Cloud bidirectional reflectance functions: a comparison of experimental and theoretical results.

Bidirectional reflectance models have been developed for clouds using radiative transfer theory. Also, empirical models have been developed using data from the visible channel of both the earth radiation budget (ERB) instrument aboard the Nimbus 7 spacecraft and the GOES spacecraft. Comparison is made between the theoretical and empirical models. Overall, the comparison is quite good, there being differences of ∼10% for solar zenith angles <70°. For larger solar zenith angles, the differences increase rapidly. These differences appear to be due to 3-D cloud effects and to smearing effects associated with averaging measurements over finite solar zenith angle ranges. Thus it is demonstrated that the theoretical model provides a good description of the radiance fields of clouds as seen by satellites. A cloud model based on GOES data is also compared with the Nimbus 7 general cloud model. Differences of 13% are noted for solar zenith angles of <70°. This result shows that bidirectional models which compare well with the Nimbus 7 ERB results can be produced using GOES data. Thus, GOES data can be used to develop bidirectional models for cases in which orbital constraints do not permit observations from the Nimbus 7 spacecraft.