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. Plain Language Summary Solar radiation warms the surface and provides energy for evaporation and biological processes, resulting in the release of heat and moisture to the atmosphere. This upward transport of warm and moist air eventually leads to the formation of clouds, which then alter the spatial distribution of solar radiation at the surface by partly reflecting and absorbing the incoming sunlight. Most previous studies that simulated these complex interactions between clouds, solar radiation, and the surface used 1-D radiation models. These are faster than 3-D radiation models but produce unrealistic surface solar radiation fields by only considering the vertical propagation of radiation. In this study, we use a relatively fast 3-D radiation model to simulate the formation of clouds and the surface heat and moisture fluxes. In our simulations, 3-D radiation results in thicker and wider clouds than 1-D radiation, predominantly because clouds no longer shade the surface beneath them when radiation propagates under an angle. Unlike in simulations with 1-D radiation, we find higher surface fluxes below clouds than under clear-sky and higher surface fluxes with increasing cloud thickness in simulations with 3-D radiation. Our results show that 3-D radiation may strongly impact the coupling between clouds and the land surface.