Use of observed ice crystal sizes and shapes to calculate mean-scattering properties and multispectral radiances: CEPEX April 4, 1993, case study

During the Central Equatorial Pacific Experiment, ice crystal sizes and sha pes were measured in an outflow anvil. A habit (i.e., column, bullet rosett e, Koch fractal polycrystal, sphere) was assigned to each particle using a self-organized neural network based on simulations of how the maximum parti cle dimension and area ratio varied for random orientations of these crysta ls. Average ice crystal size and shape distributions were calculated for 25 km long segments at six altitudes using measurements from a two-dimensiona l cloud probe for crystals larger than 90 mu m and a parameterization for s maller crystals based on measurements from the Video Ice Particle Sampler ( VIPS). Mean-scattering properties were determined by weighting the size and shape dependent single-scattering properties computed with ray-tracing alg orithms according to scattering cross-section. Reflectances at 0.664, 0.875 , 1.621, and 2.142 mu m were then calculated using a Monte Carlo radiative transfer routine. Although these reflectances agree reasonably with those m easured by the MODIS airborne simulator (MAS) above the anvil, uncertaintie s in cloud base and system evolution prevent a determination of whether ray -tracing or anomalous diffraction theory better predict reflectance. The ca lculated reflectances are as sensitive to the numbers and shapes of crystal s smaller than 90 mu m as to those of larger crystals. The calculated refle ctances were insensitive to the classification scheme (i.e., neural network , discriminator analysis, and previously used classification scheme) for as signing particle shape to observed crystals. However, the reflectances sign ificantly depended on assumed particle shape.