Worth remembering that ice can even form on an airframe in above-0 conditions if it flies into freezing rain dropping from above.
(I have a feeling of deja-news about this... Has it been discussed recently elsewhere?)
I'm not sure that's likely. The freezing rain is rain because at some point it was in above-zero air at high level. It then fell into below-zero air to become supercooled. Surely the issue is meeting it in that below-zero air, where it will freeze on contact with your airframe? I'm not sure it poses such a threat to an airframe above zero once it has dropped further into above-zero air.