Re airborne dry ice crystals. With most weather radars it is wise to tune the Gain control from "auto" to "Max" when flying at high altitudes in IMC. Because the tops of CB consist of dry ice crystals, they do not reflect on the radar screen.
Max gain gives a more powerful beam (not a very accurate description) and providing the tilt is set accurately, a CB top may show up as a tiny echo instead of no echo in Gain auto. Once the CB top is detected in this manner, then a judicious combination of gain control and tilt and range will allow the crew to "see" the weather problem.
During the investigation into the Mount Erebus (Antartic) Air New Zealand DC10 disaster, some criticism was made of the perceived failure by the crew to use their weather radar to pick up the slopes of Mt Erebus. Investigation revealed that even if the crew did have their radar operating, the mountain would not have been displayed on radar because the mountain was covered by solid ice and therefore zero reflection.