Browsing by Subject "numerical simulation"

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  • Lu, P.; Cao, X.; Wang, Q.; Leppäranta, M.; Cheng, B.; Li, Z. (2018)
    To investigate the influence of a surface ice lid on the optical properties of a melt pond, a radiative transfer model was employed that includes four plane-parallel layers: an ice lid, a melt pond, the underlying ice, and the ocean beneath the ice. The thickness H-s and the scattering coefficient sigma(s) of the ice lid are altered. Variations in the spectral albedo and transmittance T due to H-s for a transparent ice lid are limited, and scattering in the ice lid has a pronounced impact on the albedo of melt ponds as well as the vertical distribution of spectral irradiance in ponded sea ice. The thickness of the ice lid determines the amount of solar energy absorbed. A 2-cm-thick ice lid can absorb 13% of the incident solar energy, half of the energy absorbed by a 30-cm-deep meltwater layer below the lid. This has an influence on the thermodynamics of melting sea ice. The color and spectral albedo of refreezing melt ponds depend on the value of the dimensionless number sigma(s) H- s. Good agreement between field measurements and our model simulations is found. The number sigma(s) H- s is confirmed to be a good index showing that the influence of an ice lid with sigma(s) H- s Plain Language Summary Melt ponds are pools of open water that form on sea ice in the warm months of the Arctic Ocean, and they will frequently be refrozen due to loss of heat and then covered by an ice lid or snow even in summer. This lid is very important to the optical properties of melt ponds. If the ice lid is very thin, the change in the reflective characteristics of the melt pond is minimal; that is, the influence of the ice lid is negligible. If snow accumulates on the ice lid, the reflective characteristics of the melt pond change completely. How about the situation between the above two extreme cases? In this study, we find that a dimensionless number is a good index to quantify the impact of the ice lid. Visual inspections on the color of refreezing melt ponds also help to judge the significance of the influence of the ice lid. This will allow for an accurate estimation on the role of surface ice lid during field investigations on the optical properties of melt ponds.
  • Tamminen, J.; Tarvainen, T.; Siltanen, S. (2017)
    The D-bar method at negative energy is numerically implemented. Using the method, we are able to numerically reconstruct potentials and investigate exceptional points at negative energy. Subsequently, applying the method to diffuse optical tomography, a new way of reconstructing the diffusion coefficient from the associated Complex Geometrics Optics solution is suggested and numerically validated.
  • Pfau-Kempf, Yann (Finnish Meteorological Institute, 2016)
    Vlasiator – From local to global magnetospheric hybrid-Vlasov simulations Contributions 127
    The Sun is the source of the solar wind, a continuous stream of electrically charged particles and magnetic fields pervading the Solar system. Its interaction with the magnetic field of the Earth, in and around the region called the magnetosphere, controls the flow of matter and energy in near-Earth space. A fundamental understanding of the physical processes at play is crucial for the building of forecasting and warning systems, as the influence of the solar wind during space storms can harm life and technology in space and on the ground. These effects, collectively known as space weather, are one of the biggest albeit least understood natural threats to society. The research effort needed includes the development of observational methods as well as theories and models, to first describe and later predict the mechanisms and consequences of space weather. This doctoral thesis, comprising an introduction and four peer-reviewed articles, presents the hybrid-Vlasov model Vlasiator developed at the Finnish Meteorological Institute. Based on a detailed description of proton physics in space plasmas, Vlasiator allows to simulate both local contexts and the Earth’s magnetosphere on global scales. This unprecedented capability is only accessible by harnessing the power of modern supercomputers. The aim of this work is threefold. The current version of Vlasiator is documented considering physical and computational aspects, the correctness of the simulations is demonstrated by comparing to analytical theories and spacecraft observations, and new scientific results gained with this model are presented.