Lab Exercise: The Earth’s Radiation Budget: Balancing Your Heat Book
In this lab, students learn how the Earth's radiation budget works and the ways certain events or forces impact that budget. Some radiation from the sun is reflected back into space (shortwave) and some of it is absorbed. The absorbed energy warms the Earth's surface (longer wavelength). This process of absorption, reflection, and reemission establishes the global energy balance, which is fundamental to Earth's climate system.
Context for Use
Description and Teaching Materials
The structure and primary components of this lab lesson is sourced from Columbia University's Earth Environmental Systems Climate (EESC) course (Spring 2011). The instructor/TA will need to be familiar with the introductory content of the lab. There are a number of terms that need to be understood before the computer lab begins (see the first section below for a review of key concepts/terms).
I. Background Information
A. How the data were collected
The Earth Radiation Budget Experiment (more info) (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 has employed three satellites to collect this information: ERBS, NOAA-9, and NOAA-10. Each satellite was equipped with special instruments (scanners) that measured radiation. Radiation was measured in three wavelength bands:
· Total: radiation in the 0.2 to 50 micron (mm; 1x10-6 meters) wavelength band.
· Longwave: radiation in the 5 to 50 mm wavelength band.
· Shortwave: radiation in the 0.2 to 5 mm wavelength band.
Technical information about the scanners and other information about the experiment can be found at the following NASA web sites.
B. The structure of the ERBE dataset, and how to access it
The ERBE data available from the RI/LDEO Climate Data Library, contains information from all three ERB satellites and their combinations (for the periods when the satellite provided overlapping observations). The data are organized by satellite and 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°, making a grid with144 longitude points and 72 latitude points. 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 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):
· clear-sky: Satellite measured radiation averaged only from satellite views that were free of clouds.
· cloud-forcing: The difference between clear-sky and cloudy-sky radiation, showing how radiation at the top of the atmosphere differs in the presence or absence of clouds.
· total: Satellite measured radiation averaged over an entire month regardless of cloud coverage.
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:
· albedo: The ratio between the shortwave radiation reflected from Earth and what is coming in from space (this is a unit-less number expressed in percent).
· longwave radiation: The longwave radiative flux emitted from Earth (in W/m2).
· shortwave radiation: The shortwave radiative flux reflected from Earth (in W/m2).
· net radiation: The difference between the shortwave radiative flux absorbed by the Earth climate system and the longwave radiation emitted into space (in 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.
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 New Views link to access the NASA ERBE Climatology clear-sky albedo data.
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 there are not enough cloud-free observations to create a reliable average.
All ERBE data are taken from January to December for annual variations to be apparent in the measured variable. 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.
Task 1: Create albedo maps for January, March, July and September.
· Spatial patterns: Which regions have > 30% albedo, and which regions have < 30% albedo?
· Patterns through time: In a few sentences, describe how the albedo varies seasonally by comparing data for the four key calendar months.
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. Unlike albedo, this is an absolute measurement and not a ratio. Thus, the albedo can be where the actual reflected radiation is low.
Task 2: Create 4 maps for reflected SW radiation. Briefly compare January albedo and January SW maps. Focus on Antarctica and northern Europe, Asia and North America. Is high albedo always accompanied by high outgoing SW? If not, where?
C. Total Incoming Radiation
There isn't an ERBE data set of total incoming radiation received at the top of the atmosphere. However, if the total amount of short wave radiation that is reflected and the albedo is known the amount of incoming solar radiation can be calculated. How is this done? Remember that:
Albedo = (reflected solar radiation) / (incoming solar radiation)
This implies that: (incoming solar radiation) = (reflected solar radiation) / albedo
Review results of from the browser interface and the maps illustrating results. Note that the lines of equal radiation are all straight and paralle to lines of latitude.
Task 3: Examine the plots of incoming solar radiation in March, June, September, and December to see the changes through time. In two sentences, describe how the incoming solar radiation varies with A) Latitude, and (B) Seasons.
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 that 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.
Study the longwave maps provided for January, March, July and September (see accompanying pdf file).
· In no more than 3 sentences, describe the regions on the globe that emit the least and most longwave radiation (you can be general here – wide latitude bands, "desert," "Northern Africa," etc.).
· Look at your January map, at 40˚N latitude. Compare emitted longwave radiation for oceans and continents (North America is the best example in this case). Also broadly compare Northern and Southern hemispheres – which emits more longwave?
· In two sentences, describe the outstanding changes you see in the 4 maps that occur as you move through the year.
1. Why does reflectivity vary across latitudes, and within continents such as Africa and North America?
2. Some regions (like Greenland) have high albedo, but low outgoing shortwave radiation. Why?
3. Why does incoming radiation vary with latitude and time of year?
II. Hands-on Experiments
Experiment A: Measuring Incoming Solar Radiation as a Function of Latitude
In this experiment, students 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. 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 students will demonstrate this effect in a simple laboratory setting.
The experiment equipment consists of a circular globe section, a solar cell mounted on this globe section, and a current meter connected to the solar cell. Set the digital current meter to mA (milliamps). The solar cell generates electrical energy proportional to the amount of light it receives. The class 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.
· 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 (about 10 – 15 data points are good). Report these in a table.
· 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).
1. What kind of a trigonometric function should this curve trace? Does it? If not can you explain the source of errors?
Experiment B: Albedo
This experiment should give students an appreciation for where on Earth albedo might be high or low.
Students 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 on the table below the photometer and measure the current output of the photometer (use the unpainted back of the granite colored metal panel to do this).
Students assume that the aluminum perfectly reflects all the light shone upon it and thus the photometer picks up all the light emitted by the source. This is therefore a case of perfect reflectivity, or an albedo of 1 (100%). Write down the corresponding current output by the photometer. Now replace the plain aluminum panel with the white painted panel. Write down the current. The ratio between the current now and that recorded by the bare aluminum is the albedo of the white panel. Any light lost is either absorbed by the white panel or scattered such that it does not reach the photometer. The albedo is the percentage of energy that is not absorbed or scattered.
Measure the albedo for white panel and the remaining five colored mental panels (black, tan, green, blue, and granite).
· Make a bar graph of albedo vs. color using Excel.
· In one or two sentences, compare the albedos of the black and white panels.
DISCUSSION (In 1/3 of a page or less – be concise)
1. What regions or materials on Earth might be represented by the six painted panels?
2. Do you think there are any significant regions on earth that are not represented by these six colors?
3. Are your answers to 4 and 5 supported by the albedo values from the ERBE data that you examined? Do they make sense?
Below are the links for source material and resources:
· EESC course page:
· http://eesc.columbia.edu/courses/ees/climate/syllabus.html (updates forthcoming for publicly accessible site)
· Computer lab or moveable laptops with Internet access and Excel.
· LCD projector
· Handouts - lab instructions
· "Writing a Lab Report" (may have already been disseminated)
· Circular globe section,
· Solar cell
· Current meter
· Desk lamp
· Metal desk
The Earth's Radiation Budget (LAB)
The complete module in a Word version
Radiation - Maps
Maps of incoming solar radiation during different periods; to accompany the data lab portion