Airbus A320 Series FCOM Bulletin 36/2 dated Apr 99 entitled 'Radio Altimeter Anomalies During Adverse Weather Conditions' goes into this. Unfortunately, I don't have the technology or the know how to post an extract, but suffice it to say that you can expect returns to be generated due to reflection from 'hail clouds or heavy rain. If energy received by the radalt is powerful enough, it will be validated and a height lower than the distance to the ground will be measured and sent to system users.'
Perhaps somebody could post a fuller extract from CD ROM.
Hope this helps.
Cheers
mcdhu