One- and Three-Dimensional radiative effects in shallow cumulus cloud fields: Large-Eddy Simulations and Observations

Clouds strongly regulate how much radiation reaches the earth’s surface. Due to scattering and absorption processes within clouds, surface radiation can be highly variable both in space and time. Accurately capturing the high surface heterogeneity of solar radiation is important, for example for grid operators who need to ensure the safe ingestion of solar energy into the energy grid. In Numerical Weather Predictions and Large-Eddy-Simulations, radiation is often computed using one-dimensional radiative transfer, even though this can produce unrealistic surface irradiance fields. Applying three-dimensional radiative transfer is much more accurate because of the possibility of the horizontal transfer of radiation. However, applying three-dimensional radiative transfer in simulations is for most applications still too expensive.

In this study we investigated cloud-radiation interactions by applying either one-dimensional or three-dimensional radiative transfer using Large-Eddy-Simulations. The performance of the respective simulations were weighted against high quality surface observations from the Baseline Surface Radiation Network at Cabauw. We simulated/ observed a case representing a summer’s day in the Netherlands where shallow cumulus developed late in the morning and dissipated again later in the afternoon.

Simulations where one-dimensional radiative transfer was applied did not reproduce the observations well. Although being over a factor ten slower, simulations with three-dimensional radiative transfer applied did resemble observations much better. Still, for the simulations with three-dimensional radiative transfer the underestimation of diffuse radiation was considerable.   

To speed up the three-dimensional radiative transfer simulations we investigated an option that allowed the radiative transfer calculations to not fully converge. Using this incomplete solve option we found that biases in global radiation, sensible and latent heat remained smaller than for one-dimensional radiative transfer simulations, when we compared both to a fully solved three-dimensional radiative transfer run.   

With this work, we aimed to provide an insight into the relevance of three-dimensional radiative transfer application in shallow cumulus cloud fields emphasizing the need for further development of computationally efficient three-dimensional radiative transfer solvers in LES.