Jani Tyynelä

Radar Backscattering from Snowflakes: Comparison of Fractal, Aggregate, and Soft Spheroid Models

AbstractThe sensitivity of radar backscattering cross sections on different snowflake shapes is studied at C, Ku, Ka, and W bands. Snowflakes are simulated using two complex shape models, namely, fractal and aggregate, and a soft spheroid model. The models are tuned to emulate physical properties of real snowflakes, that is, the mass–size relation and aspect ratio. It is found that for particle sizes up to 5 mm and for frequencies from 5 to 35 GHz, there is a good agreement in the backscattering cross section for all models. For larger snowflakes at the Ka band, it is found that the spheroid model underestimates the backscattering cross sections by a factor of 10, and at W band by a factor of 50–100. Furthermore, there is a noticeable difference between spheroid and complex shape models in the linear depolarization ratios for all frequencies and particle sizes.

Interpretation of single-particle negative polarization at intermediate scattering angles

We study the interrelation of the internal field of irregular particles to the far-field scattering characteristics by modifying the internal field of dipole groups. In this paper, we concentrate on the longitudinal component, i.e., the internal-field component parallel to the incident wave vector. We use the discrete-dipole approximation to determine the internal field and switch off the longitudinal component from the dipoles that have the highest energy density above a preset cutoff value. We conclude that only a relatively small number of core dipoles, about 5% of all dipoles, contribute to the negative linear polarization at intermediate scattering angles. These core dipole groups are located at the forward part of the particles. The number of core dipoles in the group becomes greater as particle asphericity increases. We find that the interference between the scattered waves from the core dipole groups, which was studied previously for spherical particles, is preserved to a large extent for nonspherical particles.

Coherent backscattering in planetary regoliths

Atmosphereless solar-system objects exhibit two ubiquitous light-scattering phenomena at small solar phase angles (sun.object.observer angle α): first, the opposition effect in the intensity of scattered sunlight (e.g., [1]); and, second, the negative degree of linear polarization (I ┴ − I ║)/(I ┴ + I ║). Here I ║ denotes the intensity component parallel to the scattering plane defined by the Sun, the object, and the observer and I ┴ denotes the component perpendicular to that plane [2].

Polarized backscattering by clusters of spherical particles.

The linear and circular polarization ratios for clusters of spherical particles averaged over multiple orientations show a systematic pattern as a function of the refractive index and the size parameter. We show that, at backscattering, the depolarizing behavior of orientation-averaged clusters of spheres can be approximated by second-order scattering of bispheres. The pattern is relatively invariable in terms of the number of particles. We also demonstrate the significance of the near-field effects for polarization at backscattering.

Modeling melting layer radar observations at GPM frequencies; comparison to measurements

The melting layer attenuation is studied by fitting the modeled reflectivity factor and the reflectivity-weighted fall velocity to measured values at both Ku- and Ka-bands. In the fitting process the optimized snow density and velocity parameters are defined and the dependence of the modeled attenuation on these parameters is investigated. The study shows that for Ku-band the implemented model can describe the melting layer with sufficient accuracy, but for Ka-band there is discrepancy between the measured and modeled values despite the fitting process. At Ka-band the attenuation of water vapor and cloud water influence more than at Ku-band and this should be considered also in the model. The sensitivity study of the modeled attenuation shows that in the snow velocity power-law function V = αDβ the changes in the factor α induces a significant deviation in the modeled attenuation. This is true for all of the tested effective permittivity models and at both frequency bands. The permittivity model of a coated sphere solution is out of the tested models the most susceptible on changes of snow related parameters.

Modeling attenuation of melting hydrometeors with a method based on volume integral equations

The attenuation of spheroidal melting hydrometeors is simulated in C-, Ku- and Ka-band utilizing a microphysical melting layer model. The scattering properties are obtained with Mie scattering solution. In C-band the polarimetric radar parameters are computed utilizing a method based on volume integral equation. Polarization difference is detectable, but reflectivity values are regularly smaller than those calculated with Mie solution. This is dependent on the process of formatting the particle structure according to the change in liquid water mass fraction.