Impact of model grid spacing on the feedback between shortwave three-dimensional radiative transfer and an isolated nonprecipitating cumulus

This study investigates the dependency of the shortwave three-dimensional radiative transfer (3D RT) effects upon the grid width when simulating an isolated cumulus. A meteorological model coupled with a 3D RT model enables us to investigate the dependency through simulations which consider feedback of the 3D RT to the atmosphere. Using the coupled model, simulations of the isolated cumulus were conducted for grid widths ranging between 25 m and 500 m. The same series of simulations were also conducted using a conventional one-dimensional (1D) radiative transfer model and were compared with the results by the 3D RT model. The comparison clarifies that the discrepancy in liquid water path between the two RT schemes becomes more prominent with smaller grid widths. The comparison also indicates that the difference in the radiative heating is negligible for grid spacing of 500 m but was substantial for finer grid spacings. These results suggest that the 3D RT model is required to accurately simulate the feedback between clouds and shortwave radiation for an isolated cumulus with grid widths of at least 250 m or smaller, when the lateral edge of the cloud is spatially resolved.