  We present simulations of cumulus convection over land with shortwave
radiation computed by a Monte Carlo ray tracer coupled to the flow solver. Ray
tracing delivers very realistic in-cloud heating rates and global horizontal
irradiance fields. The necessary performance of the ray tracer has been enabled
by the raw power of GPU computing and from techniques for accelerating data
lookup and ray tracer convergence. We used a case study over a grassland in the
Netherlands to compare simulations with a coupled ray tracer to those with a
conventional two-stream solver, and to test ray tracer convergence. We
demonstrate that the simulated cloud evolution is insensitive across a wide
range of samples sizes in the ray tracer. Furthermore, simulations with a
coupled ray tracer produce surface irradiance patterns that resemble
observations and that strongly feed back to the evolution of clouds via locally
enhanced surface heat fluxes.
