A cloud-resolving model with the radiation scheme based on the Monte Carlo method

A fully compressible cloud-resolving model was developed based on the MacCormack numerical scheme with a shortwave radiation scheme based on the Monte Carlo method. On the basis of the results from a numerical experiment with a buoyant bubble in the neutral atmosphere, the model exhibited close correlation with other similar compressible models. The radiation scheme was evaluated using the data reported by Nicholls [Nicholls, S., 1984. The dynamics of stratocumulus: aircraft observations and comparison with a mixed layer model. Q. J. R. Meteorol. Soc. 110, 783–820.]. Use of the model permitted accurate reproduction of the structure of measured shortwave radiation fluxes. Various accuracy and sensitivity tests were conducted. It was found that more than 10,000 photons were needed to provide sufficient accuracy. The present Monte Carlo radiation scheme appeared to be computationally more efficient than the Streamer model with the eight streams option. The sensitivity tests indicated that the optical thickness was the most critical parameter in the calculations of shortwave radiative fluxes. A comparison of radiation flux fields for broken- and solid-cloud cases showed significant differences confirming that the Monte Carlo method is the most appropriate for flux calculations in a broken-cloud case.