The ray tracer won't help your terrain in the slightest. Using RTE is best for scenes that; You cannot see the ground or, the ground is completely covered with objects or, you are rendering very distant terrain that you don't need to see great detail on.
In general, Planetside's advice in the RT advice thread is good. Use RTO for a scene like this, and, you really don't even need that checked, since you haven't any objects in it at all.
The rasterizer is your best bet here. Higher cloud quality samples will remove the grain from the clouds, higher atmosphere samples will remove the grain from the atmosphere.
The main problem I see here is that the shadowed areas of the clouds are a bit noisy on the rasterizer render. To remove this, atmosphere samples are required, not more cloud samples.