Ice particles in cirrus clouds exhibit a strong spatial and temporal variability in particle size and shape as is shown in numerous results from in-situ measurements. The state of a cirrus parcel is determined by the (often rapidly changing) ambient meteorological conditions during its entire trajectory. Thus, for a specific situation, cirrus microphysical properties are little correlated with the actual conditions of the ambient air. For practical applications in remote sensing or radiation budget calculations it follows that ice particle size and shape must be regarded as random variables with certain probability density functions.

The present paper tries to quantify the uncertainties in the solar radiative fluxes (reflection, transmission, absorption) which result from such random microphysical properties. To this end, solar spectral Monte Carlo radiative transfer calculations have been performed for two situations: 1) Plane parallel homogeneous cirrus clouds with varying ice particle shapes (hexagonal columns, irregular polycrystals) and size distributions from aircraft measurements. 2) Two-dimensional inhomogeneous cirrus clouds from Raman lidar measurements with both constant and noisy microphysical properties.

The radiative fluxes differ stronger for different particle shapes than for different size distributions. For irregular shaped particles the reflectivity varies around 4%, the transmissivity around 2 - 3% and the absorptivity from 9 to 25% with increasing optical thickness. Hexagonal columns are more sensitive to ice particle size and here reflectivities vary around 7%, transmissivity around 1% and absorptivity around 20 to 6% with increasing optical thickness. The uncertainties are largest for absorption also in absolute numbers ranging from 15 to 20 Wm^-2 depending on particle type.

Inhomogeneous cloud extinction coefficients affect the radiative fluxes one order of magnitude stronger than additional fluctuations in the scattering and absorption properties of the ice particles.