Most Rad Alt antennae are susceptible to moisture and should be protected from such. The proper operation of an antenna is dependent on there being good electrical bonding between the antenna and the aircraft. Moisture can affect this bonding. A ring of aerodynamic sealant is put around the antenna after installation to prevent water ingress, but this may deteriorate with age.
When pilots log known false GPWS warnings, the engineer usually removes the appropriate antennae and inspects the antennae and fuselage for corrosion and other signs of water ingress. After the inspection, and assuming the corrosion is minimal, the antennae-fuselage contact surfaces are cleaned and treated with a cocktail of chemicals and the antenna is refitted. Aerodynamic sealant is then applied around the edges of the antenna/e. The sealant may later be painted with non-metallic paint (so as not to interfere with the radio signals)... Noting here that there have been been cases of painters not using the correct paint.
It may be possible that the transmitted RA microwave signals reflect off heavy rain just as weather radar would. However, I haven't, as yet, seen anything in writing from the manufacturer of these systems relating to this.
Rgds.
Q.