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We present a new remote sensing technique to infer the average  asymmetry parameter of ice crystals near cloud top from  multi-directional polarization measurements. The method is based on  previous findings that (a) complex aggregates of hexagonal crystals  generally have scattering phase matrices resembling those of their  components; and (b) scattering phase matrices systematically vary  with aspect ratios of crystals and their degree of microscale  surface roughness. Ice cloud asymmetry parameters are inferred from  multi-directional polarized reflectance measurements by searching  for the closest fit in a look-up table of simulated polarized  reflectances computed for cloud layers that contain individual, randomly oriented  hexagonal columns and plates with varying aspect ratios and  roughness values. The asymmetry parameter of the hexagonal particle  that leads to the best fit with the measurements is considered the  retrieved value. For clouds with optical thickness less than 5, the  cloud optical thickness must be retrieved simultaneously with the  asymmetry parameter, while for optically thicker clouds the  asymmetry parameter retrieval is independent of cloud optical  thickness. Evaluation of the technique using simulated measurements  based on the optical properties of a number of complex particles and  their mixtures shows that the ice crystal asymmetry parameters are  generally retrieved to within 5%, or about 0.04 in absolute  terms. The retrieval scheme is largely independent of calibration  errors, range and sampling density of scattering angles and random  noise in the measurements. The approach can be applied to  measurements of past, current and future airborne and satellite  instruments that measure multi-directional polarized reflectances of  ice-topped clouds

atmospheric measurement techniques

Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 1: Methodology and evaluation with simulated measurements