The impact of natural variations of ice crystal aspect ratios on single-scattering radiative properties

The most common building blocks of the non-spherical crystals occurring in ice clouds are hexagaonal prisms which occur in plates, columns and bullet rosettes. Because the fundamental scattering properties of ice clouds strongly depend on the assumed aspect ratio of such shapes, it is necessary to build a database how these single particle properties depend on cloud and environmental parameters (e.g., temperature), and vary with geographical location. By applying a new analysis tool developed at the University of Illinois to high-resolution images of ice crystal silhouettes obtained by a Cloud Particle Imager (CPI) in the Arctic, mid-latitudes and Tropics, a database on the length (L') and width (W') of the silhouettes has been developed. In combination with simulations of the projections of randomly oriented ice crystals on two-dimensional planes, the measures of L' and W' are used to derive possible relations between the actual length (L) and actual width (W) of ice crystals in different environmental conditions, which are expressed as L = a W^b. The relevant a and b parameters are expressed not as most likely fit parameters, but rather as surfaces of equally realizable solutions in (a,b) phase space with measured uncertainties in L' and W' determining the size of the ellipse encompassing the possible solutions. Single-scattering simulations conducted with Monte Carlo ray-tracing codes assuming geometric optics are conducted using the range of possible L' and W' to determine the plausible ranges of calculated single-scattering properties (e.g., asymmetry parameter, single-scatter albedo, scattering phase function) for different environmental conditions and geographic locations. Implications of expressing single-scattering properties as probability distribution functions rather than as single values for calculations of radiative heating profiles and for estimates of cloud radiative forcing are discussed.