To study the radiation in the Earth's atmosphere and its interaction with water and ice clouds, aerosols, and molecules, we are developing and using several radiative transfer models, including a Matrix-Operator model (MOM), the libRadtran package, and the three-dimensional MYSTIC code.
LibRadtran is a comprehensive radiative transfer package which has been jointly developed by Bernhard Mayer (IPA) and Arve Kylling (NILU, Norway) over the last 10 years. It covers the complete solar and terrestrial spectral ranges (see Figure) and is our main tool to study the interaction between clouds and radiation and to develop new remote sensing methods. It includes several radiative transfer solvers for different purposes, e.g. the well-characterised DISORT code (Stamnes et al., 1988), a polarisation dependent code polRadtran, a fast two-stream model, and finally the three-dimensional MYSTIC solver described below. It provides a flexible and user-friendly interface to define a wide range of input properties, in particular, trace gas profiles, surface properties, water and ice cloud microphysics, and several others more. It has been extensively used by the authors for simulations of ultraviolet radiation under clear and cloudy conditions (Mayer et al., 1997, 1998a), for studies of the influence of inhomogeneous surface conditions on ultraviolet radiation (Mayer and Degünther, 2000; Kylling et al., 2000; 2001), for the determination of UV irradiance and actinic flux from satellite observations (Mayer et al 1998b; 1998c), to test and improve the radiation parameterisation in a climate model (Marquart and Mayer, 2002), and for the development of cloud remote sensing algorithms (Gonzalez et al., 2002). libRadtran is Open Source under the GNU Public License and is freely available at http://www.libradtran.org. It has been used by several groups for a variety of applications and has been validated in several model intercomparisons (Koepke et al., 1998; van Weele et al, 2000; DeBacker et al., 2001) and by comparison with experimental data (Mayer et al, 1997).
The Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres (MYSTIC) has been developed by Bernhard Mayer (Mayer, 1999, 2000). MYSTIC considers inhomogeneous clouds as well as any kind of surface heterogeneity, that is, a 2D distribution of the surface albedo or BRDF(bidirectional reflectance distribution function) and even topography (Mayer and Degünther, 2000; Kylling and Mayer, 2001). MYSTIC incorporates the physics of scattering, absorption by molecules and aerosol and cloud particles, and reflection at the surface without any simplifying assumptions. Hence, the result can be considered exact for the given input conditions, within some statistical noise which can be accurately determined. All radiation quantities are calculated by tracing many photons (typically between 10,000 and 100,000,000) on their random paths through the atmosphere. MYSTIC has been compared to other three-dimensional radiative transfer models in the Intercomparison of 3D Radiation Codes (I3RC) where it belonged to a group of a few models that agreed with in 1% in every case.