О сфероидальной модели рассеяния света несферическими частицами

We have constructed a spheroidal model to solve the problem of light scattering by nonspherical particles. The semiaxes of the model spheroid are determined based on the requirement that the volumes of initial and model particles are equal, as well as the ratios of their longitudinal and transverse dimensions. This ensures the closeness of the optical properties of initial and model particles. This approach has been applied to prolate and oblate parallelepipeds, cylinders, and cones with the ratios between their larger and smaller dimensions equal to 2 or 10. The direction of propagation of the incident TE or TM plane wave was either parallel or perpendicular to the symmetry axis of particles and model spheroid. The particle size has been determined by dimensionless parameter $${{x}_{{v}}}$$ = $$2\pi {{r}_{{v}}}$$ /λ, which depends on the particle volume, since $${{r}_{{v}}}$$ is the radius of the equivolume sphere. In calculations, this parameter has been varied from small values to fairly large ones, $${{x}_{{v}}}$$ = 10. The applicability range of the model has been determined by comparing the results of numerical calculations performed by the rigorous separation of variables method for spheroids and the method of discrete dipoles for other nonspherical particles. It has been shown that the applicability range of the model for parallelepipeds, cylinders, and cones is wide enough for different parameters of the problem, in particular, if the parameter $${{x}_{{v}}}$$ ≤ 6, then the relative error of the model does not exceed 10–15%. To a large extent, this is related to the fact that the first maximum of the dependence of scattering factor Q _sca on $${{x}_{{v}}}$$ is similar for particles of different shapes approximated by one and the same model spheroid.

Modelling light scattering by mineral dust using spheroids: assessment of applicability

We study the applicability of spheroidal model particles for simulating the single-scattering optical properties of mineral dust aerosols. To assess the range of validity of this model, calculations are compared to laboratory observations for five different dust samples at two wavelengths. We further investigate whether the best-fit shape distributions of spheroids for different mineral dust samples have any similarities that would allow us to suggest a generic first-guess shape distribution for suspended mineral dust. We find that best-fit shape distributions vary considerably between samples and even between wavelengths, making definitive suggestions for a shape distribution difficult. The best-fit shape distribution also depends strongly on the refractive index assumed and the cost function adopted. However, a power-law shape distribution which favours those spheroids that depart most from the spherical shape is found to work well in most cases. To reproduce observed asymmetry parameters, best results are obtained with a power-law shape distribution with an exponent around three.

On realistic size equivalence and shape of spheroidal Saharan mineral dust particles applied in solar and thermal radiative transfer calculations

Realistic size equivalence and shape of Saharan mineral dust particles are derived from in-situ particle, lidar and sun photometer measurements during SAMUM-1 in Morocco (19 May 2006), dealing with measured size- and altitude-resolved axis ratio distributions of assumed spheroidal model particles. The data were applied in optical property, radiative effect, forcing and heating effect simulations to quantify the realistic impact of particle non-sphericity. It turned out that volume-to-surface equivalent spheroids with prolate shape are most realistic: particle non-sphericity only slightly affects single scattering albedo and asymmetry parameter but may enhance extinction coefficient by up to 10 %. At the bottom of the atmosphere (BOA) the Saharan mineral dust always leads to a loss of solar radiation, while the sign of the forcing at the top of the atmosphere (TOA) depends on surface albedo: solar cooling/warming over a mean ocean/land surface. In the thermal spectral range the dust inhibits the emission of radiation to space and warms the BOA. The most realistic case of particle non-sphericity causes changes of total (solar plus thermal) forcing by 55/5 % at the TOA over ocean/land and 15 % at the BOA over both land and ocean and enhances total radiative heating within the dust plume by up to 20 %. Large dust particles significantly contribute to all the radiative effects reported. They strongly enhance the absorbing properties and forward scattering in the solar and increase predominantly, e.g., the total TOA forcing of the dust over land.

On realistic size equivalence and shape of spheroidal Saharan mineral dust particles applied in solar and thermal radiative transfer calculations

Realistic size equivalence and shape of Saharan mineral dust particles are derived from on in-situ particle, lidar and sun photometer measurements during SAMUM-1 in Morocco (19 May 2006), dealing with measured size- and altitude-resolved axis ratio distributions of assumed spheroidal model particles. The data were applied in optical property, radiative effect, forcing and heating effect simulations to quantify the realistic impact of particle non-sphericity. It turned out that volume-to-surface equivalent spheroids with prolate shape are most realistic: particle non-sphericity only slightly affects single scattering albedo and asymmetry parameter but may enhance extinction coefficient by up to 10%. At the bottom of the atmosphere (BOA) the Saharan mineral dust always leads to a loss of solar radiation, while the sign of the forcing at the top of the atmosphere (TOA) depends on surface albedo: solar cooling/warming over a mean ocean/land surface. In the thermal spectral range the dust inhibits the emission of radiation to space and warms the BOA. The most realistic case of particle non-sphericity causes changes of total (solar plus thermal) forcing by 55/5% at the TOA over ocean/land and 15% at the BOA over both land and ocean and enhances total radiative heating within the dust plume by up to 20%. Large dust particles significantly contribute to all the radiative effects reported.