A New Spectrally Integrating 3D Monte Carlo Radiative Transfer Model

A process level understanding of the interactions between clouds, aerosols, and radiation is integral to understanding our changing climate. Some work has already been done to show the importance of 3D effects in redistributing radiative heating in the atmosphere and biasing satellite retrieval algorithms of quantities such as cloud effective radius and aerosol properties. However, there is much more still to be done.

The Intercomparison of 3D Radiation Codes Community Monte Carlo model (I3RC) has been extended from its original monochromatic solar source form to include monochromatic thermal emission and broadband integration. A common method of spectral integration is via band models or k-distributions. This new model's unique feature is a Monte Carlo approach to spectral integration, randomly sampling frequencies from a probability distribution determined by either the solar source function, or the thermal emission function unique to the emitting properties of the prescribed surface and atmosphere. This provides a statistically exact solution with unbiased random error whose magnitude is dependent on the number of contributing samples. Benchmarks have been performed against analytical solutions and numerical solutions from independent models.

Potential applications of the model include simulating retrieved radiances by convolving a specific instruments' spectral response function with the solar or thermal emission spectral probability distributions and simulating heating rates in complex atmospheres. Another unique feature of the model is the planned public release, making it one of the only 3D broad-band Monte Carlo radiative transfer models freely available for use and development.