Ice crystal habits and solar radiation

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 shape. A novel parameterization has been used to explore the differences in broadband 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 Wm^-2 for mid-latitude and up to some 100W m^-2 for tropical cirrus. The largest differences are found for an intermediate range of optical depths and the optical thickness of natural cirrus happen to be in this intermediate range. The absorptivity of the cirrus cloud is independent of the crystal habit, nevertheless, the change in the cloud’s reflectivity has an impact on the tropospheric absorption and, thus, the energy absorbed by the troposphere depends on the crystal habit. The sensitivity of the results with respect to surface albedo and solar zenith angle has been investigated. A lower zenith angle enhances the differences found for the various habits, and this effect 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 large differences in the modelled fluxes are comparable to the uncertainty in the measurements. Despite this observational difficulty, the large differences in SW fluxes emphasise the importance of the crystal habit for radiation schemes.