Coupled 3D radiation deepens cumulus clouds without changing the mean surface solar irradiance

Most atmospheric models consider radiative transfer only in the vertical direction (1D), as 3D radiative transfer calculations are too costly. Thereby, horizontal transfer of radiation is omitted, resulting in incorrect surface radiation fields. Previous work on 3D radiative effects mainly used uncoupled 3D radiative transfer. In our current work, we study the impact of coupled 3D radiative transfer on the development of clouds, and the resulting impact on the domain average surface solar irradiance.

We performed a series of realistic Large-Eddy simulations with MicroHH. We developed the option to use aerosol data from the CAMS global reanalysis to include the interactions between aerosols and radiation in our LES simulations. This makes sure our simulated radiation is in line with observations. To investigate the impact of 3D radiative transfer, we selected 12 days on which shallow cumulus clouds formed over Cabauw, the Netherlands. For each day, we performed simulations with 1D radiative transfer and with a coupled ray-tracer. The simulations with the coupled ray-tracer also include the results of uncoupled 1D radiative transfer. This allows us to compare the differences between 1D and 3D radiative transfer when the clouds are the same.  

In general, our simulations with coupled 3D radiative transfer have a higher domain average liquid water path compared to our simulations with coupled 1D radiative transfer. The cloud cover is similar in both simulations, but the cloud size is increased in the simulations with coupled 3D radiative transfer. For the domain average radiation, we find that 3D radiative transfer in general decreases the direct radiation and increases the diffuse radiation, but the net effect is on average less than 1 W m^-2. We can explain the differences in radiation when we look separately at the direct and diffuse radiation, the uncoupled 3D effects, and the impact of the change in the clouds. The uncoupled effect of 3D radiative transfer is an increase in global radiation, which is counteracted by a decrease is global radiation caused by the change in clouds.