The asymmetry parameter, g, is a very important component in climate model radiative transfer calculations. The value of g determines how much radiation is scattered in the forward and backward hemispheres. If g has a value of -1 then all the radiation is scattered in the backward hemisphere, if the value is 1 then all the radiation is scattered in the forward hemisphere if it is zero then the radiation is scattered equally in both hemispheres. In climate models the influence of cirrus is strongly determined by the choice of g. The most appropriate value of g to represent cirrus at non-absorbing wavelengths is still an open question. Theoretical calculations of g for different single crystal shapes tend to predict g values, which are usually between 0.75 and 0.85 at non-absorbing wavelengths. If, however, the crystal has air inclusions within it then the g value can be as low as 0.55 for maximum air density. Clearly, the range of g that theoretical models predict is significant, and the impact of such an uncertainty on broadband short-wave calculations can be as large as 20 Wm-2. In order to improve the parametrization of cirrus within climate models and hence reduce the uncertainty in short wave forcing then g must be constrained; this can be achieved through experiment.

In this paper we present new preliminary estimates of the asymmetry parameter based on single crystal light scattering experiments at the University of Hertfordshire, England. In the experiment ice analogues are produced which resemble rosettes and aggregates that are known to exist in cirrus clouds. The refractive index of the ice analogue is virtually the same as that of ice at visible wavelengths. The crystal is levitated electrodynamically and the scattered intensity is measured between the scattering angles of 3 deg and 177 deg at a wavelength of 0.488 microns. In order to estimate the asymmetry parameter the intensities between 0 deg and 3 deg are interpolated using Mie theory based on the equal area sphere. Since the functional form of the scattering pattern is relatively flat at backscattering angles, the intensity is assumed to be constant between the scattering angles of 177 deg and 180 deg. Based on these experiments we find for the rosette an asymmetry parameter value of 0.72 whilst for the aggregate an asymmetry parameter of 0.82±0.03 is found. These experiments confirm that the asymmetry parameter of non-spherical particles is lower than their equivalent spherical counterparts. The estimated asymmetry parameter values are also compared and contrasted with theoretical predictions based on improvements to geometric optics, and future experiments will be outlined which will constrain the asymmetry parameter further. The results presented in this paper will be of interest to those concerned with remote sensing of cirrus, and radiative transfer calculations in climate models.