Although I can't answer that query, I have had false returns from a rad alt from very heavy rain. I had an occasion near Tangier back in the 80s, in a 737-200, when at around FL150 in torrential rain the rad alt was giving wildly fluctuating readings for several minutes. We had GPWS fitted but I can't remember whether it reacted. It does show that the rad alt will see things other than the surface.