The Earth's Radiation Budget, Part I.

I. Introduction

The global energy balance is important for Earth's climate. When visible radiation from the Sun reaches the Earth, some of it is reflected or scattered directly back into space as shortwave radiation (the percent reflected is known as albedo) and some of it is absorbed. In the absence of clouds, absorption happens mainly at the surface. The absorbed energy warms the Earth's surface, which, in turn, emits this energy at a longer wavelength (infrared rather than visible light). Of course, clouds complicate the balance, so we will address them next week. Today, we start with a simplified view.

The purpose of this lab is to get you thinking about the Earth's radiation budget and the sorts of phenomena which may influence that budget.

A. 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.

  1. The Earth Radiation Budget Experiment.
  2. The NASA Educational Resources website - the Trading Card page (click on radiation budget).
  3. JPL Quick-Look at ERBS site.
  4. A NASA Fact Online ERBE page.

B. The structure of the ERBE dataset, and how to access it

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 shortwave (solar) radiation reflected by the Earth's surface and longwave radiation emitted by the Earth. 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 the 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:

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.

More information about the ERBE dataset can be accessed by clicking on the "NASA ERBE documentation" link in the blue IRI box in the upper left corner of the browser window.

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 is given in the total dataset but the variable list under cloud-forcing is somewhat different. In the next section, we will work with the latter data sets to study the effects of clouds on the Earth radiation budget.

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

II. Computer Lab

A. Clear-Sky Albedo

The instructions below assume you arrived at the radiation budget data web site following the instructions above.

Go to the open viewer window displaying the NASA ERBE Climatology clear-sky albedo data. Clear sky albedo is the light reflected back only from cloudless areas of the Earth's surface (this is calculated by ERBE scientists by identifying cloud-free regions during each satellite's observations and averaging their data separately). If the maps you are examining of clear sky albedo have white patches, these patches are areas so often covered by clouds that we do not have enough cloud-free observations to create a reliable average.

All ERBE data run through the calendar months from January to December so that annual variations in the measured variable can be appreciated. Use the drop-down 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.

Task 1: Study the albedo data by concentrating on the months of January, March, July and September.

  1. Identify the latitudinal and longitudinal boundaries of those parts of the Earth's surface that are highly reflective and identify those that are not (implying they are strongly absorbing the solar radiation). (Results)
  2. Describe how (Results) and why (Discussion) the albedo varies seasonally by comparing data for the four key calendar months.
  3. Explain why there are variations of reflectivity from high to low latitudes and within continents such as Africa and North America. (Discussion)

B. Short Wavelength Solar Radiation Reflected from the Earth

Reflected short wavelength radiation (SW) is a direct measurement of short wavelength radiative flux reflected from the Earth's surface and is expressed in watts per meter squared. Unlike albedo, this is an absolute measurement and not a ratio. Thus, the albedo can be high where the actual reflected radiation is low.

To appreciate the difference, compare the fields of Climatology clear-sky shortwave radiation for January with the abedo data for the same month that you have been studying earlier.

Task 2: As before, outline the continents, set colored contours, and set the short wavelength range from 0 to 400. Look at SW and albedo over Antarctica and northern Europe, Asia and North America. These locations have high albedos but the clear-sky short wave radiation values there are very different. Why is this?

C. Total Incoming Radiation.

We do not have an ERBE data set of total incoming radiation received at the top of the atmosphere. However, if we know the total amount of short wave radiation that is reflected and we know the albedo we can calculate the amount of incoming radiation. How would we do this? Remember that:

albedo = (reflected solar radiation) / (incoming solar radiation)

This implies that:

(incoming solar radiation) = (reflected solar radiation) / albedo

We have used the browser interface to perform this calculation. Look at total incoming radiation to view incoming solar radiation in units of W/m2. Notice that a small window opened above the row of "view" icons. In it is a list of instructions to the program that accesses the data, telling it to divide the reflected solar radiation by the albedo. Set the viewer such that: you see the outline of continents; the incoming shortwave radiation scale runs from 0 to 520; and the data are displayed contoured and colored. Click "Redraw". Note that the lines of equal radiation are all straight and parallel to latitudinal circles.

Task 3: Examine the plots of incoming solar radiation in March, June, September, and December to see the changes between seasons.

  1. Write a paragraph or two on how (Results) and why (Discussion) incoming radiation varies only in latitude and time of year.

D. Clear-Sky Long Wavelength Radiation

This last data set is for the radiation that the Earth emits in response to being warmed by the Sun. Since the Earth is much colder than the sun, its radiation to space peaks in the infrared (long wavelength) band of the electromagnetic spectrum. Because some components of Earth's atmosphere trap longwave radiation (the greenhouse effect), emission to space occurs not at Earth's surface, but at a higher level in the atmosphere which varies depending on the concentration of greenhouse gases (mainly water vapor) at that location.

Geographic variations in this data set are a result of differences in the effective temperature (the temperature at which the planet is emitting radiation to space) at various locations. Effective temperature depends both on the temperature at the surface, and on the concentrations and vertical profiles of greenhouse gases.

Task 4: Go to the NASA ERBE Climatology data page again and select the clear-sky longwave radiation data. Remember to "draw coasts," select "colors |contours," and click "Redraw." Study the data for January, March, July and September.

  1. Write down the areas of each hemisphere that emit the least and most longwave radiation. (Results)
  2. Use the Stefan Boltzmann relationship (I = σT4 where σ = 5.67*10-8 Wm-2K-4) to convert the minimum and maximum radiation values (for the northern and southern hemispheres in January and July) to temperatures first in Kelvin, then in Celsius (°C = K - 273.15). (Results)
  3. Describe the outstanding changes that occur as you move through the year. (Results)
  4. Summarize how the radiation from continents and oceans varies, and how these variations compare between the two hemispheres, based on your results from part a. Remember that these changes are due to changes in surface properties and water vapor content of the atmosphere – the effect of clouds is excluded. (Discussion)

III. Hands-on Experiments.

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

In this experiment, you will try to demonstrate the effect of Earth's spherical shape on the change of solar radiation with latitude. In most places on the Earth, sunlight does not strike perpendicularly to the surface, but at some oblique angle, even at local noon. Do you know where perpendicular radiation does occur at local noon? You know from experience that this affects the ground temperature. In general, the warmest part of each day occurs when the sun is most directly overhead (i.e. closest to a 90 degree angle). Now you will demonstrate this effect in a simple laboratory setting.

The experiment equipment consists of:

Set the digital current meter to mA (milliamps). The solar cell generates electrical energy proportional to the amount of light it receives. We will be able to tell how much short-wave (visible) energy reaches the solar cell by looking at the strength of the electrical current.

Start by turning the globe so that the solar cell is exactly perpendicular to the line between the light source and the globe section. This is analogous to standing on the Equator at noon during equinox (or to standing at 23.5°N during the summer solstice). Write down the current shown on the current meter. Now rotate the globe section 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 41 degrees north latitude, that is analogous to measuring the angle of the sun while standing in Manhattan at noon on the equinox.

  1. Write down the currents for various latitudes. It doesn't matter which "latitudes" you use, as long as you include the equator and you find a range of values. (Results)
  2. Enter your values in Excel and make a plot of current (analogous to solar intensity) vs. latitude. Make sure you label your plot well (that is, give your graph a title and label the axes). (Results)
  3. What kind of a trigonometric function should this curve trace? Does it? If not can you explain the source of errors? (Discussion)

B. 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 should give you an appreciation for where on Earth albedo might be high or low.

We mount a photometer about 20 cm above the table pointing down. The light source is a desk lamp, which also is pointing down 20 cm above the table. Make sure that the reflected light from the light source can reach the photometer. To find the intensity of light being reflected, we place a piece of aluminum foil on the table below the photometer and measure the current output of the photometer.

We are making the assumption that the aluminum foil perfectly reflects all the light shone upon it and thus the photometer picks up all the light emitted by the source. This thus, is a case of perfect reflectivity, or an albedo of 1 (100%). Write down the corresponding current output by the photometer. Now replace the aluminum foil with a plain white piece of paper. Write down the current. The ratio between the current now and that recorded by the aluminum foil is the albedo of the white paper. Any light lost is either absorbed by the paper or scattered such that it does not reach the photometer. The albedo in this case is the percentage of energy that is not absorbed or scattered.

Measure the albedo for white paper and different color papers, and make a bar graph of albedo vs. color using Excel.

  1. How does dark paper compare to white paper? (Results)
  2. Did you see similar differences in albedo in the ERBE data? (Discussion)
  3. What materials on Earth might have large or small albedos? (Discussion)

C. Radiative Energy Flux.

You have learned that the radiation received at an arbitrary target depends upon the square of the distance between it and the source of the radiation. The equation for this is:

I(r1) = I(r2) (r22 / r12)

where I(r1) is the intensity at a distance r1 from the source and I(r2) is the intensity at a distance r2 from the source.

Now you're going to try to confirm that equation by a simple experiment. The setup is similar to the albedo experiment except that you need to mount the photometer horizontally and the desk lamp needs to point sideways shining light directly into the photometer. You should set the photometer to the less sensitive setting. Then measure the current at various distances from the light source.

Make a graph of the current at various distances from the light source.

  1. Does your graph look like the graph of the above equation (that is, showing a 1/r2 relationship)? (Results)
  2. Assume that your mesurements I(r2) are always at a distance r2 and calculate what the desk lamp's radiative energy flux I(r1) at a distance of r1 = 1 cm would be for all cases. I(r1) should be a constant dependent only on the desk lamp's physical properties. Is it constant in your case? (Results) If not, what could be the sources of error? (Discussion)

IV. References

V. Lab Report Instructions

Write a lab report (as per the Lab Report Format) summarizing the major findings of your investigation. When writing your lab report, consider your answers to the three tasks specified in the data viewing section and address the questions posed below:

Over time (at least a full year), the amount of incoming radiation has to be balanced by the outgoing radiation (longwave emission plus shortwave reflection). Otherwise, the Earth as a whole would either warm or cool until a balance is reached. Yet, in today's lab, when we examined the distribution of incoming, reflected, emitted, and net radiation, it was clear that locally and monthly, there is no balance between incoming and outgoing radiation.

  1. What do you expect are the implications of this imbalance? Consider the implication of heat coming in at an excess in one location compared to another location; for instance, in a room being heated in winter by a hot water radiator on one end while the windows and wall on the other end are loosing heat to the outdoors. (Discussion)
  2. Does the Earth as a whole have to balance the radiation coming in from the sun on a seasonal basis or even over a few years? Consider the effect of heat storage within a system and try to identify parts of the climate system where heat can be stored below the surface from one season to another or even over a number of years. As we shall see later in this course, heat storage in the climate system leads to interesting annual and interannual climate variations. (Discussion)

Updated July 9, 2007
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