Development of three-dimensional Monte Carlo radiative transfer model for analysis of cloud-resolving radiation budget

Clouds have greenhouse effects that prevent cooling of ground surface and lower atmosphere by absorbing terrestrial infrared radiation, along with cooling effects by blocking the solar radiation. Those effects play an important role in determining the Earth's radiative energy budget which varies regionally and seasonally. Especially, complex geometry and inhomogeneity of clouds affect significantly on the radiative energy distribution of the solar and terrestrial radiation. Modeling of three-dimensional radiative processes is key issue for reliable simulations of cloud-resolving radiation budget. In this study, three-dimensional atmospheric radiative transfer model has been developed for the purpose of evaluating the cloud-resolving radiation budget. Many broadband calculations covering solar and terrestrial radiations in complex geometry and inhomogeneous cloud system are required for the reliable evaluation. In addition, multiple-scattering, absorption, and emission effects should be taken into account properly to radiative transfer process. Those requirements to radiative transfer model tend to make it complicated and time-consuming scheme. Monte Carlo method has been employed as a basic scheme in this study because the method is easily applicable to complex three-dimensional system rather than explicit analytical radiative transfer scheme. Especially, the dependent sampling approach enables simultaneous calculations at multi-wavelength, which is suitable to broadband calculation based on the correlated-k distribution method. Performance, validation, and application of the Monte Carlo radiative transfer model will be discussed from the point of view of evaluating cloud-resolving radiation budget.