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  • Author or Editor: Craig F. Bohren x
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Timothy J. Nevitt
and
Craig F. Bohren

Abstract

Shape can strongly affect scattering by particles at wavelengths near bulk absorption bands. An example is scattering by many components of the atmospheric aerosol at wavelengths in the transmission window centered around 10 μm. An exact solution to the problem of scattering by an irregular particle, even if it were available, would yield more detail than necessary. An alternative approach is to formulate the irregular particle scattering problem in terms of the kinds of averages inherent in the optical properties of an ensemble of particles. Such a statistical method has been applied to an ensemble of small irregular particles by averaging over a range of electromagnetic microstates, in this instance randomly oriented anisotropic oscillators (i.e., Rayleigh ellipsoids). Infrared backscattering spectra calculated by this method agree better with laboratory-measured spectra for ammonium sulfate particles than those calculated using Mie theory.

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Louis J. Battan
and
Craig F. Bohren

Abstract

Calculations have been made of the radar backscattering and attenuation cross sections of dry and spongy ice spheres. One set of calculations was the cross sections of spheres with diameters exponentially distributed. As expected, attenuation cross sections are greater at a wavelength of 3.21 cm than at 5.05 and 10.0 cm. Calculations were also made of attenuation by monodisperse distributions of spheres composed of spongy ice and having diameters as large as about 8 cm. Attenuation of 3-cm radiation by dry ice and spongy ice spheres can be very large. At most diameters and water volume fractions, the one-way attenuation of 10-cm radiation by monodisperse spheres, in concentrations giving a radar reflectivity of 60 dBZ, is negligibly small (i.e., <0.1 dB km−1), but at a few diameters and water fractions, attenuation can be substantially larger. Although in most circumstances attenuation increases as wavelength decreases, there are exceptions at some diameters and water volume fractions. These calculations may explain observations that C-band attenuation in hailstorms is not always larger than S-band attenuation.

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Louis J. Battan
and
Craig F. Bohren

Abstract

No abstract available

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