Lab Instructions: The Earth's Radiation Budget, Part II.
Go to the window that links you to the total fields. Look at the January map of total albedo (adjust your graphical interface so continental outlines are shown and the colored values are contoured). Access clear-sky albedo from last week's lab on your other opened window. Note which areas have persistent cloud cover (high albedo) and which areas of the Earth are generally clear of clouds. Move through February and March to April and note if the patterns change and how they change if they do. Note how the total albedo changes as you move through May and June to July. Pay particular attention to the northern Indian Ocean and South Asia. Compare the July albedo in this area with February. Can you explain the changes you observe? Also pay attention to the eastern sides of the North and South Pacific.
In January which hemisphere reflects the most of the short wavelength energy? Note again the persistent belt of clouds along the equator reflecting short wave length energy. Look at the month of March, do you think clouds or the Earth's surface accounts for most of the Earth's reflected energy? Note again where clouds are and are not in the belt between 55 degrees north and 55 degrees south.
We can calculate the globally averaged amount of reflected solar radiation in the clear-sky case and in the total. To do that go back to the point where you were viewing the data (just before you clicked on one of the small figure to view the field of total shortwave radiation). Click on the expert mode link. Let us first average all the data over time, to look at the annual average. In the expert mode window type:
[T] average
This tells the software to average the data over all time points. Each point in space is averaged separately. Then type:
Y cosd mul
This multiplies every grid point in space by the cosine of the latitude angle (Y is the latitude angle in degrees, and cosd is a calculation of cosine when the angle is given in degrees). We need to do that so that our grid points will be properly weighted with respect to the geographical areas they represent (why is that necessary?) Then do:
[X Y] average
and click the OK button to the right of the expert window. The viewer will return a single number just below the expert window (in bold letters). That number is the amount of reflected shortwave energy averaged over the entire globe in W/m2. Do the same operation with clear-sky shortwave radiation and compare the numbers.
What latitudes in January are strong radiators of infrared Earth radiation? Compare the January map of long wavelength Earth radiation with the January reflected short wavelength solar radiation. How did you interpret the reflective zone along the equator? Compare the longwave radiation along the equator with that between 10 and 30 degrees on the eastern sides of the oceans (particularly the Pacific). These regions, generally referred to as the subtropics, have similar albedo (confirm) yet emit quite different longwave radiation. Why do you think the equator doesn't radiate as much long wavelength radiation as the zones between 10 and 30 degrees north and south latitude? (Hint: remember the intensity of the longwave earth radiation will be a function of the absolute temperature of the radiating surface from which it is emitted).
The net radiation is the difference between the radiation coming into the Earth from the sun and the energy radiated by the Earth to space. For the planet as a whole what comes in must equal what goes back out, however more comes in the tropics than goes out in these latitudes and more is radiated from the higher latitudes (north and south of the tropics) than comes in. this difference provides the energy to drive the circulation of the atmosphere and ocean.
We can use ERBE data to check if the Earth as a whole maintains a radiative balance. Calculate the annual and global average of net radiation using the expert mode and look at the value. It is not exactly zero probably due to measurement errors and incomplete coverage.
Using the clear-sky and total datasets, ERBE scientists calculated the effect of clouds on the radiation budget. These fields are available under the cloud-forcing� link in the Climatology dataset (try to get there on your own in one of the other open windows). The fields in this dataset show how much clouds affect the amount of radiation emitted into space by comparing the data from the same locations during cloudy and non cloudy days (in some places where clouds are either present all the time or never present this calculation can not be done, of course). To make the study of these fields easier, lets look at ANNUALLY AVERAGED data. So each time we link to a specific variable we will go to expert mode and add to the code the lines:[T] averageand also
X Y 1 SM121and click OK. The second line adds a bit cleaner look to the data by smoothing it in space.
The longwave fields here show the amount by which clouds reduce the outgoing longwave radiation. Positive values mean less longwave cooling of the planet. Open the cloud forcing longwave field, calculate the annual average as above, and look at the tropics over land and over the western Pacific. Compare these areas to the eastern subtropical Pacific and Atlantic oceans. The difference between these regions comes from the different types of clouds. Along the equator cumulonimbus* clouds reach deep into the top of the troposphere and their tops are cold while subtropical stratocumulus* clouds are shallow and their tops are warm (can you explain that in terms of atmospheric stability?).
*For more information about clouds and their types, and for pictures of what they look like, you can link to: University of Illinois WW2010 or the Cloud Atlas provides by Prof. Houze at the University of Washington.Cloud free areas in the subtropics emit the largest amounts of longwave radiation. Can you identify these regions? Are they distinguishable from the stratocumulus covered areas in the eastern subtropical oceans? Compare the Sahara to the eastern subtropical Pacific near California and Mexico. The Albedo field clearly shows the difference between these two regions and yet the cloud effects on longwave radiation are similar.
Now examine the shortwave cloud forcing. Here the fields show what clouds add to the solar radiation available to Earth (negative values mean that clouds reduce the amount available to earth). These fields are mostly negative (clouds reflect the solar radiation). There are some some small regions near the poles where shortwave cloud forcing is positive, what could be the reason for that?
Calculate the annual averaged field. Can you see a general difference between the oceans and the land areas? Which is more cloudy in general? Identify the cloud free regions over the central subtropical regions. Why are these areas cloud free?
Move to the last dataset: net. Here you can appreciate the fact that overall clouds cool the Earth's climate, in particular over the oceanic regions in middle and high latitudes. This seems to be an expected result, but not so obvious because clouds have a strong warming effect as well (see below). In the polar regions and over the tropics clouds either do not affect the radiative balance or even have a small positive effect. This is particularly interesting in the tropics, where there are regions with extensive cloud coverage (cumulonimbus clouds). The reason for that is the cancellation between the longwave and shortwave effects in deep tropical clouds.
Physical experiment:
Test the Stefan Boltzmann Law:
From the class lecture, you saw the Stefan-Boltzmann law. However, before you take it for granted that the law works, it may be better to test the law in an experiment. Hopefully, you will be convinced that the law works.
In this experiment we will use a lamp, a thermometer and two pieces of paper, one black and one white, both of which can be treated as black bodies. We will do the experiment on the two papers at the same time under the same conditions. Tape the thermometer sensors to the pieces of paper and turn the lamp on. You will find that the temperatures immediately rise. Why? After a few minutes, the temperatures stop growing and stay constant, which means the papers have reached an equilibrium state. Now write down the temperatures. Why is there a big difference? Use the albedo values you measured in the first lab and the Stefan-Boltzmann law to calculate the energy emitted by the lamps. Are they the same?
Radiation spectrum from different light sources:
According to the l~1/T relationship, the energy emitted by different light sources, which have different temperatures, are concentrated on different wavelengths. A spectroscope is a device that allows you to see the spectrum of the incoming lights. Point the spectroscope at different light sources (sunlight, fluorescent light, halogen or whatever you want). Can you see the difference in the spectra? Try to make a qualitative spectrum plot with several spectra. Compare your spectra.
The amount of longwave radiation emitted by a black body is related to its temperature by the Stefan Boltzmann Law. Thus we can calculate the effective temperature of Earth as determined by its total longwave radiation emitted into space, and compare it with the surface temperature. To do that let us go back to the total longwave radiation dataset (you might want to close all other windows first).
In the expert mode we can calculate the temperature corresponding to the radiation by first dividing by the Stefan Boltzmann constant, and taking the square root of the result twice. We do that as follows:
5.67E-08 div
sqrt sqrt
273.15 sub
X Y 1 SM121
(we converted the results to degree C and add some smoothing in space).
View the results in colors and contours. Do the values agree with your notion of temperature on Earth? Why are they so cold?
Now the JONES surface temperature climatology dataset. These temperature data were carefully compiled from land station measurements and from ship observations. Compare the January field from the Jones data to that calculated from ERBE. At the surface temperatures generally drop uniformly from equator to pole. This is not true about the effective temperature calculated from longwave radiation. Point at the outstanding differences and try to explain them in light of all the material covered in class and in this lab.
We can look at these temperatures differently, by comparing the amount of longwave radiation emitted from the Earth's surface, and that emitted into space. Go back to the longwave radiation dataset and in the expert mode replace the Stefan Boltzmann calculation by a calculation of the annual averaged longwave radiation as described before. Then go to the Jones surface temperature dataset and use the Stefan Boltzmann Law to calculate the longwave radiation emitted from the surface assuming that surface emissivity is 1 (this is not accurate but for our purpose it would be OK to assume that). To do that type the following into the expert window:
273.15 add
dup mul dup mul
5.67E-08 mul
[T] average
(dup creates a copy of the data which is then multiplied by the original using mul. This is done twice to create the 4th power of temperature in degree K).
Now compare the annual averaged longwave radiation coming from the surface with that going out to space. Which is larger? What does the difference represent? Where is the effect (difference) largest. Where is it smallest?
Data