Was there rainbows before the biblical flood? NO! God said the bow in the sky was a contract and so its impossible that it existed before the flood. it was a new creation otherwise it makes no sense to use a existing thing as a new contract expression.
This suggests there was no rain before the flood. it suggests there might of been a different system in the earth that didn't allow rainbows even at waterfalls of any way we now can create them.
some strange different way the earth systems was held in.
it also suggests the rainbow being seen while the rain had long stopped indicates the air was very misty still.
Genesis provides the data to see that the world has not been in a constant equation of its present system. tHINGS changed.