Three-Dimensional Radiative Effects By Shallow Cumulus Clouds on Dynamic Heterogeneities Over a Vegetated Surface

One-dimensional radiative transfer solvers are computationally much more efficient than full three-dimensional radiative transfer solvers but do not account for the horizontal propagation of radiation and thus produce unrealistic surface irradiance fields in models that resolve clouds. Here, we study the impact of using a 3-D radiative transfer solver on the direct and diffuse solar irradiance beneath clouds and the subsequent effect on the surface fluxes. We couple a relatively fast 3-D radiative transfer approximation (TenStream solver) to the Dutch Atmosphere Large-Eddy Simulation (DALES) model and perform simulations of a convective boundary layer over grassland with either 1-D or 3-D radiative transfer. Based on a single case study, simulations with 3-D radiative transfer develop larger and thicker clouds, which we attribute mainly to the displaced clouds shadows. With increasing cloud thickness, the surface fluxes decrease in cloud shadows with both radiation schemes but increase beneath clouds with 3-D radiative transfer. We find that with 3-D radiative transfer, the horizontal length scales dominating the spatial variability of the surface fluxes are over twice as large as with 1-D radiative transfer. The liquid water path and vertical wind velocity in the boundary layer are also dominated by larger length scales, suggesting that 3-D radiative transfer may lead to larger convective thermals. Our case study demonstrates that 3-D radiative effects can significantly impact dynamic heterogeneities induced by cloud shading. This may change our view on the coupling between boundary-layer clouds and the surface and should be further tested for generalizability in future studies.