Spatial-scale Characteristics of Three-dimensional Cloud-resolving Radiation Budget by Monte Carlo Radiative Transfer Simulations

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, in cloud-resolving scale, complex geometry and inhomogeneity of clouds affect significantly on three dimensional radiative energy budget of the solar and terrestrial radiation. Modeling of three-dimensional radiative processes and its spatial-scale characteristics is key issues for reliable simulations of cloud-resolving system.

In this study, three-dimensional atmospheric radiative transfer model has been developed for the purpose of evaluating the cloud-resolving radiation budget. Broadband calculations covering solar and terrestrial radiations are required for the reliable estimates of radiative energy budget. 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 both broad- and narrow-band calculation based on the correlated-k distribution method. The Monte Carlo radiative transfer model was applied to cloud scenes calculated by large eddy simulation model, and cloud-resolving radiative energy budgets were estimated for several different spatial-scales. Performance of the Monte Carlo radiative transfer model and the spatial-scale characteristics of three dimensional radiation effects will be discussed from the point of view of cloud-resolving radiation budget.