How about a randomised computer program that will turn sprinklers on anywhere from a light drizzle to heavy rain and at a random time within the race. Have sprinklers around the circuit that are also random so you could have rain on one part of the circuit and not another.
This way you could have a race with anything from no rain at all during the event, to constant heavy rain. Teams would start to design for that, but at least that design would tend to be for a general good performer in all conditions.
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