Cars will use LIDAR, ultrasonic, and video for first party sensing.
But a lot of sensing will come from their party. Other cars reporting their position on the road. Their intent and upcoming moves. Details about the environment that they sense. Additional the infrastructure can provide info. The roads can report if cars are present. If there is ice, etc.
Anyways, the issue isn't if there is ice or snow on it's sensors. That will be easy to mitigate. The problem is the ice and snow in the environment that it needs to see through.