Iain_W
These approaches will reduce the clutter in the returned signal but they do not change the attenuation of the radar outbound or reflected signal as it passes through rain. If the rain is heavy enough the radar signal will literally not get through. There are things that can be played with like changing the polarization of the radar signal but they don't solve the attenuation problem. An analogy is dense fog - To radar rain is like dense fog you can play with yellow headlights or blue headlights but if the fog is dense enough you will not be able to increase the visibility by a lot.
The approaches suggested in research are variants of multisensor tracking where a complete 4D picture is built up using ground radars. See NSSL Projects: Multi-Radar/Multi-Sensor System (MRMS). These pictures may then be sent to the flight deck through Aircraft Access to SWIM (System Wide Information Management). This is in the Continental United States. It may be that other areas could do the same but it is expensive.
Other approaches have looked at taking all the aircraft radars and mosaicing them into a 4D picture. But again someone has to do it and then find someone willing to pay for it. Guess what the beancounters won't hear of it.
Earned a bookmark for the next time we get weather here. Another promising option would be space based RADAR eliminating the need for additional RADAR equipment on the planes.
http://cup.aos.wisc.edu/will/im_and_durden2005.pdf
Every new technology faces the cost vs benefit decision, but if accidents prove to have been avoidable the cost has a way of becoming affordable.