCULATION AND PRECIPITATION PATTERNS
Now we will explore differences in the atmospheric circulation and precipitation patterns by creating some plots using an
“interactive” tool available at the National Oceanic and Atmospheric Administration’s (NOAA), NCEP (National Center for
Environmental Prediction) division website (http://www.cdc.noaa.gov/cgi-bin/data/composites/printpage.pl). This website will
allow us to plot many, many climatic parameters for any place on the globe.
The interface is a little clumsy, but once you enter the information one time it will be pretty easy to modify for the next steps. It
is important that you set the Scale plot size to 150% because the best way to examine the differences is to copy the pictures and
paste them into PowerPoint WITHOUT rescaling the images, this way you will be able to flip back and forth to see the changes.
The settings in the graphic (Figure 2) are the ones that should be used for looking at winds. You may choose either South
America or Asia. Atmospheric scientists generally look at summer and winter conditions, in their literature you see that graphs
are made for DJF (December, January, February) and JJA (June, July, August). You will do the same for the work today. If you
pick South America you remember that DJF is summer and JJA is winter!
Figure 2.
Lab 4D – Atmospheric Circulation
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1. Examine the winds at 925 mb. Based on the graph from Part 1, how high up in the atmosphere are these winds located?
2. Create graphs of vector wind for DJF and JJA winds for the region you have selected and describe where and how wind
patterns differ seasonally?
3. Now create graphs of “precipitation rate” for DJF and JJA. What are the differences or similarities during the different
seasons?
4. Looking at all four graphs together, what can you say about the seasonal wind patterns and the precipitation patterns? Is it
possible to identify monsoonal circulation? Where?
5. Create graphs of the pressure at sea level. Based on what you have learned, is it possible to reconcile the changes in wind
direction and precipitation with these diagrams of surface pressure. Where do the seasonal pressure variations occur?
Please take a moment to speculate on why they occur and suggest some possibilities for this variation.
PART 3 – POINTS TO PONDER (TAKEN AND SOMEWHAT MODIFIED FROM WHAT APPEARS IN KUMP, KASTING AND CRANE)
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6. What is latent heat? Explain why latent heat is important for the redistribution of energy.
7. Describe three processes that produce uplift in the atmosphere and are important in causing precipitation.
8. Saturation vapor pressure is where the rate of condensation equals the
evaporation rate and the gas is at equilibrium. In this scenario, the saturation
vapor pressure depends only on the rate at which molecules are transferred from
liquid to gas and back again. Here is a graph from our textbook that illustrates the
relationship between temperature and saturation vapor pressure.
Explain why the information shown in the plot is useful for understanding the
relationships between atmospheric circulation and precipitation.
Lab 4D – Atmospheric Circulation
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