The single-scattering properties of cirrus clouds depend not only, upon the
ice water path and the size of the crystals, but also upon the crystal sha
pe. A novel parameterization has been used to explore the differences in br
oadband short-wave radiation due to various crystal habits. The differences
in the net-fluxes are found to be appreciable, both at the tropopause and
at the surface. They reach values up to the order of several 10 W m(-2) for
mid-latitude and up to some 100 W m(-2) for tropical cirrus. The largest d
ifferences are found for an intermediate range of optical depths and the op
tical thickness of natural cirrus happen to be in this intermediate range.
The absorptivity of the cirrus cloud is independent of the crystal habit, n
evertheless, the change in the cloud's reflectivity has an impact on the tr
opospheric absorption and, thus, the energy absorbed by the troposphere dep
ends on the crystal habit. The sensitivity of the results with respect to s
urface albedo and solar zenith angle has been investigated. A lower zenith
angle enhances the differences found for the various habits, and this effec
t is even amplified by a high amount of water vapour as it may be found in
the tropics. A comparison with observations from ICE'89 reveals that the la
rge differences in the modelled fluxes are comparable to the uncertainty in
the measurements. Despite this observational difficulty, the large differe
nces in SW fluxes emphasise the importance of the crystal habit for radiati
on schemes.