Radiative transfer

About: Radiative transfer is a research topic. Over the lifetime, 43287 publications have been published within this topic receiving 1176539 citations.

Microwave Emissivity and Accumulation Rate of Polar Firn

Abstract: Radiative transfer theory is formulated to permit a meaningful definition of emissivity for bulk emitting media such as snow. The emissivity in the Rayleigh-Jeans approximation is then the microwave brightness temperature T B divided by an effective physical temperature 〈T〉. The 〈T〉 is an average of the physical temperature, T(z), weighted by a radiative transfer function ƒ(z). Similarly, where e(z) is the local emittance. An approximate ƒ(z) is used to determine analytically the effects of various absorption coefficients, of scattering coefficients that vary with depth, and of the seasonal variation of T(z). It is shown that a mean emissivity, which is equal to the mean annual T B divided by the mean annual surface temperature T m, is a useful quantity for comparing theory and observations. Snow-crystal size measurements, r(z), at seven locations in Greenland and Antarctica are used to determine the Mie/Rayleigh scattering coefficient γs (z) and to calculate the mean emissivities. The observed mean emissivities are determined by a which is the average of 12 monthly Nimbus-5 (1.55 cm) microwave observations, and the Tm measured at the same locations. The calculated emissivities are about one-half of the observed values. The assumption that each snow crystal is an independent and equally effective scatterer, and the use of an approximation to ƒ(z), tend to over-estimate the effect of scattering. Therefore, a parameter multiplying γs (z) is used. The emissivities calculated with a single value of this empirical parameter for all seven locations agree well with the observed emissivities, showing that the microwave emissivity variations of dry polar urn can be characterised as a function of the crystal sizes. One optical depth corresponds to a typical fini depth of 5 m, but significant radiation emanates from up to 30 m. Since r(z) depends on the snow accumulation rate A and T m. the sensitivity of the emissivity to changes in T m or A are estimated using this semi-empirical theory. The results show that a one degree change or uncertainty in Tm is approximately equivalent to a 10% change in A, and that such a change will affect the emissivity by 0.003 to 0.014 or the T B by about 0.6 K to 3 K, depending on the location.

Semi-implicit scheme for treating radiation under M1 closure in general relativistic conservative fluid dynamics codes

Abstract: A numerical scheme is described for including radiation in multi-dimensional generalrelativistic conservative fluid dynamics codes. In this met hod, a covariant form of the M1 closure scheme is used to close the radiation moments, and the radiative source terms are treated semi-implicitly in order to handle both optically t hin and optically thick regimes. The scheme has been implemented in a conservative general relativistic radiation hydrodynamics codeKORAL. The robustness of the code is demonstrated on a number of test problems, including radiative relativistic shock tubes, static radiation p ressure supported atmosphere, shadows, beams of light in curved spacetime, and radiative Bondi accretion. The advantages of M1 closure relative to other approaches such as Eddington closure and flux-limited di ffusion are discussed, and its limitations are also highlighted.

Radiation driven winds of hot luminous stars - XIV. Line statistics and radiative driving

Abstract: This paper analyzes the inter-relation be- tween line-statistics and radiative driving in massive stars with winds (excluding Wolf-Rayets) and provides insight into the qualitative behaviour of the well-known force-multiplier parameters kCAK; and , with special emphasis on . After recapitulating some basic properties of radiative line driving, the correspondence of the local exponent of (almost) arbitrary line-strength distribution functions and , which is the ratio of optically thick to total line-force, is discussed. Both quantities are found to be roughly equal as long as the local exponent is not too steep. We compare the (conventional) parameterization ap- plied in this paper with the so-called Q-formalism in- troduced by Gayley (1995) and conclude that the latter can be applied alternatively in its most general form. Its \strongest form", however (requiring the Ansatz Q = Qo to be valid, with Qo the line-strength of the strongest line), is justied only under specic conditions, typically for Supergiants with Te > 35 000 K. The central part of this paper considers the ques- tion concerning the shape of the line-strength distribution function, with line-strength kL as approximate depth in- dependent ratio of line and Thomson opacity. Since kL depends on the product of oscillator strength, excitation- and ionization fraction as well as on elemental abundance, all of these factors have their own, specic influence on the nal result. At rst, we investigate the case of hydrogenic ions, which can be treated analytically. We nd that the expo- nent of the dierential distribution is 4=3 corresponding to =2 =3, as consequence of the underlying oscillator strength distribution. Furthermore, it is shown that for trace ions one stage below the major one (e.g., Hi in hot winds) the equality+ 1 is valid throughout the wind. For the majority of non-hydrogenic ions, we follow the statistical approach suggested by Allen (1966), rened in