## Help

This page offers help to the online version of the model for cloud radiative forcing, available at http://www.iac.ethz.ch/url/crf.
### Input variables

Six input variables are needed to estimate the shortwave and longwave cloud radiative forcing.

#### Surface albedo

The reflectivity of the Earth surface for shortwave radiation.

#### Surface temperature

The temperature of the Earth's surface.

The model was derived for surface temperatures ranging from 210 to 305 Kelvin.

#### Solar constant

The solar radiative flux incident upon the top of the Earth's atmosphere.

The mean value amounts to 1368 Wm^{-2}.
To compute CRF for daily mean conditions, enter the solar constant multiplied by the fraction of the day that the sun is above the horizon,
in combination with the daily mean solar zenith angle (see below).
The global and daily mean can be represented by a solar constant of 684 Wm^{-2}, combined with a zenith angle of 60°.

#### Zenith angle

The angle between the direction to the zenith and the direction of the sun.

For nighttime calculations, set the zenith angle to 90°.
For daily mean CRF, use the daily means solar zenith angle in combination with the solar constant multiplied by the fraction of the day that the sun is above the horizon (see above).
The global and daily mean conditions can be approximated by a zenith angle of 60° in combination with a solar constant of 684 Wm^{-2}.

#### Cloud optical depth

The optical thickness between the top and bottom of the cloud, measured vertically and in the shortwave.

The model has been developed for optical depths up to 100.

#### Cloud top temperature

Temperature of the top of the cloud.

The model is applicable for temperatures ranging from 180 and 305 Kelvin.

### How to cite

Whenever you publish research using this model, please include the following citation:

*Corti, T. and Peter, T.: A simple model for cloud radiative forcing, Atmos. Chem. Phys., 9, 5751-5758, 2009.*

In addition, please point to the online version of the model, e.g., in the acknowledgment:
"An online version of the CRF model by Corti and Peter is available at http://www.iac.ethz.ch/url/crf."

### More help

For further questions and comments, please contact Thomas Peter.