Abstract
Electromagnetic wave scattering by ice particles is commonly modeled by defining representative habits, including droxtals, columns, plates, and aggregates, although actual particles in the atmosphere can be even much more complex. In this study, we examined a superspheroidal approximation method for modeling electromagnetic wave scattering by ice crystals. Superspheroid can be associated with a shape index (SI) defined by the particle volume and average projected area. Corresponding to realistic ice crystals, suitable superspheroid models with the same SI (that means, identical volume and average projected area) and aspect ratio can be identified as surrogates for optical property calculations. We systematically compared the optical properties of ice crystals and superspheroids at 33 microwave bands in the range of 3–640 GHz and at three representative visible or infrared wavelengths (0.66, 2.13, and 11 μm). It was found that the single-scattering properties of compact ice crystal habits and their superspheroidal model particles were quite close. For an aggregate with sparse distribution of elements, a superspheroid model produces relatively large errors because the aspect ratio may not be sufficient to describe a particle shape. However, the optical similarity of a superspheroid and an aggregate is still encouraging.
Highlights
Cirrus clouds cover approximately 20% of the earth’s surface, and their radiation characteristics depend on their microphysical properties, including the shape, refractive index, and size distribution, of ice crystal particles [1,2,3,4,5]
Superspheroid models are proposed as possible surrogates for representative ice crystal habits to compute the optical properties
The shape index (SI) was found to be quite useful for finding a superspheroid that has high optical similarity to realistic ice crystal shapes. In both the microwave bands and the visible and the infrared wavelengths, a suitable superspheroid model was identified for each ice crystal, and the single-scattering properties of the superspheroid were close or similar to those of a realistic ice crystal habit
Summary
Cirrus clouds cover approximately 20% of the earth’s surface, and their radiation characteristics depend on their microphysical properties, including the shape, refractive index, and size distribution, of ice crystal particles [1,2,3,4,5]. Attempts at modeling electromagnetic wave scattering by ice crystals were based on an assumption of spherical particle shapes [1,12], followed by nonspherical approximations including simple geometries, such as spheroids and cylinders [18,19,20,21,22]. Mishchenko and Macke [27] exploited the optical similarity between hexagonal column and circular cylinder to determine how big the particle size should be to have 46◦ halo These approximations have gradually disappeared because of the drawback of shape approximations being hard to evaluate, and a variety of computational methods have been developed that can deal efficiently with more realistic ice crystal habits. A discussion is given in Section 5, and Section 6 concludes this study
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