RE: WX Radar
I'm an old radar guy (not a pilot) and I'd like to offer the following.
Display Color Rainfall Density
RED 12mm/hr or greater (about 1/2 inch/hr)
YELLOW 4 - 12mm/hr
GREEN 1 -4mm/hr
The Doppler mode detects the mezzo-cyclones within a cell by measuring the horizontal motion of the raindrops. The RDR-4A used a dual pulse signal for doppler. It measured the change in spacing (time) between the transmitted pulse pair and its reflected pulse pair. It could detect anything greater than 6 meters per second and display that in MAGENTA.
For those of you who like to play with GAIN control (if you have one) should know that when you change gain your display colors are no longer CALIBRATED!!
Regarding AF447, if they did encounter ice, it's possible that ice may have accumulated on the radome, and potentially may have attenuated the radar signal. That could explain how the radar could become inaffective without an actual failure.
I flew with a DC-10 Captain some years ago who asked me why the radar quit just when he needed it the most. He had attempted to land at IAH and hit what he said was a solid wall of water. He told me that the radar displayed a narrow band of noise and nothing beyond. Well, a solid wall of water would certainly attenuate the radar's signal, hence no returns beyond.
Hope this helps a little.