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The Cloud-radiative Forcing
The cloud-radiative forcing is simply the difference between the radiative energy budget of clear-sky and all-sky conditions. The all-sky radiative energy values include clear and cloudy observations. A major research problem addressed with the NASA ERBE program was how clouds affect the radiative energy balance of the planet, and thereby climate change. To determine cloud-radiative forcing, the ERBE program had to separate clear-sky scenes from all others.
Clear-sky observations
Below are the ERBE derived monthly mean clear-sky albedo, outgoing longwave radiation, and net radiation budgets. From these maps you can observe the effects of changing vegetation on the energy balance of the planet. Observations of cloud radiative forcing are given below.Loop of Monthly Mean Clear-sky Albedo
Loop of Monthly Mean Clear-sky OLR Loop of Monthly Mean Clear-sky Net Radiation
Cloud radiative forcing
The solar and terrestrial properties of clouds have offsetting effects in terms of the energy balance of the planet. In the longwave, clouds generally reduce the radiation emission to space and thus result in a heating of the planet. While in the solar (or shortwave), clouds reduce the absorbed solar radiation, due to a generally higher albedo than the underlying surface, and thus result in a cooling of the planet.
The latest results from ERBE indicate that in the global mean, clouds reduce the radiative heating of the planet. This cooling is a function of season and ranges from approximately -13 to -21 Wm-2. While these values may seem small, they should be compared with the 4 Wm-2 heating predicted by a doubling of atmospheric concentration of carbon dioxide.
In terms of hemispheric averages, the longwave and shortwave cloud forcing tend to balance each other in the winter hemisphere. In the summer hemisphere, the negative shortwave cloud forcing dominates the positive longwave cloud forcing, and the clouds result in a cooling.
View the maps of cloud forcing given below to answer the following questions:
- Does the presence of low level clouds over oceans heat or cool the planet?
- What about the convective clouds over the oceans?
- Do deserts have a large or small cloud radiative forcing?
These images, and more, are available on a video tape. If you are interested in obtaining this video contact stevea@ssec.wisc.edu.