Light scattering by Gaussian rough ice crystals: experimental and modelling results

It is qualitatively well understood that ice and mixed-phase clouds have an important role in the radiation balance and climate of the Earth [1,2]. However, quantitative understanding of these clouds is poor, and is one of the biggest uncertainties within climate models [3]. Efforts to solve this problem are particularly focusing on light scattering by ice crystals within these clouds[1], particularly on why the haloes that should be a regular occurrence with crystal that are hexagonal prisms are so rare. One of the primary candidates to explain the lack of haloes is ice crystal roughness, but the structure of ice crystals in clouds cannot be directly determined and so indirect methods have to be used.
This presentation will describe the generation of Gaussian rough[4] crystal geometry and cover the applications of these roughened crystals, as well as results obtained through their use. Previous results show that ice crystals found in cirrus have similar surface roughness to mineral dust grains[5]; atomic force microscope scans of these grains are used to derive roughness parameters that can then be used to generate realistic crystal geometries. Light scattering simulations are then performed using the RTDF[6] and ADDA[7] scattering models with these realistic geometries and the phase function, degree of linear polarisation and asymmetry parameter can be determined and compared to results from simulations using smooth and moderately rough geometries. The Gaussian roughness used in light scattering simulations can also be applied to ice analogue crystals[8] using the microfabrication technique of focused ion beam milling[9]. This enables laboratory light scattering measurements of these rough geometries to be taken, and these can be compared to the simulation results.