Lab Instructions: The Earth's Radiation Budget, Part I.

I. The Data.

  1. How the data were collected.

    The Earth Radiation Budget Experiment (ERBE) was designed to collect information about sunlight reaching the Earth, sunlight reflected by the Earth, and heat released by the Earth into space. Since October 1984 ERBE employed three satellites to carry the instruments which collected this information: ERBS, NOAA-9, and NOAA-10. Each satellite was equipped with special instruments (scanners) that measured radiation along the satellite track and from space. Radiation is measured in three wavelength bands:

    Technical information about the scanners and other information about the experiment can be found in the following NASA web sites.

  2. Structure of the ERBE dataset.

    The ERBE data available from the IRI/LDEO Climate Data Library contains information from all three ERB satellites and their combinations (for the period when the satellite provided overlapping observations). The data are organized by satellite, and by variable.

    Open the ERBE dataset. (Note that you just opened a new browser window. Please move that browser aside so you can continue to access it later).

    As indicated above, the ERBE data include radiation measured from Earth in the short-wave (solar) and long wave (Earth) radiation bands. These data are processed by month for the duration of the satellite flight, and are provided on a grid of latitude and longitude lines. On this grid, longitude varies from 1.25°E to 1.25°W by intervals of 2.5°, and latitude varies from 88.75°N to 88.75°S by intervals of 2.5°. Thus there are 144 grid points on each latitude and 72 latitudes overall. You can read the information on the time and space grids when you click on a satellite name in the viewer. For example, in ERBE dataset page you opened earlier, click on the link Climatology . This is a time averaged set created by using data from the NOAA 9 and NOAA 10 satellites. Each calendar month was averaged for four full years of available data (February 1985 to January 1989).

    The Climatology dataset is divided again into three data types (as are all other ERBE datasets as well):

    For each of these data types, the "data tree" branches off further, as you can see by clicking on their links. For example, on the NASA ERBE Climatology page, click on clear-sky. Now you can see the different variables measured by the satellites, and provided by NASA in the ERBE dataset:

    All variables are given in units of W/m2.

    Also on this page (titled NASA ERBE Climatology clear-sky), under the section Grids, you can find the Latitude and Longitude grid information described above. Note that the Time grid for this dataset is the period of overlap between the two satellites, NOAA 9 and NOAA 10.

    Click on albedo. Notice that the page no longer contains dataset links. You are now ready to access the actual albedo data month by month and to view them using the different buttons on the page.

    The same set of variables are given in the total dataset but the variable list under cloud-forcing is somewhat different. In the next section, we will work with the different data set to study the details of the Earth Radiation Budget.

    Click on the Views link to access the NASA ERBE Climatology clear-sky albedo data.

II. The Assignment.

  1. Incoming Solar Radiation.

    1. Calculating Incoming Solar Radiation at the Top of the Atmosphere.

      Before we play with physical models or look at the data, it will be useful to use what you already know to calculate the incoming solar radiation at the top of the atmosphere.  This will give you an understanding of the theory behind the globe experiment.  We will do this for noon at an equinox and the summer and winter solstice. This is easy to do with EXCEL. So launch EXCEL and in the first worksheet label the first column "latitude." In the following rows, enter the numbers "-90" to "90" in increments of 2.5. These numbers will represent the latitudes of the earth from the south pole to the north pole.

      During an equinox neither the northern nor the southern hemisphere is tilted toward or away from the sun. If the earth were a flat surface, the amount of radiation at the top of the atmosphere would be a constant 1367 Watts/m2. However, due to the curvature of the earth, this radiation is incident on larger and larger areas as the latitude increases. Therefore, as the latitude increases, the amount of radiation incident at the top of the atmosphere decreases proportional to the latitude. Based on the diagram below, use Excel functions to calculate the dependence of the incoming solar radiation on latitude. (Hint: you may need to convert your latitudes into radians to use the excel functions.)

      During the solstice, the earth is inclined 22.5 degrees with respect to the earth-sun plane. Therefore, during the northern hemisphere summer solstice, for example, the incoming radiation will be strongest at 22.5 north! In two new columns in your Excel spreadsheet, add a correction to your calculations which accounts for this effect during the northern hemisphere summer and winter solstices.

      Create an excel scatter plot which shows the equinox, summer and winter solstice TOA radiation values versus latitude. Remember to include units and labels on the axes! Explain the differences between the lines.

    2. Measuring Incoming Solar Radiation as a Function of Latitude.

      This experiment tries to mimic conditions found on Earth.  The angle of incoming solar radiation strikes the Earth's surface at a 90 degree angle only at one latitude at a time.    Thus for most places on the Earth presently in daylight, sunlight is not striking perpendicularly, but at some oblique angle (the angle is 0 degrees right when the sun drops below the horizon).  This should be no surprise after completing the previous section.  You know from experience that this affects the ground temperature, as for the most part, the warmest part of each day roughly coincides with when the sun is most directly overhead (i.e. closest to a 90 degree angle).  Now you will find out why this is true.

      The experiment consists of a globe, a solar cell mounted on the globe, and a current meter connected to the solar cell.  We will be able to tell how much short-wave (visible) energy is reaching the solar cell by looking at the current the solar cell produces when it is inundated with light.  Start by turning the globe so that the solar cell is exactly perpendicular to the line between the light source and the globe.  This is analogous to standing on the Equator at noon on either equinox, or standing at 22.5 N on the summer solstice.  Write down the current shown on the voltmeter.  Now move the globe so that the solar cell moves to various "latitudes."  This will be analogous to standing anywhere but directly under the sun.  For instance, if you move the solar cell to 40 degrees north latitude, that is analogous to measuring the angle of the sun while standing  in Manhattan at noon on the equinox.

      Write down the currents for various latitudes.  It doesn't matter the exact "latitudes" you use, as long as the Equator is one latitude and you find a range or values.  Enter your values in Excel and make a plot of current (analogous to solar intensity) vs. latitude.  Make sure you label your plot well!!  How does this plot compare with the plot you made in part A1?  Whether or not they compare well is not the point, rather how you explain the similarities or differences is of most importance.  If you expected them to compare well and they did not, or vice versa, explain that as well.

  2. Clear-Sky Albedo and Reflected Short Wavelength Radiation.

    1. Clear-Sky Albedo.

      You must read the data section above before you start this section.  The methods here assume you are already on the data web site arrived at as you walk through the data set in the outline above.

      Albedo is the term used to describe the reflectivity of a non luminous surface. Values range from 0 for total absorption (a black body) to 1, total reflection. It is usually expressed as a percent. The Earth for example has an albedo of about .35, or 35 percent. So 35 percent of the solar radiation intercepted by the Earth is either reflected or scattered back to space.

      Clear sky albedo is the light reflected back only from cloudless areas of the Earth's surface. If the maps you are examining of clear sky albedo have white patches, these patches are areas continuously covered by clouds during the time the data was being gathered.

      Go to the open viewer window displaying the NASA ERBE Climatology clear-sky albedo data. These data run through the calendar months from January to December. Use the pop up menus to look at the data as colored contoured values, outline the continents by "drawing coasts," and then set the range of the albedo from 0 to 90. Click the "Redraw" button.

      Study this data set by moving month by month through the year. Note which parts of the Earth's surface are highly reflective and which are not (these latter are strong absorbers of solar radiation). Can you explain the variations of reflectivity from high to low latitudes, and within continents such as Africa and North America. Note how the albedo varies seasonally, are these variations in step with changes in incoming radiation?

    2. Short Wavelength Solar Radiation Reflected from the Earth.

      Reflected short wavelength radiation (sw) is a direct measurement of short wavelength radiation reflected from the Earth's surface and is expressed in watts/meter squared. Albedo, as you will remember, is the percent of the total incoming radiation that is reflected or scattered back to space.

      Now look at Climatology clear-sky short-wave radiation so you can compare the short wavelength radiation data set with the albedo data set you have been studying. As before, outline the continents, set colored contours, and set the short wavelength range from 0 to 400. Compare the albedo and short wavelength radiation data sets for the months of January through March. Note how the data sets differ for any given month and how they change with the progression from January to March. Can you explain these differences?

  3. Irradiance.

    1. Albedo.

      It is one thing to think about the concept of albedo or to look at a dataset of albedo values of the Earth.  It is another to gain an appreciation of albedo by performing experiments.  This experiment consists of the same materials as the globe experiment, expect without the globe.  Now we want to know what albedo really is.  This should give you an appreciation for where on Earth albedo might be high or low.

      We mount the solar cell about a half meter above the table, with the light source also a half meter above the table.  We will use a mirror on the table surface to find the intensity of light emitted by the light source by measuring the current output of the solar cell.  We are making the assumption that the mirror perfectly reflects all the light shone upon it and thus the solar cell picks up all the light emitted by the source.  This is a case of perfect reflectivity, or an albedo of 100%.  (Naturally, a case of perfect absorption would be an albedo of 0%.)  Note the current output by the solar cell with the mirror in place.  Now replace the mirror with a plain white piece of paper.  Write down the current and divide by the mirror current.  The result is the albedo of the white paper.  Why?  Because we know how much energy the light emits (measured by the solar cell current as the light was reflected in the mirror) and we can measure how much energy the solar cell picks up if the light must first reflect off a surface that does not reflect 100% like the mirror.  Any light lost is either absorbed by the paper or scattered.   The albedo is thus the energy not absorbed or scattered.

      Do the same for paper of various colors and even other objects if you want.  Enter your results in Excel and make a bar graph of the various papers and materials. If you are already thinking ahead, you might guess that snow has a similar albedo to the white paper.   How does dark paper compare to white paper?  Did you see this difference in the data in Part B2 above?  What materials on Earth might have large or small albedos.

    2. Radiative Energy Flux.

      You have learned in lecture yesterday that the radiation received at a point depends upon the square of the distance from the source of the radiation. The equation for this is I(r) = I(s)/r2 where I(r) is the intensity at a distance r from the source and I(s) is the intensity at the source. But you shouldn't just believe everything you read, so now you're going to try to confirm that equation. The experiment is really easy: using the setup from the albedo experiment, just measure the current at various distances from the light source. Make a graph of your results. Did it work? Does your graph look like the graph of a exponential function, or that of something else?

  4. Clear-Sky Long Wavelength Radiation.

    This last data set is Earth radiation, the result of the absorption of solar radiation at the Earth's surface heating the Earth, and the Earth then radiating to space. Since the Earth is cool relative to the sun its radiation to space peaks in the infrared (long wavelength) band of the electromagnetic spectrum. Geographic variations in this data set are therefore a result of temperature differences at the Earth's surface. Go to the NASA ERBE Climatology data page again and select the clear-sky long wave radiation data. Remember to "draw coasts," select "colors | contours," and click "Redraw." Study the data month by month through the course of the year. Note the areas of each hemisphere that emit the least and most long wave radiation. Use the Stephan Boltzman relationship to convert these minimum and maximum radiation values to temperatures first in Kelvin, then in Celsius. Note, as you move through the year, how the radiation from continents and oceans varies, and how these variations compare between the two hemispheres.


Lab Report Instructions

Write a lab report (as per the Lab Report Format) summarizing the major findings of your investigation. Don't forget to answer the questions that were asked in each section. Incorporate your answers to the following questions into your lab report:

  1. Your calculation of the amount of seasonal incoming solar radiation tells a lot about the reason for a thermal (temperature) gradient between the pole and the equator but it is not the whole story. From your observations of data sets in this lab and the physical experiments, provide a fuller explanation. Think about why the north polar regions are not as warm or warmer in July than the tropics.

  2. As you have seen, the seasonal patterns of incoming solar radiation are very simple while the patterns of albedo, reflected short wavelength radiation and long wavelength Earth radiation are much more complex due to the properties of the Earth's surface. From your observations, describe how the properties of the Earth surface modify the simple pattern of incoming radiation.

  3. How might modifying these properties cause climate change both regionally and globally?