Lab Instructions: Climatological Structure of the Atmosphere

    Part I: Latitudinal temperature structure and baroclinic instability

  1. Use the viewer to examine the zonal mean January temperature. Is the North Pole or the South Pole colder in January? Why? Is the coldest spot anywhere in the atmosphere located at either pole, or somewhere else? Now make a time-latitude plot to display the seasonal cycle of temperature as a function of latitude. Select "Time" from the horizontal axis pop-up menu, and select "Latitude" from the vertical axis pop-up menu; click "Redraw". In what season is the equator-pole temperature difference largest/smallest in the Northern Hemisphere? Is it largest/smallest in the same season or a different season in the Southern Hemisphere? In which season then do you expect the most vigorous synoptic-scale storms, given the nature of baroclinic instability and the Earth's need to transport excess heat poleward to where the largest deficits exist? Transfer the January temperature and January humidity tables to Excel.

  2. Open the January temperature Excel table. Each column of data corresponds to a different latitude band, in 2.5 degree latitude increments, and each row gives the temperature (in degrees Celsius) at a different pressure level. The rows are arranged in order of decreasing pressure (increasing altitude) because that is the sequence in which data are recorded by a rising weather balloon; thus the first row of data at the top of the table corresponds to the Earth's surface. We are used to having the surface at the bottom, so sort the data according to pressure in ascending order.

  3. Make charts (line graphs) of temperature vs. latitude at the 200 mb, 500 mb, and 1000 mb pressure levels. When using Excel, always select the "Scatter" plot option rather than the "Line" plot option, even when you are asked to make a line graph. You can easily connect the dots on a scatter plot to make it a line graph. (To make all 3 graphs at the same time on one chart, select row 1, which contains the latitude numbers for the x-axis, and then, while holding down the Control key, select each of the rows for the three pressure levels, before clicking on the Chart Wizard.) At the Earth's surface, is the meridional temperature gradient larger in magnitude in the Northern or Southern Hemisphere? At approximately which latitude is this temperature gradient largest? Does the magnitude of the temperature gradient increase or decrease as one moves from the surface to the tropopause? Considering that baroclinic instability needs strong meridional temperature gradients to enable storms to grow, are synoptic weather systems more likely to be found in the lower/middle, or the upper, troposphere? Excluding thunderstorms (a different type of storm; we'll get to them in step #4), how high in general should airplanes fly in order to get above the weather and have a smooth flight?

    Part II: Vertical temperature and moisture structure and convective stability

  4. Make new line graphs showing temperature vs. pressure at three latitudes: One near the equator, a second in northern midlatitudes, and a third at the North Pole. (As in the previous step, put all three graphs on a single chart to make them easy to compare; but this time you have to select columns rather than rows.) On these graphs, altitude increases toward the left, i.e., Earth's surface is on the right and the lower stratosphere is on the left. At what pressure does the tropopause (defined as the pressure level of minimum temperature) occur near the equator? In midlatitudes? Focus now on the vertical temperature structure near the surface. At which latitude is near-surface air warmest relative to air at slightly higher altitudes? What part of the world should therefore be most prone to thunderstorms? Do you notice anything qualitatively different about the near-surface vertical temperature structure at the North Pole compared to the other two latitudes? Make a note of this for future reference. Now delete the chart and make a new one in which you plot temperature vs. pressure at two locations (again on the same chart for ease of comparison): One at a Northern Hemisphere midlatitude location, and the other at the identical latitude but in the Southern Hemisphere. In January, in which hemisphere is it winter? Summer? Comparing the vertical temperature structure in the two hemispheres, in which season are thunderstorms most likely to occur?

  5. Select and copy (using the Edit menu) the equatorial temperature profile. Now open the January humidity Excel table; this shows water vapor mixing ratios (in grams of vapor per kilogram of dry air) in the same format as for the temperature table. Sort the data into ascending order as you did for temperature. Paste the equatorial temperature profile into the humidity table and make a scatter plot of temperature vs. humidity at the equator. Is moisture positively-correlated, negatively-correlated or not-correlated with temperature? Is the behavior linear, or does moisture change with temperature faster at either warm or cold temperature (i.e., for a given difference in temperature, is the difference in humidity larger at the warm end or the cold end of the diagram)? Considering that warm, moist air near the ground is conducive to thunderstorms, does the correlation of water vapor concentration with temperature help or hinder the formation of thunderstorms?

    Part III: The zonal mean general circulation

  6. Return to the viewer and display the January Zonal wind field. At what pressure level and latitude do the maximum mean zonal winds (the jet streams) occur? Does the latitude of the jet stream correspond more closely to that of the maximum or minimum temperature, or the latitude of maximum or minimum latitudinal temperature gradient? Consider the tropics (equator to 30 degrees latitude). What is the direction of the zonal wind near the surface? (To make this easier to see, display the data as a black-and-white contour plot; click and hold on the pop-up menu currently displaying "colors"; select "contours"; click "Redraw". The thick zero contour separates positive (eastward) zonal winds on one side of the contour line from negative (westward) zonal winds on the other side.) Poleward of about 10 degrees latitude, what is the direction of the zonal wind near the tropical tropopause? The zonal wind in the tropics is produced primarily by the Coriolis force acting on air parcels moving toward higher or lower latitudes. For air moving poleward, in what direction does the Coriolis force deflect it? What about for air moving equatorward? Based on this, infer the meridional wind direction in the upper and lower tropical troposphere from the observed zonal wind direction at these altitudes.

  7. Now return to "colors," choose "Time" and "Latitude" for a time-latitude plot, and click "Redraw." Now type 200 (ie. 200mb) in the pressure selection box and click "Redraw" again. This will display the seasonal cycle of the zonal wind at the tropopause in color. In what season do the strongest jet stream winds occur? Given that weather systems are carried by the jet stream, do you expect that weather changes faster in general in winter or summer? Is the strength of the jet stream correlated primarily with maximum/minimum temperature or temperature gradient?

  8. Use the viewer to display the January Meridional wind field. Are mean meridional winds weaker, stronger, or about the same strength as mean zonal winds? Considering that mean meridional winds occur when the Coriolis force on the mean zonal wind is not in geostrophic balance with the meridional pressure gradient, and that the larger this imbalance is, the larger the meridional wind will be, is geostrophic balance a good (10% or better accuracy) or a poor approximation for the climatological features of our atmosphere? Since it is harder to accurately measure weak winds than strong winds, and since sampling in the Southern Hemisphere is poor, errors in the mean meridional wind in the Southern Hemisphere data are large, so concentrate for now on the Northern Hemisphere. Is your expectation from Question #6 about the sign of the meridional wind at the tropical tropopause and surface borne out by the observations? As you move from the tropics to midlatitudes, what happens to the meridional wind at the surface? At the tropopause? As you cross the equator into the Southern Hemisphere tropics, what happens to the meridional wind at the surface? Make a note of each of these features for future reference.

    Part IV: Physical Experiment

  9. All atmospheric and oceanic movement we observe on the Earth is observed not from a stationary vantage point, but from one that is moving. The coordinate system itself is rotating with the Earth, giving the coordinate system acceleration. The most satisfactory way to include the effects of coordinate acceleration is to introduce "apparent" forces in Newton's second law of motion. The Coriolis force is one of these apparent forces. The Coriolis force is really the phenomenon that makes fluid motion on the Earth sophisticated. Under its influence a number of interesting phenomena occur on our planet that are beyond our imagination at first glance.

    In this experiment, we will try to set up a system that allows us to see rotational dynamics at work. We take a cylindrical fish tank, filled with clear water. In the center of the tank we place a thin cylinder, sealed at the bottom, filled with red-colored fluid that is denser than the surrounding fluid. If we then remove the seal at the bottom of the cylinder, thus allowing the two fluids to come into contact, what happens? Observe how the water moves from the cylinder to the rest of the tank and how long it takes to reach a final equilibrium. Sketch what the pressure gradient force would be at the moment the bottom seal is removed and the fluid of different densities come into contact. No, you will not "see" the pressure gradient force, but knowing the densities of the two fluids, you should be able to sketch it.

    Compared with real atmospheric circulation, the scale of motions in the tank is so small that the Coriolis force does not matter much. The previous experiment therefore gives you an idea of how atmospheric or ocean circulation might behave on a non-rotating planet. But what will happen on one that is rotating? Let's repeat the previous experiment on a rotating table. The rotation of the table introduces the "Coriolis effect." What will happen this time? Observe the differences and sketch the final equilibrium state as well as the pressure gradient force. If you have any background in mechanics or physics, think about the force balance.


Lab Report Instructions

  1. Write a lab report (as per the Lab Report Format) summarizing the major findings of your investigation. Use the questions posed in the lab instructions as a guide for a scientific text that describes the data fields you are observing and the connection between them and the material you studied in class. Incorporate your answers to the following questions into your lab report:

    1. Are summer-winter temperature differences (an example of a natural, rather than an anthropogenic, climate change) generally larger in the Northern or Southern Hemisphere? What is the major geographic difference between the two hemispheres, and how might that explain this result? Now consider current attempts to detect the signature of global warming due to increasing greenhouse gas concentrations; the greater (and faster) the warming, the easier it is to detect. Should we expect to see the clearest evidence of global warming in the midwest U.S. and central Asia, in coastal locations such as NYC, or out over the open ocean? Explain.

    2. Consider the qualitative difference between the lower troposphere lapse rate in January at the North Pole vs. midlatitudes and the tropics. What major thing (aside from the extreme cold) differentiates the North Pole in January from lower latitudes? How might this cause the observed difference in lapse rate? (Hint: Why is the surface in general the warmest part of the atmosphere? What would happen to the surface if the source of this warmth were removed?)

    3. Consider the locations of sign changes of the mean meridional wind with latitude near the surface (in both hemispheres), and the sign of the tropopause meridional wind in the Northern Hemisphere tropics. Make a schematic pressure- latitude cross section of the Northern Hemisphere tropics and midlatitudes, indicating by arrows the direction of the meridional wind in the different locations. At what latitudes does the surface meridional wind converge? Diverge? What about the tropopause meridional wind? What must be the sign of the vertical velocity between the surface and tropopause at these latitudes? Indicate this by arrows as well. Following the arrows, will a parcel of air originating near the equatorial surface eventually return to its point of origin? Considering the effect of rising and sinking motion on the saturation of a moist air parcel, which latitudes do you expect to be cloudy and rainy? Clearer and drier? How does this correspond to the appearance of the satellite images and precipitation maps we saw in lecture?