Evaluation of electromagnetic scattering modeling techniques for irregular ice hydrometeors

Weather radar measurements provide useful information on the microphysics of clouds and precipitation if interpreted correctly. Direct modeling of hydrometeor back-scattering properties is one of the essential steps to a successful quantitative analysis of these data. In general, hydrometeor scattering properties depend on their composition (usually ice, water and air or a mixture), shape, size , mass, and the radar wavelength. If at least one of the target's dimensions is comparable to about half the wavelength (or larger), the usual approximations to the electromagnetic models aren't valid anymore and a greater care must be given to the level of detail used in the model. This is the case for millimeter wave radar systems (e.g., MMCR, Cloudsat), which are essential tools for cloud remote sensing. The consideration of target size relative to the wavelength raises questions about what are the essential microphysical properties that determine the target's electromagnetic properties and thus what is the proper electromagnetic model for each class of hydrometeor. In many cases these questions are far from being answered, making it necessary to broaden our general knowledge of the issue and of the possible solutions. One approach is to explore these solutions by comparing results obtained through several scattering methods for the same conditions (i.e., hydrometeor microphysics) at several different wavelengths. Using multiple wavelengths is in fact essential because the hydrometeor scattering properties can vary significantly as a function of the wavelength.

This work is focused on electromagnetic scattering models by irregular ice hydrometeors, particularly pristine ice crystals and their aggregates. Studies are available in literature with focus on electromagnetic techniques such as FDTD, DDA, CDA, T-Matrix, Mie, GMM, for both pristine ice crystals and aggregates (Aydin, 1999; Liu, 2008; Petty and Huang, 2010). A summary of these models and comparisons of the results are given in this work. One of the results of this work relates to the effect of a “soft sphere” approximation on the scattering properties of a target, in particular ice aggregates. The soft sphere approximation consists of modeling a given target with a sphere or a spheroid with an appropriate effective dielectric constant. Several results show that this approximation fails to correctly model ice aggregates when the size of the sphere or spheroid along the propagation direction is roughly half the wavelength and larger. In fact, the assumption of a solid shape instead of a more complex one leads to characteristic interference effects in the scattered field, showing a typical resonating behavior leading to an underestimation of the back-scattering cross section. It has been shown that the use of other methods leads to large differences for aggregates (Botta et al., 2010; 2011). For example, considering a low density ice aggregate with a maximum dimension of 2 mm and a wavelength of 3.19 mm (W band), using the soft sphere approximation leads to an underestimation of the back-scattering cross-section between 5 and 15 dB, depending on the parameters of the soft sphere, if compared to the methodology developed by Botta et al. (2010; 2011). The scattering behavior of a “soft spheroid” approximation also depends on the choice of the aspect ratio and on the radiation incidence angle. A commonly used aspect ratio of 0.6 leads to an underestimation close to 5 dB at vertical incidence, and reaches 15 dB at side incidence. This work further analyzes the issue and compares several different electromagnetic models.

References

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