Abstract
Abstract. Many natural land surfaces, such as sand or snow, consist of densely packed grains, often covered by dust, water droplets, contaminated with other materials such as possible oil leaks, hoar frost, and can also be internally cracked, porous, and heterogeneous. Most scattering models ignore these complications, but here a more detailed approach is taken to test all these effects. The current model is composed of three techniques: 1) Monte Carlo-based electromagnetic volume integral equation technique for non-spherical wavelength scale dust particles, 2) Monte Carlo ray tracing for stochastic-shaped grains much larger than the wavelength, with optional point scattering from dust cover, internal inclusions, and liquid surface layer, in a layer of an optical depths of few units, and 3) adding-doubling to combine smaller layers into an arbitrary, thick and vertically inhomogeneous medium. The model allows the medium to be built in a modular way, and after initialisation, rather complicated layered structures can be computed quickly and flexibly. The computed results are compared against experimental measurements of snow and sand. The model agrees with measurements usually within the measurement accuracy (∼ 0:05). The scattering is observed to depend significantly on grain size, shape, orientation, composition, fine structures, dust, and some other properties that need to be defined. Both, measurement and modelling, require much deeper attention to these properties.
Highlights
Many Earth and planetary surfaces are covered by granulate material, such as snow, sand, dust, and regolith
Several things complicate the modelling: the particles are packed densely compared to the free path, they can be sintered together, there can be multi-scale horizontal and vertical structures, layers can be thick and scattering order high, larger particles may be covered or contaminated by fine dust, and there are structures in the wavelength scale
The model is composed of three parts: ray-tracer for thin densely packed media (Peltoniemi, Lumme, 1992, Peltoniemi, 2007), adding-doubling radiative transfer programme (Peltoniemi, 1993) for thick layered media, and electromagnetic volume integral equation scattering solver for small dust particles (Peltoniemi, 1996)
Summary
Many Earth and planetary surfaces are covered by granulate material, such as snow, sand, dust, and regolith. The rays propagate directly in space, reflect or refract from flat surfaces, and scatter to selected or random directions from diffuse scatterers This requires that all effective length scales of the medium be much larger than the wavelength. Ray-tracing has been applied to solve scattering from many kinds of particles and media (Muinonen et al, 1989, Peltoniemi et al, 1989, Peltoniemi, Lumme, 1992, Peltoniemi et al, 2007). This technique allows 3-dimensional structures to be modelled well. High-quality experimental data to validate models are still rare Proper validation needs both maximally simplified experiments to study different parts of the models, and realistic experiments to test their true applicability.
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