Solar radiative transfer simulations in Saharan dust plumes: particle shapes and 3-D effect

Radiative fields of three-dimensional inhomogeneous Saharan dust clouds have been calculated at solar wavelength (0.6 μm) by means of a Monte Carlo radiative transfer model. Scattering properties are taken from measurements in the SAMUM campaigns, from light scattering calculations for spheroids based on the MIESCHKA code, from Mie theory for spheres and from the geometric optics method assuming irregular shaped particles. Optical properties of different projected area equivalent shapes are compared. Large differences in optical properties are found especially in the phase functions. Results of radiative transfer calculations based on the Monte Carlo method are shown exemplarily for one dust cloud simulated by the cloud resolving atmospheric circulation model LM-MUSCAT-DES. Shape-induced differences in the radiation fluxes are pronounced, for example, the domain averaged normalized radiance is about 30% lower in the case of a dust plume consisting of spheroids or irregular particles compared to spheres. The effect of net horizontal photon transport (3-D effect) on the reflected radiance fields is only notable at the largest gradients in optical thickness. For example, the reflectance at low sun position differs locally about 15% when horizontal photon transport is accounted for. ‘Sharp edges' due to 1-D calculations are smoothed out in the 3-D case

Light Scattering Properties of Higher Order Chebyshev Particles and Implications for Aerosols with a Weak Surface Roughness

Chebyshev particles of comparatively low orders n are used in the past to study the effects of nonspherical but concave geometries in remote sensing applications. Their shape is given by r(theta) = r0 * [1 + a * cos(n * theta)] where r0 is the radius of the underlying sphere and a denotes the deformation parameter. We present results of light scattering computations for several Chebyshev particles characterized by higher orders n. Accurate results can be obtained for such particles within a T matrix approach. Moreover the scattering characteristics converge to stable results if the order is increased. Essential differences between, e.g., the phase functions of the higher order Chebyshev particles and the underlying regular scatterers can be observed. In particular an increased backscattering due to the surface roughness is obtained. This behavior is even more pronounced in the case of highly absorbing particles. The effects obtained agree with results of other approaches and correspond to expectations for particles with a weak surface roughness. This demonstrates that, on one hand, higher order Chebyshev particles can be used to estimate the influence of a weak surface roughness on the light scattering behavior of atmospheric aerosols. On the other hand, the consideration of roughness effects is important in the retrieval of aerosol properties

Considerations to Rayleigh's hypothesis

In 1907 Lord Rayleigh published a paper on the dynamic theory of gratings. In this paper he presented a rigorous approach for solving plane wave scattering on periodic surfaces. Moreover he derived explicit expressions for a perfectly conducting sinusoidal surface, and for perpendicular incidence of the electromagnetic plane wave. This paper was criticized by Lippmann in 1953 for he assumed Rayleigh's approach to be incomplete. Since this time there have been published several arguments, proofs, and discussions concerning the correctness and the range of validity of Rayleigh's approach not only for plane wave scattering on gratings but also for light scattering on nonspherical structures, in general. In the paper at hand we will discuss the different point of views on what is called ''Rayleigh's hypothesis'' as well as the relevance of a found theoretical limit for its validity. Furthermore we present a numerical treatment of the original scattering problem of a p-polarized plane wave perpendicularly incident on a perfectly conducting sinusoidal surface (i.e., the scalar Dirichlet problem). In doing so we emphasizes the near-field solution especially within the grooves of the grating up to points on the surface, and below the surface. Two different Green's function formulations of Huygens' principle are used as starting points. One of this formulation results in the general T-matrix approach which is considered to be affected by Rayleigh's hypothesis especially for near-field calculations. The other formulation provides a conventional boundary integral equation which is in accordance with Lippmann's point of view and free of problems with Rayleigh's hypothesis. But the obtained results show that Lippmann's argumentation do not withstand a critical numerical analysis, and that the independence of least-squares approaches from Rayleigh's hypothesis, as understood and proven by Millar, seems to hold also for certain methods which does not fit into such an approach

Scattering database for spheroidal particles

We present a database containing light scattering quantities of randomly oriented dielectric spheroidal particles in the resonance region. The database has been generated by using a thoroughly tested T-matrix method implementation. The data possess a defined accuracy so that they can be used as benchmarks for electromagnetic and light scattering computations of spheroids. Within its parameter range the database may also be applied as a fast tool to investigate the scattering properties of nonspherical particles and to verify assumptions or statements concerning their scattering behaviour. A user interface has been developed to facilitate the data access. It also provides some additional functionalities like interpolations between data or the computation of size averaged scattering quantities. A detailed description of the database and the user interface is given followed by examples illustrating their capabilities and handling. On request, the database including the documentation are available, free of charge, on a CD-ROM (email to [email protected] or [email protected])

Case study about the accuracy behavior of three different T-matrix methods

In this paper we discuss the influence of two different sets of weighting functions on the accuracy behavior of T-matrix calculations for scalar scattering problems. The first set of weighting functions is related to one of Waterman’s original approaches. The other set results into a least-squares scheme for the transmission problem. It is shown that both sets of weighting functions produce results with a converse accuracy behavior in the near and far fields. Additional information, such as reciprocity and the fulfillment of the boundary condition, are needed to choose the set of weighting functions that is most appropriate for a certain application. The obtained criteria are applied afterward to an iterative T-matrix approach we developed to analyze scattering on regular particle geometries with an impressed but slight surface irregularity. However, its usefulness is demonstrated in this paper by analyzing the far-field scattering behavior of Chebyshev particles of higher orders

Electromagnetic scattering on Janus spheres

Janus spheres are objects of growing interest in different fields of technology. For example, they can be applied in medicine for a precise drug positioning. They have moreover an enormous potential for the development of new and active materials. Some Janus spheres are already manufactured industrially. Nevertheless, there is a lack of knowledge about the scattering behavior of such objects although it is important not only for diagnostic purposes but also for discovering new applications. In this paper we present the application of the well known T-matrix method to solve the scattering problem of a plane electromagnetic wave on a dielectric sphere whose surface is partially covered with a perfect metal. The application of this approach to a specially oriented Janus sphere reveals an interesting effect for size parameters less than