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Welcome to the IREX question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
The type of ice that forms depends to a large extent on how rapidly the supercooled droplets freeze after impact. Large droplets take a little longer to freeze especially if they are just below freezing and not extremely cold. The extra time taken to freeze allows the droplets to spread and flow across the wing before freezing. The question describes clouds with stronger updrafts (thick) that support larger droplets with temperature just below freezing. That's clear icing.