I'm not sure if Polyphony has ever explained in technical detail how they create their rain and weather options, or how complex the water accumulation is simulated in GT7.
I suppose they use some kind of "water depth map," like a texture layer that tells the game where water builds up the most. The game engine probably "blends" between the dry and wet conditions. This is how they create the dangerous areas for aquaplaning, and it's also why the main racing line dries out faster than the rest of the track. This depth map is probably painted by artists following the advice of drivers who have raced on these tracks under heavy rain conditions.
But for a track like Yas Marina, I don't think they would even know where to begin. How would they create that "water depth map"?
I doubt there is much detailed data available, simply because rain isn't normal there. This means you can't rely on drivers' real-life experience in those situations to help create the water map manually.
Furthermore, simulating the water buildup from scratch must be very complex. You would have to consider how different surfaces soak up water. For example, grass should absorb rain better than asphalt, and then there are other surfaces like gravel or even special types of asphalt that let water pass through.
Of course, you can also skip these niceties and simulate a uniform water depth and, in any case, a faster drying of the path, but I think that at Polyphony they wouldn't be satisfied with this simplification and would prefer not to have rain at all rather than have it poorly simulated.
